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Probing and Manipulating the Interlayer Coupling in 2D Structures
by Zachariah Boston Hennighausen
Bachelor of Science in Electrical Engineering, United States Coast Guard Academy
A dissertation submitted to
The Faculty of
the College of Science of
Northeastern University
in partial fulfillment of the requirements
for the degree of Doctor of Philosophy
May 02, 2019
Dissertation directed by
Dr. Professor Swastik Kar
Associate Professor of Physics
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Acknowledgments
The list of people to thank and acknowledge who have made this dissertation and work possible
is extensive to say the least. The indirect contributions have been equally as influential as the
direct contributions, and often small acts have had an outsized influence. This is in part due to
the nature of science, where progress is often non-linear and unpredictable, and unexpected
sources can stimulate significant discoveries. However, it is also due to the fact that our work
lives are not decoupled or independent from our personal lives. The people who have developed
or assisted me with non-science endeavors, both during and before starting graduate school, has
arguably been more influential than those related to science. The list is surely extensive and I am
appreciative of everyone. Although I want to thank every person in this section, an important
component of science writing is the ability to compress a large volume of data into a concise piece,
and I will attempt apply this here.
Friends and shipmates are constantly challenging me with new adventures that inspire creativity
and allow for reflection periods. Additionally, they have generously inquired into my work with
difficult and probing questions that have improved not only my own understanding, but also
how to convey and explain the work. Dr. Albert Einstein is credited with saying,
“If you can’t explain it simply, you don’t understand it well.”
An interested audience offers scientists an opportunity to explain and teach their research, a
process that can improve the scientist’s own understanding. One such notable occasion was in
Tahoe, CA in 2018 where very late into the night after an extreme day, some friends asked to hear
my APS March Meeting presentation on my work, and honored me by actively engaging with
my presentation. These friends and shipmates include: Travis Murray for teaching me of the
wealth of opportunities that exist globally; Emily Theresa Sonner Kehrt for engaging discussions
that probe the soul and foundation of my research; Kip Hutchinson for inspiring creativity using
unlikely sources; and Kevin Sorrell for highly technical discussions on a spectrum of topics. I
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would also like to thank Luke Bruns, Jason Acuna, Nicole Bredariol, Tommy Adams, Nicholas
Custer, and, to a very indirect extent, Rossi.
Both my extended and immediate family have provided guidance, mentorship, sage advice, and
counsel on a range of questions and obstacles directly related to my research, but also outside of
it. By solving problems and challenges more quickly, it freed-up energy, time, and mental stamina
for me to focus on research. There is no doubt that without them this dissertation would not be
where it is today. In particular, I would like to thank Molly Hennighausen and Hannah
Hennighausen for highly engaging discussions on a range of topics, the fruitful feedback, and
wise advice. I would also like to thank Ingrid Butler, Frank Butler, Maija Butler, and Lisa Butler
for providing feedback and smart insight on a spectrum of topics. In particular, it was Ingrid
Butler who alerted me to the history of the moiré pattern and its relation to the fabric (a fact that
shaped my writing), and Frank Butler for providing unique insight into the properties of bulk
MoS2.
My parents, Dr. Lothar Hennighausen and Dr. Prof. Priscilla Furth, deserve not only their own
paragraph acknowledging their support, but an entire dissertation. However, they likely would
advise me not to invest too much time thanking them, and to keep this paragraph practical and
short. They have built the foundation for the person I am today, and they have continued to guide
me throughout every step of my life. I have only achieved what I have due to their guidance and
hard work.
I have worked with numerous scientific groups spanning a variety of disciplines throughout my
Ph.D., and have sent samples to many laboratories located across the globe. Frequently these
collaborations led to a fruitful exchange of bold ideas, and in a few instances we were able to
produce exciting results that are currently published or will be soon. I am very thankful to all my
collaborators. In particular, I would like to thank Dr. Prof. Arun Bansil at Northeastern University
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and his group, which focuses on theoretical condensed matter physics. I believe that some of the
strongest scientific discoveries are when theory and experiment collaborate, and I feel very
fortunate I was able to collaborate with such an incredible theory group. They have been not only
very supportive of my experiments, but they also played a critical role in developing me as a
physicist. I worked mostly with Christopher Lane and Dr. Gianina Buda, and will cherish their
friendship forever.
During my Ph.D. I was fortunate enough to learn a large number of techniques and how to
operate a variety of complex scientific equipment, and this was only possible due to the hard
work of my lab mates. I would like to thank the following people for assistance with various
experimental techniques: Dr. Fangze Liu, Dr. Anthony Vargas, Dr. Ismail Bilgin, Dr. Daniel
Rubin, Colin Casey, Monika Eggenberger, Kevin Mendez, Dr. Pradeep Waduge, Dr. Wentao
Liang, and Vineet Mathur.
I would argue that I might have had one of the best Ph.D. advisors that exist, and am forever
exceptionally grateful for all his hard work, guidance, mentorship, patience, and friendship. Dr.
Prof. Swastik Kar always strived to go above and beyond, and put being a good mentor above all
else. He provided me with all the resources, freedom, and opportunities needed to be successful.
If I began to stray too far into the scientific jungle and was in danger of getting lost, he made sure
to guide me back.
Lastly, I would like to thank Kristi Butler (Buthausen) for being the force that made all this
possible. On a daily basis she provides sage wisdom for challenges large and small, and has gone
to impressive lengths to help me. She has been nothing less than an irreplaceable teammate and
partner, and there is no doubt most of the thanks and acknowledgment goes to her. And, finally,
our two furry roommates, Madame and Landau, deserve acknowledgment; they were always
there during both the sunny days and the colder nights.
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Abstract of Dissertation (English)
In 2004, researchers reported the synthesis of planar graphene in the free state, overturning a
decades-old prediction that 2D crystals were too thermodynamically unstable to exist in ambient.
Since then, dozens of new monolayer crystals have been demonstrated, each with their own
properties and capabilities. Very interestingly, as these monolayer materials are vertically stacked
to create 2D structures, the properties of the structure are not simply a sum of the parent crystals,
but a product of the interlayer coupling, which can redistribute charge, modify the band
structures, and induce new properties unique to either parent crystal. The term interlayer
coupling refers to the communication and interaction that exists between the parent materials,
where the absence of a coupling suggests the parent materials are electronically independent.
Understanding and controllably manipulating the interlayer coupling holds promise to not only
engineer new capabilities, but to also retain desired properties that only exist in monolayer
materials. This dissertation explores the interlayer coupling in a variety 2D structures, with an
emphasis on a particular class of 2D structures: monolayer bismuth selenide (Bi2Se3) grown on
monolayer transition metal dichalcogenides (TMDs), using vapor-phase chalcogenization. The
physical charge redistribution induced by the interlayer coupling, and its effect on the band
structure, was studied in as-grown samples. The interlayer coupling was then probed and
manipulated using both electron beams in a vacuum, and the controlled absorption and
desorption - possible intercalation and deintercalation - of oxygen. The results suggest that the
properties could be drastically and controllably altered by varying the interlayer coupling
strength.
Chapter 1 is an introduction and background to monolayer materials and 2D structures, as well
as bismuth selenide (Bi2Se3) and transition metal dichalcogenides (TMDs), the parent materials
for the primary class of 2D structures studied in this dissertation.
Chapter 2 is instrumentation and synthesis, where all the important methods and equipment are
discussed, so as to enable reproduction of the work.
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Chapter 3 demonstrates that the interlayer coupling between dissimilar 2D heterostructures can
be probed in-situ by manipulating the twist angle and structure using the focused electron beam
of a transmission electron microscope (TEM). The electron beam imparts energy that induces the
Bi2Se3 to break-up into grains, and for those grains to twist relative to the underlying monolayer
TMD.
Chapter 4 demonstrates oxygen-induced in-situ manipulation of the interlayer coupling and
exciton recombination in Bi2Se3/MoS2 2D heterostructures. This chapter studies the possible
intercalation and deintercalation of oxygen using both experimental and theoretical methods,
providing justification for the claims, as well as other possible explanations for the manipulation
of the interlayer coupling.
Chapter 5 demonstrates the tunable photoluminescence in Bi2Se3/TMD 2D heterostructures for
write-read-erase-reuse applications. Mono- and few-layer Bi2Se3 was grown on four different
TMDs – MoS2, MoSe2, WS2, and MoSe2-2xS2x – and the interlayer coupling was manipulated
possibly using the controlled intercalation and deintercalation of oxygen. Potential technologies
include ultra high-density information storage, and tunable photoluminescing pixels (PLPs).
Chapter 6 demonstrates evidence of a purely electronic two-dimensional lattice at room
temperature that resides between the parent layers of a Bi2Se3/TMD 2D heterostructure. The
results suggest that the interlayer coupling is inducing significant charge redistribution, a
surprising result considering that that interlayer bonding in their bulk counterparts is “weak”
van der Waals.
Chapter 9.1 studies and briefly discusses work related to the interlayer coupling in Non-Bi2Se3 2D
structures that include, graphene/MoS2, graphene/graphene, Sb2Se3/MoS2, and Bi2Te3/MoS2. The
results suggest that while interlayer coupling is important in other 2D structures, the Bi2Se3/TMD
2D structures have a relatively strong interlayer coupling.
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Abstract of Dissertation (Deutsch)
Im Jahr 2004 publizierten Forscher über die Darstellung von planarem Graphen im freien
Zustand, und darbei widerlegte eine jahrzehntealte Vorhersage, dass 2D-Kristalle nachweislich
thermodynamisch instabil sind um im Außenumgebung zu existieren. Seitdem wurden
Dutzende neuer Monoschichtkristalle mit jeweils eigenen Eigenschaften und Fähigkeiten
demonstriert. Da diese Monoschichtmaterialien zur Erzeugung von 2D-Strukturen vertikal
gestapelt sind, sind die Eigenschaften der Struktur nicht einfach eine Summe der Stammkristalle,
sondern auch ein Produkt der Zwischenschichtkopplung, die Ladung umverteilt, die
Bandstrukturen modifiziert, und neue Eigenschaften erzeugt die nicht in die Stammkristalle zu
finden sind. Der Begriff Zwischenschichtkopplung bezieht sich auf die Kommunikation und
Interaktion zwischen dis Stammkristallen, wobei das Fehlen einer Kopplung darauf hindeutet,
dass die Stammkristallen elektronischunabhängig sind. Der Zwischenschichtkopplung zu
verstehen und kontrolibar zu manipulaieren verspricht, nicht nur neue Fähigkeiten zu
entwickeln, sondern auch gewünschte Eigenschaften beizubehalten, die nur in
Monoschichtmaterialien vorhanden sind. Diese Dissertation untersucht die Kopplung zwischen
Schichten in verschiedenen 2D-Strukturen (bzw. Zwischenschichtkopplung), wobei einer
bestimmten Klasse von 2D-Strukturen liegt unter dem Brennpunkt: Monoschicht-Bismutselenid
(Bi2Se3), das auf Einschicht-Übergangsmetalldichalcogeniden (TMDs) mit Dampfphasen-
Chalcogenisierung gewachsen ist. Die physikalische Ladungsumverteilung durch die
Zwischenschichtkopplung und ihre Auswirkung auf die Bandstruktur wurden in
selbstgewachsenen Proben untersucht. Die Zwischenschichtkopplung wurde dann unter
Verwendung von beider Elektronenstrahlen im Vakuum, und der kontrollierten Absorption und
Abscheidung - mögliche Interkalation und Deinterkalation - von Sauerstoff untersucht und
manipuliert. Die Ergebnisse legen nahe, dass die Eigenschaften durch Variation der
Zwischenschichtkopplungsstärke drastisch und kontrollierbar verändert werden könnten.
Kapitel 1 ist eine Einführung und Hintergrund zu Monoschichtmaterialien und 2D-Strukturen,
sowie Monoschicht-Bismutselenid (Bi2Se3) und Übergangsmetalldichalcogeniden (TMDs), die
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Stammkristalle für die 2D-Strukturen die in dieser Dissertation hauptsächlich untersuchte
werden.
Kapitel 2 ist Instrumentierung und Synthese, in der alle wichtigen Methoden und Geräte
besprochen werden, um die Reproduktion der Arbeit zu ermöglichen.
In Kapitel 3 wird gezeigt, dass die Zwischenschichtkopplung zwischen unähnlichen 2D-
Heterostrukturen in-situ durch Manipulation des Verdrehungswinkels und der Struktur unter
Verwendung des fokussierten Elektronenstrahls eines Transmissionselektronenmikroskops
(TEM) untersucht werden kann. Der Elektronenstrahl vermittelt Energie, die das Bi2Se3 dazu
veranlasst, sich in Körner aufzubrechen, und diese Körner verdrehen sich relativ zur
darunterliegenden Monoschicht- Übergangsmetalldichalcogeniden (bzw. Monoschicht-TMD).
Kapitel 4 zeigt die sauerstoffinduzierte In-situ-Manipulation der Zwischenschichtkopplung und
der Exziton-Rekombination (bzw. Elektron-Loch-Paar-Rekombination) in Bi2Se3/MoS2-2D-
Heterostrukturen. In diesem Kapitel wird die mögliche Interkalation und Deinterkalation von
Sauerstoff anhand experimenteller und theoretischer Methoden untersucht, um die Ansprüche
zu begründen, sowie andere mögliche Erklärungen für die Manipulation der
Zwischenschichtkopplung.
Kapitel 5 demonstriert die abstimmbare Photolumineszenz in Bi2Se3/TMD-2D-Heterostrukturen
für Schreiben-Lesen-Löschen-Wiederverwenden Anwendungen. Monoschicht und
Multischichten-Bi2Se3 wurde auf vier verschiedenen Übergangsmetalldichalcogeniden (TMDs) -
MoS2, MoSe2, WS2 und MoSe2-2xS2x - gezüchtet, und die Zwischenschichtkopplung wurde
manipuliert, möglicherweise unter Verwendung der kontrollierten Interkalation und
Deintercalation von Sauerstoff. Mögliche Technologien und Anwendungen umfassen ultradichte
Datenspeicher, und abstimmbare Photolumineszierendepixeln (PLPs).
Kapitel 6 zeigt den Nachweis eines rein-elektronischen zweidimensionalen Gitter bei
Raumtemperatur, das sich zwischen den Stammschichten einer Bi2Se3/TMD-2D-Heterostruktur
befindet. Die Ergebnisse deuten darauf hin, dass die Zwischenschichtkopplung eine signifikante
Ladungsumverteilung bewirkt, ein überraschendes Ergebnis, wenn man bedenkt, dass die
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Zwischenschichtbindung in ihren Massengegenstücken "schwach" van der Waals ist. Die Arbeit
könnte sich auch mit Zweidimensionalelektronkristalle befassen.
In Kapitel 9.1 werden die Arbeiten zur Zwischenschichtkopplung in Kein-Bi2Se3-2D-Strukturen,
darunter Graphene/MoS2, Graphene/Graphene, Sb2Se3/MoS2, und Bi2Te3/MoS2, beschrieben. Die
Ergebnisse legen nahe, dass die Zwischenschichtkopplung zwar in anderen 2D-Strukturen
wichtig ist, die Bi2Se3/TMD-2D-Strukturen jedoch eine relativ starke Zwischenschichtkopplung
aufweisen.
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Table of Contents
Acknowledgments .................................................................................................................................... 2
Abstract of Dissertation (English) ......................................................................................................... 5
Abstract of Dissertation (Deutsch) ........................................................................................................ 7
Table of Contents .................................................................................................................................... 10
List of Tables ............................................................................................................................................ 15
List of Equations ..................................................................................................................................... 16
Chapter 1: Introduction ..................................................................................................................... 17
1.1 Monolayer materials: the building blocks for 2D structures ............................................. 17
1.2 2D Materials: overview of specific monolayer materials ................................................... 20
1.3 2D Structures: combining monolayer crystals to build something new .......................... 23
1.4 2D Structures: a product of the interlayer interaction ........................................................ 30
1.5 Methods to manipulate the interlayer coupling in 2D structures ..................................... 35
Chapter 2: Instrumentation and Synthesis .................................................................................... 43
2.1 Growing 2D materials using Chemical Vapor Deposition (CVD) .................................... 43
2.2 Mechanically transferring 2D materials to create 2D structures ....................................... 44
2.3 Optical setup for Raman and photoluminescence spectroscopy ...................................... 45
2.4 Manipulating interlayer coupling in Bi2Se3/TMD 2D heterostructures using laser
exposure and thermal annealing ....................................................................................................... 46
2.5 Atomic Force Microscope (AFM) ........................................................................................... 48
2.6 Scanning Electron Microscope (SEM) ................................................................................... 48
2.7 Lithography: E-Beam, Photo, and Laser ............................................................................... 49
2.8 Transmission Electron Microscope (TEM) ........................................................................... 50
2.9 Electron Beam Treatment (EBT) Studies: Manipulating the twist angle and interlayer
interface ................................................................................................................................................. 50
2.10 Computational Methods ......................................................................................................... 52
2.11 Device Fabrication.................................................................................................................... 53
Chapter 3: Probing the interlayer interaction between dissimilar 2D heterostructures by in-
situ manipulation of twist angle and structure ................................................................................. 54
3.1 Monte Carlo Simulations Reveal Statistical Significance of Histograms ......................... 81
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3.2 Material Transformation vs. Twist Angle Manipulation .................................................... 84
3.3 Negligible Role of Potential Defects on the Interlayer Interaction ................................... 85
3.4 Discussion Grain Rotation Mechanisms ............................................................................... 87
Chapter 4: Oxygen-Induced In-Situ Manipulation of the Interlayer Coupling and Exciton
Recombination in Bi2Se3/MoS2 2D Heterostructures ....................................................................... 89
4.1 Discussion of Possible exciton recombination pathways ................................................. 112
4.2 Discussion on interaction of Bi2Se3 with oxygen ............................................................... 118
Chapter 5: Tunable Photoluminescence in Bi2Se3/TMD 2D Heterostructures for Potential
Write-Read-Erase-Reuse Applications .............................................................................................. 121
5.1 Laser-Annealing in Air, and Strong Stability of As-Grown Configuration ................... 139
Chapter 6: Evidence of a purely electronic two-dimensional lattice at room temperature 142
6.1 Diophantine equation and guidance on mathematically calculating the electronic moiré
lattice commensurate unit cell. ......................................................................................................... 150
6.2 Data demonstrating that the electronic moiré lattice was observed in other 2D
heterostructures .................................................................................................................................. 151
6.3 Brief explanation of double diffraction with respect to electron diffraction ................. 157
Chapter 7: Conclusion and Discussion ......................................................................................... 159
Chapter 8: Dissemination ................................................................................................................ 164
8.1 Publications ............................................................................................................................. 164
8.2 Patents ...................................................................................................................................... 165
8.3 Presentations - Conferences .................................................................................................. 165
8.4 Presentations – Invited Talks ................................................................................................ 165
Chapter 9: Appendix ........................................................................................................................ 167
9.1 Non-Bi2Se3 2D Heterostructures........................................................................................... 167
9.1.1 Graphene/MoS2 Heterostructures ................................................................................ 167
9.1.2 Graphene/Graphene Heterostructures........................................................................ 169
9.1.3 MoS2/Sb2Se3 Heterostructures ....................................................................................... 171
9.1.4 MoS2/Bi2Te3 Heterostructures ....................................................................................... 176
9.2 Programming Code ............................................................................................................... 178
9.2.1 Fitting Lorentzian Functions to Exciton Photoluminescence Spectra .................... 178
9.2.2 Raw Search for Commensurate Unit Cells and Moiré Patterns .............................. 182
References............................................................................................................................................... 185
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List of Figures
FIGURE 1.1: ARTISTIC DIAGRAM OF A GENERAL 2D MATERIAL ................................................................................. 17
FIGURE 1.2: (A) GRAPHITE AND (B) DIAMOND CRYSTAL STRUCTURES. ..................................................................... 19
FIGURE 1.3: ARTISTIC DIAGRAMS OF (A) GRAPHENE AND (B) A MONOLAYER TRANSITION METAL
DICHALCOGENIDE (TMD).................................................................................................................................. 20
FIGURE 1.4: EXPERIMENTAL AND THEORY BAND STRUCTURE FOR THE TOPOLOGICAL INSULATOR BI2SE3. ............. 22
FIGURE 1.5: ARTISTIC DIAGRAM SHOWING A REPRESENTATIVE 2D STRUCTURE. ...................................................... 24
FIGURE 1.6: GRAPHENE BAND STRUCTURE, AND DEMONSTRATION OF THE EVOLUTION OF THE BAND STRUCTURE
AS GRAPHENE LAYERS ARE STACKED. ................................................................................................................ 25
FIGURE 1.7: DEMONSTRATION HOW THE PL IS MODIFIED BY SIMPLY INCREASING THE LAYER COUNT OF MOS2 .... 27
FIGURE 1.8: ANGLE RESOLVES PHOTOEMISSION SPECTROSCOPY (ARPES) OF BI2SE3 ............................................... 29
FIGURE 1.9: ARTISTIC DIAGRAM ILLUSTRATING THE POSSIBILITIES OF DESIGNING VERTICALLY STACKED 2D
STRUCTURES ........................................................................................................................................................ 30
FIGURE 1.10: DENSITY FUNCTIONAL THEORY (DFT) CALCULATED BI2SE3/MOSE2 2D HETEROSTRUCTURE ............ 32
FIGURE 1.11: DFT CALCULATED 2D STRUCTURE WITH ONE WS2 LAYER ON THE BOTTOM AND ONE BI2SE3 LAYER
ON THE TOP ......................................................................................................................................................... 34
FIGURE 1.12: TUNING THE INTERLAYER INTERACTION AND COUPLING USING THE INTERCALATION OF MOLECULES
AND ATOMS. ....................................................................................................................................................... 36
FIGURE 1.13: VARIOUS METHODS THAT MODIFY THE INTERLAYER COUPLING IN 2D STRUCTURES. ........................ 38
FIGURE 1.14: DFT CALCULATED 2D STRUCTURES WITH ONE LAYER WS2 AND ONE LAYER BI2SE3 .......................... 40
FIGURE 1.15: TWO POWERFUL METHODS TO CONSTRUCT AND MANIPULATE 2D STRUCTURES ............................... 41
FIGURE 2.1: OPTICAL PICTURE OF BI2SE3 2D HETEROSTRUCTURES TRANSFERRED ONTO A TEM GRID .................. 51
FIGURE 3.1: ELECTRON BEAM PROBES INTERLAYER INTERACTION IN 2D HETEROSTRUCTURES. .............................. 56
FIGURE 3.2: AS-GROWN 2D HETEROSTRUCTURES PRIOR TO ELECTRON BEAM TREATMENT (EBT). ........................ 58
FIGURE 3.3: REPRESENTATIVE BI2SE3/TMD HETEROSTRUCTURES. ............................................................................ 59
FIGURE 3.4: TEM SAED PATTERNS OF FOUR 2D HETEROSTRUCTURES STUDIED, AS WELL AS BI2SE3 GROWN ON
SIO2. .................................................................................................................................................................... 60
FIGURE 3.5: TEM IMAGES OF DIFFERENT 2D HETEROSTRUCTURES. .......................................................................... 60
FIGURE 3.6 TUNING THE INTERLAYER INTERACTION THROUGH CHEMISTRY. HISTOGRAMS OF THE TWIST ANGLES
............................................................................................................................................................................ 62
FIGURE 3.7 MANIPULATING THE TWIST ANGLE IN-SITU USING AN ELECTRON BEAM. .............................................. 65
FIGURE 3.8 IN-SITU CRYSTAL MANIPULATION OF BI2SE3/MOSE2 USING AN ELECTRON BEAM. ................................. 68
FIGURE 3.9 BI2SE3/WS2 CONTROLLABLE CRYSTAL DISRUPTION, ANNEALING, AND ROTATION IN RESPONSE TO EBT.
............................................................................................................................................................................ 69
FIGURE 3.10 CONTROLLABLE CRYSTAL REARRANGEMENT IN-SITU USING AN ELECTRON BEAM. ............................ 70
FIGURE 3.11 SUMMARY OF DIFFERENT 2D HETEROSTRUCTURE’S RESPONSE TO AN EBT INTENSITY. ....................... 72
FIGURE 3.12 MANIPULATION OF THE OPTICAL AND PHOTOLUMINESCENCE PROPERTIES USING AN ELECTRON
BEAM. .................................................................................................................................................................. 74
FIGURE 3.13 MANIPULATION OF THE OPTICAL AND PHOTOLUMINESCENCE PROPERTIES USING AN ELECTRON
BEAM. .................................................................................................................................................................. 77
FIGURE 3.14 BI2SE3/MOSE2 AFFECTED WITH ELECTRON BEAM TREATMENT (EBT). .................................................. 78
FIGURE 3.15 TEXTURE AND WORK FUNCTION INDUCED CHANGES FROM EBT. ........................................................ 79
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FIGURE 3.16 MONTE CARLO SIMULATION: INSTANCE NOISE FLOOR VALUES. ........................................................ 82
FIGURE 3.17 MONTE CARLO SIMULATION DATA TO IDENTIFY THE INSTANCE NOISE FLOORS (INF) ....................... 83
FIGURE 3.18 DATA FROM THE MONTE CARLO SIMULATIONS USED TO CALCULATE THE INSTANCE NOISE FLOOR . 84
FIGURE 3.19: REPRESENTATIVE RAMAN SPECTRA OF A BI2SE3/MOS2 2D HETEROSTRUCTURE ................................. 86
FIGURE 4.1: FIGURE BRIEFLY SUMMARIZING THE CAPABILITIES DEMONSTRATED IN CHAPTER 4. ............................ 90
FIGURE 4.2: AS-GROWN BI2SE3/MOS2 VERTICAL 2D HETEROSTRUCTURES. ............................................................... 92
FIGURE 4.3: MANIPULATING THE INTERLAYER COUPLING USING OXYGEN AND ENERGY. ....................................... 93
FIGURE 4.4: ATOMIC FORCE MICROSCOPE (AFM) IMAGES OF 2D HETEROSTRUCTURE THERMALLY TREATED IN AIR.
............................................................................................................................................................................ 95
FIGURE 4.5: AFM ANALYSIS OF 2D HETEROSTRUCTURE: AS-GROWN VS. LASER EXPOSED IN AIR. ............................ 95
FIGURE 4.6: OXYGEN-INDUCED MANIPULATION OF EXCITON DYNAMICS USING LASER SETUP. ............................... 96
FIGURE 4.7: 2D HETEROSTRUCTURES THERMALLY TREATED IN AR (I.E. AN O2-FREE ENVIRONMENT) AT 240°C FOR
3 HOURS. ............................................................................................................................................................. 98
FIGURE 4.8: 2D HETEROSTRUCTURE THERMALLY TREATED IN DRY AIR (I.E. 79% N2 + 21% O2). ............................ 99
FIGURE 4.9: 2D HETEROSTRUCTURE THERMALLY TREATED IN AN N2+H2OVAPOR ENVIRONMENT. .......................... 100
FIGURE 4.10: 2D HETEROSTRUCTURES FIRST THERMALLY TREATED IN AIR, AND THEN IN N2.
PHOTOLUMINESCENCE SPECTRA AT DIFFERENT LOCATIONS. ......................................................................... 101
FIGURE 4.11: 2D HETEROSTRUCTURES THERMALLY TREATED IN AIR AND THEN IN N2. PL SPECTRA AT THE SAME
LOCATION. ........................................................................................................................................................ 102
FIGURE 4.12: TUNING THE INTERLAYER COUPLING AND PHOTOLUMINESCENCE INTENSITY. ................................ 104
FIGURE 4.13: HIGH TUNABILITY AND CONTROL OF INTERLAYER COUPLING AND PL INTENSITY. ......................... 105
FIGURE 4.14: DFT CALCULATIONS PREDICT INTERCALATED O2 DIMINISHES INTERLAYER COUPLING .................. 106
FIGURE 4.15: SITE-SELECTIVE MANIPULATION OF INTERLAYER COUPLING AND PHOTOLUMINESCENCE. ............. 107
FIGURE 4.16: RAMAN RESPONSE OF BI2SE3/MOS2 PRE AND POST LASER EXPOSURE. ............................................... 109
FIGURE 4.17: POSSIBLE NON-RADIATIVE ELECTRON-HOLE RECOMBINATION PATHWAYS LEADING TO PL
QUENCHING IN AS-GROWN 2D HETEROSTRUCTURES. ..................................................................................... 114
FIGURE 4.18: DFT CALCULATIONS COMPARING A BI2SE/MOS2 SUPERLATTICE WITH AND WITHOUT O2
INTERCALATED. ................................................................................................................................................ 116
FIGURE 4.19: DFT CALCULATIONS SHOWING THE LOCATION OF THE O2. .............................................................. 116
FIGURE 4.20: DIAGRAMS DEMONSTRATING THE CREATION OF GRAIN BOUNDARIES, FACILITATING O2
INTERCALATION INTO THE INTERLAYER REGION. ........................................................................................... 117
FIGURE 5.1: FIGURE BRIEFLY SUMMARIZING THE CAPABILITIES DEMONSTRATED IN THIS CHAPTER. ..................... 121
FIGURE 5.2: REVERSIBLE OXYGEN ABSORPTION AND ITS ROLE IN MANIPULATING THE PHOTOLUMINESCENCE
INTENSITY ......................................................................................................................................................... 123
FIGURE 5.3 AS-GROWN BI2SE3/TMD 2D HETEROSTRUCTURES ................................................................................ 124
FIGURE 5.4: REPRESENTATIVE SAED IMAGES OF AS-GROWN AND LASER-EXPOSED 2D HETEROSTRUCTURES ...... 125
FIGURE 5.5: BI2SE3/MOSE2(1-X)S2XALLOY SAED AND TEM IMAGES. ............................................................................. 126
FIGURE 5.6: MANIPULATING THE OPTICAL AND EXCITONIC PROPERTIES IN BI2SE3/TMD 2D HETEROSTRUCTURES
.......................................................................................................................................................................... 127
FIGURE 5.7: DEMONSTRATION THAT LASER EXPOSURE IN AIR EFFECTS ARE REVERSED THROUGH ANNEALING ... 128
FIGURE 5.8: COMPLEX PATTERNS AND GRIDS CAN BE CONTROLLABLE WRITTEN INTO BI2SE3/TMD 2D
HETEROSTRUCTURES ......................................................................................................................................... 129
FIGURE 5.9: ATOMIC FORCE MICROSCOPY (AFM) ANALYSIS OF BI2SE3/MOSE2 AND BI2SE3/WS2 ......................... 130
FIGURE 5.10: RAMAN SPECTROSCOPY STUDY OF BI2SE3/MOSE2 .............................................................................. 131
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FIGURE 5.11: RAMAN SPECTROSCOPY STUDY OF BI2SE3/MOSE2 AND BI2SE3/WS2 ................................................... 132
FIGURE 5.12: BI2SE3/WS2 SAED IMAGES AFTER LASER EXPOSURE IN AIR, AND THEN ANNEALED IN AR. ............. 133
FIGURE 5.13: TUNING THE PL INTENSITY AND PEAK POSITION ............................................................................... 133
FIGURE 5.14: MANIPULATION OF BOTH EXCITON POPULATIONS AND THEIR RECOMBINATION ENERGY ............... 134
FIGURE 5.15: REPRESENTATIVE A AND A- FITS FOR PHOTOLUMINESCENCE SPECTRA ............................................ 136
FIGURE 5.16: ANNEALING REVERSES THE CHANGES TO OPTICAL AND EXCITONIC PROPERTIES. ............................ 137
FIGURE 5.17 VERY LOW LASER POWER CAN ANNEAL BI2SE3/TMD 2D HETEROSTRUCTURES. ................................ 140
FIGURE 5.18 PROBING THE PL STABILITY OF MOIRÉ HETEROSTRUCTURES. ............................................................. 141
FIGURE 6.1 DIRECT OBSERVATION OF A NEW LATTICE IN SELECTED AREA ELECTRON DIFFRACTION OF VERTICALLY
STACKED BI2SE3/TMD 2D HETEROSTRUCTURES. ............................................................................................. 143
FIGURE 6.2 ELECTRONIC MOIRÉ LATTICE SCATTERING SITES FORMED BY THE INTERLAYER COUPLING. ................ 145
FIGURE 6.3 THE MOIRÉ SAED SPOTS ARE NOT A PRODUCT OF ONE OF THE INDIVIDUAL LATTICES. ...................... 146
FIGURE 6.4 CHARGE POOLS FORM BETWEEN NEAREST INTERLAYER NEIGHBORS. ................................................... 148
FIGURE 6.5 GUIDANCE ON CALCULATING THE MOIRÉ SUPERLATTICE COMMENSURATE UNIT CELL. ..................... 151
FIGURE 6.6 EXPERIMENTAL, MOIRÉ MODELING, AND DFT DATA FOR A BI2SE3/MOS2 2D HETEROSTRUCTURE. .... 153
FIGURE 6.7 EXPERIMENTAL, MOIRÉ MODELING, AND DFT DATA FOR A BI2SE3/MOSE2 2D HETEROSTRUCTURE. .. 154
FIGURE 6.8 EXPERIMENTAL AND MOIRÉ MODELING IMAGES FOR DIFFERENT BI2SE3/MOSE2-2XS2X 2D
HETEROSTRUCTURES AT VARIOUS TWIST ANGLES............................................................................................ 155
FIGURE 6.9 EXPERIMENTAL AND MOIRÉ MODELING IMAGES FOR A BI2SE3/WS2 2D HETEROSTRUCTURE .............. 156
FIGURE 6.10 EXPERIMENTAL AND MOIRÉ MODELING IMAGES FOR BI2SE3/WS2 2D HETEROSTRUCTURES FROM
FIGURE 2. .......................................................................................................................................................... 156
FIGURE 6.11: SIMPLIFIED DESCRIPTION OF DOUBLE DIFFRACTION DUE TO TEM ELECTRONS. ................................ 158
FIGURE 9.1 MOS2 GROWN ON TOP OF GRAPHENE TO CREATE MOS2/GRAPHENE 2D HETEROSTRUCTURES. .......... 168
FIGURE 9.2 MOS2 GROWN ON GRAPHENE USING THE SECONDARY ELECTRON SEM IMAGING MODE. .................. 168
FIGURE 9.3 ARTISTIC DIAGRAMS DEMONSTRATING MECHANICALLY TRANSFERRING ECONOMICAL GRAPHENE
PRODUCES A CONDUCTING PATH. ................................................................................................................... 170
FIGURE 9.4 IMAGES OF AS-GROWN GRAPHENE MECHANICALLY TRANSFERRED ONTO GLASS. .............................. 171
FIGURE 9.5 OPTICAL IMAGES OF MONOLAYER MOS2 PRIOR TO GROWTH OF SB2SE3. .............................................. 172
FIGURE 9.6 OPTICAL IMAGES OF HETEROSTRUCTURES WHERE SB2SE3 WAS GROWN ON TOP OF MONOLAYER MOS2.
.......................................................................................................................................................................... 173
FIGURE 9.7 PHOTOLUMINESCENCE AND RAMAN SPECTROSCOPY FROM SB2SE3/MOS2 HETEROSTRUCTURES. ....... 173
FIGURE 9.8 TEM SAED IMAGES AND TEM IMAGES OF SB2SE3/MOS2 HETEROSTRUCTURES. ................................. 175
FIGURE 9.9 OPTICAL IMAGES OF MONOLAYER MOS2 AND BI2TE3 GROWN ON MONOLAYER MOS2. ...................... 176
FIGURE 9.10 PHOTOLUMINESCENCE AND RAMAN SPECTRA OF BI2TE3/MOS2 HETEROSTRUCTURES. ..................... 177
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List of Tables
TABLE 2.1 MONOLAYER TMD GROWTH PARAMETERS USING VAPOR-PHASE CHALCOGENIZATION (VPC). .......... 44
TABLE 2.2: LOWEST LASER EXPOSURE POWERS FOR ALL BI2SE3/TMD 2D HETEROSTRUCTURES TESTED. ................. 46
TABLE 2.3: DETAILED LASER EXPOSURE PARAMETERS FOR EXPERIMENTS DESCRIBED IN CHAPTER 4. ..................... 47
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List of Equations
EQUATION 1.1: SCHRÖDINGER EQUATION ................................................................................................................ 18
EQUATION 3.1: ELECTRON BEAM DOSING DENSITY (GENERAL) ............................................................................. 76
EQUATION 3.2: ELECTRON BEAM DOSING DENSITY (VALUES) ................................................................................. 76
EQUATION 6.1: LENGTH OF MOIRE SUPERLATTICE ................................................................................................. 150
EQUATION 6.2: LATTICE RATIO OF 2D HETEROSTRUCTURES .................................................................................. 150
EQUATION 6.3: TWIST ANGLE OF 2D HETEROSTRUCTURES .................................................................................... 150
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Chapter 1: Introduction
1.1 Monolayer materials: the building blocks for 2D structures
2-Dimensional (2D) materials have demonstrated a range of impressive and unique capabilities
that have allowed them to impact a broad spectrum of fields, including transistors, DNA
sequencing, particle physics, radiation detection, superconductivity, neural regeneration,
spintronics, and body armor, just to name a few. Their diversity in properties is due not only to
the large number of monolayer materials grown and characterized, each with their own distinct
properties, but also the extreme number of 2D structures possible from combining the monolayer
materials, where each 2D structure can have properties vastly distinct from the parent crystals.
More specifically, 2D structures are not simply a sum of the parent crystals, but are a product of
their interaction as well. This incredible effect is surprising and highly unique, being confined to
nanomaterials and not observed in the 3D class. With this in mind, let us begin with the questions:
What makes 2D materials ‘2D’? And how come 2D structures are a product of the interaction,
vice simply a sum?
Figure 1.1: Artistic diagram of a general 2D material, where it is only one atom tall (i.e. in the z-axis), but is many more
in the x- and y-axes. This confines the electrons in the material to a 2D plane, thereby significantly altering the
properties.
The field of 2D materials is relatively new having only started in 2004 with the report of graphene,
where multiple definitions of what makes a material “2-Dimensional” have arisen. However, at
its core, a 2D material is ‘2D’ because the electrons have been confined to a plane (i.e. confined to
two axes), and are thereby significantly restricted from traveling along a 3rd axis. More
specifically, 2D materials are frequently only one to a few atoms high in the z-axis, but are
thousands of atoms long in the x- and y-axes (see Figure 1.1), which confines the electrons through
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boundary conditions. This may seem unusual at first glance because in the world of classical
physics, the world we mostly experience every day, materials are not expected to change
properties simply because they are made atomically thin. So why do electrons care that they are
confined, and why does changing the dimensionality significantly alter the properties? The
answer lies within quantum mechanics and the fact that electrons have wave-nature, where their
stable state needs to satisfy the Schrödinger equation (Eq. 1). Simply said, in the quantum
mechanical world dimensionality and relative size are critical in determining properties. Below
this effect is discussed in more detail.
−ħ2
2𝑚𝛻2𝜓 + 𝑉(𝑟)𝜓 = 𝐸𝜓 Equation 1.1
The properties of a material are determined not just by the type of atoms present and
stoichiometry, but also by the crystal structure. For example, both graphite and diamond are
constructed only of carbon atoms, yet their properties could not be more distinct in many ways.
Graphite is commonly found as the “lead” in pencils, where it is black, flakes easily, is breakable,
and leaves a residue when wiped on paper. Diamonds, on the other hand, are clear, diffract
colors, extremely hard, and will often scratch other materials before breaking themselves.
Further, graphite is conductive, while diamonds are good insulators. The only different between
graphite and diamond is their crystal structure (Figure 1.2). Why is it that the crystal structure
can have such a dramatic impact on the properties? The answer lies in how the electrons perceive
the material they exist within, and then how they interact with it. By changing the crystal
structure, they will perceive the material different and respond differently.
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Figure 1.2: (a) Graphite and (b) diamond crystal structures. Reproduced from Universität Kiel.1
As stated earlier, electrons must satisfy the Schrödinger equation while they reside inside a
material. The potential term (i.e. V(r) shown in Eq. 1.1) represents objects that impart potential
energy, as well as their relative positions in space, which includes the nuclei of the atoms. Hence,
as the crystal structure changes, so does the solution to the Schrödinger equation and the electron
response, and thereby the material’s properties. More specifically, solutions to the Schrödinger
equation determine what energies (or momentums) the electrons are allowed to have. These
allowed and disallowed states are often visualized using band structure diagrams, which provide
a tremendous amount of information and can be used to predict exotic effects, such as super
conductivity, 2D electron gases, Rashba splitting, and topological insulators. Along these lines,
one important consequence of crystal structure is whether a band gap exists, and, if so, how large
it is. The band gap of a material is critical in determining the electronic and optical properties,
along with other properties. For example, graphite has a very small band gap while diamond has
a very large one, thereby bestowing conducting and insulating properties, respectively. The band
gap results because electrons are forbidden from existing at certain energy (or momentum)
ranges. It is beyond the scope of this dissertation to explore this extensive topic, which can be
found in numerous condensed matter textbooks.2 For the readers of this dissertation, the primary
takeaway is that the crystal structure is critical in determine the material’s band structure, and
thereby their properties.
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2D materials are so ubiquitous and have had such a tremendous impact on a diverse number of
fields that it is easy to forget how surprising their history is. Although numerous properties and
layer-dependent effects were surprising, one of the most surprising findings was that 2D
materials could even exist without highly specialized environments. Up until the report of
graphene in 2004, the scientific community believed that 2D materials were too unstable to exist
in ambient.3 In fact, the first report of graphene was considered so controversial and unlikely that
the manuscript was rejected twice before being accepted. Once published, the findings were
repeated by numerous groups across the globe, while expanding upon the fundamental
understanding and demonstrating applications. As a testament to the impact of graphene and the
initial research, Dr. Andre Geim and Dr. Konstantin Novoselov were awarded the 2010 Nobel
Prize in Physics, the second fastest Nobel Prize ever awarded!3
1.2 2D Materials: overview of specific monolayer materials
Up until now the dissertation has not discussed any specific materials. This section will discuss
some of the more prominent and influential 2D materials, while touching on the diversity of
properties demonstrated. This is important for discussing 2D structures later in the paper, and
understanding the influence the interlayer coupling has on the properties.
Figure 1.3: Artistic diagrams of (a) graphene and (b) a monolayer transition metal dichalcogenide (TMD). Both
materials have a hexagonal crystal structure. While graphene is only one atom tall, monolayer TMDs are three atoms
tall, where chalcogen atoms are above and below a transition metal atom. The hexagonal structure is best shown by
looking down the z-axis.
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Graphene is one layer of graphite (i.e. one layer of carbon atoms arranged in a hexagonal lattice –
see Figure 1.3). It has demonstrated the highest tensile strength recorded,4 one of the lowest room-
temperature resistivity ever recorded,5 extremely high thermal conductivity,6 Dirac-Fermions,7
and is nearly transparent.8 As far as applications, graphene is being used as optical detectors,9
biosensensors,10 massless electrodes for resonators,11 supports in transmission electron
microscopy,12 detection of explosives,13 transparent electrodes,14 flexible and wearable
electronics,15 energy storage,16 low-cost solar cells,17 single-ion detection,18 DNA sequencing,19
platform to tune the band gap through chemical doping,20 and catalysis.21 It is the most well-
known and studied of the 2D materials, and justified enormous funding initiatives, including the
European Union’s one billion euro flagship project.22 Multilayer graphene is discussed in the next
section. The opportunities and next-gen technologies possible using graphene are substantial,
where very bold and creative ventures that bridge disciplines are not as unlikely as they used to
be. For example, this dissertation posutlates that the exceptionally high tensile strength combined
with the nano-pore capabilities demonstrated by graphene, suggest that it is a good candid for
micron-sized jet injection technology, enabling localized drug deliver at micron scales.23 Neurons
with their long dendrites would be good candidates for ultra-localized drug delivery. This
dissertation also did work with graphene/MoS2 2D heterostructures, where such samples were
grown and characterized. This is discussed in Chapter 9.1.
Monolayer transition metal dichalcogenides (TMDs) are three layers of atoms arranged in a
hexagonal structure (Figure 1.3), where each layer is approximately 0.7nm in height.24 As of
writing this dissertation, dozens of monolayer TMDs have been demonstrated, each with their
own properties.25,26 Some of the properties, such as photoluminescence (PL) and being a
semiconductor, are observed in numerous materials and are considered characteristic of the
class,27–33 while other properties, such as charge density waves in NbSe2, are unique to only a few
materials.34–37 The family of monolayer TMDs is by far the largest of the monolayer and 2D
material families, and has shown promise for numerous applications across a spectrum of fields.38
This includes nano lasers,39 chemical, optical and bio sensors,31,40–43 transistors,44
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superconductivity,45 piezoelectric energy conversion,46 exciton-plasmon coupling for optical
switches and modulators,47 gating-induced Ising superconductivity,48 and nano-pores for
sequencing DNA,49 facilitating the study of signaling pathways and complex proteins such as
Stat5.50 MoS2 is discussed in the next section. This dissertation grew several MoS2-based 2D
heterostructures, which are discussed in different chapters.
Figure 1.4: Experimental and theory band structure for the topological insulator Bi2Se3. High-momentum-resolution
angle-resolved photoemission spectroscopy (ARPES) measurements along the (a) Γ-M and (b) Γ-K momentum-space
cuts. (c) The momentum distribution curves corresponding to (a). This work shows a representative band structure
from a topological insulator, including the Dirac cones. This work was reproduced with permission from 51.
Monolayer bismuth selenide (Bi2Se3) has five layers of atoms arranged in a hexagonal
rhombohedral structure, where each layer is approximately 1nm in height. A majority of this
dissertation discusses Bi2Se3 in monolayer and few layer form, although in Chapter 9.1 it also
discuss research concerning two similar compounds: bismuth telluride (Bi2Te3) and antimony
selenide (Sb2Se3). Few published works have discussed Bi2Se3 in its monolayer morphology, and
if they have, it likely involved either a support structure to stabilize it, or the Bi2Se3 samples were
nano particles.52,53 This dissertation postulates that there are two reasons that contribute to this.
The first being that Bi2Se3 is likely not very stable or unstable in its monolayer morphology in
ambient, where it requires either a surface to stabilize it or it needs to be suspended in a liquid.
Further, Bi2Se3 is well-known to be highly sensitive to air and oxygen at ambient, suggesting this
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factor exasperates and further destabilizes the compound.54–60 The second reason monolayer
Bi2Se3 likely is not researched is because the well-known topological insulating properties are not
present in the one to few layer morphology (see Figure 1.4 for more details).61 The topological
insulating properties only appear once a sufficient number of layers have been stacked to induce
band inversion, a requirement of topological insulating properties.51 Hence, without the scientific
allure to grow monolayers, which have been theoretically and experimentally shown to not have
topological insulating properties, it can be expected that such samples are less likely to be
fabricated. Similar to Bi2Se3, Bi2Te3 has been demonstrated to be a topological insulator and highly
sensitive to oxygen at ambient.51,62,63 In contrast, Sb2Se3 has a different crystallographic structure
– orthorhombic and not hexagonal - and is predicted to not have band inversion or topological
insulating properties.61 Bi2Te3 and Sb2Se3 are discussed more in Chapter 9.1. This dissertation grew
numerous Bi2Se3/TMD 2D heterostructures, which are the primary focus of the work. These are
discussed in more detail throughout.
1.3 2D Structures: combining monolayer crystals to build something new
Up until now we have only discussed monolayer materials, also known as monolayer crystals,
where a few notable examples were described (e.g. graphene, monolayer TMDs), along with a
general overview of 2D material physics. The subtle importance of the crystal structure was
explained. In summary, it determines the relative positioning of the potentials, which affect the
solutions to the Schrödinger equation (Equation 1.1) and allowed electron energies. This
contributed to the fundamental question: Why does loss of a dimension so dramatically affect the
material’s properties? The dissertation will next discuss 2D structures, materials created by
combining 2D materials. In particular, the dissertation will focus on vertically stacked 2D
structures (see Figure 1.5 for an example of a 2D structure).
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Figure 1.5: Artistic diagram showing a representative 2D structure. In this example, two different monolayer materials
(each one atom tall) were vertically stacked. The parent crystals so not need to be different for the properties to be
dramatically altered (e.g. the bright PL of monolayer MoS2 is quenched when a second MoS2 layer is placed on top).
Additionally, 2D structures can be constructed using more than two compounds, suggesting a massive number of
permutations for creating materials with application tailored properties.
Classical physics predicts that as materials are combined via vertical stacking, the new compound
will be close to a simple-summation of the properties of the parent materials, something that is
confirmed by the reader’s intuition. For example, when the body of a car is painted, the metal
does not become weaker or change its properties to a noticeable degree. The combined product
of the painted car is now a summation of both materials (e.g. it has the strength of the metal and
the shiny color of the paint). Now, why are we discussing such an example? The answer captures
one of the biggest and most important messages of this dissertation:
Vertical 2D Structures are more than a simple sum of the parent materials, but are a
product of the interlayer coupling. This effect is distinctly quantum mechanical in
nature, cannot be explained by classical physics, and is a surprise to classical intuition.
It should be noted that when we discuss “vertical stacking”, mixing or the creation of alloys is
not included in the definition. 2D Structures have distinct layers where the crystal structure and
chemistry are being maintained - atoms are not being exchanged between the layers to create
alloys, and their structure is not evolving to a new phase. Additionally, it is documented that the
interface between two 3D materials placed in contact can be changed due to quantum mechanical
effects; however, in these cases only the interface, and the not the entire 3D material itself, is being
altered. In the case of 2D structures, the properties of the individual layers can be changed and
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dramatically altered from their original state, due to the interlayer coupling. Before discussing
the impact and physics of the interlayer coupling, this dissertation will review the applications
and capabilities of 2D structures. The enormous diversity and remarkability of the effects
demonstrated in previous work will serve as foundation and guide for understanding the impact
and influence of the interlayer interaction. Below a variety of examples of 2D structures with
accompanying figures, where the information is a collection from multiple published works and
serves as a short review of the 2D structure work thus far.
Figure 1.6: Graphene band structure, and demonstration of the evolution of the band structure as graphene layers are
stacked. (a) Graphene band structure with an inset showing the Dirac cone. (b) Same graphene Dirac cone band
structure, but plotted differently. Note how the slope is approximately linear, suggesting that the electrons in graphene
have a linear energy-momentum relationship, an indication that Dirac fermions exist within the material. (c) Band
structure from bilayer graphene with an AB stacking. Bilayer graphene is two layers vertically stacked on top of each
other. Note, very interestingly, how the band structure is evolving from the monolayer morphology. The Dirac cone is
no longer sharp at the interface, but more rounded, due to the interaction of the two layers. If the layers were
electronically independent and behaved monolayer, they would each have the band structure in (a) and (b). The plots
in (b) and (c) were reprinted with permission from 64. (d) Band structure from bilayer graphene stacked in the AB
configuration, and (c) band structure of bilayer graphene stacked in the AA configuration. Very interestingly, simply
changing their stacking – how the layers are interacting – dramatically changes the band structure and opens up a
sizeable band gap. These plots were reprinted with permission from 65. Together this figure demonstrates how
significantly the band structure, and thereby properties, change when 2D materials are stacked.
Graphene is the most widely researched and well-known 2D material. This is not only due to its
incredible properties (discussed in the previous section), but also in part because it was the first
2D material identified, where it was first reported in 2004. After studying graphene in its
monolayer form, a natural next step was to start stacking it. After all, the properties of graphene
and graphite are drastically different, so the following questions naturally arise: Why are their
properties so different? Where do the transitions occur between monolayer and bulk? And is
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there a natural “linear” progression in properties from monolayer layer to bulk? Or are there
intermittent configurations with different properties from either monolayer or bulk?
This dissertation attempted to answer all those questions thoroughly; however, short answers are
as follows: (1) The properties are different due to the interlayer interaction. (2) It is not always
clear when the transitions between monolayer and bulk occur, and the transition is different for
each material, but usually it is around ten layers that the properties are close to bulk than
monolayer. (3) and (4) Very interestingly, the transition from monolayer to bulk is not linear. New
properties unique to either bulk or monolayer emerge as the layer count is changed and as the
layers are twisted relative to each other. The most prominent example is how bilayer graphene
twisted to magic-angles will be superconducting.66,67
As shown in Figure 1.6, the band structure of graphene is altered as the layer count increases.
This modifies the properties, both inducing new capabilities while removing others. Figure 1.6a
shows the band structure for graphene, where the well-known Dirac cones are visible. The fact
that six Dirac cones are shown is due to the six-fold symmetry of the crystal structure. The inset
shows a close-up of a Dirac cone, where the linear behavior of the sides is better shown. This
linear structure is very important and unique to graphene because it leads to the formation of
Dirac fermions in graphene, particles with a linear dependence on their energy, an interesting
phenotype that also photons have. Figure 1.6b-c show the band structures of monolayer and
bilayer graphene. The linear behavior and Dirac cone is clearly visible in Figure 1.6b, while Figure
1.6c has lost the sharp structure of the cone with more rounded ends.64 Further, it appears as
though a band gap is opening up. It is important to note that Figure 1.6c specifies AB stacking
because the stacking is critical in determining the band structure, as shown in Figure 1.6d-e,
where both AB and AA are shown. AA stacking has been shown to open up a band gap, while
AB retains more of the monolayer structure.65 Together this data demonstrates that stacking
graphene changes the band structure significantly, underlining the importance of the interlayer
interaction. Further, how they are stacked determines the band structure.
As a result of the interlayer interaction and layer-dependent properties, bilayer and multilayer
graphene have demonstrated the following applications: high conductivity,68 large tensile
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strength and flexibility,69 tunable infrared properties for low-cost high-performance detectors,70
platform to study phono-exciton coupled particles,70 high thermal conductivity,71 high photon
transparency,72 the impermeability to gas species,73 sensitivity to tune the properties through
chemical functionalization,74 transparent electrodes,14 terahertz electronics,75,76 thermoelectric
power harvesting,77 tunable plasmonic devices,78 high sensitivity energy detector (i.e. a
bolometer),79 battery applications with superdense lithium storage80,81, ultrafast lithium diffusion
into and out of the structure,82 reinforcing nanocomposites,83 humidity sensors,84 platform to
study van Hove singularities,85 system to probe Diract fermions and Hofstadter butterflies,86,87
platform to study superconductivity,66,67 topological insulating and transport properties,88
coherent commensurate electronic states,89 biosensors,90 and low cost solar cells,17 The following
reference reviews contain more information.91,92
Figure 1.7: Demonstration how the PL is modified by simply increasing the layer count of MoS2 , a conclusion that can
be applied to the other monolayer materials. (a)-(d) Very interestingly, as the layer count is increased, the band
structure undergoes major modifications. For example, in the case of MoS2, as well as many other TMDs, the band
structure shifts from being (a) direct band gap in the monolayer morphology to an indirect band gap as the layer count
increases – see (b)-(d). The data is reproduced with permission from 93. (e) Demonstrates this effect best using PL
mapping, where the bilayer 2D structure has significantly weaker PL. Data is reproduced with permission from 94. (f)
Shows representative PL spectra from monolayer and bilayer MoS2, including a PL with the corresponding optical
image. The large change in PL magnitude is clearly seen, a reflection of the changing band structure due to the second
layer. This work is reproduced with permission from 95. (g) Artistic diagram of monolayer MoS2 with a neutral exciton.
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Charged excitons have also been reported within the monolayer TMDs. It is believed that the excitons are very tightly
bound in large part because the electric field lines are able to extend out of the material, suggesting the dielectric
environment surrounding the monolayer TMDs affects the exciton dynamics.
One of the most striking and notorious examples of a change in properties due to layer count is
the quenching of the PL from monolayer to bilayer in certain TMDs. Certain TMDs are direct
band gap and contain a very bright PL. This includes MoS2, MoSe3, WS2 and their alloys, materials
that will be discussed in depth later in the dissertation. The bright PL is due to very tightly bound
excitons that exist within the material.30,33 These excitons have some of the highest binding
energies ever recorded, due to the monolayer nature.96,97 More specifically, as the electron-hole
pair is formed, the electric field lines will reach out of the material (see Figure 1.7g). Very
interestingly, as the layer count in increased, the band structure will begin to evolve. Numerous
changes are made; however, one of the most important ones is that the band structure will become
indirect band gap, as shown in Figure 1.7a-d. A consequence of the band structure evolution is
that the PL will quench. This is best shown in the PL mapping in Figure 1.7e-f, where the bilayer
regions have a significantly reduced PL intensity. Other changes to the PL result from the change
in layer count, including the peak position shifts and the relative ratio of the excitons changes;95
however, these changes are beyond the scope of this dissertation.
As a result of the interlayer interaction and layer-dependent properties, bilayer and multilayer
TMDs-based 2D structures have demonstrated the following applications: photovoltaic energy
harvesting,98 photoelectrochemical solar energy conversion,99,100 coupling magnified spin-orbit
effect,101 photodetectors,102 ultra high-density information storage,103 advanced oxygen sensors,104
advanced p-n diodes,105 platform to study plasmonic and phono-exciton physics,106 lithium
intercalation and storage for batteries,107 sensitive photon calorimeters,104 high-performance
spintronics,108 DNA sequencing,109 tunable Schottky device contacts,110 gate-tunable
ferromagnetism for memory and spin manipulation,111 biosensors,42 tunable magnetism and
spin,112 ultrahigh-q room temperature spin filters,113 ultrahigh “one million percent”
magnetoresistance,114 platform to study magnetic Skyrmions,115 and energy efficient artificial
synapsis.116
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Figure 1.8: Angle resolves photoemission spectroscopy (ARPES) of Bi2Se3 at different layer numbers along the Γ-K
direction. As the layer count is increased, the band gap closes and band inversion appears that allows for topologically
protected states. This experiment agrees strongly with theory and demonstrates the impact of layer count. The band
inversion and topologically protected states are not present in bulk, demonstrating that this effect is only present in
samples with the correct layer count. This also further demonstrates that the evolution of properties from monolayer
to bulk is not “linear”, where a progression from one type to another is observed, but that unique states and properties
exist in between. The following figure was reprinted with permission from 117.
Figure 1.8 shows angle resolved photoemission data (ARPES) from several different Bi2Se3
samples with different layer counts. The samples were grown using molecular beam epitaxy
(MBE), suggesting that they have low doping and defects. It can be seen that as the layer count
increases, the band gap diminishes and band inversion starts to appear, suggesting topological
states. These experimental results are in strong agreement with DFT theory calculations and
theory.51,118 This band inversion and the topological insulating properties vanish as the layer count
increases, and is only present when the correct number of layers is present. Interestingly, in the
samples with lower layer counts, where a band gap is present, sizeable Rashba-type spin–orbit
splitting is observed. This is likely due to the substrate-induced potential difference between the
two surfaces,117 further demonstrating the powerful impact of the interlayer coupling, and its
ability to influence not just the neighboring atoms, but also atoms several angstroms away.
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Figure 1.9: Artistic diagram illustrating the possibilities of designing vertically stacked 2D structures , and how the
careful control of not just the parent materials, but also the stacking order, stacking configuration, and twist angle could
lead to the engineering of materials with application tailored properties. The figure was reprinted with permission
from 38.
As shown above, 2D materials in their monolayer morphology have demonstrated incredible
properties. When these same materials are then stacked to from multilayer structures and
heterostructures, the properties and capabilities change. Combining 2D materials does not result
in a simple summation of their properties. It was shown that the research community has used
the 2D structures to both study fundamental science and design application-focused materials
and devices. The extensive volume of published results and patents is a testament to the impact
of the field and potential of this technology to solve problems. Figure 1.9 is an artistic diagram of
how the knowledge and research can be used to construct technology. By vertically stacking 2D
materials in different configurations, custom materials can be constructed. The extreme number
of permutations possible hints at the opportunities, and is a key driver and justification for further
exploration and research.
1.4 2D Structures: a product of the interlayer interaction
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The interlayer interaction is critical to understanding the properties of 2D structures. It is a very
broad term that attempts to capture the physical changes that lead to changes in properties. At a
fundamental level, the interlayer interaction describes the type and magnitude of
“communication” between the different layers. The more communication there is, the more
opportunity and probability that new properties will manifest, while, on the other hand, if there
was no communication, the materials would act monolayer and independent. With this
framework in mind we are presented with the following questions: What does “communication”
mean? What are the different types of communication”? What factors determine whether
materials are communicating? How come this effect is not observed when 3D materials are made
to communicate with one another?
The extraordinary impact of the interlayer interaction and coupling is best observed using
vertically stacked 2D materials. Although the study of interlayer physics is decade’s old and
delivered tremendous findings, the measurements were confined to studying the interlayer
region, and not the material as a whole. One of the most well-known examples of this is a highly
cited work that studied the metal-insulator transition in the interface in a silicon-based system.119
In this work bulk silicon pieces were placed in contact, and a two dimensional electron gas formed
at the interface, suggesting the materials were communicating and interacting. Despite this strong
interaction, the properties of the silicon pieces themselves were only negligibly impacted,
demonstrating that 3D materials are not significantly affected by inter-material interactions. This
effect is in agreement with our intuition, and something we observe frequently in the everyday,
classical world. For example, when a book is placed on a desk, neither the book nor the desk
change their properties. We also used the example of a painted car earlier, where the paint does
not affect the strength of the metal, allowing the combination to yield strong vehicles with a shiny
look. Now that we have established that the coupling formed between 3D materials generally
does not affect the properties of the whole material, the question is: Why does it affect the
properties of 2D materials? The answer is due to the fact that the 2D material is exceptionally
thin, so changes in the environment as able to propagate through the entire material, and that the
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interlayer separation is on the same order as the 2D material thickness, suggesting its influence is
on the same order as well.
Figure 1.10: Density functional theory (DFT) calculated Bi2Se3/MoSe2 2D heterostructure showing the side-view and z-
axis view on the left and right side, respectively. The calculated structure is one layer of MoSe2 on the bottom with one
layer of Bi2Se3 on the top, where the twist angle is 0°. Note the large interlayer separation, which is 24.9% of the total
height, indicating that it will influence the properties. For example, electrons traveling through the material will likely
interact with the massive volume of the interlayer region, and modify their behavior. To form the unit cell of the above
2D structure, four unit cells of Bi2Se3 matched closely with five unit cells of MoSe2, which is the structure used and can
be seen in the z-axis view on the right side. The formation energy density predicts that this 2D structure is significantly
more stable than the two layers individually, and a predictor that strong interlayer coupling is formed. The above
structure was calculated by students in Prof. Arun Bansil’s group and reproduced with their permission.
Figure 1.10 shows a representative 2D structure relaxed and calculated using density function
theory (DFT), suggesting the dimensions are approximately to scale and closely match
experiment. The 2D structure is one tri-layer of MoSe2 on the bottom with one quintuple-layer of
Bi2Se3 on top, where four unit cells of Bi2Se3 match with high precision five unit cells of MoSe2 (see
view along z-axis on the right side of Figure 1.10), and the twist angle was made to be 0°.
Throughout the dissertation the terms “tri-layer” and “quintuple-layer” may be shortened to just
the word “layer”, a common abbreviation in the community. The above structure was relaxed
and calculated using DFT by Dr. Gianina Buda and Dr. Christopher Lane from the Prof. Arun
Bansil group at Northeastern University. The group uses highly advanced theoretical techniques
such as strongly constrained and appropriately normed (SCAN), where their work has been
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found to be in strong agreement with experiment for bulk and 2D materials.51,120–122 They were
and are important collaborators for the DFT work discussed in the dissertation. The DFT work
discussed in the dissertation was done by this group as a part of a collaboration. As shown in
Figure 1.10, the combined structure is predicted to be 13.52Å tall, where the interlayer region is
predicted to be a massive 24.9% of the height, more than the height of the MoSe2 layer!
Considering that the interlayer region does not contain any atoms, this is a massive distance. The
large percentage suggests that it would have a large influence on the properties. As will be shown
later in the disseration, the experimental and theoretical data suggests that charge pools develop
in the interlayer region that act as scattering sites. It is reasonable to assume that these potentials
will not only affect the solutions to the Schrödinger equation and band structure, but might have
their own band structure as well. With this in mind, the large volume of the interlayer region
indicates it will have a significant influence
As will be discussed later in the dissertation, DFT calculations completed by Prof. Bansil’s group
predict numerous other changes, including a change in lattice constants for the parent materials.
This is a result of the influence of the quantum mechanics, which induce a redistribution of charge
and electrons, thereby affecting atoms several angstroms from the interlayer region. The fact that
the interlayer region induces such a large influence angstroms away is in agreement with the
claim that the properties of the parent materials are changed by the interlayer coupling. The
formation energy density on the bottom predicts that the following structure is highly stable and
preferred over two individual layers. The formation energy also implies that there is a strong
coupling because the materials are interacting - and communicating - to produce the electron
bonding required to decrease the energy.
Figure 1.10 and the above discussion demonstrated that the interlayer separation is predicted to
occupy a significant volume of the 2D structure, suggesting it should have a sizeable influence.
Further, the fact that the 2D parent crystals change lattice parameters suggests that the interlayer
interaction affects more than the nearest neighbor atoms, but also affects the atoms several
angstroms away. The far-reaching effect of the interlayer interaction indicates that its influence
extends throughout the entire 2D parent materials, or at a minimum, most of it, offering an
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explanation why 2D parent materials change their properties, but not their bulk counterparts,
when coupled together. Next we will show that the interlayer interaction induces a charge
redistribution into the interlayer region into charge pools between nearest neighbors, suggesting
the formation of a charge-based lattice. Our research suggests that the formation of this charge
lattice is central to the manifestation of new properties.
Figure 1.11: DFT calculated 2D structure with one WS2 layer on the bottom and one Bi2Se3 layer on the top , where twist
angle is 19.1°. In contrast to the 2D structure shown in Figure 1.10, the construction of this supercell is more complicated
and not a simple ratio of 4:5 of Bi2Se3:TMD (see Chapter 6 for amplifying information). This is due to the twist angle
and moiré pattern formed, which requires two lattice vectors per parent material, vice two. The left part is a view along
the z-axis, while the right image is an off-axis view to show the interlayer charge redistribution. The left image shows
the spatially varying charge density, where red corresponds to higher densities. It can be seen from both images that
charge pools form between nearest neighbors.
Figure 1.11 shows a DFT calculated 2D structure with one layer of WS2 and one layer of Bi2Se3
twisted to the angle of 19.1°. Very interestingly, the interlayer interaction induces the formation
of charge pools in the interlayer region that exist between nearest neighbors. These charge pools
are due to significant charge redistribution from the parent crystals, suggesting that the electrons
have undergone a major migration. This migration is a physical change due to the interlayer
interaction, and suggests the materials form a strong interlayer coupling. Further, the charge
distribution suggests that bonding has formed between the layers, which facilitates electron
transport, electron communication, and lattice vibrational communication. As discussed later in
the dissertation, these charge pools are often strong enough to diffract high-energy electrons, a
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very surprising finding considering that it is, as far as we know, the first instance of select area
electron diffraction (SAED) being used to reveal charge redistribution.
The physical changes due to the interlayer interaction are not well-studied, but their existence is
supported by other works. For example, the Raman vibrational modes have been shown to be
dependent on the interlayer interaction, suggesting the charge redistribution affects the structure,
k-constant of the vibrational modes, and the phonons.123–126 Some of those works have even begun
to use Raman as a fingerprint or identifier to quantify the interlayer coupling strength and
characteristics. Other works have used scanning tunneling microscopy (STM) to study the
interlayer bonding and importance of nearest neighbors.127,128 Although an STM cannot probe the
charge redistribution, it is able to demonstrate a change in band gap and tunneling energy. This
in itself demonstrates that the layers are interacting and suggests charge redistribution, since a
change in properties requires a physical change at some level. Said another way, if there were no
physical changes induced in either parent material, the interlayer region would not have charge
redistribution, there would be no new potential sites in the interlayer region, and the parent
materials would be in indistinguishable from their monolayer morphology. Hence, the materials
would be effectively independent and the interlayer coupling would be effectively zero. The
concept of interlayer coupling strength and material independence – including electronic and
vibration independence - will be discussed next and throughout the dissertation, including
methods to control the interlayer coupling magnitude.
1.5 Methods to manipulate the interlayer coupling in 2D structures
This dissertation has so far established that 2D materials can have significantly different
properties than their bulk counterparts, and has established that vertically stacking and
combining monolayer materials produces 2D structures whose properties are not a simple sum
of the parent materials. It then showed that the interaction and communication between the
parent 2D materials is what induces the new properties and changes observed. Further, it was
shown that the interlayer region is a significant volume of the overall 2D structure, and that the
interlayer interaction is able to induced significant charge redistribution, as well as changes to the
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lattice constants of the parent materials. The changes induced allow the strength or magnitude of
the interlayer interaction can be quantified and discussed as a range of values, vice a binary choice
of “on/off”. Higher values lead to more physical changes and changes to the properties, while
lesser values lead to less change overall. Taken together, these findings suggest that if the
interlayer interaction could be manipulated, so could the properties, providing a mechanism and
parameter to tune, engineer, and build new materials. Next this dissertation will demonstrate
methods that have been used to manipulate and tune the interlayer interaction.
Figure 1.12: Tuning the interlayer interaction and coupling using the intercalation of molecules and atoms. (a) and (b)
are data from a recently published highly-viewed publication where bulk, 3D materials were intercalated with various
molecules, and they showed the interlayer coupling could be diminished. More specifically, when molecules
intercalated between the layers, the interlayer coupling diminished, allowing each layer to behave more independent
and exhibit properties similar to their monolayer morphology. This work demonstrated it using several layered, van
der Waals materials, including, MoS2, phosphorous, and graphite. (a) is pre intercalation and (b) is post intercalation.
This work was reprinted with permission from 129. (c)-(g) are from a paper where the interlayer coupling in bilayer
graphene was modified using intercalated lithium. (c)-(d) show the experiment how the lithium was intercalated using
artistic representation, where (c) is pre-lithiation and (d) is post lithiation. (e)-(g) are density functional theory (DFT)
calculated results for bilayer graphene intercalated with different amounts of lithium. It can be seen that as the lithium
intercalates, it modifies the charge distribution, thereby influencing the coupling and communication between the
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layers, as well as the properties of the individual graphene layers themselves. This work was reprinted with permission
from 81.
One of the most effective methods to tune the interlayer coupling is by intercalating atoms or
molecules. The intercalation of atoms and molecules has been studied for decades, and has found
applications in chemical storage, catalysis, energy storage, material doping, and ionic
transport.130–132 The strong interlayer interaction in 2D structures as provided intercalation
research with a new platform and system to study the effects. Further, the experiments can be
repeated between 2D and 3D systems to study changes in the interlayer interaction. One of the
most striking examples is the differences in lithium intercalation between bilayer graphene 2D
structures and bulk graphite. As shown in Figure 1.12c-g, bilayer graphene is able to store lithium
in super dense configurations that have not been demonstrated in bulk graphite.81,82 This is likely
due to the changes in the interlayer coupling. Additionally, once the lithium atoms have started
intercalating, the interlayer interaction and communication will begin to be disrupted. Hence,
this method is one way to tune the interlayer coupling and effects in bilayer graphene 2D
structures. A more global and universal method was published in the summer of 2018 showing
how bulk, 3D van der Waals layered and stacked materials are able to have the interlayer
coupling manipulated by intercalating and deintercalating molecules (see Figure 1.12a-b).129 The
molecules are often specific to certain compounds. For example, MoS2 can be intercalated with
quaternary ammonium molecules with variable carbon chain lengths, and phosphorous can be
intercalated with cetyl-trimethylammonium bromide, where in each case the interlayer coupling
is disrupted the bulk crystal’s layers behave more independent and exhibit properties of their
monolayer morphology.
One of the most studied topics has been the intercalation of oxygen between graphene and its
metal substrate, where it has been shown that oxygen decouples the materials and diminishes
their communication, inducing the graphene to behave more freestanding and independent.127,133–
142 Further, it was shown that the intercalated oxygen makes the graphene domains easier to
rotate, suggesting the bonding (potentially between nearest neighbors) has been diminished and
the interlayer coupling induced charge distribution has been reversed.134 The work also showed
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that the intercalation of oxygen and other materials is an edge limited effect, where the
intercalation is at boundaries and not through the material.82,135,139 As shown later in the
dissertation, we are able to intercalate oxygen between the Bi2Se3 and the monolayer TMD,
diminishing the interlayer coupling and inducing the materials to behave more independent.104
Previous work has used the intercalation of ionic compounds to modify the interlayer coupling
as well; however, the stability is not as high, and the ionic compounds tend to induce changes to
other properties, such as charge transfer or doping.107,143
Figure 1.13: Various methods that modify the interlayer coupling in 2D structures. (a) A tool to apply extremely high
pressures in the vertical direction was used to manipulate the interlayer separation distance, where it was found that
materials pressed together were more coupled. This fits intuition in that the atoms will be close, insinuating that they
will communicate more. This work was used to show that the magic-angle induced superconductivity in bilayer
graphene can be tuned. The work was reprinted with permission from 144. (b) This is a highly original work where an
atomic force microscope (AFM) was used to remove contaminants and press materials together, which improved the
coupling. This work was reprinted with permission from 145. (c) This is a similar setup to (a) where a high-pressure
setup is used to press materials together and decrease the interlayer separation. In this study a 2D structure of different
TMDs (i.e. MoS2 and WS2) was used, and it was found that not only did the photoluminescence change, but the Raman
response and vibrational modes changed as well. This is important because it shows that not only are the electronic
properties affected, but the lattice and phonon modes are affected as well. This work was reprinted with permission
from 146. (d) Annealing has been shown to improve and strengthen the interlayer coupling in 2D structures. The exact
mechanisms are not fully understood, but it is believed that possibly contaminants are desorbing, or the materials are
gently perturbed with heat into more stable configurations. For example , it has been shown that annealing graphene
on hBN surfaces causes the graphene domains to rotate into more stable moiré patterns.147 It can be seen that the
photoluminescence peaks are modified due to annealing sessions, and that the data suggests the interlayer coupling
was modified. This work was reprinted with permission from 148.
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Figure 1.13 demonstrates numerous other methods to modify the interlayer coupling using non-
intercalation methods. Both Figure 1.13a and Figure 1.13c demonstrate that decreasing the
interlayer separation strengthens the interlayer coupling, which fits intuition. This is because the
decrease distance allows for more atomic interaction and strong interlayer bonds.144,146 The results
suggest that the interlayer separation affects both the electronic coupling and the structural
coupling, where changes to the gating behavior and the Raman modes were observed,
respectively. Figure 1.13b shows how an atomic force microscope (AFM) can be used to remove
contaminants and press materials together, thereby strengthening the coupling.145 Figure 1.13d
demonstrates that annealing is able to strengthen the coupling in 2D structures. The exact
mechanism for why this happens is unclear, but it is believed either contaminants desorb, or the
layers are gently perturbed by the heat to twist and stretch/contract into more stable
configurations and moiré patterns.147–150 The fact that entire grains more than thousands of atoms
could rotate and twist due to gentle thermal annealing is an indication of the extreme influence
and forces the interlayer coupling has. A unique method to tune the interlayer coupling was using
strain, where the 2D structure would be strained in the lateral direction, which would place force
on the in-plane direction.151 Although this method shows signs of controllably modifying the
strain, it suffers the hindrance that the strain effects are present simultaneously, clouding the
experimental results because the interlayer coupling is no longer an independent parameter. The
previous two figures methods were shown that manipulate the interlayer coupling and
interaction strength. Next the dissertation will discuss how manipulating the twist angle can
induce large changes to the band structure. For the interlayer interaction to have an effect and
produce the desired properties, both the interlayer coupling magnitude must be large enough,
and the twist angle must be correctly set to induce the correct band structure. Said another way,
the interlayer coupling facilitates electron movement and communication across the interlayer
region, while the twist angle helps determine the moiré pattern and new band structure created.
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Figure 1.14: DFT calculated 2D structures with one layer WS2 and one layer Bi2Se3 , where the twist angle has been
altered. This figure demonstrates that the twist angle and resulting moiré pattern significantly influence the stability
and crystal structure. Later the dissertation shows that the twist angle and moiré pattern also have a significant
influence on the band structure and properties. Both a side-view and z-axis view are shown of the 2D structures. (a)
Twist angle of 0° was used, where three Bi2Se3 unit cells equaled four WS2 unit cells. The side-view shows the significant
interlayer separation distance, which is, very interestingly, 0.3Å larger than the height of the WS2. Both the WS2 and
Bi2Se3 lattice constants changed as a result of the interlayer interaction. The formation energy density demonstrates
that the combined 2D structure is more stable than the layers individually. (b) The same 2D structure except twisted to
the angle of 5.1°, where interlayer separation and WS2 height have increased, while the Bi2Se3 height has decreased.
Additionally, the formation energy density is significantly diminished, in agreement with experiment that 2D
structures grown at this angle are observed significantly less frequently, suggesting this twist angle is less stable than
0°. The above structure was calculated by students in Prof. Arun Bansil’s group and reproduced with their permission.
The twist angle of 2D structures has been demonstrated to be critical in determining its properties.
This is because the twist angle determines the moiré pattern and supercell created. Although this
topic is discussed in more detail throughout the dissertation, a short explanation is provided here.
When two periodic lattices or arrangements are overlaid – or effectively multiplied – they will
form a moiré pattern.152 Said another way, moiré patterns, or moiré supercells, are a new periodic
arrangement or ordering that is a manifestation of the interaction of the parent lattices or periodic
arrangements. This is clearly an interesting mathematical artifact, but the question then emerges:
How does a new larger periodic ordering affect the properties? The answer lies in the explanation
for what determines a material’s band structure.
As discussed earlier in this dissertation, electrons exhibit wave properties, and these waves
interact with the periodically spaced potentials of a crystal. The stable states are described by the
solutions to the Schrödinger equation (see Equation 1.1). If the potential term in the equation (i.e.
the V(r) term) is changed by the change in crystal structure and the addition of new atoms, then
the solutions and band structure will evolve, and the corresponding properties.153,154 The concept
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of the band structure radically changing in response to crystallographic changes was also
discussed earlier, where the example of graphite and diamond was compared, two materials
composed solely of carbon atoms, but arranged in different configurations. A similar effect takes
place when the moiré pattern and super cell is changed, which is effectively changing the periodic
unit cell the electrons interact with.12,103,155–159 Now that we have established the significant
influence of the twist angle and moiré pattern, the question becomes how this parameter can be
manipulated to control the properties? In the next section, methods to manipulate the twist angle
are discussed. Additionally, Chapter 2 of the dissertation discusses a method using a transmission
electron microscope (TEM) to manipulate the twist angle and probe the interlayer interaction.
Figure 1.15: Two powerful methods to construct and manipulate 2D structures are to (a)-(d) mechanically transfer 2D
materials on top of each other, and (e)-(h) rotate 2D materials using an atomic force microscope (AFM). (a)-(d)
Demonstration of magic-angle induced superconductivity in bilayer graphene. When the 2D structure bilayer graphene
is twisted to magic-angles, a moiré pattern and supercell is formed that has band flattening and superconductivity. The
quality of the 2D structure to measure such sensitive effects, suggests that mechanically transferring 2D structures
produces a sufficiently strong interlayer coupling. (a) The moiré pattern formed at the magic-angle of 1.08° for a bilayer
graphene 2D structure. This is reprinted with permission from 67. (b) Artistic diagram of a device of a bilayer graphene
2D structure twisted to a magic angle and overlaid on a hexagonal boron nitride (hBN) surface. The hBN surface is
extremely flat and non-interacting with graphene structures, minimizing the effects of defects, phonons, and other
scattering sites, allowing the properties of the graphene structures to be probed. (c) Diagrams showing the temperature
vs. carrier density on the conductance. It can be seen that simply by gating the devices, the 2D structures can be shifted
between the superconducting regions and the Mott insulating region. The two different plots are for two different twist
angles and moiré patterns. (d) DFT calculated band structure of a bilayer graphene structure where only the Dirac
point is shown. The following plot demonstrates band flattening, a consequence of not only sufficiently strong
interlayer coupling, but also the correct twist angle. (b)-(d) are reprinted with permission from 66. (e) Demonstration
for how an AFM can be used to controllably manipulate the twist angle in 2D structures. Two moiré patterns are shown,
demonstrating the changing angle-dependent supercell. (f)-(h) AFM images of an hBN/graphene structure where the
upper hBN material has been controllably rotated to numerous angles. This work was reprinted with permission from 160.
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Figure 1.15 shows two methods to produce 2D structures with user-defined twist angles. The first
method is to mechanically transfer one layer on top of another layer. This is the most popular
method with numerous publications expanding on the technique.161–164 It has shown consistently
to produce clean interfaces, and advanced methods even allow high accuracy alignment.164 A
weakness of the method is that it is very difficult to scale, suggesting it is not commercially viable.
Although recent work has used advanced robotics and machine learning to scale the mechanical
transfer process for the production of 2D structures.163 Figure 1.15e-h demonstrates a very new
technique with high rotational accuracy and user-defined angles with in-situ control. It uses an
AFM to controllably rotate the top material. Previous has also shown that 2D structures can be
fabricated by growing one material on top of the other.30,103,165,166 When one layer is grown on top
of the other, the resulting 2D structures often have a strong interlayer coupling, while maintaining
chemical purity, but twist angle control is difficult to achieve. This limitation is discussed further
Chapter 3 and Chapter 4 about Bi2Se3/TMD and graphene/MoS2 2D heterostructures, respectively.
In summary, 2D structures are more than a sum of the parent materials, but are also a product of
the interlayer interaction, which can induce new properties. Not only is the communication and
exchange of electrons between the parent materials important, but so is the twist angle, which
determines the moiré pattern and moiré supercell. This interlayer coupling can be controlled to
tailor properties and engineer application-specific materials.
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Chapter 2: Instrumentation and Synthesis
This chapter discusses the methods, techniques, equipment, and instrumentation required to
complete the experiments and measurements discussed throughout the dissertation. The below
descriptions are a summary of the technique or equipment, and not intended to replace the user
manual or be a thorough guide. Additionally, the descriptions are biased toward the applicability
and interest of 2D structures (all the techniques and equipment were primarily applied to 2D
materials), and caution should be taken before generalizing the expected results to other materials
or systems.
2.1 Growing 2D materials using Chemical Vapor Deposition (CVD)
Growth of TMD and TMD-alloy 2D crystals
All 2D TMDs, as well as 2D TMD-alloys, discussed in this dissertation were grown using vapor-
phase chalcogenization (VPC), a subset technique apart of the chemical vapor deposition (CVD)
family.31 The growth setup consisted of quartz tubes [1 inch (2.54 cm) in diameter] in a horizontal
tube furnace (Lindberg/Blue M). A quartz boat, containing a thin layer of either MoO2 or WO2
powder (3 mg or 15mg, respectively) with SiO2/Si (MTI Corporation) substrates suspended over
the powder with the growth side facing down, was placed in the hot center of the furnace. Either
Sulfur, selenium, or both powders (150 mg or 50mg, respectively) were used, depending on the
material being grown. To grow the alloy MoSe2-2xS2x, both powders were used at the same time.
Sulfur was placed near the insulating edge of the furnace upstream, whereas selenium was placed
~1cm past the insulating edge of the furnace upstream. The setup was pumped down and purged
with argon gas before it was filled with an Ar atmosphere. Downstream was then opened to
atmosphere, in addition to a constant 200 standard cubic centimeter per minute (SCCM) Ar flow.
The furnace was heated to different temperatures and at variable rates, depending on the material
being grown. The growth was conducted in two stages, 1st-stage and 2nd-stage, where 2nd stage
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would start once the 1st-stage temperature was reached. See the below table for material specific
growth information. After the elapsed time, the furnace was opened and allowed to cool rapidly.
Table 2.1 Monolayer TMD growth parameters using vapor-phase chalcogenization (VPC).
2D Crystal 1st Rate (°C/min.) 1st Temp. (°C) 2nd Rate (°C/min.) 2nd Temp. (°C) Hold time (min.)
MoS2 50 500 5 712 20
MoSe2 50 200 20 740 10
WS2 50 1080 5 1150 25
MoSe2(1-x)S2x 50 100 30 750 10
Bi2Se3 growth was performed in a nearly identical vapor-phase deposition setup, except a heating
wrap was coiled around the quartz tube at the down-stream end, leaving no gap between the
furnace and the heating wrap. The Bi2Se3 powder (50 mg) was placed in the hot center of the
furnace. The 2D TMD or 2D TMD-alloy substrate was placed downstream ~0.75cm from the
boundary between the furnace and the heating wrap. The system was pumped down to a base
pressure of ~10 mtorr before a 35-SCCM Ar flow was introduced, raising the growth pressure to
~490 mtorr. The heating wrap was set to a temperature of 245°C, and a temperature controller (J-
KEM Scientific Model Apollo) ensured it remained within ±2°C. The furnace was heated at a rate
of 50°C/min to 530°C and then held there for 20-25 min depending on the desired thickness. Once
growth was completed, the furnace was opened and the temperature controller was de-
energized, allowing the setup to cool rapidly.
2.2 Mechanically transferring 2D materials to create 2D structures
2D Structures can be constructed using numerous techniques, but one of the most popular
techniques is mechanically transferring 2D materials on top of each other. Although the method
often requires significant time and skill, and is unlikely to ever be commercially attractive, its
most attractive quality is that it can be universally applied to the construction of any 2D structure,
and is likely to produce the 2D structure with sufficient effort. This is in contrast to techniques
that fabricate 2D structures by growing 2D materials on top of each other, where the 2D structure
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only forms if it is the energetically preferred state. Such methods that rely on dynamics of the
natural world usually require a higher initial time and material investment, and sometimes are
only possible inside synthetic environments (e.g. unusual temperatures or pressures). Despite
these drawbacks, if 2D structures can be grown, vice mechanically constructed, they are
significantly more like to be economically viable.
Most mechanical transfer techniques use a specific material to lift a monolayer material off the
substrate it was grown on, and then transfer it onto a new substrate. In this dissertation, all
transfers used Polymethyl methacrylate (PMMA) to lift-off and transfer the 2D materials. After
the PMMA was placed onto the new substrate, is was dissolved using acetone, leaving the 2D
materials behind. Amplifying information with sufficient detail to enable reproduction of the
technique can be found in the methods section of the following reference.167 A weakness of this
method is that dissolving the PMMA often leaves significant residue. An example of this can be
seen in Figure 5.5, where the visually-non-linear spots are possibly PMMA or acetone remaining.
Folding and tearing can also be seen in various TEM images throughout the dissertation,
suggesting that this method can damage the 2D heterostructures. An improved method that has
even shown promise to be commercially viable is the dry transfer method, which uses
viscoeleastic compounds to both pick-up and place the 2D material down, removing the need to
solvents, thereby keeping the 2D materials clean and intact.161 Viscoelastic compounds are critical
for this method, as they behave like a solid over short time frames, but a liquid over long time
frames, thereby breaking the adhesion symmetry and allowing the same compound to both pick-
up and drop-off.
2.3 Optical setup for Raman and photoluminescence spectroscopy
Instrumentation
Raman and PL spectra were measured using a Renishaw Raman microscope equipped with a
488nm laser and a grating of 1800 lines/mm. A ×100 or ×150 objective focused the laser to
diffraction-limited spot size. TEM images and SAED patterns were collected from a JEOL 2010F
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operated at 200 kV. AFM images were taken from a NanoMagnetics Instruments Ambient AFM.
All Raman, PL, AFM, and UV-Vis experiments were performed under ambient condition.
2.4 Manipulating interlayer coupling in Bi2Se3/TMD 2D heterostructures using
laser exposure and thermal annealing
Laser Exposure Experiments
The 2D heterostructures were exposed to a 488nm continuous-wave laser at ambient temperature,
pressure, and atmosphere, and the atmosphere pressure was sea level. Previous work has shown
that the response of the 2D heterostructures to laser exposure is dependent on the partial pressure
of oxygen, suggesting environments at lower pressures might yield a different response.104 The
laser exposure recipe (i.e. power density and exposure time) determine the 2D heterostructure’s
response. The laser spot was approximately 1μm in diameter. Each type of 2D heterostructure
(i.e. Bi2Se3/MoS2, Bi2Se3/MoSe2, Bi2Se3/WS2, and Bi2Se3/MoSe2-2xS2x) responds differently to the same
recipe, where Bi2Se3/WS2 was the most sensitive and Bi2Se3/MoSe2 was the least sensitive.
Additionally, the Bi2Se3 layer count dictates the response, where a great number of layers requires
a more intense recipe to get the same response. The below table provides amplifying information
of the laser power required to induce approximately the same response across 2D
heterostructures, assuming the exposure is applied continuously without interruption.
Table 2.2: Lowest laser exposure powers for all Bi2Se3/TMD 2D heterostructures tested.
2D Heterostructure – 1 Layer Bi2Se3 grown on 1 layer of TMD Laser spot power
Bi2Se3/WS2 0.91μW
Bi2Se3/MoS2 60μW
Bi2Se3/MoSe2-2xS2x 70-90μW
Bi2Se3/MoSe2 80μW
The below table expands upon the parameters used for different experiments, both for exposure
and data acquisition, shown in the manuscript. All the experiments were conducted at ambient
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temperature (~22°C), pressure, and atmosphere. When “continuous” is listed it refers that the
data was collected while the exposure portion of the experiment was taking place.
Table 2.3: Detailed laser exposure parameters for experiments described in Chapter 4.
Experiment Experiment Power Experiment Time Data acquisition power Data acquisition time
Figure 5.6b 168μW 1 - 6 seconds N/A N/A
Figure 5.6e-f 0.91μW 30 seconds Continuous Continuous
Figure 5.13a-b 3.8µW - 1430µW 30 sec – 3 minutes 0.91μW 2 minutes
Figure 5.13c 50μW 1 minute Continuous Continuous
Figure 5.16 80μW 12 minutes 0.91μW 2 minutes
It should be noted that the as-grown samples, to the best of our knowledge, have clean interfaces.
Bi2Se3 grows highly crystalline across the whole TMD (i.e. micron sized domains), suggesting the
foreign atoms are not present to impede or modify the growth. Further, the laser exposure and
Raman results have been highly consistent across independently grown samples, suggesting the
interface is the same or very similar. Since it is unlikely that the samples would be equally “dirty”,
the likeliest explanation is that the interface is clean.
Thermal treatment experiments
All thermal treatment experiments were performed between 240-245°C for 3 hours. All
experiments used a flow rate of 3 SCCM, except the thermal treatment under air, where no flow
rate was used. The five environments were pure Ar, pure N2, N2+H2Ovapor, dry air (21% O2 and
79% N2), and air. All setups, except N2+H2Ovapor, were pumped down and filled with the
respective gas prior to thermal treatment. The N2+H2Ovapor environment was created by flowing
N2 at 3 SCCM, while several boats with deionized H2O were present in the tube. The heating of
the tube caused the H2O to evaporate. The downstream side “rained” significantly during the
entire thermal treatment process and H2O was still present in most of the boats, verifying that
sufficient H2Ovapor was present throughout the thermal treatment process.
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2.5 Atomic Force Microscope (AFM)
An atomic force microscope (AFM) was used in this dissertation to determine the height of the
2D materials being studied, and therefrom infer the number of layers present. This is critical
information, as the layer count determines the properties of the material. For example, as
discussed in Chapter 1, MoS2 in direct bandgap in monolayer morphology, but becomes indirect
bandgap as the layer count is increased. Previous work demonstrates that AFMs can be used for
not just a variety of other measurements relevant to 2D materials, but also applications, including
manipulating the twist angle of 2D structures,160 removing interlayer contaminants to strengthen
the coupling,145 and measuring the tensile break strength of a 2D material by pushing on a
suspended sample until it permanently deforms.4 A review of the AFM working principles, as
well as applications to 2D materials, can be found at the following reference.24
2.6 Scanning Electron Microscope (SEM)
The scanning electron microscope (SEM) is used in this dissertation to image 2D materials, write
patterns for electron-beam lithography, and manipulate the properties of Bi2Se3/TMD 2D
heterostructures. SEMs illuminate samples with a focused beam of electrons, and then measure
the electrons that emanate from the material. It is able to control the area illuminated by the
electron beam with exceptional precision using magnetic coils that steer the charged electrons.
Although the electrons interact with the sample in a variety mechanisms, only two types of
mechanisms are primarily used: backscattered electrons, and secondary electrons. Backscattered
electrons are reflected from the material. The size of the nucleus is the primary determinant for
the amount of backscattered electrons. Hence, this technique can be used to probe the atomic
composition of materials. Secondary electrons are not reflected from the material, but are actually
ejected from the material due to the energy of the incoming electron beam. More specifically,
when the electron beam interacts with the material, it imparts energy to the electrons in the
material, which will eject from the material if sufficient energy is imparted. The lower the ejection
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energy barrier is (i.e. the work function), the higher the probability an electron will be ejected,
suggesting that samples which appear brighter have a lower work function. Previous work has
shown that the SEM can be used to measure the work function of 2D materials.168 As shown in
Chapter 3, an SEM was used to probe the comparative work functions of Bi2Se3/MoS2 and
Bi2Se3/MoSe2 2D heterostructures before and after SEM-treatment, where the results suggest SEM-
treatment lowers the work function of the 2D structures. An explanation of the working principles
and physics of an SEM can be found at the following reference.169
2.7 Lithography: E-Beam, Photo, and Laser
Lithography was used extensively in this dissertation to build devices and put down markers for
wafers used to grow 2D materials. Additionally, it was found that both gold and titanium
promote the growth of 2D materials. Hence, lithography was also used to fabricate substrates
more favorable to the growth of 2D materials.
The exact instructions to apply lithography are extensive and beyond the scope of this
dissertation. Further, the instructions vary depending on the photoresist used, the operating
parameters of the equipment being used, and 2D material being work with. More specifically,
sensitivity of 2D materials to chemicals (e.g. photoresist, and Polymethyl methacrylate – PMMA)
varies, and specific processes need to be found for each material. For example, graphene is highly
sensitive to PMMA, and it is very difficult to remove it from the graphene surface. A consequence
is applying PMMA to graphene significantly dopes it and modifies its properties.
Laser lithography was developed during this dissertation with significant advantages over both
electron-beam lithography and photolithography. The 457nm laser in the laboratory coupled to
piezoelectric stages, allowing the laser spot location to be controlled with micron-scale precision.
Programs in LabView were written that allowed the writing of electrodes with micron-feature
size, and large pads to allow wires to be attached to the sample. The technology allows devices
on micron-scale 2D materials to be fabricated without the resource constraints demanded by
electron-beam lithography, or the pattern/mask constraints of photolithography. I demonstrated
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that titanium and gold could be deposited into the micron-scale channels, and it will remain on
the SiO2 wafer after stripping the photoresist away. This technique used chemicals readily
available in industry, suggesting it can be economically scaled. Unfortunately, the photoresist
showed indications it was damaging the Bi2Se3 layer of Bi2Se3/MoS2 2D heterostructures,
suggesting that further research is required to make it viable for device fabrication.
2.8 Transmission Electron Microscope (TEM)
A transmission electron microscope (TEM) was used in this dissertation to image 2D materials,
determine crystallinity using diffraction, and manipulate the twist angle and structure using
concentrated electron-beam exposure. The operation and physical concepts of a TEM are highly
complex and beyond the scope of this dissertation. The TEM used in this dissertation is the JEOL
2010F, where the manual can be found freely available online; however, the exact operation is
unique to each setup and institution. The general working principles and physics of a TEM can
be found at the following reference.170
Section 2.9 and Chapter 3 discusses the methods and techniques used to manipulate the twist
angle and crystal structure of 2D heterostructures using a focused electron-beam. Chapter 6
discusses the principles of selected area electron diffraction (SAED) in greater detail.
2.9 Electron Beam Treatment (EBT) Studies: Manipulating the twist angle and
interlayer interface
Prior to select area electron diffraction (SAED) EBT, the 2D heterostructures needed to be
transferred to a transmission electron microscope (TEM) grid. The TEM grids used were Pelco
QUANTFOIL® Holey Carbon film. PMMA C4 was first spin-coated at 4000 rpm for 60 s on the
SiO2 wafer with the 2D heterostructures and baked 180 C for 1:30 min. Then the chip was
immersed in 1 M KOH solution for 4 hours. Obtained PMMA and heterostructure film transferred
to new substrate. This was followed by acetone and IPA cleaning to remove PMMA residues. An
optical picture 2D heterostructures transferred onto the TEM grid are below.
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Figure 2.1: Optical Picture of Bi2Se3 2D heterostructures transferred onto a TEM grid
Once the grid was inside the TEM, a heterostructure suspended over a hole was found, zoomed-
in, and then focused. Then the 200nm-diameter aperture was inserted and TEM was switched to
SAED mode. Then the brightness was decreased to the lowest value, and an SAED image was
taken. To EBT the 2D heterostructure, the brightness knob was rapidly increased, while
maintaining the focus, until the desired value was obtained. The brightness was maintained for
the desired length of time as instructed by the recipe. After the recipe completion, the brightness
was rapidly diminished while maintaining focus. The operator had the phosphorous screen
lowered throughout the entire evolution and observed the SAED pattern carefully. The pattern
did not change as the brightness was rapidly increased or decreased, verifying that the imaged
location did not shift. Once the brightness was restored to its lowest value and the image focused,
the screen was raised and an image was taken. This process allowed snap-shots of the EBT
induced structural rearrangement to be observed.
The movement of the SAED spots would be demonstrated best by taking a movie of the SAED
pattern while EBT was being applied; however, this was not possible with the TEM used in this
work (JEOL 2010F). The TEM camera is exceptionally sensitive, limiting the exposure time to one
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second and the current density to the lowest value. EBTs are not allowed while the camera is
imaging. A phosphorous screen was lowered to block the electron beam while EBTs were applied,
allowing the crystal’s reaction to be observed and recorded by the TEM operator. At periodic
intervals the operator would decrease the beam current, restore identical imaging settings, and
record an SAED pattern, thereby documenting snap-shots of the EBT-induced changes.
The SEM EBT studies were done on the SiO2 wafer that the 2D heterostructures were grown on.
No transferring needed. Once inside the SEM, the SEM was zoomed-in and focused until near
the desired recipe, and pictures were taken. Once ready for the EBT recipe, the accelerating
voltage was modified and an area was scanned. The scanned area was calculated using the
equations in Chapter 3 to ensure that the electron beam current density was being applied. More
specifically, by modulating the area being scanned, the EBT current density was modulated. After
applying the EBT, the SEM scanning location was moved to a different part of the SiO2 wafer for
~20 minutes to allow for the 2D heterostructure to discharge. After ~20 minutes, the SEM imaged
the same area with the same settings.
2.10 Computational Methods
Lorentzian Fitting of the Photoluminescence Spectra for Exciton Analysis
The PL spectra were fit with two Lorentzian functions and a linear background, following
guidance in previously published work,33 where the Lorentzian’s represent the A exciton and A-
trion. The fitting was accomplished using the Matlab Fit() function. The quality of the fit was
accessed using the R-squared value and visual assessment.
Monte Carlo Simulations described in Chapter 5
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Monte Carlo simulations were done using Matlab software by MathWorks. 151 bins were made
representing the possible number of twist angles, which was 0-30° at increments of 0.2°. Then a
specific number of points were randomly assigned a twist angle and placed inside one of the bins.
The number of points assigned a twist angle was dependent on the number of experimental
points in the histogram (discussed in Chapter 3 and Chapter 9.2). 4-sigma criteria were applied
to determine whether a bin was statistically significant. The simulations were run 1,000,000
instances.
2.11 Device Fabrication
Bi2Se3/MoS2 2D heterostructures grown on 285 nm Si/SiO2 were transferred to an identical chip
that had titanium/gold markers, by the PMMA mechanical transfer method. First, PMMA C4 was
spin coated at 4000 rpm for 60 s and baked 180 C for 1:30 min. Then the chip was immersed in 1
M KOH solution for 4 hours. Obtained PMMA and heterostructure film transferred to new
substrate. This was followed by acetone and IPA cleaning to remove PMMA residues.
The field effect transistor (FET) devices were made on 285 nm Si/SiO2 substrate by Electron-beam
lithography using PMMA C4 or A4. The electrodes (5 nm Ti/50 nm Au) were deposited by e-
beam evaporator with rate deposition 1 and 3 Å/s, respectively. Lift off process was performed
with acetone followed by IPA cleaning.
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Chapter 3: Probing the interlayer interaction between
dissimilar 2D heterostructures by in-situ manipulation of
twist angle and structure
2D heterostructures are more than the sum of the two parent crystals - the interlayer interaction
facilitates the exchange of electrons and charge that can induce new properties.66,67,103,149,155,171 These
artificial materials hold enormous promise for new science and tailorable applications, due in
part to their diversity in properties.172–174 They often contain properties from both parent layers,
as well as new ones that result from the interlayer interaction.66,67,103,149,155 Recent works have
shown that the effect of the interlayer interaction can be substantial, and even dominate the
effective electronic and optical properties. Despite its importance, the interlayer interaction is not
fully understood and there are a limited number of tools to probe it. It has been shown to create
interlayer excitons,175,176 change the perceived color,150 shift e-h recombination paths,103 modify the
band structure,144 and even induce superconductivity.66 The importance of the interlayer
interaction has given rise to research that attempts to tune and manipulate it;149,177–179 however,
this is challenging to accomplish experimentally, and even theoretical descriptions are
incomplete. This work can be found published at the following reference.180
“Interlayer interaction”, sometimes called “interlayer coupling”, are broad terms that encompass
the known and unknown parameters affecting the exchange of electrons and charge. The atomic
registry (or interlayer nearest neighbors),159,181 interlayer spacing,144 interlayer charge
redistribution,103,182 intercalation of atoms,81,183 and interlayer electron energy barrier184 are some
parameters that affect the interlayer interaction and the 2D heterostructures properties. When
quantifying and studying this interaction, an important metric is the heterostructure’s stability
(i.e. the energy required to disrupt the current configuration, including twist angle and crystal
structure). Higher stability implies increased bonding (or charge redistribution) between the
layers, which facilitates increased electron and charge transfer, the primary “communication”
medium that leads to the manifestation of new properties. Hence, studying the stability will
reveal information about the interlayer interaction. Two notable parameters that affect the
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stability are the layer-to-layer atomic registry, and the chemical compositions. These parameters
determine how strongly one of the layers adhere to the other. Conversely, the ease with which
one could manipulate one layer with respect to the other, would be a measure of their interlayer
strength. However, there is currently no facile method for directly manipulating one layer of a
bilayer structure with respect to the other, for example in-situ relative twisting. There are several
impressive methods to manipulate the twist angle, including optically aligned mechanical
transfers,161,163 thermal annealing induced self-alignment,147,185 growing 2D heterostructures at
preferred twist angles,103,186–188 and AFM controlled twisting;160 however, each of these methods
has limitations. The relative orientation obtained by the first three methods cannot be altered once
the 2D heterostructure is fabricated, and the fourth has only been demonstrated for bulk-
monolayer heterostructures (i.e. one material is bulk). A method for in-situ modification of twist
angles would provide valuable insights into the nature of the interlayer interactions in 2D
heterostructures.
In this work, we investigate the interlayer interaction between two chemically and
crystallographically-dissimilar 2D layered materials, using statistical analysis of both as-grown
and in-situ “twist-angle” manipulated samples. We grew 1-2 layers of Bi2Se3 on top of several
different TMDs (MoSe2, MoS2, and WS2) and a TMD alloy (MoSe2-2xS2x), to test the how far the
chemical composition of the nearest and next-nearest interacting layer of atoms at the interface
impact the interaction strength. We found that the family of 2D heterostructures involving pure
TMDs all prefer (to varying degrees) to grow rotationally aligned at or near a twist angle of 0°,
with TMD-specific angular-distribution patterns. While the rotationally-aligned growth is a
possible consequence of van der Waals epitaxy, the angular distribution between 0-30° is
dependent on both interfacial surface atoms (i.e. chalcogens) and sub-surface atoms on the TMD
side (i.e. the transition metals), suggesting that the interlayer interactions penetrate beyond the
interfacial chalcogen layers. Additionally, the as-grown heterostructures corresponding to the
TMD alloys showed little or no affinity towards the 0° twist angle. The alloy vs. non-alloy TMD
results provide compelling evidence that long-range chemical order (i.e. of the same atomic
species) is critical for obtaining van der Waals epitaxy.
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Figure 3.1: Electron beam probes interlayer interaction in 2D heterostructures. (a) A vertically incident electron beam
treatment (EBT) imparts thermal energy into a 2D heterostructure that – at sufficiently high current densities – induces
it to overcome the interlayer interaction and rearrange. (b) Low current densities break only the weak interlayer bonds
associated with semi-stable grains, resulting in recrystallizing (i.e. grain migration to stable configurations). (c)
Intermediate current densities overcome most interlayer bonding, but cannot disrupt the stronger intra-layer bonding,
enabling significant grain rotation (twist). (d) Highest current densities break both the inter- and intra-layer bonding,
disrupting the crystallographic order and leading to the formation of nano-crystals. The interlayer stability was
quantified by varying the EBT recipe parameters (i.e. current density, time interval, and accelerating voltage) and
observing when the structure began to morph (or rearrange). By varying the beam current and analyzing the
parameters that induce each type of behavior, the interlayer interaction and configurational stability could be
compared.
More interestingly, we show for the first time that a tightly focused beam of electrons on
suspended Bi2Se3/TMD 2D heterostructures can selectively rotate the Bi2Se3 layer in-situ, and that
it is possible to do so on a large number of samples to obtain statistical relevance. This approach
enables a direct visualization and quantification of twist-angle-dependent interlayer bonding and
charge redistribution.182,189 We note that in an earlier study, we had seen the first evidence of Bi2Se3
layers rotating on MoS2 monolayers, triggered by a focused laser under ambient conditions.103
This present investigation was done using focused electron beams under high-vacuum,
circumventing any role of chemicals, and hence the observations are purely intrinsic measures of
the interlayer interactions. Figure 3.1a schematically summarizes our electron-beam treatment
(EBT) method, performed inside a JEOL 2010F high-resolution TEM. The intensity of the EBT was
controlled using the “brightness” feature of the TEM, and the crystallographic orientation of the
two layers in of the TMDs were determined using selected area electron diffraction (SAED)
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pattern spots. Under the application of an e-beam, the TMD lattice remains unchanged (except
for very high-density e-beams, discussed later), while the Bi2Se3 lattice showed signs of “rotation”
with respect to their original positions, the spots visibly “morphing” through continuous,
discrete, or diffused rotations. The interlayer stability could be quantified by varying the EBT
recipe parameters (i.e. current density, time interval, and accelerating voltage) and observing
when the structure began to morph (or rearrange). By investigating 12-16 samples, we were able
to categorize the behavior of these SAED spots into three broad categories: disruption, twisting,
or recrystallizing. More specifically, depending on the heterostructure being investigated, “low”,
“medium”, and “high” intensity EBT’s resulted in either breaking down of the Bi2Se3 layer into
smaller “grains”, twisting of these “grains” with respect to the underlying crystallographic axis
of the TMD, or recrystallizing of multiple grains into a lesser number. Figure 3.1b-d schematically
outlines these categories. Using this novel approach, we find that among the Bi2Se3/pure-TMD
junctions, the Bi2Se3/MoSe2 heterojunction appeared to be the most stable and Bi2Se3/WS2 the least;
while the Bi2Se3/TMD-alloy system shows the weakest interlayer interactions. Finally, we show
that sufficiently high doses of EBT appear to break the interlayer bonds and induce the Bi2Se3 to
form nanoparticles with a broad photoluminescence centered between 1.95-2.1eV.
The behavior of these 2D heterostructures is dependent on their interaction; however, what
comprises “weak” interactions between layers – the hallmark of layered materials – has no clear
definition or demarcation in the community. Electronically weak interactions would be an
outcome of a low overlap of electronic orbitals between the two participating layers, leading to
only incremental changes in the electronic properties, while mechanically it would be a measure
of how much energy is lowered when two free layers are brought into contact. These two
approaches are addressing the same question from two different perspectives. This work directly
addresses the mechanical interaction by allowing the interfaces go out of their lowest energy
configuration under EBT. We believe that by comparing the “ease” with which we are able to do
so should be a measure of their relative strengths, and that is presented here.
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Figure 3.2: As-grown 2D heterostructures prior to Electron Beam Treatment (EBT). (a) Optical image of a Bi2Se3/MoSe2
vertical heterostructure on SiO2, where 1-2 layers of Bi2Se3 were grown on a monolayer MoSe2 crystal (see Figure 3.3)
using vapor phase deposition. (b) Typical TEM SAED pattern of a Bi2Se3/MoSe2 heterostructure with the corresponding
diffraction spots labeled. The orange line shows the Bi2Se3 grew near-aligned on the MoSe2. The distinct dots indicate
well-formed crystals. (c) AFM scan of a Bi2Se3/MoSe2-2xS2x heterostructure, and (d) the corresponding line profile (left
to right). The scan is typical of a Bi2Se3/TMD 2D heterostructure: it has near-continuous monolayer Bi2Se3 with bilayer
islands. MoSe2-2xS2x grows crystalline with a lattice constant between MoSe2 and MoS2 (Figure 3.4), suggesting the
selenium and sulfur are randomly placed. Despite the loss of chemical periodicity, Bi2Se3 still grows crystalline (Figure
3.4).
Figure 3.2a shows an optical image of a Bi2Se3/MoSe2 2D heterstructure on SiO2, where 1-2 layers
of Bi2Se3 were uniformaly grown on top of a monolayer MoSe2 flake using vapor-phase
deposition.31 The original monolayer MoSe2 crystal (Figure 3.3) was grown using vapor-phase
chalcogenization (VPC), described in the methods section. Figure 3.2b is a typical TEM selected
area electron diffraction (SAED) pattern with the two parent crystals labeled. The distinct dots
indicate both materials are highly crystalline, and the orange line shows the Bi2Se3 grew aligned
along the 0° with respect to the underlying MoSe2 crystal. Figure 3.2c is an AFM scan of a
Bi2Se3/MoSe2-2xS2x heterostructure, and Figure 3.2d is the corresponding line profile. The AFM
scan is typical of any of the Bi2Se3/TMD 2D heterostructures: it contains near-continuous
monolayer Bi2Se3 with islands of bilayer regions.
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Figure 3.3: Representative Bi2Se3/TMD heterostructures. (a)-(b) Optical images of monolayer MoSe2 and a Bi2Se3/MoSe2
vertical heterostructure, respectively. Figure S1a shows an optical image of a monolayer MoSe2 flake grown on SiO2
using vapor-phase chalcogenization (VPC),31 its well-defined geometry suggesting a single-crystal structure. (b) shows
the same crystal after 1-2 layers of Bi2Se3 were uniformly grown on top using chemical vapor-phase deposition (see
methods section). The purple spots surrounding the 2D heterostructure are Bi2Se3 nanocrystals, which has been
previously demonstrated.52,190 (c) Side profile of a heterostructure with 1L Bi2Se3 and 1L TMD.
We are also able to infer information about the TMD-Bi2Se3 interlayer interaction by comparing it
to Bi2Se3 growth on SiO2, a material commonly used as a platform to grow 2D heterostructures.
First, when lower concentrations of vapor-phase bismuth and selenium atoms are generated,
Bi2Se3 grows only on the TMD (Figure 3.2c); however, at higher concentrations, the Bi2Se3 begins
to grow on the SiO2 as well (purple “spots” surounding heterostructure in Figure 3.2a). Second,
Bi2Se3 grows as multilayered nanocrystals on SiO2 (Figure 3.4e), but prefers to grow monolayer
and highly epitaxial/crystalline on TMDs (Figure 3.4a-d). Together the data suggests there is a
substantial interlayer interaction that stabilizes the Bi2Se3 in its few-layered morphology.
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Figure 3.4: TEM SAED Patterns of four 2D heterostructures studied, as well as Bi2Se3 grown on SiO2. (a) Bi2Se3/MoSe2,
(b) Bi2Se3/MoSe2-2xS2x, (c) Bi2Se3/MoS2, (d) Bi2Se3/WS2, and (e) Bi2Se3 grown on SiO2. The distinct dots indicate both
materials are highly crystalline. Interestingly, the MoSe2-2xS2x grows highly crystalline despite the fact that it is an alloy.
The both photoluminescence peak center, as well as the lattice constant, was between the MoS2 and MoSe2 values,
strongly suggesting that an alloy was grown, vice only a single material. We find the interesting fact that although
Bi2Se3 grows as multilayered nanocrystals when grown directly on the SiO2 (Figure 3.4e); however, it prefers to grow
as monolayers, and highly epitaxial/crystalline when grown on TMDs (Figure 3.4a-d), suggesting there is a substantial
interlayer interaction that stabilizes the Bi2Se3 in its few-layered morphology
Figure 3.5: TEM images of different 2D heterostructures. (a)-(c) TEM images of the same Bi2Se3/WS2 crystal from
different perspectives. The blemish in the photo (see top of (c)) is due to laser treatment, a method used to gently
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disrupt the crystal structure.103 Laser treatment was affected in order to disrupt the crystal structure and break the
Bi2Se3 into smaller grains, enabling the demonstration that EBT can anneal. (d) Bi2Se3/MoSe2 2D heterostructure. (e)
Bi2Se3/MoS2 2D heterostructure.
Figure 3.5 shows a variety of TEM images of different 2D heterostructures. The islands of Bi2Se3
can be seen, suggesting the electrons undergo a phase change or change in scattering between
different regions. Figure 3.5a-c show a 2D heterostructure that was exposed to a laser in an air in
a local region, where the change is evident. The laser appears to have induced tearing. It was
found that a 2D material suspended on a TEM grid was significantly more suscentible to tearing
than one grown on an SiO2/Si substrate. This is likely a result of the fact that these 2D materials
are suspended across micron-sized holes, but only approximately two nanometers tall,
suggesting that the bonding is exceptionally strong to hold the materials togetehr. When a laser
is applied, the materials heats locally and the strain from the suspended sample causes tearing.
The change in contrast of the region indicates changes in the material, possibly due to the
intercalation of oxygen or the formation of new materials due to oxygen absorption. Figure 3.5d-
e show that tearing and folding can occur during the transfer process.
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Figure 3.6 Tuning the interlayer interaction through chemistry. Histograms of the twist angles (Bi2Se3 vs. the TMD) in
as-grown 2D heterostructures (as labeled), with the following interlayer chemistry: (a) Se-Se interface with
molybdenum. (b) Se-S interface with molybdenum. (c) Se-S interface with tungsten. (d) Se-Se/S-alloy interface with
molybdenum. Despite their large crystallographic dissimilarities, Bi2Se3 prefers to grow near 0° when chemical
periodicity is present in the underlying material (i.e. non-alloy heterostructures), but prefers 12°-20° when it is absent
(i.e. MoSe2-2xS2x), suggesting chemical periodicity strengthens the interlayer interaction at 0°. The frequency at 0°
decreases from (a)-(c) suggesting Se-Se interfaces and molybdenum-based TMDs form stronger interlayer interactions
at this angle. Although Monte Carlo simulations with a “4-σ” criteria (see Section 3.1 later in this chapter for more
details) reveal that most of the data outside of 0° is statistically insignificant quantitatively, we find qualitatively that
the interlayers with TMDs “weaker” interactions were found to have larger angular spread in their “twist” angles. As
discussed later, experimental data indicates that the defect density is low.
The growth of 2D materials is governed by the interlayer interaction, and we use the twist angle
of the as-grown heterostructures as our first parameter to indicate the interlayer interaction
strength. In other words, if Bi2Se3 grows at preferred, statistically significant twist angles, they
represent energy-minimized configurations; conversely, if the twist angle distribution is more
random (i.e. “spread out”), we will infer weaker, non-direction specific interactions. The
measured twist angle distributions for each type of heterostructures grown were collected from
75-150 “measurements” taken across 15-40 flakes, for each heterostructure type. The resulting
angular distribution are shown as histograms in Figure 3.6a-d, corresponding to Bi2Se3/MoSe2,
Bi2Se3/MoS2, Bi2Se3/WS2, and Bi2Se3/MoSe2(1-x)S2x heterostructures, respectively. The insets in each
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figure show the relative arrangement of atoms in the nearest and next-nearest layers at the
interface. We see that as the TMD and corresponding interlayer interaction change, so does the
angular distribution. Bi2Se3 has a 79.7% probability of growing at a 0° twist angle on MoSe2 (Figure
3.6a), whereas it only has a 41.0% probability on MoS2 (Figure 3.6b), a 24.5% probability on WS2
(Figure 3.6c), and a near 0% probability on MoSe2(1-x)S2x. The twist angles outside of 0° were found
to be statistically insignificant based on Monte Carlo simulations (see Section 5.1 later in the
chapter for more information), suggesting the stability landscape outside of 0° is relatively flat
with only weak preference, although we can see some qualitative correlation between how weak
the response is, and how far “spread out” the distribution is. The stability and energy of the
interlayer interaction is in part determined by both the spatial density and energy of the interlayer
bonds which mediate the amount of charge redistribution/transfer between the layers. Also, the
bond energy is dependent on the interatomic distance, where a sufficiently long distance prevents
bond formation or significantly weakens it, thereby limiting bond formation to nearest
neighbors.191–193 The relative positioning, or atomic registry, of the atoms is instructed by the twist
angle. In summary, angles with greater stability are more likely to be observed, suggesting a 0°
twist is the most stable configuration for the non-alloy heterostructures.
Comparing the distribution in Figure 3.6a and Figure 3.6b, it is clear that the Se-Se nearest-layer
interface (Bi2Se3/MoSe2, Figure 3.6a) results in a much stronger inter-layer interaction compared
to the Se-S interface (Bi2Se3/MoS2, Figure 3.6b), as inferred from the near halving of the
rotationally-oriented (0°) percentage population of the latter. The more surprising result,
however, is how much the chemistry of the “next-nearest-layer” atoms affects the interlayer-
interactions, as seen by comparing the distributions in Figure 3.6b and Figure 3.6c, which
represent dissimilar “next-nearest-neighbors” only: i.e. -Mo-S-Se-, vs. -W-S-Se-. Once again, the
percentage population of rotationally-aligned heterostructures fall roughly by half, denoting that
W-based TMDs interact less strongly with Bi2Se3, compared to Mo-based ones. We note that when
molybdenum is replaced with tungsten, the in-plane lattice constant increases by less than 1%
(Figure 3.4), and hence it is unlikely that the relatively small change to the in-plane lattice constant
could alone induce such a large change to the stability at 0°, suggesting next-nearest-neighbor
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chemistry is strongly influential in determining the interlayer interaction in these
heterostructures.
Finally, we find that the Bi2Se3/MoSe2-2xS2x distribution (Figure 3.6d) is distinctly different from all
other heterostructures, despite the fact that the same atomic compounds were used (i.e. Mo, Se,
and S), and that the TMD-alloy has long-range crystallographic order using lattice constants
comparable to the other heterostructures (Figure 3.4). Comparing Figure 3.6d, we see that the
Bi2Se3 layer showed no tendency for forming rotationally aligned heterostructures (although the
underlying alloy-TMD had a well-defined, uniform crystallographic orientation), with the
orientations spread widely across the entire measured range. Since the only noteworthy
difference is that the chemical nature of the TMD alloy was random, i.e. the Se and S atoms are
likely randomly distributed with the TMD lattice, our results suggest that long-range chemical
periodicity, not just crystallographic periodicity, is critical for strong interlayer interaction, and
possible van der Waals epitaxy. Raman spectroscopy results (Figure 3.19), as well as recent
publications by our group,30,31,103,104 indicate the 2D heterostructures studied in this work had a
low number of defects, which would affect the interlayer interaction (see Section 5.2 of this
Chapter for more information). Taken together, the data from as-grown heterostructures suggests
the interlayer interaction (at 0°), arranged from strongest to weakest, is: Bi2Se3/MoSe2, Bi2Se3/MoS2,
Bi2Se3/WS2, and Bi2Se3/MoSe2-2xS2x, and we will use this result to test the efficacy of the inference
drawn from the EBT-treatment, and compare how far they are in agreement, as discussed next.
Our findings indicate that the similarity of the touching layers, i.e. Se-Se interactions leads to a
stronger interaction between the two parent crystals compared to Se-S layers. On the other hand,
for the Se-S case, the Mo-based TMD interacted more strongly than the W-based TMD. It was not
possible to pinpoint exactly which aspects (e.g. size, electronegativity, lattice parameters, density
of nearest-neighbor atoms within a moiré supercell) play the most dominant role in these two test
cases. We found little or no interaction in the case of the alloy, possibly due to its chemical
randomness. From these, we conclude that the so called “weak” interaction that is supposed to
be a hallmark of layered materials is at best a loose description, since similar categories of
materials can have vastly different interactions, as revealed by our work.
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The effect of e-beam on the heterostructures was investigated using selected area electron
diffraction (SAED) patterns of the samples in situ. SAED is conventionally used to probe a
material’s crystal structure in reciprocal space, where well defined patterns of bright spots
indicates a sample of high crystallinity.170,194 The material’s crystal structure determines the
pattern generated, while the lattice constant determines the pattern’s size - smaller lattice
constants produce larger patterns.170,194 When probed along the z-direction, Bi2Se3 and the TMDs
produce similar hexagonal SAED patterns; however, Bi2Se3 has a smaller pattern because its
lattice constant is larger (see Figure 3.2and Figure 3.4). The bright spots are spaced every 60° along
the same radii; however, due to rotational symmetry (i.e. –θ = θ), only twist angles between 0-30°
are unique.
Figure 3.7 Manipulating the twist angle in-situ using an electron beam. (a)-(d) TEM SAED images of a Bi2Se3/MoS2
heterostructure subject to consecutive TEM electron beam treatments (EBTs). (a) Initially two Bi2Se3 spots are observed,
where the spot at 27° is 2.58× times brighter than the spots at 21°. (b) Post 1st EBT: The spot at 21° splits into 3 smaller
spots, and a new spot appears at 3.7°. (c) Post 2nd EBT: The 3 spots near 21° combine into two, and the 27° spot splits
into two. (d) Post 3rd EBT: Both sets of Bi2Se3 spots settle at 24°, and are nearly equally bright - 8.4 vs. 8.0 photon counts
for orange and blue, respectively – suggesting the 24° twist angle is more stable than either 21° or 27. The 3.7° spot
disappeared, suggesting the grain was perturbed out of the imaging area. A vertically-incident electron beam can twist,
split, and merge Bi2Se3 grains grown on a TMD (more demonstrations in Figure 3.8 and Figure 3.9).
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When the TEM e-beam is incident on the heterostructures, it induces structural changes in the
SAED spots, and the nature of these changes depend on the e-beam current density (and hence
the rate of energy transfer to the crystals). Higher densities break bonds and disrupt the
crystallinity, whereas lower densities recrystallize by gently perturbing the material into a more
stable state with greater crystallinity. Densities in between exhibit more complex behavior
because the power is sufficient to loosen weak bonds (e.g. interlayer and van der Waals), but not
strong bonds (e.g. intra-material). Appropriately-tuned densities tend to loosen the interlayer
bonding while simultaneously perturbing Bi2Se3 grains to rotate relative to the TMD. The
movement can be observed in-situ using a TEM in SAED mode, revealing information about the
interlayer interaction landscape (e.g. the relative stability of the twist angles). It is reasonable to
assume that under EBT treatment, the cumulative time spent at each twist angle configuration
correlates to their relative stability, i.e. more time equates to higher stability.
We first present results for EBT treatment for a fixed beam current, using multiple exposures. The
SAED images in Figure 3.7 show the evolution of a Bi2Se3/MoS2 2D heterostructure subject to
multiple EBTs of the same recipe (e.g. the same electron beam current density was applied),
demonstrating in-situ manipulation of the twist angle. Figure 3.7a shows the initial configuration,
where two Bi2Se3 spots at 21° and 27° are identified by orange and blue arrows, respectively. Here,
the angles were measured with respect to the nearest stable MoS2 spot, shown by similarly-
colored dashed lines, resulting in the 21° spot measured clockwise and the 27° spot is counter-
clockwise from their nearest reference MoS2 spot. We associate the relative brightness of spots
with the relative areas of a material associated with the spot, where increased brightness
correlates to increased amounts.194 Initially the 27° spot is 2.58× brighter than the 21° spot,
suggesting the size of the Bi2Se3 crystallite under the TEM e-beam corresponding the 27°is bigger.
Under the EBT, the spots moved in real-time. Although these spot-movements could be seen real-
time on the phosphorescent screen of the TEM, unfortunately, they could not be recorded
videographically owing to an instrument limitation. Hence, we present snapshots of the system
between EBT treatments.
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The first EBT (Figure 3.7b) disrupted the Bi2Se3 spot at 21°, splitting it into three closely aligned
spots (18°, 21°, and 24°) that appeared to move within narrow angular ranges, indicating they
were only at semi-stable twist angles. The spot at 27° did not appear to move. Interestingly, a spot
at 3.7° appeared during the EBT; however, due to its rapid emergence, we are not sure of its
origins. This uncertainty led us to question whether the imaged location shifted, which is possible
if either the electron beam or the sample moved during the EBT. We tested for this, but found the
TEM to be probing the same area before and after EBT, leading us to conclude the changes are
due to movement of Bi2Se3 grains. We believe that a grain moved into the imaging field of view
and settled at a twist angle of 3.7°. This is a probable scenario since the electron beam
simultaneously treats areas inside and outside of the imaged location. While operating the TEM
in SAED mode, an aperture is inserted below the sample to diminish the spot intensities and
protect the camera. The aperture only reduces the surface area being imaged, and does not affect
the electron beam, its interaction with the sample, or the area being subject to EBT.
The second EBT (Figure 3.7c) merged two of the orange spots (the left spot at 18° merged with
the middle spot at 21°), and disrupted the spot at 27° to split into two spots (24° and 27°,
respectively). The third EBT (Figure 3.7d) merged the two orange spots (21° merged with 24°),
and merged the two blue spots (27° merged with 24°), resulting in only two spots that are both at
24°, but at opposing rotations. More specifically, the orange spot is twisted 24° to the right, and
the blue spot is twisted 24° to the left. Very interestingly, both spots at 24° are nearly equally
bright, where the orange spot is only 1.05× brighter (8.4 vs. 8.0 photon counts), respectively. This
is a significant change from the initial SAED image, where the blue spot was 2.58× brighter,
suggesting the system evolved into a statistical equilibrium where both the right- and left-circular
24° twist angles contain the same amount of Bi2Se3 crystal. These results are in strong agreement
with the fact that these 2D heterostructures have rotational symmetry (i.e. –θ = θ). Without any
external forces to break the symmetry, it appears that the EBT evenly split the Bi2Se3 crystals
between the two 24° twist angles. The spot at 3.7° also disappears, possibly because it moved out
of the imaging area. Our investigations hence reveal that although there were some quasi-stable
configurations (located at 18°, 21°, and 24°), the fact that both spots settled at 24° suggests that
24° is the most stable twist angle in the Bi2Se3/MoS2 heterostructure.
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Figure 3.8 In-situ crystal manipulation of Bi2Se3/MoSe2 using an electron beam. A Bi2Se3/MoSe2 2D heterostructure was
transferred to a TEM grid and laser treated, a previously demonstrated method to gently disrupt the crystal structure.103
The laser treatment disrupted the heterostructure enabling the demonstration that the heterostructure can be annealed
using an electron beam. A high current density was applied to the above heterostructure. Instead of becoming more
polycrystalline as seen with Bi2Se3/MoS2 (Figure 3.7) and Bi2Se3/WS2 (Figure 5), the heterostructure was annealed,
suggesting Bi2Se3/MoSe2 has a stronger interlayer interaction than the other heterostructures. Additionally, in (d)-(f)
the smaller MoSe2 grains cease to anneal into the large crystal, and oscillate near their current angle, suggesting those
grains are at a relatively stable twist angle.
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Figure 3.9 Bi2Se3/WS2 controllable crystal disruption, annealing, and rotation in response to EBT. (a) Initial
measurement. 2D heterostructure was disrupted using laser-treatment, a process where a focused laser beam is applied
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to the sample in an environment with O2. (b)-(f) Eight electron beam treatments (EBTs) were consecutively applied.
Precisely determining the current density through the sample is difficult; however, the brightness can be controlled,
where 0 is the lowest and 10 is the highest. All measurements were taken at the lowest brightness. The ratio of spot
brightness is used, vice the absolute brightness, to mitigate equipment and operator error. It can be seen that the
heterostructure behavior can be controlled and is determined by the current density. Higher densities disrupt the
crystallinity [(b), (d), (h), and (f)], whereas lower densities anneal [(c), (e), and (g)]. Very small values do not affect
noticeable changes (f). (g) The Bi2Se3 grain rotated into the 0° relative angle, a configuration believed to be more stable
than the other angles based on epitaxial growth studies (Figure 3.6). As mention in this Chapter, determining accurate
current density values is beyond the scope of this project due to the large number of variables present in focusing the
electron beam, and limitations in the equipment sensitivity. As purely a guide, we provide the values measured with
no aperture or sample inserted: the lowest value was 0.0pA/cm2 and highest was 33.4pA/cm2. Inserting the aperture
diminishes the beam intensity where it is no longer within range of the sensor.
Figure 3.10 Controllable crystal rearrangement in-situ using an electron beam. Electron beam treatments (EBTs) were
consecutively applied to a Bi2Se3/WS2 2D heterostructure, demonstrating the structural rearrangement possibilities. The
electron beam current density was adjusted across EBTs to demonstrate controllable structural rearrangement. (a)
Initial SAED image of a Bi2Se3/WS2 2D heterostructure, where the two Bi2Se3 spots and the WS2 crystal are identified
with green, red, and blue arrows, respectively. The relative intensities (i.e. photon counts) of the Bi2Se3 spots compared
to the WS2 spot are shown in the corresponding graph panel. (b) SAED image after two consecutive EBTs (60% followed
by 20%), where the red-arrow spot has vanished and the green-arrow spot is noticeably more intense (see
corresponding panel). (c) SAED image after applying a third EBT (60%), which disrupted the structure, inducing the
green-arrow spot to split into two. (d) SAED image after applying three additional EBTs (20%, 10%, and 20%), which
recrystallized the heterostructure and twisted the red-arrow spot into the 0° angle, a known stable twist angle (Figure
3.6). Further info can be found in Figure 3.8 and Figure 3.9, which also shows how very high current densities will
significantly disrupt the Bi2Se3/WS2 heterostructure. Approximate current density values are shown in Figure 3.9.
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We next present results for variable EBT beam currents on the same sample. Figure 3.10a-d shows
SAED images of the same location on a Bi2Se3/WS2 heterostructure, where multiple consecutive
EBTs of varying electron beam current density were applied, demonstrating controllable
structural rearrangement of the 2D heterostructure. The current density was controlled using the
TEM “brightness” knob, and is expressed as a percentage of the “brightness” knob’s possible
range. Higher brightness values correspond to increased current density, which disrupts the
heterostructure’s crystallinity. Lower brightness values recrystallize the heterostructure and
increase the crystallinity. Due to equipment limitations, the exact current densities are difficult to
measure; however, a guide of approximate values can is shown in Figure 3.9b. Figure 3.10a-d
present snapshots of SAED patterns following representative EBT treatments, while the graph in
Figure 3.10e compares the variation of the Bi2Se3 spot intensity (i.e. the photon counts) values
relative to the corresponding WS2 spot.170,195 Numerous EBTs of various current density values
were consecutively applied to the same location. The EBT current density used is stated below
each panel as a percentage of the possible range. The complete set of SAED images after each
treatment dose has been presented in Figure 3.8 and Figure 3.9.
Figure 3.10a is the SAED image of the pristine sample, where the two weaker Bi2Se3 spots and a
stronger spot corresponding to the WS2 crystal are identified by green, red and blue arrows,
respectively. The corresponding panel in Figure 3.10e shows the relative intensity values of both
Bi2Se3 spots to the WS2 spot, where the green spot is nearly twice as intense as the red spot. Figure
3.10b shows the result after two EBTs were applied (60% and 20%, respectively), which perturbed
the red-arrow spot to merge with the green-arrow spot.
A single high-density (60%) EBT was then applied (Figure 3.10c), which split the green spot into
two. The new spot (clockwise from the green spot) was labeled with the red arrow. This was done
to facilitate analysis, even though the Bi2Se3 crystals/atoms that correspond to this spot are likely
not the same as the original red-arrow spot. Next, as shown in Figure 3.10d, three low-density
EBTs were applied (20%, 10%, and 20%, respectively), perturbing the red spot to rotate into the
0° twist angle and become brighter, as seen in the corresponding Figure 3.10e panel. 0° is a known
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stable twist angle for Bi2Se3/WS2 2D heterostructures (Figure 3.6), suggesting that the low-density
EBTs recrystallized the 2D heterostructure into its most stable twist angle.
Figure 3.10e demonstrates several important results. When EBTs of 60% current density intensity
are applied, the combined brightness of the Bi2Se3 spots decreased, suggesting the
heterostructure’s crystallinity is being disrupted,170,195 since amorphous, or non-crystalline,
materials do not produce well defined SAED spots or rings. Conversely, when lower EBT current
density values are applied (20%), the combined brightness increases, suggesting the
heterostructure’s crystallinity is being recrystallized and increased. EBTs of 10% did not appear
to have a noticeable effect, suggesting the power was too low to affect change. As EBTs were
applied, the relative brightness between the red and green-arrow spots changed, demonstrating
structural reconfiguration and movement of crystals. The most interesting reconfiguration is
shown in Figure 3.10d where the red spot rotated to the 0° twist angle, a known stable twist angle,
a strong indication that the interlayer interaction significantly influences the behavior, and
suggests the efficacy of using this method to study the interlayer interaction. As shown in Figure
3.8 and Figure 3.9, EBTs of 100% current density values will rapidly disrupt the heterostructure’s
crystallinity, and even showed the ability to disrupt the WS2. In this manner, it was possible to
discern values of EBT-brightness that could controllably manipulate SAED spots, in-situ.
Figure 3.11 Summary of different 2D heterostructure’s response to an EBT intensity. Lower EBT intensities (i.e.
“brightness” knob settings) tend to recrystallize the 2D heterostructures and increase the crystallinity, while higher
EBT intensities tend to disrupt. The EBT intensity is correlated to the current density and the amount of power imparted
to the 2D heterostructure. It can be seen that Bi2Se3/MoSe2 is the most stable, while Bi2Se3/WS2 is the least.
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Figure 3.11 shows the responses of three different 2D heterostructures (Bi2Se3/MoSe2, Bi2Se3/MoS2,
and Bi2Se3/WS2) when different EBT intensities were applied, allowing their interlayer
interactions to be probed and compared (further info in Figure 3.7, Figure 3.8, Figure 3.9, and
Figure 3.10). These results are in agreement with the epitaxial growth studies (Figure 3.6), which
indicate that Bi2Se3/MoSe2 has the strongest interlayer interaction and Bi2Se3/WS2 the weakest.
While high current densities disrupted both Bi2Se3/MoS2 and Bi2Se3/WS2 (Figure 3.10), they
recrystallized Bi2Se3/MoSe2 heterostructures (Figure 3.7, Figure 3.8, and Figure 3.9), rotating a
significant portion of the Bi2Se3 spots into the 0° twist angle and increasing the crystallinity, in
agreement with earlier results that showed the 0° twist angle is very stable for Bi2Se3/MoSe2
heterostructures (Figure 3.6).
Previous work has demonstrated thermal heating is able to rotate and merge monolayer MoS2
nanoparticles,196 suggesting the EBT dose is being converted into thermal energy and inducing
the observed changes. Additionally, it has been previously shown that vertically incident TEM
electron irradiation is able to rotate and move crystallites and nano-particles.197 Together this
suggests that the mechanism for recrystallization, twisting, and disruption is due to heating by
electron beam irradiation.
To the best of our knowledge, this is the first demonstration of the in-situ manipulation of a 2D
heterostructure’s twist angle inside a TEM, a capability with significant potential when combined
with TEM measurement methods. More specifically, electron scattering inside a TEM has
demonstrated numerous impressive capabilities, including probing the interlayer van der Waals
electronic structure,182 measure chemical induced charge redistribution,198 probe interlayer energy
in 2D heterostructures,189 measure the interlayer distance,12 and probe crystallographic ordering
and charge redistribution of intercalated atoms.81 Current methods have not demonstrated the
ability to manipulate the twist angle on the TEM grid, limiting the amount of data that can be
reasonably collected. In contrast, this method allows the twist angle to be manipulated in-situ
facilitating numerous data points from the same sample. Further, this method can be done
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rapidly, highly locally, it is site selectable, and even nanoscale samples can be probed. Combining
these methods with the ability to control the twist angle in-situ can hence enable significant
progress in our understanding of 2D heterostructures whose properties can be engineered using
the twist angle.
As discussed in more detail in Section 5.2 of this chapter, we do not believe that EBT induces a
material transformation. Since SAED spots remain distinct before and after the EBT treatment, it
is difficult to assign this transformation to any “permanent” changes during the EBT.
Additionally, any intermediate “phase transition” is unlikely as the “re-oriented” spots represent
identical lattice parameters after the EBT, and we never see any other crystal structure no matter
what the extent of EBT was. Lastly, chemical modification such as oxidation is unlikely since all
the experiments were done at ultra-high vacuum and does not explain why the Bi2Se3 crystal
reappears after EBT.
Figure 3.12 Manipulation of the optical and photoluminescence properties using an electron beam. (a)-(b) Optical and
SEM images, respectively, of Bi2Se3/MoS2 heterostructures. Arrows mark locations where electron beam treatment
(EBT) was applied using a scanning electron microscope (SEM). Red arrows correspond to higher energy densities
(≥8.44x106 J·m-2) and green arrows to lower energy densities (<3.05 x106 J·m-2). While lower densities do not appear
to induce any changes, higher doses shift the heterostructure’s perceived color from purple to green and induce a new
PL peak between 1.95-2.1eV, shown in (c) and Figure 3.14). They also appear to alter the material’s texture and work
function based on SEM image analysis (Figure 3.15). (c) PL of as-grown Bi2Se3/MoS2, and the PL of a Bi2Se3/MoS2
heterostructure after high-dose EBT was applied. High-dose EBT induces a broad peak centered between 1.95-2.1eV,
which corresponds to Bi2Se3 nanoparticles. It was established in Figure 3.7, Figure 3.8, Figure 3.9, Figure 3.10, and
Figure 3.11 that high density EBT rearranges the structure and disrupts the Bi2Se3 to become nano-crystalline. All this
evidence suggests that high density SEM-EBT induces the Bi2Se3 to form nanoparticles. Here we demonstrate a
technique that allows for submicron pattern writing using focused electron beams.
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As discussed in Figure 3.12, EBT recipes contain three important parameters: current density,
accelerating voltage, and time interval. Using a TEM to apply EBT allows the crystal structure to
be observed in-situ; however, precisely measuring the electron beam current density or
manipulating the accelerating voltage are difficult, limiting our ability to quantify the beam
current. To overcome this, we also applied EBT using a scanning electron microscope (SEM),
whose electron beam parameters can be precisely measured and manipulated. Moreover, SEM
beams allow us to “write” complex patterns with nanoscale dimensions,199 and raster over a
greater spatial region, thereby allowing us to EBT-modify samples with regions large enough for
their properties to be investigated optically (i.e. under an optical microscope). The three
parameters (current density, accelerating voltage, and time interval) can be independently
controlled using an SEM. Increasing any of these parameters raises the energy dosage delivered
and, thereby, the likelihood of structural modification and the emergence of new properties. We
next show that sufficiently intense SEM-EBT recipes induce three notable changes: the perceived
color under broad-band illumination shifts from purple to bluish-green, emergence of a new,
broadband photoluminescence, and a possible modification of work function of the treated area.
Figure 3.12a-b are optical and SEM images, respectively, of Bi2Se3/MoS2 2D heterostructures (1-2
layers of Bi2Se3 CVD grown on monolayer MoS2) subject to different SEM-EBT recipes, where the
current density was manipulated, while the time interval (60 seconds) and accelerating voltage
(5keV) were held constant. The exposed areas are identified by red and green arrows,
representing recipes using higher vs. lower current densities, respectively, with values noted in
the figure caption. Optical and SEM images of the locations prior to EBT can be seen in Figure
3.13. Changes were only observed for the recipes using higher current (or energy) densities
(≥8.44x106 J·m-2, red arrows) and not for the lower densities ((<3.05 x106 J·m-2, green arrows),
suggesting a threshold current density, or energy dose, is required to induce change, which is in
agreement with the results in Figure 3.10, where threshold current densities (>10% “brightness”)
were required to induce structural rearrangement.
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The beam current increases slightly with increasing accelerating voltage. Example values are:
152.8pA, 157.3pA, and 160.2 for 5keV, 5.5keV, and 6keV, respectively. The values in Equation 1
and Equation 2 were used to calculate the dosing densities in Figure 3.10. We found treating
Bi2Se3/MoSe2 for 3 minutes at ~19.7A·m-2, changes were observed at accelerating voltages greater
than 5.5keV (2.01x107 J·m-2), and no changes were observed at 5keV (1.78x107 J·m-2). We found
treating Bi2Se3/MoS2 heterostructures for 60 seconds at 5keV, changes were observed to start
occurring at values greater than 8.44x106 J·m-2, and no changes were observed at densities less
than 3.05x106 J·m-2. As demonstrated in the movie in the SI, Bi2Se3/MoSe2 was affected with SEM-
EBT. It can be seen that the structural modification is not instantaneous, but happens slow and
steady over a set period of time. In the case of the movie, the change was observed over the course
of a minute. All the experiments came to the same conclusion: changes are only induced when
the electron beam current density crosses a certain threshold, and hence we can conclude that this
represented intrinsic energy barriers to changes.
𝐸𝑙𝑒𝑐𝑡𝑟𝑜𝑛 𝐵𝑒𝑎𝑚 𝐷𝑜𝑠𝑖𝑛𝑔 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 = 𝐵𝑒𝑎𝑚 𝐶𝑢𝑟𝑟𝑒𝑛𝑡
𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝐴𝑟𝑒𝑎∗ 𝑇𝑖𝑚𝑒 ∗ 𝐴𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑛𝑔 𝑉𝑜𝑙𝑡𝑎𝑔𝑒 Equation 3.1
𝐸𝑙𝑒𝑐𝑡𝑟𝑜𝑛 𝐵𝑒𝑎𝑚 𝐷𝑜𝑠𝑖𝑛𝑔 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 = 152.8𝑝𝐴∗60𝑠∗5𝑘
𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝐴𝑟𝑒𝑎=
4.58𝑥10−5𝐽
𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝐴𝑟𝑒𝑎 Equation 3.2
Figure 3.12c shows representative PL spectra from as-grown and EBT-affected Bi2Se3/MoS2
heterostructures, respectively (see Figure 3.14 for similar results in Bi2Se3/MoSe2) respectively.
Very interestingly, high-dose EBT induces the emergence of a new broad PL peak centered
between 1.95-2.1eV, and which corresponds to those seen previously reported Bi2Se3
nanoparticles.52,190 It was shown in Figure 3.7, Figure 3.8, Figure 3.9, Figure 3.10, and Figure 3.11
that high-dose EBT can disrupt the heterostructure’s crystalline order and induce the formation
of smaller Bi2Se3 crystallites. Both these observations suggest the emergence of the PL might be
due to the formation of Bi2Se3 nanoparticles. Low-dose EBT does not appear to have an effect on
the PL, which is in agreement with the optical and SEM images in Figure 3.12a-b, where no
changes were detected. Figure 3.15 shows an SEM image of as-grown vs. EBT-affected
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Bi2Se3/MoSe2, demonstrating high-dose EBT modifies the texture and contrast on the 2D
heterostructure’s SEM image, suggesting possible changes to the grain size, structure or work
function. Materials with a lower work function will emit more electrons and appear brighter.168,200
Figure 3.13 Manipulation of the optical and photoluminescence properties using an electron beam. (a)-(b) Optical and
SEM images of as-grown Bi2se3/MoS2 heterostructures, respectively. (c)-(d) Arrows mark locations that were electron
beam treated (EBT), where red corresponds to higher current densities (≥8.44x106 J·m-2) and green to lower (3.05 x106
J·m-2.
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Figure 3.14 Bi2Se3/MoSe2 affected with electron beam treatment (EBT). (a) Photoluminescence spectra for 1 layer MoSe2,
1-2 layer Bi2Se3 grown on MoSe2, post high dose EBT, and the same location after being laser treated. It can be seen that
EBT induces a new broad PL peak centered about 2.1eV. The new broad PL peak corresponds to previously reported
Bi2Se3 nanoparticles, suggesting that high-dose EBT induces the formation of Bi2Se3 nanoparticles.52,190 Laser treatment
diminishes the PL intensity, suggesting that the nanoparticles are metastable, and when perturbed by a laser, they will
morph into a structure that has a less intense PL. (b) SEM image of as-grown Bi2Se3/MoSe2. (c) Same heterostructure
after low-dose EBT (5keV), where no texture or work function changes were observed (green arrow identifies the
location). (d) Same heterostructure after a higher dose EBT (5.5keV), where texture and work function changes were
observed (red arrow), suggesting the EBT recipe determines whether the heterostructure is altered, as well as the rate
of change. More specifically, when treating Bi2Se3/MoSe2 for 3 minutes at ~19.7A·m-2, changes were observed at
accelerating voltages greater than 5.5keV (2.01x107 J·m-2), and no changes were observed at 5keV (1.78x107 J·m-2).
The time interval is an important parameter of EBT because it reveals the rate of structural
rearrangement and change. Nearly all the changes observed for the EBT experiments, both SAED
and SEM, were slow and steady enough to be observed by in real time, occurring over time
periods of several seconds to several minutes. Hence, in case of the SEM-EBT experiments, it was
possible to obtain a videographic representation of the changes; a movie of a Bi2Se3/MoSe2 2D
heterostructure undergoing SEM-EBT is provided at the following reference (Probing the
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Interlayer Interaction between Dissimilar 2D Heterostructures by In-Situ Rearrangement of their
Interface in 2D Materials), demonstrating the steady rate of change.
Figure 3.15 Texture and work function induced changes from EBT. Bi2Se3/MoSe2 affected with electron beam treatment
(EBT). Green lines show the area that was integrated over to produce the Brightness/Texture curve as a function of
distance (above the image). The area that was affected by EBT is the black-bordered box. It can be seen that the middle
looks “melted” with larger grains, compared to the undisturbed, as-grown areas outside the box. High-dose EBT
“smooths” the heterostructure’s texture, suggesting the Bi2Se3 grains increase in size by merging. This is reflected in
the curve where the EBT area is smoother and contains three broad peaks (red arrows), whereas the as-grown areas
(blue brackets) have more noise and contain only one broad peak (left side). These differences suggest EBT modifies
the structure. The EBT affected area has a diminished brightness compared to the as-grown area, suggesting EBT
increases the work function of the material. It has been shown that the work function can be measured by comparing
the relative brightness in an SEM image: Materials with a lower work function will emit more electrons and appear
brighter, while materials with higher work functions will eject less electrons, making them appear dimmer on the SEM
image.168,200 The effects of beam charging were mitigated by allowing the material to “rest” and discharge uninterrupted
for 20 minutes after the EBT. It should be noted that recipes with higher doses would occasionally create a black border
around the area affected, likely due to increased treatment time while the beam is “turning-around” and preparing to
move in the opposite direction. We believe that it is either the result of more intense material modification, carbon
build-up,201 or charging.202
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To understand the formation of the nanocrystals, we note that Bi2Se3 prefers to grow poly/nano-
crystalline on the SiO2 (Figure 3.4); however, it prefers to grow crystalline on the TMD, suggesting
the TMD provides a surface that stabilizes the Bi2Se3 through the interlayer interaction. We believe
that as the interlayer bonding it broken, the Bi2se3 becomes metastable and begins to breaks into
smaller grains, in agreement with the TEM-EBT experiments in Figure 3.7, Figure 3.8, Figure 3.9,
and Figure 3.10. Further, the quantity and relative brightness of the Bi2Se3 spots in those figures
suggests that high-dose EBT is able to break the Bi2Se3 into nano-scale particles. Previous work
demonstrated that Bi2Se3 particles tens of nanometers across have a PL,52,190 suggesting high-dose
EBT is able to disrupt the Bi2Se3 layer into forming nanoparticles that induce a different perceived
color (Figure 3.12a and Figure 3.13), work function (Figure 3.15), and PL spectra (Figure 3.12c and
Figure 3.14). Our results indicate that the mechanism for the grain rotation is that energy is
imparted to the grains via the electron beam, and once sufficient energy is imparted to overcome
bonding, the grains will rotate and move. Although this may appear to be thermal motion, we
are cautious to label it as such, since the interlayer interaction applies a different set of forces and
constraints than a typical “Brownian motion particle” would experience.
In conclusion for this section, mono- to few-layer Bi2Se3 can be grown epitaxial on a family of
TMDs (including an alloy), suggesting an influential interlayer interaction is present between the
2D materials. Comparing the angular distributions between the heterostructures provides insight
into their interlayer interaction, and the influence of certain parameters (e.g. twist angle, atomic
registry, chemistry). For example, when chemical periodicity is present (i.e. non-alloys), the most
stable twist angle is at 0°. We then demonstrated a new method to study and quantify the
interlayer interaction in-situ by varying the current density of focused electron beams in either a
TEM or SEM. While in SAED mode, the heterostructure could be disrupted using higher
densities, recrystallized using lower densities, or rotated in-situ into different twist angles using
values in between. EBT measurements indicate Bi2Se3/MoSe2 has the strongest interlayer
interaction and Bi2Se3/WS2 the least, results that are in agreement with the epitaxial growth study
(Figure 3). An SEM allows for greater control of the electron beam parameters (e.g. current density
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and accelerating voltage), enabling improved quantification of the EBT and subsequent changes.
Specific dosing thresholds were measured for when heterostructure disruption begins,
experimental results that can improve bonding strength estimates. Very interestingly, the recipe
can be tuned to change the optical properties, as well as induce the formation of Bi2Se3
nanoparticles, allowing the writing of submicron patterns that photoluminesce. This work
provides insight into the interlayer interaction in 2D heterostructures, an effect that has been
shown to dramatically transform the properties of 2D materials, but is not understood well due
to the limited number of experimental tools to probe it. This work demonstrates a facile technique
to manipulate the twist angle in-situ, which when combined with previously published TEM-
based techniques, is a platform to rapidly collect electronic structure information of 2D materials
facilitating statistically significant results, and supports the development of 2D heterostructures
whose properties are engineered using the twist angle. Lastly, the demonstration of submicron
pattern writing using SEM-EBT holds unique applications for information storage and photonics.
3.1 Monte Carlo Simulations Reveal Statistical Significance of Histograms
The following section explains in more detail the Monte Carlo simulations done to measure the
statistical significance of the histogram results in Figure 3.6. It can be seen that the instance
noise floor was found, which is the boundary established by the four-sigma criteria. When the
instance noise floor is one, the histogram is above the noise floor if one value is above it,
suggesting that the interlayer interaction is inducing a preference at that value. The two
instance noise floor means that when two bars are above that value, then the spectrum is above
the noise floor and the values are not random. The figure captions below describe the Monte
Carlo simulations, where additional information can be found in Chapter 2: Instrumentation
and Synthesis.
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Figure 3.16 Monte Carlo Simulation: Instance Noise Floor Values. Tuning the interlayer interaction through chemistry.
Histograms of the angular separation (Bi2Se3 vs. the TMD) in 2D heterostructures. (a) Se-Se interface with molybdenum.
(b) Se-S interface with molybdenum. (c) Se-S interface with tungsten. (d) Se-alloy interface with molybdenum. Bi2Se3
prefers to grow near 0° when chemical periodicity is present (i.e. non-alloy heterostructures), but prefers 12°-20° when
absent (i.e. MoSe2-2xS2x), suggesting chemical periodicity strengthens the interlayer interaction at 0°. The frequency at
0° decreases from (a)-(c) suggesting Se-Se interfaces and molybdenum-based TMDs form stronger interlayer
interactions. Monte Carlo simulations with a 4-sigma criteria reveal the data outside of 0° is statistically insignificant,
with the exception of 3.85% and 4.30% in (c) and (d), respectively. The relative positions of the interlayer atoms at 0°
might allow for stronger bonding.
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Figure 3.17 Monte Carlo Simulation data to identify the instance noise floors (INF) and statistical significance of the
histograms showing the as-grown 2D heterostructure twist angles. The table shows the values used for running Monte
Carlo simulations, as well as calculating the noise floor for each material. Simulations were run using the data points
outside of zero (right column); however, the results were divided by the total number (left column) to calculate the
percentage values shown in this chapter. To get the percentage we divided the number in each bin by the total number.
(a) Representative simulation and distribution from randomly placing 216 points into 151 bins. The bins represent the
range from 0°-30° at increments of 0.2°. This simulation represents a potential distribution if there was no interlayer
interaction to break the angular symmetry and induce angular preference. (b) 1 million simulations like that in (a) were
run, and this graph shows the simulation’s tallest bin vs. frequency. For example, the tallest bin was 9 for 2,287 of the
distributions.
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Figure 3.18 Data from the Monte Carlo simulations used to calculate the instance noise floor and statistical significance
of the histograms showing the as-grown 2D heterostructure twist angles. The instance noise floor (INF) sets a threshold
when data is statistically significant, assuming 4-sigma criteria. For example, 1 INF – 3.50% means individual peaks at
or below this percentage are expected to occur at least 63 times out of 1 million. Multiple INFs were calculated to verify
the validity of percentages that had numerous instances, such as 4.30% in (d) which is below the 1 INF, but has a
sufficiently high number of instances to make it statistically significant.
3.2 Material Transformation vs. Twist Angle Manipulation
In this section we address the question whether EBT is a material transformation, vice a twist
angle manipulation. If there is some kind of “Material Transformation” taking place during the
EBT, we ask ourselves, what is the nature of the transformation? Specifically, what physical
process could take a portion of the Bi2Se3 crystal (the part under the electron beam), transform it
into “something else”, and then transform it back into a Bi2Se3 crystal again, but with a different
orientation? Since SAED spots remain distinct before and after the EBT treatment, it is difficult to
assign this transformation to any “permanent” changes during the EBT, for example
amorphization. Also, any intermediate “phase transition” is unlikely as the “re-oriented” spots
represent identical lattice parameters after the EBT, and we never see any other crystal structure
no matter what the extent of EBT was. The other possibility to be considered is chemical
modification such as oxidation: This is unlikely since all the experiments were done at ultra-high
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vacuum and does not explain why the Bi2Se3 crystal reappears after EBT – since oxidation is a
thermodynamically-drive process that can go only in one direction under EBT.
The two possible candidates would be (a) if the EBT was somehow causing the crystal to non-
equilibrium non-crystalline phase, perhaps akin to 2D liquid like phase where the atoms re-orient
themselves randomly, but do not leave the zone, and then recrystallize when the EBT is turned
off, and (b) the sub-crystallite (under the electron beam) rotates as a whole, much like how a tiny
gold nanoparticle can rotate under EBT of a TEM.197 Both of these possibilities can allow the
recovery of the Bi2Se3 phase post EBT, and the material transformation as a whole could be a
combination of these two changes, different at different EBT treatment doses etc.
We do note, however, that the cumulative Bi2Se3 spot brightness was diminished, possibly
because it starts to become polycrystalline, reducing the efficiency with which constructive
interference can happen within each SAED spot. And finally, we also found that when sufficiently
high current densities are used, the 2D heterostructure appears to lose its highly crystalline
structure and break into nanocrystals, an indication that the sample is being structurally altered
and possibly degraded. With this in mind, we concur that at high current densities the 2D
heterostructures undergo crystallographic degradation.
3.3 Negligible Role of Potential Defects on the Interlayer Interaction
Chemical vapor deposition (CVD) has previously been shown to grow 2D crystals with more
defects than exfoliated samples, and that these defects can substantially influence the
properties.203,204 Further, defects at the surface would likely have a some impact on the interlayer
interaction because the chemistry would be different, for example grain boundaries could result
in the periodicity being interrupted. Hence, it is important to address what impact defects might
have on the system. Although we are not able to provide the defect density generated in either
the monolayer TMDs or the Bi2Se3, we are able to provide insight into the crystal quality and
consistency of the samples, suggesting that our observations are most likely dominated by the
chemistry of the materials and not that of the defects.
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Crystal Quality: Based on SAED, TEM images, Raman spectroscopy, and photoluminescence
measurements, the quality of our crystals is comparable to published results, suggesting a low
density of defects, and that a majority of the interlayer interaction is mediated by the non-defect
ordering. Previous work on 2D materials has shown there is a relationship between the defect
density and the Raman response. The Raman response of our samples (see representative spectra
below) is in agreement with that of numerous exfoliated samples, and published work studying
this question, suggesting we have a defect density less than LD=24nm (i.e. one defect every
24nm).205,206
Additionally, previous publications from our group have completed a thorough characterization
of the different monolayer TMDs grown in our laboratory, where the results suggest there are
minimal defects in our samples.30,31,103,104,187 We are proud to report we were able to observe very
subtle effects that are known to be sensitive to the sample quality. For example, the Raman spectra
observed for monolayer MoSe2 matched strongly with the density functional theory (DFT)
predictions.30 In another paper, high-resolution TEM imaging is able to observe the moiré pattern
and identify atomic number in MoS2/MoSe2 2D heterostructures.187
Figure 3.19: Representative Raman spectra of a Bi2Se3/MoS2 2D heterostructure , where all the peaks have a strong
intensity and can be clearly identified, suggesting that the defect density is low. This Raman spectra can also be seen
in an upcoming work in ACS Applied Materials & Interfaces.
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Growths Yield Reproducible Samples: The 2D crystals grown in this lab are relatively
reproducible and consistent, suggesting a majority of the samples formed equivalent interlayer
interactions. If this is the case, the primary variable would be the electron beam, suggesting the
conclusions for the effect of an electron beam are reproducible.
Hence, although it was not possible to measure or identify the nature of defects at the interface,
Raman spectroscopy, TEM imaging, SAED, and PL suggest the defect density is low. Further, the
samples are reproducible, suggesting the interlayer interaction was equivalent across the 2D
heterostructures probed. This is important because it means, although the defect density cannot
be probed, its effect on the 2D heterostructures is at least consistent.
And finally, if we assume that the defect density remains consistent for all the samples, then by
comparing the heterostructures with different nearest and/or next-nearest neighbors
automatically eliminates the possibility that defects are playing a major role in the observations.
This is perhaps most strongly demonstrated when we observe noticeable differences even when
the next-nearest neighbor layer is changed (i.e. the interface along with defects remain the same).
From this, we conclude that the role of defects is not dominant in our observation.
3.4 Discussion Grain Rotation Mechanisms
Our results indicate that the mechanism for the grain rotation is that energy is imparted to the
grains via the electron beam, and once sufficient energy is imparted to overcome bonding, the
grains will rotate and move. Although this may appear to be thermal motion, we are cautious to
label it as such, since the interlayer interaction applies a different set of forces and constraints
than a typical “Brownian motion particle” would experience.
As shown in Figure 6, the current density determines the response of the 2D heterostructures. For
example, Bi2Se3/WS2 appears to have the weakest interlayer interaction, while Bi2Se3/MoSe2 the
strongest. Applying a certain current density will have differing effects. Further, it can be seen
that each material has a range where the 2D heterostructure is neither annealed nor disrupted,
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but is twisted. This is because the powers are sufficient to overcome the interlayer bonding, but
not intense enough to overcome the intra-layer bonding.
We note that as the crystal is made smaller its properties might change. For example,
photoluminescence dependent on the size of Bi2Se3 nanoparticles has been reported. 52,190 This is
a possible explanation for the change in color and properties, suggesting that the interlayer
coupling may play only a partial role, or no role at all.
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Chapter 4: Oxygen-Induced In-Situ Manipulation of the
Interlayer Coupling and Exciton Recombination in
Bi2Se3/MoS2 2D Heterostructures
Research in 2D heterostructures continues to gain enormous interest for their potential to advance
both fundamental and application-oriented research. They have contributed to fields as diverse
as transistors,207–209 optoelectronics,210–212 information storage,103,213,214 plasmonics,215–217
photocatalysis,218–220 capacitors,221,222 biosensors,109 spintronics,113,223 high-density lithium
storage,81,82 and superconductivity.66,67,144 The far-reaching success of 2D heterostructures is in part
due to the large spectrum of properties they have demonstrated. A 2D heterostructure’s
properties are more than a sum of the parent 2D materials, but are also a product of the interlayer
interaction, which can be manipulated to engineer new capabilities. It has been shown that the
twist angle,224 interlayer spacing,144 thermal annealing,148,150,225 and intercalation of
molecules127,129,135 all influence the interlayer coupling. There are no tools to directly probe the
interlayer coupling strength; however, its impact can be inferred by the extent it influences the
properties. In this paper we demonstrate that the interlayer coupling in Bi2Se3/MoS2 2D
heterostructures can be tuned by regulating the oxygen presence in the atmosphere, while
applying controlled laser or thermal energy doses. The coupling strength is inferred using
changes in the photoluminescence (PL) intensity, where lower PL corresponds to higher coupling.
Our data suggests the interlayer coupling is modulated by diffusively intercalating and de-
intercalating oxygen molecules. The work shown in this chapter can be found at the following
reference.226
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Figure 4.1: Figure briefly summarizing the capabilities demonstrated in chapter 4. Note that oxygen is likely
intercalating between the layers to induce the change in photoluminescence, a finding not shown pictorially above.
The interlayer coupling in 2D materials is considered a promising parameter for designing
materials with tailored properties; however, despite the significant interest, the interlayer
coupling is not well understood, which is in part due to the lack of experimental techniques with
the ability to precisely manipulate it. Some previous experimental work has focused on using
global thermal annealing to manipulate the coupling; however, this method has not
demonstrated precision, and often requires several hours to complete.148,150,225 Ion irradiation,227
in-plane strain,151 and lateral pressure144,146 have all been shown to increase the interlayer coupling
by decreasing the interlayer separation; however, ion irradiation has also been shown to damage
2D materials and induce defects228–230, and applying strain or pressure alters the 2D material’s
lattice parameters and properties,231–233 thereby introducing uncertainty. Work to manipulate the
interlayer coupling via electric fields has only been theoretical, and follow-up experimental work
is needed to confirm it.234–237 In this paper we demonstrate a facile method to tune the interlayer
coupling in-situ with high-spatial resolution, all using ambient conditions and tools commonly
found in a 2D research laboratory.
Monolayer MoS2 is known, among other things, for having a bright PL due to the formation of
tightly-bound excitons.33 However, when only one layer of Bi2Se3 is grown on the monolayer MoS2
using vapor-phase deposition, the bright PL is >99% quenched because the interlayer coupling
induces a non-radiative exciton recombination pathway.103 In this paper we demonstrate that the
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PL can be controllably increased and decreased at small increments of only a few percent,
suggesting that the coupling is being diminished or strengthened, respectively, at the same small
increments. This is done by applying energy to the material in either an oxygen-present or
oxygen-absent environment, which we believe facilitates the diffusive intercalation or de-
intercalation of oxygen molecules. Our results suggest that the diffusive intercalation of oxygen
molecules induces the monolayer MoS2 to behave more electronically independent, thereby
restoring the radiative recombination pathway and facilitating the emergence of the signature PL
peak.
The intercalation of molecules between layers has been shown to disrupt their coupling, leading
to change in the properties.81,127,129,133–135,142,238–241 For example, it has been shown that when several
layer MoS2 is intercalated with lithium or quaternary ammonium molecules, the signature
monolayer photoluminescence emerges, suggesting the intercalated molecules induce the
MoS2 layers to behave electronically independent, as if they were monolayer.107,129,238 It has also
been shown that oxygen can easily intercalate between 2D crystals and their substrates,
decoupling the two materials and inducing them to behave more “freestanding” (i.e.
electronically independent).127,133–135,142,239,240
A natural question arises as to why oxygen, vice other molecules in the atmosphere, is
manipulating the interlayer coupling in Bi2Se3/MoS2 2D heterostructure. Bi2Se3 is well-known for
its oxygen-affinity, where even trace amounts of oxygen dramatically affect its exotic topological
properties.60 Not only can oxygen react with Bi2Se3 to form a native oxide and new bonding,55,59
but it has even been shown that oxygen is able to passively diffuse through multiple layers of
Bi2Se3.242 The interaction of Bi2Se3 and oxygen is complex and has been well-studied. A thorough
literature review and how previous work relates to our findings is provided later.
In this work, we obtain highly controllable, reversible, and site-selectable switching between
direct (i.e. radiative and photoluminescent) and indirect (i.e. non-radiative) exciton-
recombination pathways in Bi2Se3/MoS2 2D heterostructures (1-3 layers of Bi2Se3 grown on
monolayer MoS2). Our approach allows permanent, in-situ, electrode-less, and use-specific
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programming of the interlayer coupling and PL intensity. Exciton recombination dynamics could
be switched (between radiative and non-radiative) by thermally treating the heterostructures in
oxygen-present (e.g. air) vs. inert atmospheres (i.e. Ar or N2). Alternately, the switching could be
localized in a highly controllable manner at ambient temperatures using a continuous wave
488nm laser (as before, in air or under N2), which allowed site-selective reversible manipulation
of different regions of the same 2D heterostructures. Additionally, our results suggest that these
2D heterostructures might have applications as standard temperature-pressure high-density
oxygen storage devices, potentially storing 69 kg/m3 (a factor of 52 times the density of O2 gas at
1 atm).
Figure 4.2: As-grown Bi2Se3/MoS2 vertical 2D heterostructures. (a) Optical image of a Bi2Se3/MoS2 vertical 2D
heterostructure, where 1 layer of Bi2Se3 was uniformly grown on a monolayer MoS2 crystal using vapor-phase
deposition. (b) Side-view diagram of the heterostructure. (c) Representative atomic force microscope (AFM) scan of a
Bi2Se3/MoS2 2D heterostructure, demonstrating monolayer Bi2Se3 (with trilayer islands) grew uniformly across the
entire MoS2 crystal. (e) Typical drain current (IDS) vs. gate voltage (VG¬¬) in back-gated bare MoS2 and Bi2Se3/MoS2
devices, respectively. The bare MoS2 device is n-doped into the conduction band, while the right-shifted data-curve of
the heterostructure suggests reduction of n-type carriers and lowering of the Fermi level into the band gap. (f)-(g)
Bi2Se3/MoS2 2D heterostructure with the lattice parameters labeled of both parent crystals. The distinct dots indicate
both materials are highly crystalline. The Bi2Se3 tends to grow crystallographically aligned (i.e. twist angle is 0°). (b) A
laser was applied while in an oxygen-present environment (i.e. air), and it was found to induce the Bi2Se3 to become
nano- and poly-crystalline. As discussed later, at higher energy doses the system undergoes more robust changes that
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are not easily reversed. This is possibly due to the Bi2Se3 becoming nano-crystalline, which creates more grain
boundaries. Oxygen is known to more easily diffuse through grain boundaries (discussed thoroughly later), which
would explain the variation in the PL’s rate-of-change.
Figure 4.2a shows an optical image of Bi2Se3/MoS2 2D heterostructure (1 layer MoS2 + 1 layer
Bi2Se3) on SiO2, where the monolayer MoS2 was grown using vapor-phase chalcogenization
(VPC)31 and the Bi2Se3 was grown on top using vapor-phase deposition. Figure 4.2b is a side-view
diagram of a typical Bi2Se3/MoS2 2D heterostructure. Despite the huge lattice mismatch (e.g. 2.74Å
to 3.57Å, see Figure 4.2f), uniform layers of Bi2Se3 grow with high regularity on top of the MoS2
crystal, suggesting strong van der Waals epitaxy-mediated growth between the two component
layers.243 Figure 4.2e shows the transfer characteristics of back-gated monolayer MoS2 and
Bi2Se3/MoS2 FET devices. The right-shift of the threshold gate voltage in the 2D heterostructure
indicates a relative downshift of the effective Fermi level due to reduction of excess n-type carriers
(by about ~9.50x1012 cm-2). The device response looks neither like monolayer MoS2, nor few layer
Bi2Se3 (a semi-metal),244 suggesting there is sufficient interlayer coupling to modify the electronic
structure of both materials.
Figure 4.3: Manipulating the interlayer coupling using oxygen and energy. (a) Optical image of a Bi2Se3/MoS2 vertical
2D heterostructure where energy was applied locally using a focused laser, while the sample was in an oxygen-present
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environment (in air). The optical properties of the affected area were altered going from purple to white, allowing
affected locations to be easily identified. (b) PL spectra from the same spot before and after energy was applied,
demonstrating how the interlayer coupling can be manipulated. Initially, the PL spectra was flat; however, after
applying a focused laser for 8 minutes at 168μW, a PL spectra corresponding to monolayer MoS2 appeared, suggesting
the interlayer coupling was disrupted, allowing the MoS2 layer became more electronically independent. Electronically
independent monolayer MoS2 has a signature PL peak due to its tightly bound excitons. (c) The laser was now applied
to a different location (orange arrow) on the same sample at the same power (168μW for 12 minutes), but the
environment was changed from oxygen to nitrogen. Interestingly, the color change seen in (a) was not observed. The
inset shows the setup where N2 gas was flown across the sample to displace the oxygen. (d) PL spectra from before
and after energy was applied show no perceivable appearance of PL, suggesting the interlayer coupling was not
affected. Thermal treatment experiments discussed later demonstrate applying heat in an oxygen environment disrupts
the interlayer interaction, and that the other components in air (e.g. N2, H20, CO2) do not appear to affect the interlayer
coupling. (e) PL spectra of a Bi2Se3/MoS2 2D heterostructure before energy was applied, as well as after several
consecutive doses, demonstrating how the PL intensity is dependent on the total dose.
Figure 4.3a is an optical image of a Bi2Se3/MoS2 vertical 2D heterostructure, where energy was
applied locally using a focused laser (168μW for 8 minutes), while the sample was in an oxygen-
present environment (i.e. air). The red arrow identifies the affected location, which underwent a
change in color from purple to white. Figure 4.3b shows its PL spectra before and after the energy
dose was applied. Characteristic B2Se3/MoS2 2D heterostructures do not have a PL, or only a very
weak PL, because the interlayer coupling creates a non-radiative recombination path for the
tightly-bound excitons that exist in the monolayer MoS2.103 However, if energy is applied to the
heterostructure in the form of either thermal treatment or a laser, while the heterostructure is in
an O2-present environment, a PL spectrum emerges that corresponds to that of monolayer MoS2,
suggesting the interlayer coupling was disrupted and the signature MoS2 excitons are
recombining along radiative pathways. Further, Raman spectroscopy measurements show the
intensity of the monolayer MoS2 modes increasing after laser exposure in air, suggesting the
interlayer coupling has been diminished (see Figure 5.10, Figure 5.11, and Figure 4.16). Previous
work has demonstrated a correlation between the intensity of the Raman modes and the
interlayer coupling strength.103,123,126,140
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Figure 4.4: Atomic force microscope (AFM) images of 2D heterostructure thermally treated in air. The images show
that thermally treating in air does not induce obvious changes to the Bi2Se3 topography or step height despite the fact
that there are large optical changes. A Bi2Se3/MoSe2-2xS2x 2D heterostructure was used above.
Figure 4.5: AFM analysis of 2D heterostructure: as-grown vs. laser exposed in air. Laser exposure in air does not remove
the Bi2Se3, a key finding demonstrating the exciton recombination pathway switching is more subtle and not simply
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because the Bi2Se3 is being removed. Interestingly, the effect of the laser exposure on the topography is difficult to
discern, where the pre- and post-air-laser-treatment AFM images look nearly identical. No clear changes were detected,
despite the fact that the optical and photoluminescence properties are drastically altered. A Bi2Se3/MoSe2-2xS2x 2D
heterostructure was used above.
Atomic force microscope (AFM) measurements shown in Figure 4.4 and Figure 4.5 demonstrate
that the Bi2Se3 remains on the MoS2 after a color change has been induced and the PL remerges,
verifying that the change is not due simply to the removal of Bi2Se3. A different spot on the same
sample (Figure 4.3c, yellow arrow) was exposed to the same power (168μW for 12 minutes), while
being continuously purged with N2 gas (inset Figure 4.3c), thereby removing oxygen from the
vicinity of the exposed spot. We find that even with an increased laser dose (12 min. vs. 8 min.
exposure), there was neither a perceivable color change, nor emergence of the PL peak (Figure
4.3d), suggesting the interlayer coupling was not affected. Figure 4.3e demonstrates how the PL
intensity is dependent on the energy dose applied, suggesting that the interlayer coupling can be
manipulated in small increments.
Figure 4.6: Oxygen-induced manipulation of exciton dynamics using laser setup. (a) Optical image of a Bi2Se3/MoS2
heterostructure. The arrow indicates a spot whose color changed after being air-laser-exposed. (b) PL spectra from the
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same spot before and after the air-laser-exposure, demonstrating the predicted behavior (i.e. PL recovery). (c) A
different location (orange arrow) on the same sample was N2-laser-exposed by flooding the environment with N2 gas
flow and displacing the air. N2-laser-exposure inhibits the color change observed in (a). Inset shows the setup used. (d)
PL spectra from before and after N2-laser-exposure, showing no perceivable appearance of PL after N2-laser-exposure.
Laser exposure and thermal treatment experiments were separately performed in synthetic dry O2+N2 mixture and
moisture-saturate N2 (shown later) from which any role of the other components of air, e.g. N2, H20, CO2 could be
eliminated. (e) The same spot in (c) was air-laser-exposed. The color-change is perceivable, along with the appearance
of the PL spectrum, shown in (f), demonstrating that extended N2-laser-exposure had no obvious deleterious effect on
the switching of the exciton recombination pathways.
After demonstrating that removing the partial pressure of oxygen disrupts change, the
atmosphere was switched back to air without changing the laser power or moving the sample -
thereby re-introducing oxygen without changing the specific location being probed - resulting in
a color change and a strong PL peak recovery (shown in Figure 4.6), confirming that the
atmosphere plays a critical role in the observed changes. Below are numerous experiments that
unequivocally demonstrate that oxygen is required to induce the changes, and not a different gas
in the atmosphere.
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Figure 4.7: 2D heterostructures thermally treated in Ar (i.e. an O2-free environment) at 240°C for 3 hours. 1-3 layers of
Bi2Se3 were grown on monolayer MoS2 using chemical vapor deposition (CVD). The photoluminescence was not only
quenched after each thermal treatment session, but it was also controllably increased using laser exposure in air,
demsonstrating write-read-erase-reuse capabilities.
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Figure 4.8: 2D heterostructure thermally treated in dry air (i.e. 79% N2 + 21% O2). The behavior is similar to that of the
air-annealing experiments in that the PL intensity increases when dry-air-annealed and then decreases again after N2-
annealing. However, it is different in two subtle points: (1) after dry-air-annealing, air-laser-treatment does not affect
the PL; and (2) a possible new PL signal emerges in the upper energies (~2.0eV), where the PL plateaus to a higher
intensity than that seen at 1.7eV (i.e. the PL spectra is not symmetric, showing higher intensities at higher energies).
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Figure 4.9: 2D heterostructure thermally treated in an N2+H2Ovapor environment. This experiment established that water
vapor does not react with the heterostructures to induce the optical and photoluminescence changes observed during
laser exposure in air (i.e. water vapor is not necessary to switch exciton recombination pathway). In fact, the
heterostructure will reverse both the air-laser-treatment induced optical and photoluminescence changes, when
annealed in an N2 environment saturated with water vapor. As seen above, five locations were air-laser-treated prior
to annealing, and an optical change was induced in each on. Annealing reversed the optical change in all five spots.
The two larger spots seen in the right two optical images were induced post-anneal, demonstrating it was still possible
to switch the recombination pathway. A Bi2Se3/MoSe2-2xS2x 2D heterostructure was used above.
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Figure 4.10: 2D heterostructures first thermally treated in air, and then in N2. Photoluminescence spectra at different
locations. The heterostructures are monolayer MoSe2-2xS2x with 1-2 layers of Bi2Se3 CVD grown on top. (Top Optical
Images) Note how the air-annealed heterostructures appear a lighter shade of purple and have less contrast, compared
to the as-grown heterostructures. The heterostructures were then N2-annealed, resulting in the air-annealed optical
changes being partially reversed (i.e. there is a darker shade of purple and more contrast). (Bottom PL spectra) Left to
right are the as-grown, air-thermally-treated, and N2-thermally-treated (following air-thermal-treatment), respectively.
Each graph has the pre- and post-air-laser-exposed PL spectra. The data between annealing sessions was taken at
different locations (specified by the red arrows). Air-annealed samples will have a brighter PL than the as-grown
samples, but it will be weaker than a laser-treated as-grown heterostructure. The PL intensity of air-annealed
heterostructures will decrease when air-laser-treated. A Bi2Se3/MoSe2-2xS2x 2D heterostructure was used above.
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Figure 4.11: 2D heterostructures thermally treated in air and then in N2. PL spectra at the same location. The
heterostructures are monolayer MoSe2-2xS2x with 1-2 layers of Bi2Se3 CVD grown on top. (Bottom PL spectra) The data
between annealing sessions was taken at the same location. The air-laser-exposed location maintained approximately
the same intensity; however, the peak blue-shifted. The PL intensity of air-thermally-treated heterostructures will
decrease when air-laser-treated, which is the same behavior observed when recipes with very high powers and energies
are used. After N2-thermal-treatment, the PL will be quenched, and air-laser-exposure induces the PL upward, which
is the same behavior observed in as-grown heterostructures. A Bi2Se3/MoSe2-2xS2x 2D heterostructure was used above.
Detailed investigations in different gas environments shown in Figure 4.7, Figure 4.8, Figure 4.9,
Figure 4.10 and Figure 4.11 establish that oxygen (O2), and not nitrogen, H2O vapor, or carbon
dioxide, is required to induce the changes and manipulate the interlayer coupling. Thermal
treatment at 240°C was able to modulate the interlayer coupling, suggesting that the mechanism
actuates when sufficient heat is present, and illumination with a particular wavelength is not
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required. Later in the paper we demonstrate that the interlayer coupling can be tuned with high
precision by controlling both the energy applied and the partial pressure of oxygen in the
surrounding atmosphere.
Photo-excited e-h pairs in mono-layered transition metal dichalcogenides (TMDs) form tightly-
bound neutral and charged excitons.33 In direct-gap monolayer TMDs, they recombine
radiatively, producing well-known PL spectra. In few-layered and thicker TMDs the quasiparticle
band gaps are indirect, hence the KΓ indirect (non-radiative) recombination pathway becomes
more favorable, resulting in progressively suppressed PL.245 Indirect recombination in certain
heterostructures can similarly be non-radiative, if the excitons formed near a k
-vector in the
reciprocal lattice of one layer finds the most favorable recombination pathway via a lower-
energy-state that is located at a different k
-point in the reciprocal lattice of the second layer. In
all the as-grown Bi2Se3/MoS2 2D heterostructures, the PL spectra were strongly quenched,
suggesting the interlayer coupling was inducing non-radiative recombination pathways for the
excitons. Upon application of energy (either heat or laser) in an oxygen-present environment, all
heterostructures recovered their radiative recombination pathways, suggesting the interlayer
coupling was disrupted, thereby inducing the MoS2 layer to behave more electronically
independent. Next we show that the PL can be quenched again by applying energy (via either
thermal treatment or laser exposure) in an O2-free environment, demonstrating that the effects
could be reversed and the interlayer coupling restored.
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Figure 4.12: Tuning the interlayer coupling and photoluminescence intensity. (a) Variation of PL intensity under
alternating air and nitrogen environments while a focused laser (i.e. energy) is applied, demonstrating that a nitrogen
environment is able to restore the interlayer coupling. The solid black arrow shows how the baseline PL reading
monotonically grows after repeated environment cycling, and the PL’s rate-of-change varies, suggesting the 2D
heterostructure may not return to its initial (as-grown) state. The dashed blue arrow shows the overall PL intensity
growth factor, up to 215×, achievable by this cycling approach. (b) Correlation between laser (i.e. energy) exposure (in
air) and the resulting PL intensity of a Bi2Se3/MoS2 2D heterostructure using a calibrated recipe (50 µW, 6 s doses
followed by 1 µW, 60 s for collecting data), demonstrating a controlled disruption of interlayer coupling. Under this
dose recipe, the radiative recombination was found to grow approximately logarithmically, and a vast majority of
exposures result in a <5% change in intensity (see inset). See Figure 4.13 for other possible behaviors when the recipe
is changed. Inset: histogram of the change steps under each exposure. Both (a) and (b) taken together demonstrate an
unprecedented degree of controlled manipulation of the interlayer coupling and PL achievable. Later we demonstrate
this is possible in a site-selectable manner as well.
Remarkably, not only are we able to disrupt the interlayer coupling and increase the PL intensity,
but we can also restore the interlayer coupling and decrease the PL intensity, which we believe is
due to the intercalation and de-intercalation of oxygen. Figure 4.12a shows the change in PL
intensity of a Bi2Se3/MoS2 2D heterostructure that is oxygenated and deoxygenated several cycles
by switching the ambient atmosphere between air and nitrogen. This controllable switching of
radiative and non-radiative exciton recombination pathways, demonstrates the ability to both
disrupt and restore the interlayer coupling. This process can be cycled several times,
underscoring the fact that at the initial stage, under identical laser power, the oxygenation is
reversed by the mere removal of the O2 partial pressure in the ambient. This indicates that at least
initially, the oxygenation process is diffusive and does not form chemical bonds. After a few
cycles, the maximum PL intensity grows by as much as 215× and stops quenching fully,
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suggesting that other more permanent changes occur at higher energy dosage (discussed later in
the paper).
Figure 4.12b shows the variation of PL intensity measured after repeated doses (t=6s) of combined
exposure to air (at ambient pressure) and laser power (at 50 µW). At this dose-value, the PL was
found to grow approximately logarithmically with combined exposure (along with incident
energy), suggesting the interlayer coupling can be tuned with high precision. The logarithmic
shape is in agreement with Fick’s law of diffusion, which states the diffusion flux will decrease
as the system reaches equilibrium, and whose general solution has an exponential form.
Additionally, Fick’s law has been used to describe the intercalation process, including the
intercalation of lithium into vertically stacked bilayer graphene.82 The inset shows that the
intensity-change (IPL) can be as low as~5%, suggesting possible applications as low-cost
atomically-thin laser calorimeters or photon-counters. Figure 4.13 demonstrates other possible
behaviors when the recipe is changed.
Figure 4.13: High Tunability and control of interlayer coupling and PL intensity. Rate of change of PL is dependent on
the recipe of energy applied. A Bi2Se3/MoS2 crystal was air-laser-treated using five different recipes, where the power
was held constant power, but time was changed. Different locations on the same Bi2Se3/MoS2 crystal were studied,
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allowing the results to be better compared. The results show that 6 sec. at 48.6µW produced the most consistent results.
The difference in curve shapes is due to competing factors: too much air-laser-treatment results in degradation of the
heterostructure; however, too little power does not induce sufficient change to be permanent. Additionally, using very
low powers (~0.91µW) causes the PL intensity to decline. I believe this is because the powers perturb the
heterostructure into a better coupling, which squeezes the intercalated oxygen out. It has been shown previously that
changing the interlayer coupling will change the rate of diffusion of oxygen intercalating between graphene and the
surface (discussed more in detail later).
Figure 4.13 demonstrates that changing the recipe changes the response of the Bi2Se3/TMD 2D
heterostructure. The power was held constant while the time was exposure time changed. The
same 2D heterostructure crystal was used, where only different locations on the same crystal were
probed, thereby mitigating different samples as a variable. The red arrows show the different
locations that were probed, where the laser and oxygen induced color changes can be seen. The
lack of clear logarithmic behavior when time intervals outside of six seconds were used, suggests
that the one minute data collection period began to partially reverse the effects and disrupt the
predictable logarithmic behavior.
Figure 4.14: DFT calculations predict intercalated O2 diminishes interlayer coupling (a) DFT calculations of a
rotationally aligned (i.e. twist angle is 0°) Bi2Se3/MoS2 superlattice predict significant charge redistribution into the
interlayer region, and an influential interlayer coupling. This is discussed further in Chapter 6. (b) However, when O2
molecules are placed in the interface between the layers, DFT calculations predict the average interlayer separation
increases from 3.57Å to 4.18Å (17% increase), diminishing the interlayer coupling. These results are in agreement with
previous studies where intercalated O2 between a 2D material and the substrate induces the 2D material to behave
electronically independent (i.e. “freestanding”). Further, intercalation and de-intercalation are diffusion-dependent
processes, offering an explanation for why the PL intensity rises and falls so quickly as the environment changes (Figure
4.12). DFT calculations predict an interlayer separation ~4.7 times larger than the diameter of an oxygen atom,
suggesting sufficient space exists for O2 molecules to reside. The above structure was calculated by students in Prof.
Arun Bansil’s group and reproduced with their permission.
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The rapid changes observed in Figure 4.12 by simply switching the environment between air (i.e.
O2-present) and nitrogen (i.e. O2-absent) suggest that oxygen is diffusing in and out of the 2D
heterostructure. As seen in Figure 4.14, the O2 molecule is relatively small compared to the
interlayer spacing. Further, it has been shown that O2 molecules are able to permeate into Bi2Se3,
demonstrating they are able to fit between the atoms.55,59,242 Density functional theory (DFT)
calculations predict that the interlayer coupling in a Bi2Se3/MoS2 2D heterostructure induces
significant charge redistribution in the interlayer region, hybridizing nearest neighbor atoms to
form bonds, and that it is influential in modifying the bandstructure.103 Interestingly, when O2
molecules are placed in the interlayer region, DFT calculations predict that the materials will
begin to separate, diminishing the interlayer coupling, and inducing the layers to behave more
electronically independent. It can be seen in Figure 4.14 that the interlayer spacing increases on
average by 17% when five O2 molecules are intercalated. Together these results hint at the
interesting possibility that these atomically-thin layers may be excellent candidates for oxygen
storage devices, potentially storing 69 kg/m3 (a factor of 52 times the density of O2 gas at 1 atm).
Figure 4.15: Site-selective manipulation of interlayer coupling and photoluminescence. (a) Schematic of a method to
write patterns with site-selected interlayer coupling strength on Bi2Se3/TMD 2D heterostructures using a focused laser.
(b) Optical image of a laser-written pattern on a Bi2Se3/MoS2 2D heterostructure. The letters “NEU” were “drawn”
using different exposure times. (c) Fluorescence microscope image of the same sample (excitation λ=488 nm). We note
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that using a focused laser beam, excitons of selected regions could be programmed to recombine radiatively (bright
regions) or non-radiatively (dark regions), where the size of the affected area is dependent on the laser spot geometery
and recipe used (i.e. the power and exposure duration). The smallest “radiative” regions were below a micron in
diameter. This method allows for the rapid manipulation and measurement of the interlayer coupling with high spatial
resolution, facilitating experiments that produce statistically significant results.
Figure 4.15 demonstrates a low-cost method to rapidly and precisely manipulate the interlayer
coupling of Bi2Se3/MoS2 2D heterostructures with submicron spatial resolution. Figure 4.15a is a
schematic showing how a focused laser beam can be used to apply calibrated energy doses to
precisely manipulate the interlayer coupling with high (submicron) spatial resolution. Figure
4.15b shows the optical image where the method in Figure 4.15a was used to trace the letters
“NEU” using different exposure times, demonstrating a facile method to manipulate the
interlayer coupling. The laser-written sample was then imaged using a fluorescence microscope
(Figure 5c, λex=488 nm), demonstrating the ability to rapidly measure the degree of interlayer
coupling disruption. The ability to rapidly write and read patterns with varying interlayer
coupling strength, facilitates experiments that produce statistically significant results. The
fluorescence image also demonstrates site-selective light emission with a spatial size close to that
of the incident laser spot, enabling photoluminescing pixels (PLPs) tailored down to sub-micron
diameters. These site-programmable, color-selectable, atomically-thin, micron-scale PLPs (with
effective volumes ~10-21 m3) are attractive for optical and optoelectronic applications that require
ultra-small form-factors.
We now discuss the various possible mechanisms that could lead to the observed phenomena.
As mentioned earlier, we tested the effect of all the primary gaseous species present in air and
found it to be limited to oxygen (Figure 4.7, Figure 4.8, Figure 4.9, Figure 4.10, and Figure 4.11),
suggesting only oxygen is able to intercalate. While attempting to understand why oxygen would
have such a dramatic effect, vice the other gases, we found an large amount literature that
discusses the impact of oxygen on Bi2Se3 and other materials in its class (e.g. Bi2Te3), suggesting,
under different circumstances, both diffusive behavior as well as bond-forming mechanisms.55,58–
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60,242,246–249 In sharp contrast, we were unable to find any literature that demonstrates evidence
of absorption or chemical reaction with nitrogen, argon, or carbon dioxide, suggesting these
molecules are significantly less reactive/interactive with Bi2Se3. The collective body of work
appears to indicate the energetics facilitate these interactions with oxygen, vice the other gases.
From this, we conclude that interaction of oxygen, mediated by the bismuth selenide layer is the
most likely manner the changes are initiated. Further, oxygen has been shown to easily intercalate
between 2D crystals and their substrates, decoupling the two materials, and inducing them to
behave more “freestanding” (i.e. electronically independent).127,133–135,140–142,239,240 Indeed, our DFT
calculations of a Bi2Se3/MoS2 2D heterostructure predict that intercalated O2 molecules force the
materials apart, enlarging the interlayer separation by 17%, thereby diminishing their interlayer
hybridization (see DFT calculations later in the chapter - Figure 4.18). This is further supported
by our Raman measurements, which show that laser exposure in air increases the intensity of the
monolayer MoS2 modes (see Figure 5.10, Figure 5.11, and Figure 4.16).
Figure 4.16: Raman response of Bi2Se3/MoS2 pre and post laser exposure. The Bi2Se3 peaks decrease, suggesting the
modes are being suppressed. This is possibly related to the observation that laser exposure in air breaks the Bi2Se3
into nanocrystals, thereby diminishing its crystallinity. The MoS2 peaks, very interestingly, increase, suggesting that
the coupling has been diminished, in agreement with previous observations.103 Amplifying information and Raman
studies of other 2D heterostructures can be found in Chapter 3, Figure 5.10 and Figure 5.11.
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Previous work has shown that the intercalation of oxygen can diminish the interlayer coupling
and modify the Raman modes.140 Furthermore, intercalation is a diffusive process that has been
shown to be reversible,134,135,142,240 which could explain why laser exposure in an O2-free
environment is able to rapidly reverse the changes. Most notably, though, despite the large
change in the observed return of the MoS2 PL, the Raman peaks for both bismuth selenide and
molybdenum disulfide are near-identically retained before and after the laser treatment,
suggesting very little structural, electronic or chemical changes to each of these layers,
individually. For this reason, physical separation of the two layers due to diffusion of oxygen into
the interlayer region appears to be a very reasonable underlying mechanism.
If in fact oxygen is diffusing into the interlayer, what path(s) does it take? A number of past works
have explored the intercalation of atoms in micron-scale graphene and other 2D materials, where
it has shown to be highly dependent on the number of grain boundaries.81,82,127,134–139,142,250,251
More specifically, a single grain boundary of these systems was sufficient to enable large-scale
intercalation in micron-scale samples, strongly suggesting that, a high concentration of
edges/grain boundaries under the laser spot might not be needed for the observed rapid
intercalation/deintercalation. Bi2Se3 grows highly crystalline on the underlying MoS2, and when
the 2D heterostructure is then exposed to a laser in air, the Bi2Se3 becomes polycrystalline (Figure
4.2), suggesting the formation of grain boundaries. Together these results suggest (1) Intercalation
via grain boundaries is well-known, and do not necessarily require a high density of grain
boundaries to produce observable effects, and (2), these intercalations result in a de-coupling of
the parent layers electronically. While that is not a direct evidence that intercalation of oxygen is
taking place, in our case, these are strong indicators of the most possible mechanism (more
detailed explanation is below).
Other possibilities include the formation of chemical bonds, trapped oxygen at edge sites, or new
materials, however, these are less likely, at least at the initial stages of our experiments. It is well-
established that Bi2Se3 is sensitive to oxygen, including the partial pressure of oxygen in air.55,58–
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60,246–248 Studies have also shown that oxygen interacting with Bi2Se3 can form a native oxide
layer. It is unlikely that native oxide growth is the primary mechanism, at least initially, because
of the rapid rate of change and the easy reversibility of PL. We see quite easily that the oxygen
insertion and release could be achieved (keeping all other parameters constant) by simply
changing the partial pressure of oxygen in the surrounding environment. Additionally, previous
studies measured topographical changes due to native-oxide growth in Bi2Se3, 55 whereas we
found no such height changes (Figure 4.4 and Figure 4.5). Oxidation of Bi2Se3 has also been shown
to reveal new Raman spectral peaks.252–254 In contrast, our Raman spectroscopic measurements
show no new peaks (Figure 5.10, Figure 5.11, and Figure 4.16). These, taken together with the
oxygenation/release time-scales point more strongly in favor of a diffusive mechanism. Further,
the logarithmic behavior when low powers are applied (Figure 4.12b) is in agreement with Fick’s
law of diffusion, which also has a logarithmic shape.
In conclusion for this chapter, the interlayer coupling in vertically stacked 2D materials has
demonstrated the ability to dramatically alter their properties, and is considered a promising
parameter for designing materials with tailored capabilities. The Bi2Se3/MoS2 2D heterostructure
an ideal platform to study the interlayer interaction of 2D materials because it offers the ability to
tune the interlayer coupling in-situ and with high-spatial resolution. From a fundamental
perspective, in addition to rich excitonic physics, this system interplays strong spin-orbit coupling
in non-centro-symmetric crystal structures, and hence could potentially demonstrate novel
correlated, spin and valley physics.255–257 In addition, as site-programmable, color-selectable,
atomically-thin, micron-scale and intensity-tunable photoluminescing pixels (PLPs), this system
could be attractive for ultrathin and flexible optical information storage devices, color converters,
micro-cavity-lasers, and other photonic, plasmonic and optoelectronic applications.39,258–260 The
strong oxygen-selectivity of these heterostructures could also be potentially used as low-cost
oxygen-sensors and photon/power meters. We also showed data that suggested these materials
could be used as O2 storage devices, potentially storing 69 kg/m3 (a factor of 52 times the density
of O2 gas at 1 atm).
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4.1 Discussion of Possible exciton recombination pathways
This family of 2D heterostructures has produced intriguing data that speaks both to the promising
applications, as well as the complexity of the underlying science. There are a number of well-
studied mechanisms in literature that may apply because they are in agreement with the data.
Out findings indicate the Bi2Se3/TMD 2D heterostructure family potentially has multiple
competing mechanisms. Below we describe the mechanisms that we think could be present, and
then justify our conclusions with our observations and references. To best convey our thought
process, we list all the key observations (KOs) first.
Key Observations
1. The PL of the TMD is over 99% quenched when only one layer of Bi2Se3 is CVD grown on
the TMD. One layer of Bi2Se3 is not thick enough to reflect or absorb the incoming and
outgoing photons, meaning the Bi2se3 introduces a non-radiative electron-hole
recombination path.
2. Laser exposure in an air induces several changes :
a. PL intensity increases.
b. PL intensity can be precisely tuned (i.e. recovered with high control) over several
orders of magnitude.
c. Affected regions undergo a perceived color change, appearing brighter and with
less contrast, compared to as-grown heterostructures.
d. PL peak position is in the same location as the monolayer TMD’s PL peak position
from pre-Bi2Se3 growth, strongly indicating that the recovered PL observed is from
the excitons and trions in the TMD. This would indicate that air-laser-treatment
removes the non-radiative electron-hole recombination pathway, allowing the
excitons and trions to recombine at the K-point in the TMD, where it is direct
bandgap.
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e. The laser exposure recipe (i.e. laser power and time interval) will affect the rate of
change of the PL intensity and peak position shift.
3. The laser induced changes can be spatial determined with submicron precision (i.e. the
laser spot size).
4. Oxygen is required to induce changes when a laser or thermal treatment are applied.
5. Thermal treatment or laser-exposure in an oxygen-free environment reverses the oxygen-
induced changes.
6. Bi2Se3/MoS2 2D heterostructures are p-type compared to pristine monolayer MoS2.
7. Laser exposure does not remove Bi2Se3 from the surface, as seen by AFM scans.
8. Higher energy doses appear to induce more permanent changes.
9. Bi2Se3 grows crystalline and with long-range order on the TMD, suggesting strong van
der Waals epitaxy-mediated growth between the two component layers.
10. Density functional theory (DFT) calculations of the Bi2Se3/MoS2 2D heterostructure predict
that intercalated O2 molecules will increase the interlayer separation, disrupt the
interlayer bonding, and diminish the interlayer coupling, thereby inducing the two
materials to behave more “free-standing”.
Exciton recombination pathways that may be present
#1 –A straddled or staggered bandgap induces the photoluminescence quenching
Well-coupled 2D heterostructures have been shown to have overlapping bandgaps,149,261 which
we believe is happening when Bi2Se3 is grown on the TMDs, for reasons explained in key
observation 9 (KO-9). There are three different types of overlapping bandgaps: broken, straddled,
and staggered. Device data showed that Bi2Se3/MoS2 is p-type, meaning the bandgap is not broken
(broken bandgaps behave metallic), but is forming either a straddled or a staggered bandgap. All
four TMD’s studied in this work are wide bandgap compared to Bi2Se3 (>1.6eV to ~0.3eV),60
meaning both a straddled and a staggered bandgap would introduce a non-radiative electron-
hole recombination pathway.
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Mechanism #1 is in agreement with the PL quenching (KO-1) and why Bi2Se3/MoS2 is p-type
(KO-6).
Figure 4.17: Possible non-radiative electron-hole recombination pathways leading to PL quenching in as-grown 2D
heterostructures. Both straddled and staggered bandgaps are in agreement with the data and explain the
photoluminescence quenching observed because they introduce a non-radiative recombination pathway. Although a
broken bandgap also has a non-radiative recombination pathway, it disagrees with KO-6; broken band gaps are
metallic, whereas our device was p-type.
#2 – O2 intercalates between the monolayer MoS2 and the Bi2Se3, and then diminishes the
interlayer coupling
Laser exposure in air induces numerous dramatic optical and PL changes; however, the Bi2Se3 is
not removed (KO-7), as shown with AFM. We know that whatever the mechanism is, it is highly
local (KO-3) and that O2 is required for the process (KO-4).We believe that O2 intercalates into the
interlayer spacing of the heterostructure, where it disrupts the interlayer coupling, leading to the
changes in properties observed. It has been shown that O2 intercalating between 2D materials and
their surface decouples the 2D material, making quasi free-standing.127,134,135,137,142,239,240 In this case,
it decouples the materials, allowing the TMD to regain the radiative recombination pathway.
Additionally, each O2 molecule is only able to disrupt the interlayer coupling locally, meaning
the amount of disruption in a certain area is dependent on the number of O2 molecules; the more
O2 there is, the greater the interlayer coupling disruption. This mechanism (or radiative
recombination pathway) is in agreement with numerous key observations, stated below:
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KO-2a & KO-2d: As the interlayer coupling is disrupted, the non-radiative e-h recombination
path will no longer be allowed, permitting e-h excitonic pairs to radiatively recombine in the
TMD.
KO-2b: The amount of recovery is dependent on the amount of O2 that intercalates, explaining
the high control.
KO-2e: Changing the laser-exposure recipe will affect the diffusion of O2, thereby affecting the
rate of change.
KO-5: The changes can be reversed by thermal treatment or laser exposure in an O2 free
environment, potentially due to deintercalation.
KO-8: Laser exposure breaks the Bi2Se3 up into small grains. It has been previously shown that
oxygen intercalation is highly edge-limited and dependent on the number of grain
boundaries,127,134,135,137,239,240 suggesting increasing the number of grain boundaries will affect the
rate of diffusion.
KO-10: DFT calculations predict intercalated O2 will disrupt the interlayer interaction.
A number of past works have explored the intercalation of oxygen and other atoms in micron-
scale graphene and other 2D materials, where it has shown to be highly dependent on the number
of grain boundaries, and consistently shown to have an interlayer decoupling effect.81,82,127,134–
139,142,250,251 More specifically, a single grain boundary of these systems was sufficient to enable
large-scale intercalation in micron-scale samples, strongly suggesting that, a high concentration
of edges/grain boundaries under the laser spot might not be needed for the observed rapid
intercalation/deintercalation. As shown in Figure 4.2, Bi2Se3 grows highly crystalline on the
underlying TMD, and when the 2D heterostructure is then exposed to a laser in air, the Bi2Se3
becomes polycrystalline, suggesting the formation of grain boundaries. Together these results
suggest (1) Intercalation via grain boundaries is well-known, and do not necessarily require a
high density of grain boundaries to produce observable effects, and (2), these intercalations result
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in a de-coupling of the parent layers electronically. While that is not a direct evidence that
intercalation of oxygen is taking place, in our case, these are strong indicators of the most possible
mechanism.
Figure 4.18: DFT calculations comparing a Bi2Se/MoS2 superlattice with and without O2 intercalated. The calculations
predict that O2 intercalation will increase the interlayer separation, disrupt the interlayer bonding, and diminish the
interlayer interaction. Note how the interlayer separation increases after adding O2 molecules, going from an average
separation of 3.57Å to an average separation of 4.18Å, a 17% increase. Interestingly, the O2 molecules create an uneven
landscape of selenium atoms at the interface, pushing different atoms to different separation values. The above 2D
heterostructure is rotationally aligned (i.e. twist angle is 0°), where 3 Bi2Se3 unit cells are the same length as 4 MoS2 unit
cells
Figure 4.19: DFT calculations showing the location of the O2. These atomically-thin layers may be excellent candidates
for oxygen storage devices, potentially storing 69 kg/m3 ¬(a factor of 52 times the density of O2 gas at 1 atm). The
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Bi2Se3/MoS2 superlattice has a predicted volume of 1.936nm3 (1.247x1.086x1.43 nm), and the combined mass of five O2
molecules is 1.328x10-25 kg, yielding a density of 68.6 kg/m3.
Figure 4.20: Diagrams demonstrating the creation of grain boundaries, facilitating O2 intercalation into the interlayer
region. The upper diagram shows the heterostructure in an O2 environment prior to laser-exposure, and the lower
diagram is during laser-exposure.
Heterostructure prior to air-laser-treatment
Heterostructure after and during air-laser-treatment
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#3 – Potential of edge chemistry to affect the interlayer coupling
We do not believe that oxygen forms chemical bonds, at least initially, since this would contradict
the easy release by simple reversal of the oxygen partial pressure, reversal timescale (Figure 4.12)
and the absence of any oxide-related Raman peaks (Figure 4.16). On the other hand, edges or
edge chemistry might provide shallow trap sites that arise out of complex interplay between the
edge energy and surface energy of the underlying layers. Such shallow trap sites might provide
the reversible behavior seen in our system, without the need for formation of bonds. However,
the likelihood that the mere physical presence of trapped oxygen at the grain boundaries can
(without forming any chemical bonds) completely change the optical properties of the Bi2Se3
layers is possibly low.
4.2 Discussion on interaction of Bi2Se3 with oxygen
We consider why we believe that the intercalation should be limited to oxygen and not occur for
the other gaseous species. To address this, we note that the possible effect of all these gases,
including water vapor, were tested for their effect on the 2D heterostructures, by investigating
our samples dry air (79% N2 +21% O2), humid nitrogen (H2Ovapor+N2), nitrogen, argon, and air. By
process of elimination it was found that only O2 had the observed effect.
While attempting to understand why oxygen would have such a dramatic effect, vice the other
gases, we found an overwhelming amount literature that discussed the impact of oxygen on
Bi2Se3 and other materials in its class (e.g. Bi2Te3), suggesting, under different circumstances, both
diffusive behavior as well as bond-forming mechanisms.55,58–60,242,246–249 In sharp contrast, we were
unable to find any literature that demonstrates evidence of absorption or chemical reaction with
nitrogen, argon, or carbon dioxide, suggesting these molecules are significantly less
reactive/interactive with Bi2Se3. The collective body of work appears to indicate the energetics
facilitate these interactions with oxygen, vice the other gases.
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On one hand, some of the above-mentioned findings suggest that oxygen is able to readily absorb
into Bi2Se3. In particular, Bansal et al.242 showed that oxygen can diffuse through even 12 QLs of
Bi2Se3 epitaxially grown on pure Si under atmospheric exposure, and will completely oxidize the
underlying Si substrate into its amorphous oxide. Remarkably, no structural deformation of Bi2Se3
was observed (i.e. no change in lattice parameters or appearance of new bonds etc.). The
experiment was done using high resolution cross-sectional TEM. At the same time, no such
diffusive behavior is known to occur for oxygen in TMDs. Taken together, this suggests that the
possibility that oxygen, diffusing through the Bi2Se3 layers, can diffuse all the way till it meets the
Bi2Se3 interface. Our DFT calculations were utilized to interrogate the stability of such
intercalation and their effect, if any, on inter-layer separation distance.
On the other hand, the other possibility, i.e. that of the formation of chemical bonds was less
likely, at least at the initial stages of our experiments. It is well-established that Bi2Se3 is sensitive
to oxygen, including the partial pressure of oxygen in air.55,58–60,246–248 Studies have also shown that
oxygen interacting with Bi2Se3 can form a native oxide layer. It is unlikely that native oxide
growth is the primary mechanism, at least initially, because of the rapid rate of change and the
easy reversibility of PL. We see quite easily that the oxygen insertion and release could be
achieved (keeping all other parameters constant) by simply changing the partial pressure of
oxygen in the surrounding environment. Further, oxidation of Bi2Se3 has been shown to reveal
new Raman spectral peaks.252–254 In contrast, our Raman spectroscopic measurements which show
no new peaks. These, taken together with the oxygenation/release time-scales point more strongly
in favor of a diffusive mechanism. Further, the logarithmic behavior when low powers are
applied (Figure 4.12b) is in agreement with Fick’s law of diffusion, which also has a logarithmic
shape.
If in fact oxygen is diffusing into the interlayer, what is the mechanism(s), and what path(s) does
it take? To answer this question as best we can, we have conducted considerable literature
research and compared the conclusions to our data. Below we will lay out the possible
mechanisms we found for how oxygen might intercalate into the interlayer region, and then
explain why we think the most likely mechanism is through grain boundaries.
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Previous research has shown that atoms are able to diffuse into material through line-defects (in
the body of the crystals, not grain boundaries) using a process called defect-assisted
diffusion.262,263 Although the defects facilitate diffusion, the process of atoms diffusing into
crystals is still relatively slow occurring mostly over timescales of hours, especially if the diffusion
is required to travers several nanometers into the crystal and only assisted with little heating.
In contrast, we observed relatively fast rates of change in PL, suggesting a relatively fast rate of
diffusion. Further, we are able to affect these changes at relatively low laser powers (83.1μW),
and we can even affect them when more than five layers of Bi2Se3 are grown on top. Together this
data would suggest line-defect-assisted diffusion is unlikely to be the primary mechanism, and
grain-boundary assisted intercalation is the most likely mechanism.
Lastly, ARPES measurements have shown that under very low exposures (<0.1 Langmuirs),
oxygen inclusion hole-dopes ultraclean Bi2Se3 samples.60 However, the exposure level in our
system is ~109 orders of magnitude larger, and so we don’t expect doping to be a dominant cause
behind the observed switching of recombination pathways.
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Chapter 5: Tunable Photoluminescence in Bi2Se3/TMD
2D Heterostructures for Potential Write-Read-Erase-Reuse
Applications
In recent times, atomically-thin, layered 2D transition metal dichalcogenides (TMDs) have
attracted enormous attention both due to their remarkable electronic,258 optical,26 spin and valley
properties,264 and their potential for diverse flexible nanoelectronic,265 optoelectronic,266 sensing,40
and energy applications.100 Carriers in these systems form tightly-bound neutral and charged
excitons,33,267 and monolayer TMDs show strong photoluminescence (PL) peaks which result from
a direct-recombination of these excitons. Controlling the population and recombination energy
of these excitons is extremely important for both fundamental and applied research, and hence,
methods for direct manipulation of excitons is a topic of rapidly-growing interest. This work can
be found at the following reference.268
Figure 5.1: Figure briefly summarizing the capabilities demonstrated in this chapter.
In this chapter, we demonstrate highly controllable, reversible, and site-selectable switching
between radiative (i.e. photoluminescent) and non-radiative exciton-recombination pathways in
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Bi2Se3/TMD 2D heterostructures, possibly due to the regulated intercalation and deintercalation
of oxygen (Figure 5.2 and Chapter 4).104 Further, our approach allows permanent, in-situ,
electrode-less, and use-specific programming of both the radiative exciton populations and their
recombination energy values. To demonstrate this, we have directly synthesized a family of 2D
vertical heterostructures consisting of mono/bi/tri-layers of Bi2Se3 grown on top of several
monolayer TMDs (WS2, MoSe2, and MoS2) and a TMD alloy (MoSe2-2xS2x). The monolayer TMD’s
signature PL is quenched in all as-grown 2D heterostructures, indicating the interlayer coupling
is inducing non-radiative recombination pathways for the TMD’s excitons. However, the PL
intensity can be recovered at small increments by exposing them to calibrated laser doses in air,
suggesting the interlayer coupling is being controllably disrupted. Interestingly, not only does
the intensity increase, but the PL peak position shifts with increasing laser exposure as well,
suggesting multiple excitonic parameters are being manipulated. Our work indicates that both
the neutral and charged exciton binding energies shift, as well as the ratio of their populations,
suggesting these parameters can be tuned within a range. Finally, all the laser-induced changes
can be reversed by thermally annealing the 2D heterostructures in Ar or N2, suggesting the
interlayer coupling can be restored. Taken together, this family of 2D heterostructures offers a
wide range of emission energy values in the visible (1.5eV<Eph<2eV), where complex patterns
with submicron features and user defined the exciton parameters (e.g. PL intensity, peak position,
binding energies, relative populations) can be “written” into the material, suggesting interesting
optoelectronic applications.
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Figure 5.2: Reversible oxygen absorption and its role in manipulating the photoluminescence intensity (a) Optical
image of a Bi2Se3/MoS2 vertical 2D heterostructure exposed to a laser in an oxygen-present environment (i.e. air), where
the optical properties of the affected area were altered from purple to white, identified by a red arrow. (b) PL spectra
from the same spot before and after laser exposure. (c) The laser was now applied to a different location (orange arrow)
on the same sample at the same power (168μW), but the environment was changed from oxygen to nitrogen.
Interestingly, the color change seen in (a) was not observed. (d) Spectra before and after the laser was applied showing
no perceivable PL or change, suggesting the sample was not affected. (e) Optical image of the setup where N2 gas was
flown across the sample to displace the air and oxygen. (f) Variation of PL intensity under alternating air and N2
environments while a focused laser is applied, demonstrating that the PL intensity is dependent on the environment,
and can be reversed by simply switching the environment.
Our previous work demonstrated that when Bi2Se3 is grown on monolayer MoS2, the resulting
2D heterostructure forms a strong interlayer coupling that facilitates electron exchange and leads
to the formation of a new band structure.103 More specifically, Bi2Se3 grows at preferred twist
angles and moiré patterns that form supercells with their own band structure distinct from those
of Bi2Se3 and MoS2. Density functional theory (DFT) calculations predict the formation of
interlayer hybridization and an indirect bandgap, providing an explanation for the PL
suppression observed in the Bi2Se3/TMD 2D heterostructures studied in this work. The laser
exposure induced PL recovery in Bi2Se3/MoS2 2D heterostructures has appears to result from a
reversible absorption/desorption of oxygen, which electronically decouples the Bi2Se3 and MoS2
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layers, and allows the MoS2 to regain PL (Figure 5.2).104 In this work, we show that this
phenomenon can be extended to a family of Bi2Se3/TMD structures.
Figure 5.3 As-grown Bi2Se3/TMD 2D heterostructures (a) and (b) Optical images of monolayer MoSe2 and a
Bi2Se3/MoSe2 vertical 2D heterostructure, respectively. (c) AFM image of the red box in (b), demonstrating monolayer
Bi2Se3 (with bilayer islands) grew uniformly across the entire MoSe2 crystal, completely covering it. Inset: Line profile
of location shown by black line. (d) Side-view diagram of a heterostructure. Vapor-phase chalcogenization (VPC) is
used to grow monolayer TMDs on SiO2, and then vapor-phase deposition (VPD) is used to grow Bi2Se3 on the
monolayer TMDs.
Figure 5.3a shows an optical image of monolayer MoSe2 grown on SiO2 using vapor-phase
chalcogenization (VPC).31 Figure 5.3b shows a Bi2Se3/MoSe2 vertical 2D heterostructure, where 1-
2 layers of Bi2Se3 were uniformly grown on top of the TMD (see atomic force microscope (AFM)
image in Figure 5.3c), using vapor-phase deposition. Figure 5.3d is a side-view schematic of a
typical Bi2Se3/TMD 2D heterostructure. Despite the huge lattice mismatch between Bi2Se3 and
each of the TMDs studied (e.g. 3.62Å and 2.89Å for Bi2Se3 and MoSe2, respectively), uniform 1-2
layers of Bi2Se3 grow with high regularity and crystallinity on top of all the TMDs (see diffraction
images in Figure 5.4), suggesting strong van der Waals epitaxy-mediated growth between the
two component layers.243
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Figure 5.4: Representative SAED images of as-grown and laser-exposed 2D heterostructures : Bi2Se3/MoSe2 and
Bi2Se3/WS2. The laser-exposure makes the Bi2Se3 nano-crystalline and polycrystalline. The TMD will also become nano-
and polycrystalline if a sufficiently high laser power is used.
As shown in Figure 5.4, the highly crystalline growth of the Bi2Se3, as well as the consistent
response to laser exposure across independent samples, suggests that the as-grown 2D
heterostructure interface is clean and absent of foreign atoms. Bi2Se3 grows highly crystalline
across the whole TMD (i.e. micron sized domains), suggesting the foreign atoms are not present
to impede or modify the growth. Further, the laser exposure and Raman results have been highly
consistent across independently grown samples, suggesting the interface is the same or very
similar. Since it is unlikely that the samples would be equally “dirty”, the likeliest explanation is
that the interface is clean.
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Figure 5.5: Bi2Se3/MoSe2(1-x)S2xalloy SAED and TEM images. The SAED image shows how both the monolayer MoSe2-2xS2x
and the Bi2Se3 grow as crystals, indicating that there is strong epitaxial mediated growth, and an interaction between
the materials. Bi2Se3 seems to prefer to grow at 15°; although, other angles have been recorded. The TEM image shows
how MoSe2-2xS2x grows as a triangle, further evidence that it grows as a crystal. The square in the middle of the image
is an MoO2 crystal that became the seed for the MoSe2-2xS2x crystal. We are uncertain what the darker spots on the 2D
heterostructure are, but believe they are either the Bi2Se3 (it possible grew as an incomplete growth) or they are
undissolved PMMA (PMMA absorbs electrons, which is why they are so dark).
Figure 5.5 shows a TEM diffraction image of a Bi2Se3/MoSe2-2xS2x 2D heterostructure, where the
clear spots suggest that well-formed crystals are present. It should be noted that the Bi2Se3 did
not grow crystallographically aligned, but grew at a twist angle. As discussed in Chapter 5 in
more detail, the lack of chemical periodicity (i.e. the Se and S atoms are randomly distributed)
appears to remove the aligned state (i.e. 0° twist angle) as the lowest energy twist angle, where
most growths appear to be centered around 15°. The TEM image is to show that the atoms
maintain their triangular shapes after being mechanically transferred onto the TEM grids,
suggesting that they maintain their crystallographic order and structure. The dark spots on the
TEM image are likely reminiscent PMMA and not 2D heterostructure induced phase changes to
the electrons.
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Figure 5.6: Manipulating the optical and excitonic properties in Bi2Se3/TMD 2D heterostructures (a) Optical image of
the same Bi2Se3/MoSe2 2D heterostructure from Figure 5.3b after exposure to a laser showing a perceptible change in
color. (b) Optical image of a different Bi2Se3/MoSe2 2D heterostructure with a laser-written grid of “bytes”,
demonstrating the ability to write complex patterns with submicron features using a low-power laser. The signature
PL peaks of monolayer TMDs, which are suppressed in all as-grown heterostructures, appear strongly at these laser-
exposed spots, and re-quench when annealed in N2 or Ar (discussed later). (c)-(d) Normalized PL spectra of the entire
range of 2D heterostructures studied. (c) The monolayer TMDs on SiO2 prior to the Bi2Se3 growth. (d) The
corresponding 2D heterostructures with both as-grown and laser-treatment spectra, demonstrating the wide range of
photon energies over which the PL can be strongly tuned. Since the PL is a direct measure of the radiative excitons and
trions - whose recombination energies are indicated with arrows in (c) - these samples reflect a family of radiative
exciton population-tunable 2D materials, with wide-band selectable values in the visible photon energies. Diamond-
and circle-capped arrows correspond to as-grown and laser exposed 2D heterostructures, respectively. (e) PL spectra
from a Bi2Se3/WS2 2D heterostructure exposed to calibrated energy doses, as well as the PL spectrum from the
corresponding monolayer WS2 crystal prior to Bi2Se3 growth, demonstrating the PL intensity and peak position are
tunable. (f) Bi2Se3/WS2 2D heterostructure PL peaks from (e), where the dose-dependent peak position evolution is
better displayed.
Figure 5.6a is the same Bi2Se3/MoSe2 2D heterostructure as in Figure 5.3b after being exposed to a
laser at multiple locations (identified by the dark purple to light purple color change). Similar
optical changes were observed in all 2D heterostructures tested (Figure 5.7 and Figure 5.16).
Interestingly, the color change remains even after the 2D heterostructure has been kept
unprotected in air for several months, suggesting the laser-induced modifications are stable. AFM
scans from before and after laser exposure demonstrate that the changes are not due to Bi2Se3
simply being removed (Figure 5.9). Complex patterns with submicron features, including a grid
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of “bytes” (Figure 5.6b) and alphabetical letters (Figure 5.8), can be written using a low-power
laser.
Figure 5.7: Demonstration that laser exposure in air effects are reversed through annealing in either Ar or N2 (i.e. an
O2-free environment) for Bi2Se3/MoSe2 and Bi2Se3/WS2 2D heterostructures. The optical and PL effects from laser-
treatment in air can be reversed by annealing samples at 240° for 3 hours in an O2-free environment. After annealing
samples exposed to a low laser dosage, they often appear and behave indistinguishable to pristine samples; if higher
doses are applied, the annealing may not fully reverse the effects, possibly due to changes in the crystal structure. The
samples above were a monolayer TMD with 1-2 layers of Bi2Se3 deposited using vapor-phase deposition (VPD).
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Figure 5.8: Complex patterns and grids can be controllable written into Bi2Se3/TMD 2D heterostructures with
submicron precision. (a) The letters “NEU” are written into Bi2Se3/MoS2 using a more elegant font. (b) The letters
“NEU” were written into Bi2Se3/MoSe2, as well as an elongated “N” to show how submicron spots are used to write
the patterns.
Four different Bi2Se3/TMD 2D heterostructures were synthesized, where the normalized
representative PL spectra of each are shown in Figure 5.6c. Interestingly, when only one layer of
Bi2Se3 is grown on any of the monolayer TMDs using vapor-phase deposition, the bright PL
quenches >99% (Figure 5.6d and Figure 5.6e), suggesting the interlayer coupling between the
Bi2Se3 and the TMD induces a non-radiative recombination path for the excitons. Previous work
demonstrated this in Bi2Se3/MoS2 2D heterostructures, and used DFT calculations to predict the
formation of interlayer hybridization and an indirect bandgap, providing an explanation for the
PL suppression.103
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Figure 5.9: Atomic Force Microscopy (AFM) analysis of Bi2Se3/MoSe2 and Bi2Se3/WS2 : as-grown vs. laser exposed in air
vs. O2-free annealed. (a) AFM image of the as-grown Bi2Se3/MoSe2 sample shown in (e), Figure 1b, and Figure 1c. The
2nm step height reveals that there is one tri-layer of MoSe2 with 1-2 continuous quintuple-layers of Bi2Se3 on top. The
nanostructure was then exposed to a laser in air (see (f)) and then it was O2-free annealed in Ar (see (b) and (g)). (b)
AFM image of the Bi2Se3/MoSe2 sample shown in (g), which was exposed to a laser in air and then annealed in Ar. It
can be seen that the step height is still 2nm, indicating that the Bi2Se3 remains on the TMD after laser exposure and
annealing. (c) AFM image of the laser exposed Bi2Se3/MoSe2 2D nanostructure shown in (h). This AFM image reveals
two important findings: (1) the Bi2Se3 is not removed through laser exposure, and (2) the Bi2Se3 independently grows
at multiple nucleation sites. The large optical change induced by laser exposure (blue arrow) is clearly visible in (h);
however, the AFM image shows no step downward, indicating that the Bi2Se3 is still present. The Bi2Se3 is not uniformly
and continuously spread on the MoSe2, which is what is seen in (a) and (b), but is instead distinct islands between 1-3
layers thick. The Bi2Se3 deposits at multiple nucleation sites independently, and then the grains will begin to growth
together to form a continuous layer. (d) AFM image of the as-grown Bi2Se3/WS2 sample shown in (l). The sample has
>98% Bi2Se3 coverage, with islands of bilayer. The line profiles in (d), (j) and (k) verify this. (e) As-grown Bi2Se3/MoSe2,
which was then then exposed to a laser in air (see (f)). Laser exposure induces a perceived color change in the sample,
which is very remarkably reversed when the 2D nanostructure is O2-free annealed (see (g)). (h) Optical image of the
laser exposed Bi2Se3/MoSe2 nanostructure in (c), where the laser-treated area is recognized by a distinct change in
color (blue arrow). The lines on the side are markers used to identify the sample. (j), (k), and (l) are line profiles and an
optical image, respectively, associated with (d).
Very interestingly, when any of the Bi2Se3/TMD 2D heterostructures are exposed to a laser in air,
the PL intensity increases and the peak position shifts, due to a disruption of the interlayer
coupling, which induces the monolayer TMD to behave more electronically independent,
facilitating radiative exciton recombination. Figure 5.6d shows the large change in PL intensity
and recombination energy between as-grown and laser-exposed Bi2Se3/TMD 2D heterostructures.
All the spectra have been normalized, and the markers show the change in peak position. Figure
5.6are representative PL spectra from a Bi2Se3/WS2 2D heterostructure exposed to increasing laser
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energy doses, as well as from that of a monolayer WS2 crystal prior to Bi2Se3 growth. The 2D
heterostructure’s PL is over 99% quenched compared to the initial monolayer WS2 crystal. As
low-energy doses are applied, the PL intensity increases at a controlled rate and the peak position
shifts, due to changes in the exciton dynamics (discussed later).
Figure 5.10: Raman spectroscopy study of Bi2Se3/MoSe2 : as-grown vs. laser exposure in air vs. O2-free annealing.
MoSe2 and Silicon peaks. The Bi2Se3, MoSe2 and silicon Raman peaks are labeled above. An analysis of how the Bi2Se3
Raman peaks evolve as a function of laser exposure in air and O2-free annealing is shown in next. Lase exposure in air
causes both the MoSe2 and silicon peaks to increase. This is in agreement with our claim that laser exposure in air
diminishes the interlayer coupling, and induces each material to behave more “free-standing”. When the TMD is more
“free-standing”, its atoms will vibrate more freely – they will be less confined by the Bi2Se3 – thereby increasing the
intensity of the Raman peaks. The silicon peaks likely increase because the laser exposure causes the nanostructure to
become more transparent, previously shown to the case with Bi2Se3/MoS2 2D heterostructures. Annealing will either
have a negligible effect on the Raman response (see first annealing), or it can reverse the effects of the laser-treatment
(see second annealing). The acronym “LT” refers to laser-treatment (LT), the process where a laser is exposed to the 2D
nanostructure.
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Figure 5.11: Raman spectroscopy study of Bi2Se3/MoSe2 and Bi2Se3/WS2: as-grown vs. air-laser-treatment vs. O2-free
annealing. Bi2Se3 peaks. (Left Graphs) The Bi2Se3 Raman response contains two outer peaks, and a middle peak
between the two outer peaks. When the sample is exposed to a laser in air, the two outer peaks will decrease in intensity
and the inner peak will increase. Similar to the MoSe2 and silicon peaks, the first O2-free annealing had a negligible
effect on the Bi2Se3 peaks; however, the second O2-free annealing partially reversed the effects of the laser exposure by
increasing the intensity of the outer peak. These changes in the Bi2Se3 Raman modes might be related to the structural
changes shown next. (Right Graph) Raman spectroscopy study of Bi2Se3/WS2: as-grown vs. air-laser-treatment. Laser
exposure in air on Bi2Se3/WS2 has a similar impact as on Bi2Se3/MoSe2: the Bi2Se3 peaks decrease in intensity, and the
WS2 and silicon peaks increase in intensity.
It has been previously shown that the Raman modes are influenced by the interlayer
coupling.123,126 We found that laser exposure enhances the monolayer TMD’s Raman peaks,
suggesting the interlayer coupling is being diminished and the TMD is more independent (Figure
5.10 and Figure 5.11), in agreement with our PL findings. The changes in the Raman modes could
possibly be related to the introduction of oxygen in the interlayer region (Figure 5.2), or possibly
related to the changes in structure induced in the Bi2Se3 (Figure 5.12).
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Figure 5.12: Bi2Se3/WS2 SAED images after laser exposure in air, and then annealed in Ar. Laser exposure (or laser-
treatment) in air induces the Bi2Se3 to become more nano- and polycrystalline. We show that annealing in Ar or N2 (i.e.
an O2-free environment) tends to reverse the laser induced structural changes to Bi2Se3 (i.e. the Bi2Se3 becomes more
crystalline); however, a near full reversal was only observed in nanostructures treated with a low dosage, indicating
that large structural changes are difficult to fully reverse. The TEM image shows the location that was laser-exposed
and annealed.
Figure 5.13: Tuning the PL intensity and peak position (a) PL intensity and (b) PL peak position of a Bi2Se3/MoS2 2D
heterostructure exposed to 68 energy doses (via laser) of varying recipes (i.e. varying time increments and laser
powers), demonstrating the wide range of PL intensities and peak positions obtainable. The PL intensity increased by
a factor of 64x, while the peak position varied by 37meV. The experiment was conducted such that after applying a
pre-determined recipe, the laser power was reduced to the lowest setting to collect the 2D heterostructure’s entire PL
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spectrum for exciton analysis (discussed later). (c) When a laser is continuously applied at low-power (e.g. 50 µW) to
a Bi2Se3/MoSe2-2xS2x 2D heterostructure, the PL intensity increases consistently along a logarithmic curve shape before
plateauing, demonstrating that the exciton dynamics can be tuned with high precision.
By exposing the 2D heterostructures to discrete laser doses of varying recipes (i.e. varying time
increments and laser powers), the PL intensity (Figure 5.13a) and peak position (Figure 5.13b) can
be modified over a wide range. Conversely, if the laser in continuously applied without
interruption at a low power (50 µW), the range of PL intensity was found to be smaller, but also
increase approximately logarithmically (Figure 5.13c), suggesting the response is dependent not
only on the total energy dose, but also how it is applied. As shown next, the shift in PL peak
position is likely due to changes in both the exciton binding energies and the ratio of the excitonic
populations, suggesting the exciton dynamics can be manipulated.
Figure 5.14: Manipulation of both exciton populations and their recombination energy (a) Deconvolution of a typical
PL peak into its two Lorentzian contributions - which represent the exciton and trion quasiparticles - from a Bi2Se3/MoS2
2D heterostructure exposed to a laser in air. The Lorentzian’s peak position is a measure of the recombination energy
(i.e. binding energy), while the peak intensity represents the population of quasiparticles that radiate. (b)-(e) are
extracted by analyzing the PL spectra from the experiment in Figure 5.13a-b. (b) Relative variation of the radiative A-
exciton population versus laser dose. During the experiment, the laser power had to be increased from 3.8µW to
1430µW to cover the large range (3 decades) of dosing within reasonable time. Applying these changes resulted in
sharper increases, as well as decreases, in the trends. (c) Variation of the A exciton recombination energy under the
same range of doses. A large variation of 35 meV could be obtained in this sample that included both red shifts and
blue shifts. (d) Relative variation of the radiative A- -trion population versus laser dose. (e) Variation of the A- -trion
recombination energy under the same range of doses. Like the exciton, a large variation of about 35 meV could be
obtained in this sample that included both red shifts and blue shifts. The altering trends in both exciton and trion
population suggests multiple mechanisms may cause the changes seen, with the possibility of significant sample
degradation at the highest doses.
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Analysis of the PL spectra provides a broader context for the laser-induced changes within the
encapsulated TMD films under Bi2Se3. Owing to the possibility that both the population of
radiative excitons, as well as the value of their recombination energy (i.e. Eph, the energy of the
emitted photons) can be manipulated by external probes, 2D materials have become extremely
attractive for atomically-thin and tunable functional components in LEDs and lasers,39 optical
modulators,260 photonic crystals,260 and optical resonators.259 In the presence of applied electrical,33
optical,269 mechanical231 and magnetic fields,270 the excitonic populations and recombination
energy values can be manipulated; however, the changes vanish when the fields do. In contrast,
our approach allows permanent, in-situ, electrode-less, and use-specific programming of both
radiative exciton population and recombination energy values. Figure 5.14a shows a typical PL
peak arising from two known excitonic contributions (A exciton and A- trion) in the Bi2Se3/MoS2
2D heterostructure.33
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Figure 5.15: Representative A and A- fits for photoluminescence spectra shown in Figure 5.14, showing the good quality
of the fits. All R-squared values are >0.98, and 56/60 of the fits have values >0.995, a quantitative demonstration they
are a good fit. Two randomly chosen intervals (every 6 and every 10) show the progression. Fitting was done using
Matlab. Guidance for this method can be seen in 29.
Figure 5.15 shows additional examples of the deconvolution of the PL spectra. Figure 5.14b-e
show the variation peak amplitudes and recombination energies of the A exciton and A- trion for
the experiments described in Figure 5.13a-b. In addition to the 16× - 18× change in the population
of radiative excitons, there is a large (up to ~35 meV) shift of the excitonic recombination energies.
This reflects a significantly larger degree of manipulation compared to some of the previously-
reported approaches,33,259,267,271,272 suggesting sizable changes in the electronic behavior of the
heterostructure. Specifically, the recombination energy for excitons (as measured using PL
spectroscopy) is )1(
bgph EEE , where gE is the quasi-particle band gap of the heterostructure,
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and the ground-state binding energy1
bE is given by22
41 2
eEb ,273 where is the dielectric constant
of the surrounding medium, and )/(1 11 he mm is the reduced mass of the e-h system. From
this formalism, we conclude that laser-exposure imparts large changes to either: (i) the band gap
and/or (ii) the binding energy of the excitons. In our heterostructures, the overall changes
reflected both red and blue shifts in the excitonic energy values, suggesting that multiple
competing mechanisms could be present. The question arises, what are these mechanisms?
Indeed, previous work has shown that the surrounding dielectric environment influences the
exciton binding energy.272 Additionally, our concurrent work reveals that that oxygen is critical
to induce the expected changes when a laser is applied to a Bi2Se3/MoS2 2D heterostructure, and
suggests oxygen molecules might be intercalating between the layers.104 Together this suggests
diffusive oxygen might be modifying the surrounding dielectric, thereby shifting the exciton
binding energies.
Figure 5.16: Annealing reverses the changes to optical and excitonic properties. (a) Sequentially-acquired PL spectra
representative of the same location, and (b) the corresponding optical images for: the initial monolayer MoSe2-2xS2x
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crystal, the same monolayer crystal with 1-2 layers of Bi2Se3 grown on top, and the same 2D heterostructure subject to
alternating laser exposure and thermal annealing. Laser exposure in air induced the expected PL reemergence and
color change in each instance, while thermal annealing in Ar or N2 reverses those changes. The process can be repeated
numerous iterations with little to no noticeable degradation to the 2D heterostructure. The red arrows indicate the
locations that were exposed to a laser.
Figure 5.16 demonstrates that thermal annealing of the samples in either Ar or N2 environments
can reverse the laser induced changes to the optical and excitonic properties, where little to no
degradation was observed when sufficiently low-energy doses were applied, suggesting the 2D
heterostructures have repeat-use capability. Figure 5.16a shows sequentially-acquired PL spectra
representative of the same location, and Figure 5.16b shows the corresponding optical images.
Initially a monolayer MoSe2-2xS2x crystal was grown with a bright PL spectra centered at 1.74eV,
which is in between monolayer MoS2 (~1.83eV) and monolayer MoSe2 (~1.53eV), suggesting it is
an alloy of the two. 1-2 layers of Bi2Se3 were then grown on top, inducing a color change and
quenching the monolayer TMD’s PL. SAED images suggest both the monolayer MoSe2-2xS2x and
the Bi2Se3 grown on top are highly crystalline (Figure 5.5). The 2D heterostructure as then exposed
to a laser while in air, inducing the expected color change and facilitating reemergence of the
monolayer TMD-alloy’s signature PL. The 2D heterostructure was then annealed in an Ar
environment at 240° for 3 hours, reversing the changes to the optical and excitonic properties.
This sequence of laser exposure followed by annealing was repeated twice more, where each
iteration produced the anticipated results, and there appeared to be little to no noticeable
degradation, suggesting the 2D heterostructures have repeat-use capability.
In conclusion for this chapter, the Bi2Se3/TMD 2D heterostructure family is a complex system that
has demonstrated promise as both a research tool and a material with a direct connection to real-
world applications. To our knowledge, this is the first demonstration of a family of semiconductor
2D heterostructures whose photoluminescence can be reversibly tuned in such a controllable
manner, in-situ, and over such a wide range of photon energies (1.5 eV<Eph<2eV). The in-situ
tuning and wide range of photon energies allows researchers to fabricate customized excitons.
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We demonstrated that a focused laser can be used to “write” complex patterns with submicron
features, and that annealing “erases” the laser-induced changes. There is little to no noticeably
degradation when low-powers are controllably applied, suggesting these materials have repeat-
use capability. These 2D heterostructures might be useful as ultrathin and flexible optical
information storage devices, color converters, micro-cavity-lasers, and other photonic, plasmonic
and optoelectronic applications.39,258–260 Additionally, this system interplays strong spin-orbit
coupling in non-centro-symmetric crystal structures, and could potentially be used to study
correlated, spin and valley physics.255–257 Hence, we believe our work paves the way for rich new
science and technology research.
5.1 Laser-Annealing in Air, and Strong Stability of As-Grown Configuration
The below figures demonstrate that a laser applied at very low powers (0.91μW) continuously
for several minutes is able to anneal the 2D heterostructures and reverse the PL intensity changes
induced by higher laser powers. What is very surprising is that this can even be accomplished in
air, which contains oxygen. These results suggest that the very low powers are able to slowly
perturb the oxygen to leave the interlayer region (or wherever it is residing) and increase the
interlayer coupling strength. This might be because the laser perturbs the Bi2Se3 into a more stable
twist angle and configuration, which then is more likely to squeeze the oxygen out. It has been
previously shown with graphene on metal substrates that the twist angle affects the rate of
oxygen absorption, and certain twist angles that are particularly stable will even encourage the
oxygen to de-intercalate.
The other figure below shows how the PL changes over a very long experiment where very low
powers are used to decrease the PL and higher powers are used to slowly raise it. Interestingly,
the PL will plateau at unexpected intervals, suggesting that a stable configuration was reached.
Additionally, and very interestingly, the as-grown configuration appears to be more stable than
the other post-laser-exposure configurations, in agreement with the assumption that these
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materials would grow at their most energetically stable state. We found that a higher power was
required to break-out of the initial as-grown state.
Figure 5.17 Very low laser power can anneal Bi2Se3/TMD 2D heterostructures. The H-MoS2 heterostructure was laser-
treated upward sufficiently, and then the power was turned down, such that a very low power was being applied to
the sample. It was observed that the PL intensity began to consistently decrease, until it plateaued. Then an impulse of
high power was applied and the PL increased, which confirmed that the PL intensity did not simply decrease because
the TMD was damaged. Next, the same low power laser-treatment recipe was applied, and the PL intensity once again
began to decrease until it plateaued; however, this time it plateaued to a higher PL intensity. Next, a lower power laser-
treatment recipe was applied, and the PL intensity increased controllably, revealing three observations: (1) the
underlying TMD was not noticeably damaged anywhere in this experiment, (2) the low-power laser-treatments are
likely annealing the sample, and (3) a laser can be used to tune the PL intensity up or down.
(Bottom right inset) The histogram counts the change in PL intensity between measurements (IN-IN-1=ΔI) that were
obtained during the first annealing session (i.e. between the blue and purple arrows). The distribution shows that the
behavior of the system is statistical, and that very low laser power does not always cause the heterostructure’s PL
intensity to decrease; however, the overall trend is downward. This can be compared to the distribution shown in
figure 3, which is shifted to the right because the laser-treatment recipe used a higher power. Comparing these two
distributions reveals that the moiré heterostructures have a statistical response to laser-treatments, and that the peak
shifts left or right depending on the laser-treatment recipe. Higher powers and longer time intervals shift the peak to
the right, and lower powers and short time intervals do the opposite.
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Figure 5.18 Probing the PL stability of moiré heterostructures. The PL of a moiré heterostructure can be tuned
through laser-treatment and annealing. When the interface between the TMD and Bi2Se3 is disrupted, the Bi2Se3 will
break into individual grains, and these grains will begin to move (i.e. rotate, strain, shift position, and shift z-distance
to the TMD). As they move, they will form different moiré patterns with the underlying TMD, which will change the
interaction between the two compounds and affect the luminescent properties (i.e. the PL will be turned ‘on’ or ‘off’
depending on how the TMD and Bi2Se3 interact). The DFT results revealed that the different moiré patterns
(commensurate unit cells) will have different formation energies, meaning some will be more energetically stable
than others, thereby making those patterns more favorable. This favoritism was observed in the histogram (figure S9)
where certain relative-rotations are statistically preferred.
(Left side) The PL intensity was quantitatively determined to be stable eight separate instances (i.e. it plateaued),
even though the laser-treatment recipe (the time interval and power) had not changed, leading to the question: why
would the PL intensity suddenly stabilize? The PL was changing in response to the laser-treatments, which means
the moiré heterostructure itself was changing. The heterostructure will change and evolve until it falls into a pseudo-
stable state, which forces the PL intensity to remain stable for several laser-treatments. The stable points were
identified using the following rigorous quantitative criteria: (1) four consecutive measurements all need to be within
1.25% of their mean value, and (2) the first and last values need to be within 1.25% of each other.
(Right side) Pristine and annealed moiré heterostructures are in the energetically most favorable configuration,
which is why they are more stable than their laser-treated counterparts. Annealing allows a crystal to find its most
energetically favorable configuration. Here the H-MoS2 was laser-treated with a certain recipe, and the PL only
increased on average by 14 counts. Then a short impulse of a higher laser power was applied to the sample, which
caused some bonds in the sample to break. After the high energy burst, the same, original laser-treatment recipe was
applied, but this time the heterostructure’s PL increased by 96 counts on average, a 586% increase from before, which
indicates that the heterostructure was more sensitive after a larger energy impulse was applied. The heterostructure
was more sensitive because the heterostructure was taken out of its most stable configuration.
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Chapter 6: Evidence of a purely electronic two-
dimensional lattice at room temperature
Interlayer coupling can trigger a variety of phenomena in vertical 2D heterostructures, including
creating new excitons (both interlayer and moiré),212,274 modifying e-h recombination pathways,103
altering Raman vibrational modes,123 shifting the absorption profile,184 enabling super-dense
storage of lithium,81 and inducing superconductivity.66 In many instances, these new properties
are strongly modulated by the formation of moiré superlattices, where the periodic variations of
atomic registry between the two layers generate a new “lattice” with significantly modified band
structure, electronic, optical and magnetic properties.66,86 Since it is possible to artificially stack 2D
layers with arbitrary twist angles, understanding and controlling the enhanced interactions at
sites of high atomic registry in twisted 2D bilayers has become the central focus of research by
many theoretical 115 and experimental efforts 156 in recent times. These superlattices can be
“visualized” in real space in high-resolution TEM or STM measurements, both of which measure
the sum of intensities (or currents) of electrons, either transmitted through or tunneled-out from
various lattice sites, but neither of which capture the electron amplitude and phase information.
In contrast, high energy selected area electron diffraction (SAED) experiments, which are
sensitive to the amplitude and phase of electrons as they scatter from various lattice sites, have
so far never been reported to show diffraction patterns that corresponding to superlattice
periodicities in 2D materials. The wavelength of electrons are orders of magnitude smaller than
the effective separation between the layers (therefore reducing the possibility of interference
between the two atomic lattices) and electronic charge redistribution between layers (if any), are
too weak to scatter high-energy electron beams. As a result, nearly all past work on 2D moiré
lattices use 2D FFT of real-space images as a means to simulate moiré “diffraction” patterns,
rather than showing their real appearance in, e.g. a SAED measurement. In sharp contrast, we
show, for the first time, direct observation of SAED patterns whose positions appear to
correspond to the moiré superlattices in a 2D heterostructure, at room temperature. We argue
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that these are not arriving from an interference of electrons from the atomic superlattice, but
rather, from a completely new periodic lattice of scattering sites that are purely electronic in
nature. This work can be found at the following reference.275,276
Figure 6.1 Direct observation of a new lattice in selected area electron diffraction of vertically stacked Bi2Se3/TMD 2D
heterostructures. (a) and (b) are optical images of monolayer WS2 and a Bi2Se3/WS2 2D heterostructure, respectively. A
contrast is observed on the 2D heterostructure because Bi2Se3 grew monolayer with bilayer islands. (c) Atomic Force
Microscope (AFM) scan showing three layers of Bi2Se3 grew evenly on monolayer MoSe2-2xS2x, an alloy of MoS2 and
MoSe2. (d) Diagram representing the four different 2D heterostructures studied in this paper: Bi2Se3 grown on MoSe2,
MoS2, WS2, and MoSe2-2xS2x. The TMD is grown on SiO2, and then Bi2Se3 is grown on top of the TMD. (e) Select area
electron diffraction (SAED) image of a Bi2Se3/WS2 2D heterostructure with the in-plane lattice parameters labeled.
Lattice parameters were calculated by measuring the spot separation. The distinct dots (labelled in blue and green)
indicate the TMD and Bi2Se3, respectively, both grow highly crystalline. A third, new set of diffraction spots (red label)
is present that follows the geometric moiré superlattice pattern of the 2D heterostructure and is ascribed to a lattice of
purely electronic charge pools residing at the interface (see text).
High energy (200keV) SAED investigations were performed on 2D heterostructures containing 1-
3 layers of Bi2Se3 grown on a variety of monolayer transition metal dichalcogenides (TMDs) -
MoSe2, MoS2, WS2, and MoSe2-2xS2x – with various crystallographic twist-angles, and some of the
results were confirmed using first-principles density functional theory (DFT). Our past and
ongoing work show quenching of photoluminescence in these heterostructures, and their
reversible behavior when laser-treated in oxygen environment.103,104 Here, we focus on the
appearance of new diffraction spots in some of these structures.
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Figure 6.1a shows an optical image of monolayer WS2 grown on SiO2 using vapor-phase
chalcogenization.31 Bi2Se3 samples were then grown on top of such WS2 flakes using vapor-phase
deposition. Figure 6.1b shows a Bi2Se3/WS2 2D heterostructure with 1-2 layers of Bi2Se3 (i.e.
monolayer with bilayer islands). Figure 6.1c is an atomic force microscope (AFM) scan of a
Bi2Se3/MoSe2-2xS2x 2D heterostructure. Interestingly, three layers of Bi2Se3 grew on the monolayer
TMD, while none grew on the SiO2, suggesting the TMD substrate stabilizes the Bi2Se3 in its few
layer morphology and is a preferred surface over the SiO2. Figure 6.1d schematically shows how
the two different materials stack up on the substrate. Figure 6.1e is a high energy SAED image of
a Bi2Se3/WS2 2D heterostructure with the diffraction patterns (with a six-fold symmetry)
corresponding to the two parent crystals Bi2Se3 and WS2 as labeled along with their corresponding
lattice parameters. In addition, a third set of spots (also with a six-fold symmetry) labeled “New
lattice” can be clearly seen with smaller reciprocal lattice dimensions (and hence much larger real-
space dimensions), which has the same lattice structure as the moiré superlattice of the parent
crystals, and which we attribute to a separately-formed purely electronic lattice. We next discuss
the justifications for such an assignment.
To begin with, the detected extra set of diffraction pattern could not be associated with any known
atomic basis, but always appear to have a real-space equivalent to the moiré superlattice structure
of the 2D heterostructures being probed. This observation is demonstrated for a typical
heterostructure in Figure 6.2 (also section 6.2 in this chapter). First, to simulate the real-space
superlattice, hexagonal lattices representing the TMD and Bi2Se3 layers scaled to experimentally
measured lattice parameters and twist angles, were digitally generated, twisted, and then
superimposed into a composite image. An example of such a simulated superlattice created
(using the lattice parameters and twist angle value from Figure 6.1e), is presented in Figure 6.2a,
where the blue lines and the red rhombus have been used to visualize and measure the lattice
spacing and unit cell of the clearly visible moiré superlattice. Next, a 2D fast Fourier transform
(FFT) was performed on this superposed image, to simulate a set of SAED spots (Figure 6.2b). We
note the digital superposition of the real-space lattice flattens them into a single plane, and
contains no layer-separation information in the perpendicular direction, which allows the 2D FFT
to retain the superlattice-induced spots. In other words, the top-down view (along the z-axis) is
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how the SAED electrons would perceive the 2D heterostructure if the two parent lattices were merged
into the same plane. Conceptually, this 2D FFT of the superimposed “heterostructure”, mimics
what the SAED patterns would look like, had there been enough overlap between the real-space
lattices such that the diffraction pattern from the first lattice could interfere with the second one.
Figure 6.2c shows an overlap of Figure 6.2b on to the actual SAED pattern shown earlier in Figure
6.1e. In all cases investigated, the 2D FFT-generated and experimentally-obtained bright spots are
in near perfect alignment – including the new lattice - suggesting that the experimentally
observed additional spots have a direct correlation with the structure of the moiré superlattice of
the parent crystals. The Diophantine equation shown in section 6.2 (Equation 6.1) was used to
calculate the corresponding commensurate unit cell (Figure 6.2d), verifying that the SAED spots
are in fact correlated to the moiré superlattice pattern, and not some other artifact.277 And finally,
the moiré lattice was absent when the TMD and Bi2Se3 lattices were individually Fourier
transformed and overlaid (Figure 6.3), demonstrating it is not an artifact from either lattice
individually.
Figure 6.2 Electronic moiré lattice scattering sites formed by the interlayer coupling. (a) Simplified real-space model of
the 2D heterostructure in Figure 6.1e. Two lattices with in-plane lattice parameters representing the WS2 and Bi2Se3
crystals were twisted and then overlaid, thereby collapsing both lattices into the same 2D plane, over-writing pixels,
and inducing the emergence of a moiré pattern (see methods section). Moiré patterns only manifest when there is an
interaction between two lattices. The moiré pattern lattice spacing and unit cell are identified by the blue and red lines,
respectively. (b) 2D Fourier transform of the same model with the lattices labeled, where the blue, green, and red
hexagons correspond to the WS2, Bi2Se3, and moiré superlattice, respectively. (c) 2D Fourier transform (orange shading)
overlaid on the SAED image seen in Figure 6.1e (white spots), where the near-perfect alignment of the spots suggests
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the model captures the system sufficiently. Interestingly, the moiré superlattice spots match the experimental new
lattice spots, suggesting the new lattice forms in congruently with sites of high atomic registry in the moiré superlattices
of Bi2Se3 and WS2. The inset with red arrows indicate patterns from the new lattice that are hidden under the red
hexagon in the main image due to overlap of the modeling and experimental spots. (d) Table with the parameters from
the mathematically calculated commensurate unit cell (see section 6.2 and section 6.3 for more information), the model,
and the experimentally measured values. All the values are nearly identical and well within the error, demonstrating
agreement between theory, modeling, and experiment.
Figure 6.3 The moiré SAED spots are not a product of one of the individual lattices. (a) The Fourier transform of a
model of a Bi2Se3 lattice, and (b) and the Fourier transform of a representative monolayer TMD lattice. (c) TMD
transform was colored in red and overlaid on the Bi2Se3 transform. None of the images contain the moiré superlattice
spots, demonstrating that an effective multiplication and interaction of the lattices is needed to produce them (see
section 6.2 for more information).
It is unlikely that the additional SAED spots are a result that electrons diffracted by the upper
crystal is further diffracted by the lower crystal. Mathematically, the formation of moiré patterns
– or moiré effects – requires an interaction (or non-linear operation) between the two parent
lattices (see section 6.2 for details).277,278 For example, when images of periodic structures are
overlaid, pixels are over-written as both images are collapsed onto the same surface. This over-
writing of pixels, vice simply summing their intensities, reflects a non-linear “superposition” that
produces a visual moiré pattern. In contrast, when 2D crystals are stacked, the different layers do
not reside in the same plane, but are separated by ~ angstroms. The possibility of interferences
between the two layers is hence remote, since the layer-center separations (~10Å) is massive
(~400×) compared to the wavelength of the electrons (0.0251Å at 200 keV), suggesting the
electrons perceive the two materials independently. Further, the very low scattering angles of
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high energy electrons in SAED (~0.6°) usually means very little chance of any "double diffraction"
for our atomically-thin Bi2Se3/TMD heterostructures since, separated at a distance of ~3.5Å, the
lateral spread of an electron scattered from the first crystal would be ~ 0.0367 Å at the second
layer, which is far smaller than the lattice parameter of the second crystal. Hence, we conclude
that observed new diffraction spots are not resulting from composite interferences of diffraction
patterns from the atomic lattice of the parent crystals, but rather, are due to diffraction from a
completely new lattice that has the same “structure” as the moiré superlattice of the parent
crystals. Since electrons scatter elastically from concentrated charge pools, this implies that a
separate lattice of charge pools must have been created in these heterostructures that induces
these new spots.
These results lead to three important questions: (1) What do the electronic moiré lattice scattering
sites look like, (2) where do they reside, and (3) why does the lattice follow the moiré pattern, vice
a different structure? We addressed these questions by performing first principles density
functional theory (DFT) calculations, which are known to provide structural information and
electronic charge distribution in solids with relatively high accuracy. Indeed, our DFT
calculations not only confirm the formation of a purely electronic 2-D lattice in the interlayer
region, but also whose unit cell size and relative rotation are in agreement with the
experimentally observed moiré pattern in our system.
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Figure 6.4 Charge pools form between nearest interlayer neighbors. (a) DFT calculated interlayer charge redistribution
due to the interlayer coupling of a twisted 1L Bi2Se3 + 1L WS2 2D heterostructure, using the lattice parameters described
in Figure 6.2d. The image is a cut of the plane lying equidistant from both materials. The nearest neighbor atoms are
included in the image. It can be seen that the charge redistribution is concentrated between nearest interlayer neighbors.
(b) Cross-section view for the same DFT calculated 2D heterostructure, showing charge pools form between nearest
neighbors. Since the atomic registry (or nearest neighbors) is determined by the moiré pattern, the charge pools form
a purely electronic lattice that follows the moiré pattern. (c) Table with the parameters from the experimentally
measured third-crystal and the DFT calculated interlayer electronic lattice, demonstrating their complete agreement.
Together the experimental and theoretical work suggest that the high energy electrons could be scattering from the
DFT-predicted interlayer charge pools.
Figure 6.4 shows DFT calculations for the predicted charge redistribution into the interlayer
region, due to the interlayer coupling in a 2D heterostructure (1-layer Bi2Se3 and 1-layer WS2 twist
to 10.9° - to match closely with the structure shown in Figure 6.2c). DFT calculations for other 2D
heterostructures, along with the experimental data, are detailed in section 6.3. Figure 6.4a is a
charge density plot of a plane in the interlayer region equidistant from the Bi2Se3 and TMD layers,
where red specifies higher charge density. The charge congregates between nearest neighbor
atoms (shown in the image), revealing possible bonding that forms between the layers. Figure
6.4b is a cross-section image of the same DFT calculation, showing both the Bi2Se3 and WS2 layers,
as well as the shape and relative displacement of the predicted interlayer charge pooling. The
charge pools form a lattice with long-range order, where the unit cell size and twist angle (see
Figure 6.4c) are in complete agreement with the experimentally observed new diffraction pattern
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in Figure 6.1e. Together these results provide strong evidence that the new set of spots seen in
our heterostructures are very likely a .planar lattice of purely electronic charge pools that reside
midway between the two parent crystals in the Bi2Se3/WS2 2D heterostructure twisted to 10.9°.
DFT calculations were also performed on two additional 2D heterostructures - Bi2Se3/MoSe2 (0°
twist) and Bi2Se3/MoS2 (0° twist). For all 2D heterostructures DFT calculations confirmed the
formation of an electronic lattice in the interlayer region, whose parameters (i.e. unit cell size and
twist angle) are in agreement with the moiré patterns of the corresponding parent crystals (see
section 6.3). Section 6.3 shows experimental and modeling data for Bi2Se3/MoSe2-2xS2x 2D
heterostructures at various twist angles, and a Bi2Se3/WS2 2D heterostructure at 4.7°, where the
experimental and modeling data are in near perfect agreement. The experimental observation of
new SAED patterns that match perfectly with various DFT-predicted electronic lattices across
multiple different 2D heterostructures and numerous twist angles, further verifies our
hypothesis, and suggests that the formation of a purely electronic 2-D lattice (at least at certain
twist angles) even at room temperature might be characteristic trait of the investigated family of
2D heterostructures.
To summarize, strong interactions at sites of high atomic registry in Bi2Se3/TMD heterostructures,
enable charges to overcome the interlayer separation and form a purely electronic lattice situated
between the two parent crystals, and concentrated at these sites, as revealed by additional SAED
patterns, and confirmed by DFT calculations. This is surprising because it is traditionally
assumed that such high energy (200 keV) SAED should only be able to reveal atomic structural
information, owing to the large scattering cross sections of the atoms,194 but are not sensitive to
bonding or weak charge distributions. Hence, the presence of the distinct diffraction spots
suggests significant charge redistribution into concentrated pools, as seen in our DFT
calculations. To the best of our knowledge, this is the first demonstration of high energy SAED
electrons being used to reveal non-atomic electronic scattering sites, and evidence of a purely
electronic lattice at the interface of 2D materials. Very likely, the formation of interlayer charge
pools between nearest interlayer neighbors play an important role in the interlayer hybridization
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and coupling in these systems. Our findings provide significant new insights into how 2D layers
interact, and access to the 2D electronic lattice could open up opportunities for researching the
physical properties of novel purely electronic periodic systems devoid of any atoms or defects.
Control of such systems will very likely lead to new developments in nanoelectronics and
optoelectronics, as well as other applications that depend on interfaces between 2D layers.
113,129,160,279
6.1 Diophantine equation and guidance on mathematically calculating the
electronic moiré lattice commensurate unit cell.
The below equations were used to mathematically calculate the moiré superlattice commensurate
unit cell parameters, where m, n, r, and s are the unit cell parameters.277,280 For the equations to be
satisfied, all the lattice parameters must be integers. Equation 1 is Diophantine and used to
calculate the moiré superlattice commensurate unit cell length. Equation 2 was used in Figure 6.5.
Equation 3 is used to calculate the relative rotation of the moiré superlattice relative to the Bi2Se3
crystal. Equation 1 and Equation 3 were used to calculate the parameters shown in Figure 2d and
all the figures in section 6.2.
𝐿𝑒𝑛𝑔𝑡ℎmoiré = 𝑎𝐵𝑖2𝑆𝑒3√𝑚2 + 𝑛2 − 𝑚𝑛 = 𝑎𝑇𝑀𝐷√𝑟2 + 𝑠2 − 𝑟𝑠 Equation
6.1
𝑙𝑎𝑡𝑡𝑖𝑐𝑒 𝑟𝑎𝑡𝑖𝑜 =𝑎𝐵𝑖2𝑆𝑒3
𝑎𝑇𝑀𝐷 Equation 6.2
𝑐𝑜𝑠(𝜑moiré,𝐵𝑖2𝑆𝑒3) =𝑚−
𝑛
2
√𝑚2+𝑛2−𝑚𝑛 Equation 6.3
Next we will discuss the mathematics of how a moiré pattern is formed because it provides
insight into the interlayer coupling. Moiré patterns only develop when two lattices interact
through an effective multiplication - simply summing two lattices does not produce a moiré
pattern.152,277,278,280 The Fourier transform is a linear operation: ℱ[𝑎𝑓(𝑥) + 𝑏𝑔(𝑥)] = 𝑎ℱ[𝑓(𝑥)] +
𝑏ℱ[𝑔(𝑥)], where 𝑓(𝑥) and 𝑔(𝑥) are functions. Summing two functions does not produce any new
frequencies. However, when two non-identical functions are multiplied, they will produce a new
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frequency: ℱ[𝑎𝑓(𝑥) · 𝑏𝑔(𝑥)] = 𝑎𝑏𝐹[𝑓(𝑥)] ∗ 𝐹[𝑔(𝑥)] ≠ 𝑎𝐹[𝑓(𝑥)] + 𝑏𝐹[𝑔(𝑥)], where ∗ is the
convolution operation. This new frequency is the moiré pattern. The presence of moiré
superlattice spots when using SAED indicates that the two materials are being “multiplied”, a
process facilitated through the interlayer coupling.
Figure 6.5 Guidance on calculating the moiré superlattice commensurate unit cell. (a) Matlab generated model of the
2D heterostructure in Figure 1e. (b) Fourier transform of the model in (a). (c) The Fourier transform was shaded orange
and then overlaid on the experimental SAED data shown in Figure 1e (white spots). The near perfect alignment
indicates that the model sufficiently captures the system. (d) Plot showing 2D heterostructure configurations that
produce smaller (m, n, r, and s are all less than 10) moiré superlattice commensurate unit cells, using the relative
rotation (or twist angle) and ratio of lattice constants (Eq.2) are input parameters.277,280 The intersection of dark or light
lines indicate regions where a commensurate unit cell is more like to found. See the corresponding references for more
thorough guidance on interpreting the graph.277,280 The red arrow highlights the location that corresponds to the 2D
heterostructure in (a), (b), and (c), indicating that the moiré superlattice parameters have been identified.
6.2 Data demonstrating that the electronic moiré lattice was observed in other
2D heterostructures
SAED is a powerful tool used to observe the reciprocal space of samples. If a periodic
arrangement of scattering sites with a sufficient high cross exists (e.g. a lattice), spots will
appear.194 Spots closer to the center correspond to crystals with a larger periodicity, which is why
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the monolayer TMD has the largest radius and the electronic moiré lattice has the smallest. Other
electron diffraction methods (e.g. convergent beam electron diffraction - CBED) have been used
to reveal bonding,281 and low energy electrons (~236eV) have revealed the moiré pattern in bilayer
graphene structures; however, both these situations involve different physics and scattering
mechanisms. CBED functions with different assumptions, and is able to reveal significantly more
information.
2D Structure moiré patterns have been previously shown using TEM imaging; however, the
electron-material interaction mechanisms between imaging and SAED are different, allowing
each method to provide unique information. TEM imaging is a different process involving both
inelastic and elastic scattering, as well as charge induced phase shifts. SAED, on the other hand,
only detects electrons elastically scattered at low angles (~0.6°), where bright spots are the product
of periodically spaced scattering sites.194 Observation of periodic bright spots in SAED is strong
evidence that a “lattice” of scattering sites exists. Further, the scattering cross section is dependent
on not only the total charge, but also the shape,194 suggesting the scattering sites have well-formed
shapes (i.e. the charge redistribution was not diffusive). These conclusions are in agreement with
previously published synchrotron X-ray diffraction experiments, which showed charge
accumulation between the layers of bulk TiS2, suggesting the “weak” van de Waals bonding
induces sufficient charge redistribution that leads to the formation of a polar bond.282
The observation of SAED spots using high energy electrons, suggests the electronic moiré lattice
has a relatively large scattering cross section. It has been previously shown that, when used in
conjunction with DFT, scattering data is able to measure the magnitude and shape of charge
redistribution.281,282 Although performing such a robust analysis is beyond the scope of this paper,
we are able to infer a relative range of scattering cross section values by comparing the relative
brightness (i.e. photon counts) of the SAED spots. The electronic moiré lattice SAED spots are 1-
6% the intensity of the monolayer TMD spots, suggesting an approximate range of values that
would be very large for non-atomic scattering sites. The scattering cross section is determined not
only by the magnitude of charge, but also by the distribution,170,194 where well-formed sites are
more likely to scatter than very diffuse clouds.
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Figure 6.6 Experimental, moiré modeling, and DFT data for a Bi2Se3/MoS2 2D heterostructure. Taken together, the data
suggests high energy electrons are diffracting off interlayer coupling induced scattering sites, which DFT predicts to
be located between nearest interlayer neighbors. (a) Real-space model of the 2D heterostructure in (b) using the
techniques described in section 6.2. (b) High-voltage (200keV) SAED experimental data with the Fourier transformed
model from (a) overlaid on top. The strong overlap of the spots suggests the model and experiment are in agreement,
and that the model captures the system sufficiently. (c) Table with the data summarized, demonstrating that theory
and experiment are in very good agreement. (d) DFT calculated interlayer charge distribution in the interlayer region
between both layers. (e) Cross-section view of a DFT calculated 2D heterostructure with the interlayer coupling
induced charge pools shown. The DFT calculated 2D heterostructure is different from the parameters in (d) due to
computational limitations; however, the calculated structure is very similar where the twist angle was made 0°, vice
1.7°, making the lattice parameters (3, 0, 4, 0 for m, n, r, and s, respectively). The DFT models do not contain the
experimentally measured twist angles of 1.4° and 1.7° for the Bi2Se3/MoSe2 and Bi2Se3/MoS2 2D heterostructures,
respectively, because the superlattice required at such twist angles was computationally excessive.
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Figure 6.7 Experimental, moiré modeling, and DFT data for a Bi2Se3/MoSe2 2D heterostructure. Taken together, the
data suggests high-voltage electrons are diffracting off interlayer coupling induced scattering sites, which DFT
predicts to be located between nearest interlayer neighbors. (a) Real-space model of the 2D heterostructure in (b)
using the techniques described in Section 6.2. (b) High-voltage (200keV) SAED experimental data with the Fourier
transformed model from (a) overlaid on top. The strong overlap of the spots suggests the model and experiment are
in agreement, and that the model captures the system sufficiently. (c) Table with the data summarized,
demonstrating that theory and experiment are in very good agreement. (d) DFT calculated interlayer charge
distribution in the interlayer region between both layers. (e) Cross-section view of a DFT calculated 2D
heterostructure with the interlayer coupling induced charge pools shown. The DFT calculated 2D heterostructure is
different from the parameters in (d) due to computational limitations; however, the calculated structure is very
similar where the twist angle was made 0°, vice 1.7°, making the lattice parameters (4, 0, 5, 0 for m, n, r, and s,
respectively).
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Figure 6.8 Experimental and moiré modeling images for different Bi2Se3/MoSe2-2xS2x 2D heterostructures at various
twist angles. The data demonstrates that the method can not only be applied to arbitrary twist angles, but can also be
used on alloy-based 2D structures. Despite the fact that the sulfur and selenium atoms appear to be randomly
distributed, the monolayer TMD still forms a crystal, and this crystal is able to form a significant interlayer coupling
with the Bi2Se3. Interestingly, the Bi2Se3 appeared to grow with slightly different lattice constants across different
monolayer TMDs, but kept the same lattice constant when growing on MoSe2-2xS2x, despite the fact that the MoSe2-
2xS2x lattice constant fluctuated. The fluctuation in MoSe2-2xS2x is believed to be due to the changing sulfur and
selenium concentrations. Photoluminescence values were always between signature monolayer MoS2 and MoSe2
values, but varied across samples, suggesting different crystals contained different ratios of sulfur to selenium, which
offers an explanation why the monolayer MoSe2-2xS2x lattice constant fluctuated. DFT calculations on the alloy 2D
heterostructures were not completed due to computational limitations. To the best of our knowledge, the sulfur and
selenium atoms are randomly distributed, therefore requiring very large supercells that can capture the random
placement.
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Figure 6.9 Experimental and moiré modeling images for a Bi2Se3/WS2 2D heterostructure (with a different twist angle
from that in the main text). The image is presented on its own to better show the moiré superlattice spots, and the
strong agreement between the model and the experimental data.
Figure 6.10 Experimental and moiré modeling images for Bi2Se3/WS2 2D heterostructures from Figure 2. A complete
explanation can be found in Figure 2. The information lattice information from solving the Diophantine equation is
included in this figure (i.e. the m, n, r, and s data).
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6.3 Brief explanation of double diffraction with respect to electron diffraction
As shown in this chapter, a third crystal was observed using SAED, and this crystal cannot be
correlated to any known atomic basis. Using supporting DFT results, it is argued that this
unknown crystal is due to periodic charge redistribution inside the interlayer region due to the
interlayer coupling. The SAED evidence is not a direct probe of the charge crystal, but an
indication that it exists. With this in mind, it could be that a different effect is responsible for the
crystal’s diffraction spots. For completeness of the work, a significant literature review was done
to search for other possible explanations, which could provide guidance as to the source of the
diffraction spots. The only report of diffraction spots that correspond to the moiré pattern found
were due to an effect called “double diffraction”. We applied this concept to our 2D
heterostructure system and determined that it does not apply. Despite the fact that it does not
apply, it is briefly discussed in this dissertation because it is the only other documented source of
moiré pattern diffraction spots, suggesting that the spots we observe are simply due to this effect.
Double diffraction is explained briefly in Figure 6.11 below. In summary, double diffraction is
when the top material of a heterostructure projects a diffraction pattern onto the bottom material.
As it projects the diffraction pattern, the bright spots could happen to overlap with certain atoms
of the lower material in a periodic fashion. An atom that is illuminated with a bright spot will
carry a weighting with a higher value than the neighboring atoms, suggesting it appears brighter
or more intense than the neighboring atoms. The periodically spaced atoms with additional
intensity will then produce their own diffraction pattern spots that correspond to their spacing.
The observation of double diffraction spots requires not only a heterostructure that is oriented at
the correct twist angle, but the electron wavelength must also correspond, so that the diffraction
onto the lower material matches (the diffraction pattern spacing is dependent on the initial
electron wavelength).
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Figure 6.11: Simplified description of double diffraction due to TEM electrons. Electrons are incident on a bulk 3D
heterostructure as a plane wave, where they begin to scatter and diffract from the upper material. If the upper material
is sufficiently large, the diffraction response will project onto the lower material (shown as the diffraction peaks in the
black box). Certain atoms are periodic intervals will overlap with the diffraction response, whereupon they will be
illuminated with an electron intensity. This illumination provides specific atoms at periodically spaced intervals with
a higher weighting, thereby inducing a disproportionate response from them and producing a new set of diffraction
spots.
Double diffraction is not possible with 2D heterostructures due to the small size of the sample
and the low scattering angle of the elastic (i.e. kinetic energy conserving) interaction, which
together prohibit neighboring atoms from interacting to produce a diffraction pattern on the
lower material. More specifically, double diffraction presents when the diffraction pattern of the
upper material is projected onto the lower material and the bright spots periodically align with
the atoms of the lower material. However, to project a diffraction pattern, the electrons scattering
from neighboring atoms must interact, which is not possible considering the atomically thin
nature of the samples. SAED only allows low scattering angles (0.6°) due to the elastic scattering
requirement, which requires approximately 100nm of distance before the scattered electrons of
neighboring atoms interact. With this in mind, it is highly unlikely that a diffraction pattern of
the upper material can be projected onto the lower material, suggesting that double diffraction is
not a probably explanation for the third crystal.
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Chapter 7: Conclusion and Discussion
This dissertation discusses methods to probe and manipulate the interlayer coupling in 2D
structures, using primarily bismuth selenide (Bi2Se3) and transition metal dichalcogenides (TMD)
2D heterostructures as a platform. Both Bi2Se3 and TMDs are layered materials with the same
hexagonal structure. Bi2Se3 is also a topological insulator whose properties are strongly
dependent on the number of layers. Monolayer TMDs are best known for being direct bandgap
with tightly bound excitons. As demonstrated in this dissertation, when Bi2Se3 and monolayer
TMDs are combined to form 2D heterostructures, a strong interlayer coupling is formed that
modifies the properties of both materials. The interlayer coupling is then manipulated using
focused electron beams in a vacuum, and, possibly, the controlled intercalation and
deintercalation of oxygen.
The properties of monolayer materials – or 2D materials - are often dramatically different than
their bulk counter parts, in part due to quantum confinement along the z-direction. For example,
monolayer MoS2 has a direct bandgap, while a bilayer MoS2 2D structure has an indirect bandgap,
indicating that simply stacking the second layer induces changes to the band structure. These
changes are a product of the interlayer coupling, which facilitates communication and interaction
between the layers. On the other hand, if the interlayer coupling could be diminished, thereby
disrupting the communication and interaction, the layers would become electronically
independent and gain the properties of their monolayer morphology. Hence, if the interlayer
coupling could be controllably modulated, the properties of materials could in principle be
manipulated and tailored for specific applications. All this inspires the following questions: What
are the physical changes induced by the interlayer coupling that induce changes to the properties?
Can the interlayer coupling be manipulated across a range, vice simply “on/off”? What are
primary factors that determine the interlayer coupling strength?
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The interlayer coupling in 2D structures induces charge redistribution, which can facilitate the
formation of bonds between the layers and affect the bonding within the material. The interlayer
coupling induced changes to bonding can be probed using Raman spectroscopy, which measures
the vibrational modes of the atoms. It was found that the intensity of the Raman modes of the
parent materials in a well-coupled 2D heterostructure are relatively, suggesting the charge
redistribution is affecting the bonding, possibly also due to the formation of new bonds. When
the interlayer coupling is diminished, the intensity of those same Raman modes increases, in
agreement with theory, and demonstrating a measurement technique to quantify the interlayer
coupling strength. The charge redistribution can also be studied using transmission electron
microscope (TEM) imaging and selected area electron diffraction (SAED), where the scattering of
electrons incident onto the sample is determined by the charge redistribution, allowing changes
induced by the interlayer coupling to be studied. These experimental results can then be
compared to density functional theory (DFT) calculations, which theoretically predict the induced
charge redistribution.
This dissertation describes two methods to controllably manipulate the interlayer coupling with
Bi2Se3/TMD 2D heterostructures. The first method uses a focused electron beam in a vacuum to
impart energy with spatial resolution. If sufficient energy is imparted, the interlayer bonds break
allowing the parent materials to become more electronically independent. The second method
uses both oxygen in the surrounding atmosphere and thermal energy to possibly controllably
intercalate and deintercalate oxygen between the layers. The intercalation of oxygen between
layers, or between 2D materials and their substrates, has been shown to diminish the interlayer
coupling strength, facilitating the 2D materials to behave “free standing” or electronically
independent. Further, this dissertation demonstrates that the interlayer coupling strength is not
simply “on/off”, but can be a range of values, suggesting it can be controllably manipulated to
engineer materials with application tailored properties.
The dissertation found that when Bi2Se3 is grown on monolayer TMDs, certain twist angles are
preferred, suggesting they are energetically more favorable. DFT calculations performed by Prof.
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Bansil’s group, collaborators specializing in theoretical condensed matter physics, predicted that
the twist angle and resulting moiré superlattice are critical in determining the energetic stability
and the formation of an interlayer charge crystal, in agreement with the experimental findings.
Further, it was found that the preferred twist angles are dependent on the underlying monolayer
TMD, suggesting that atomic chemistry is an influential factor in determining the interlayer
coupling strength.
Chapter 1 is a background and overview of monolayer materials and 2D structures. It discusses
the physics of 2D materials and provides a significant number of references for readers seeking
more detail. Chapter 1 was written for readers with no knowledge of 2D materials, and only a
general technical background, facilitating accessibility for the general population. This chapter
discuss the individual properties of monolayer TMDs, as well as monolayer to few layer Bi2Se3.
Previous work on the interlayer coupling are summarized, and noteworthy, high-impact papers
are discussed in more detail, providing a significant background into the primary topic of this
dissertation.
Chapter 2 is the instrumentation and synthesis of all the important techniques and equipment
used during this dissertation. The chapter is split into eleven sections, where each section is a
different technique or equipment operation. Each section provides a brief overview of the
method, why it is important to this dissertation, important details on how it was applied, and, in
some cases, a reference containing amplifying information.
Chapter 3 describes work using a focused electron beam to manipulate the twist angle, structure,
and interlayer interaction of Bi2Se3/TMD 2D heterostructures. A focused electron beam imparts
energy into the material, which when large enough will cause interlayer bonds to break. The
changes to the twist angle and structure were studied using SAED, while changes to the interlayer
coupling were studied using an SEM and laser setup with Raman and photoluminescence
spectroscopy capability. The interlayer coupling strength was also studied by analyzing the
statistical likelihood of different as-grown 2D heterostructures.
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Chapter 4 demonstrates that the interlayer coupling in Bi2Se3/MoS2 2D heterostructures can be
controllably manipulated using the partial pressure of oxygen in the surrounding atmosphere
and the application of energy. The results suggest that oxygen is being controllably intercalated
and deintercalated between the layers, thereby affecting the strength of the interlayer coupling.
Further, the interlayer coupling can be manipulated with submicron precision using a focused
laser.
Chapter 5 demonstrates color-tunable, photoluminescing pixels (PLPs) can be fabricated using a
family of Bi2Se3/TMD 2D heterostructures. The exciton dynamics in the monolayer TMDs can be
tuned by manipulating the interlayer coupling, where the photoluminescence peak position can
be controllably shifted. Analysis suggests that both the binding energy, as well as the ratio of the
trion/exciton, are inducing the shift in the peak position. This chapter shows that the same
behavior is observed in a family of Bi2Se3/TMD 2D heterostructures and is not confined to
Bi2Se3/MoS2, suggesting PLPs with a range of colors can be fabricated. The 2D heterostructures
demonstrated in this chapter are: Bi2Se3/MoS2, Bi2Se3/MoSe2, Bi2Se3/WS2, and Bi2Se3/MoSe2-2xS2x, an
alloy.
Chapter 6 discusses the possible formation of a charge crystal in the interlayer region, providing
insight into the interlayer coupling induced charge redistribution. Using SAED, the crystal
structure of the 2D heterostructure was studied, where three distinct crystals were observed:
Bi2Se3, the TMD, and a new larger lattice distinct from both parent materials. The new crystal
corresponded to the moiré superlattice formed by the parent crystals. The moiré pattern is
dependent on the lattice constants and the twist angle, and does not correspond to any known
atomic basis. SAED captures the elastic scattering of electrons at very low scattering angles, where
only periodic scattering sites with large cross sections are detected. Previous work indicates that
only atoms have a sufficiently large cross section to generate a signal; however, no atomic basis
corresponds to the new crystal. DFT calculations completed by Prof. Bansil’s group predict the
formation of a charge crystal in the interlayer region, in agreement with the experimental results.
Together these results suggest that the interlayer coupling is redistributing charge into
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concentrated pools in the interlayer region, which have a sufficiently large cross section to be
detected.
The appendix discusses non-Bi2Se3 2D structures fabricated, where data suggests the interlayer
coupling is weaker than those formed in Bi2Se3/TMD 2D heterostructures. The following 2D
structures are studied: graphene/MoS2, bilayer graphene, Sb2Se3/MoS2, and Bi2Te3/MoS2. The
appendix also contains programming code written in the Matlab language, and used to analyze
data in the dissertation.
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Chapter 8: Dissemination
8.1 Publications
Hennighausen Z.B., Lane C., Benabbas A., Mendez K., Eggenberger M., Champion P.,
Robinson J., Kar S. Oxygen-induced in-situ manipulation of the interlayer coupling and exciton
recombination in Bi2Se3/MoS2 2D heterostructures. ACS Applied Materials and Interfaces (11)
2019, DOI: 10.1021/acsami.9b02929
Hennighausen Z.B., Kar S. Probing the interlayer interaction between dissimilar 2D
heterostructures by in-situ rearrangement of their interface. 2D Materials (6) 2019, DOI:
10.1088/2053-1583/ab136e
Keyvani-Someh E., Hennighausen Z.B., Lee W., Igwe R.C., Krami M.E., Kar S., Fenniri H.
Organic Photovoltaics with Stacked Graphene Anodes. ACS Applied Energy Materials 2018,
DOI: 10.1021/acsaem.7b00020
Vargas A., Liu F., Lane C., Rubin D., Bilgin I., Hennighausen Z.B., DeCapua M., Bansil A., Kar
S. Tunable and laser- reconfigurable 2D heterocrystals obtained by epitaxial stacking of
crystallographically incommensurate Bi2Se3 and MoS2 atomic layers. Science Advances 2017,
DOI: 10.1126/sciadv.1601741
Poudel Y., Lim G., Moazzezi M., Hennighausen Z.B., Rostovstev Y., D’Souza F., Kar S., Neogi
A. Active control of coherent dynamics in hybrid plasmonics MoS2 monolayers with dressed
phonons. ACS Photonics 2019, DOI: 10.1021/acsphotonics.8b01436
Hennighausen, Z.B.; Lane, C.; Buda, I. G.; Mathur, V.; Bansil, A.; Kar, S. Evidence of a Purely
Electronic Two-Dimensional Lattice at the Interface of TMD/Bi2Se3 Heterostructures. Nanoscale
2019. https://doi.org/10.1039/C9NR04412D.
Hennighausen, Z.B.; Bilgin, I.; Casey, C.; Kar, S. Widely Tunable Bi2Se3/Transition Metal
Dichalcogenide 2D Heterostructures for Write-Read-Erase-Reuse Applications. 2D Materials.
2019, 6 (4), 041003. https://doi.org/10.1088/2053-1583/ab33b5.
Mathur V., M., Jaeger E., Talapatra S., Kar S., and Hennighausen Z.B. Computationally
Identified In Vivo Partial Restoration of Neural Activity Across Severed Spinal Cords Bridged
with Carbon Nanotubes. (Under Revision).
Poudel Y., Seetharaman S., Hennighausen Z.B., Kar S., D’Souza F., Neogi A. Interlayer exciton-
phonon coupling modulated absorption and emission from bulk GaN-monolayer MoS2
Heterostructure. (Under Review).
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Isherwood L., Hennighausen Z.B., Kar S., Baidak A. Heavy Ion-Induced Defect Production in
MoS2 Produced by Mechanical Exfoliation and Chemical Vapour Deposition. (Submitted).
8.2 Patents
Kar S., Vargas A., Liu F., Lane C.A., Rubin D., Bansil A., Buda G., Hennighausen Z.B.,
inventors.
Northeastern University, assignee. Tunable and Reconfigurable Atomically Thin
Heterostructures. United States of America PCT/US2017/048191. 2017 August 23
Kar S., Hennighausen Z.B., inventors. Northeastern University, assignee. Two Dimensional
Materials for Use in Ultra High Density Information Storage and Sensor Devices. United States
of America Provisional Patent Application. ADN/19815-504-INV-19032. 2018 December 18
8.3 Presentations - Conferences
Hennighausen, Z.B., et al., Direct observation and in-situ control of interfaces between
heterostructures of chemically and structurally distinct 2D materials. APS March Meeting 2019,
Session K15
Hennighausen, Z.B., et al., O2-induced in-situ manipulation of exciton recombination pathways
in 2D heterostructures: Submicron, intensity-programmable pixels with rapid write-read-erase
capability, as well as 2D O2-sensor applications. APS March Meeting 2018, Session F36
Hennighausen, Z.B., et al., Reconfigurable Electronics and Optics at the Interface between
Dissimilar 2D Materials. 2017 MRS Fall Boston, Session NM04.02
Hennighausen, Z.B., et al., 2D Heterocrystals: Vertical stacking of rotationally aligned
electronically, structurally and chemically dissimilar 2D materials. APS March Meeting 2017,
Session C32
8.4 Presentations – Invited Talks
Hennighausen, Z.B., Probing and Manipulating the Interlayer Coupling in Bi2Se3/Transition
Metal Dichalcogenide 2D Heterostructures: Site-Selectable, in-situ Tuning of
Photoluminescence. Ludwig-Maximilian-Universitaet (LMU). June 2019
Hennighausen, Z.B., Engineering the properties of 2D materials via in-situ manipulation of the
interlayer coupling in vertically stacked 2D structures. Naval Research Laboratory (NRL).
December 2018
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Hennighausen, Z.B., In-situ Interface Engineering between 2D Materials: Tunable and
reversible properties with submicron spatial control (laser pattern-writing) of vertically stacked
TMD-Bi2Se3 heterostructures. National Institute of Science and Technology (NIST). May 2018
Hennighausen, Z.B., Graduate School and Opportunities in Science for CG Officers: Cadet to
2D Materials Researcher. U.S. Coast Guard Academy Symposia. April 2017
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Chapter 9: Appendix
9.1 Non-Bi2Se3 2D Heterostructures
9.1.1 Graphene/MoS2 Heterostructures
Graphene was the first monolayer material reported,283 and monolayer MoS2 is believed to be the
first monolayer transition metal dichalcogenides reported. 44 Both these materials have incredible
properties independently and have experienced tremendous growth and research since they
were first reported.25 A natural next step has been to combine the two materials to utilize the
comparative advantages of each material.109,284–286 For example, graphene has an exceptionally
high conductance, and it is relatively transparent. Monolayer MoS2 is photosensitive due to its
direct bandgap.
During this Ph.D. (or Doktorarbeit) monolayer and bulk MoS2 was grown on top of graphene to
create MoS2/graphene 2D heterostructures. The MoS2 was found to grow epitaxial on the
graphene, suggesting that the two materials are interacting and that there is a non-zero interlayer
coupling. Previous work has shown that there is charge transfer between MoS2 and graphene that
is large enough to quench the monolayer MoS2 signature PL.287 Our results are in agreement,
where no PL was detected when MoS2 was grown on graphene. Raman spectroscopy studies
showed a clear MoS2 signature, suggesting it was present. Below are SEM images, showing clear
indications of MoS2. The differences in work function between different layers of MoS2 and
graphene, including their interaction with electrons, allows contrast to be observed and
differences to be identified. It can be seen that islands of different graphene layers grew, and that
MoS2 grew on top, where triangles are characteristic of MoS2, while graphene prefers to grow as
a hexagon.
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Figure 9.1 MoS2 grown on top of graphene to create MoS2/graphene 2D heterostructures. The darker locations are
graphene (see blue arrow), suggesting graphene interacts less with the SEM electron beam or has a higher work
function since less electrons are generated. The lighter areas are MoS2, suggesting more electrons are being scattered
or ejected. The red arrows highlight locations that are clearly MoS2 due to both their lighter shading and the triangular
nature. MoS2 generally grows as a triangular crystal. The MoS2 grew in the graphene cracks, suggesting the cracks are
nucleation sites. The green arrow shows a crack where MoS2 grew. Raman and photoluminescence spectroscopy show
clear monolayer MoS2 signatures, suggesting it is present. Backscattered imaging was used to take the following image.
Figure 9.2 MoS2 grown on graphene using the secondary electron SEM imaging mode. Blue arrow shows three layers
of graphene. The green arrow shows two layers of graphene. These are islands that grew in the chemical vapor
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deposition (CVD) process. The red arrow shows the MoS2 crystals. The remaining area is graphene. Secondary electron
imaging was used for this image.
As shown in both Figure 9.1 and Figure 9.2, the MoS2 grew in the cracks and grain boundaries
between graphene grains, suggesting that the grain boundaries a nucleation sites. The changes in
contrast and color labels between the two images is due to collecting backscattered electrons vs.
secondary electrons. Backscattered electrons are more dependent on the size of the nucleus,
which is why MoS2 is brighter in Figure 9.1. Figure 9.2 is collected secondary electrons which are
ejected depending on the work function of the material. Since graphene has a lower work function
- it is bandgap-less material – it will appear brighter, while MoS2, which has a large bandgap, will
be darker.
9.1.2 Graphene/Graphene Heterostructures
In the beginning of the Ph.D. (or Doktorarbeit) a significant amount of graphene was grown. A
project was completed in collaboration with the first author, Dr. Ehsan Keyvani-Someh, where
graphene was mechanically stacked and used to fabricate low-cost solar cells. The graphene
served as a conductive and transparent electrode. The work found that two layers of graphene
are the most ideal 2D structure when using them as a transparent electrode in the structure
fabricated. Two layers were the ideal balance between transparency and conductivity. As the
number of layers increases, the transparency decreases, while the conductivity increases. The
change in transparency and conductivity is non-linear as the layer count changes, suggesting the
interlayer coupling is influencing the properties. The work showed that low-cost solar cells can
be fabricated whose performance is lower than commercially available products, but are cost
competitive due to the low manufacturing costs. Manufacturing high-quality graphene is
difficult, whereas low-quality graphene is much easier, suggesting that research into low-quality
graphene technology is more commercially realistic. Below are images and figures taken with
permission from the following reference.17
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Figure 9.3 Artistic diagrams demonstrating mechanically transferring economical graphene produces a conducting
path. Growing economical graphene is often separated spatially and not connected between electrodes. However,
mechanically transferring two of these growths creates a connecting path between spatially separated electrodes, as it
is statistically very likely that the islands will overlap in a configuration that allows for electrons to travel between
well-spaced electrodes. The following images were taken with permission from the following reference.17
While predictably growing a single and continuous layer of graphene is difficult and expensive,
growing separated islands is more economical and can be done predictably. However, this
method produces graphene with a high-resistance as there is often not a direct path for the
electrons. In this work we showed that mechanically transferring two of these economical
growths on top of each other is statistically very likely to produce a well-connected path that has
a sufficiently low resistance. Vice the electrons traveling continuously in-plane from one electrode
to the other, this method conducts electrons through different graphene grains vertically stacked.
This method is only possible since the interlayer coupling between graphene layers is known to
be sufficiently low to facilitate electron transfer between the layers.
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Figure 9.4 Images of as-grown graphene mechanically transferred onto glass. (a) shows graphene grown as islands. (b)
Shows monolayer graphene with islands of bilayer grown on top. (c) Monolayer graphene where only Poly-methyl
methacrylate (PMMA) residue can be seen. Removing PMMA from graphene is known to be difficult as they form a
strong coupling. The scale bars are all 10μm. The following images were taken with permission from the following
reference.17
9.1.3 MoS2/Sb2Se3 Heterostructures
Antimony selenide, Sb2Se3, similar to Bi2Se3 in that antimony (Sb) is in the same column as
bismuth, suggesting that Sb2Se3 would have overlapping properties to Bi2Se3. Sb2Se3 was grown
on top of monolayer MoS2 with the objective of studying whether similar effects are observed as
the Bi2Se3/MoS2 2D heterostructures. It was found that despite the fact several nanometers of
Sb2Se3 were grown on top, suggesting that the monolayer MoS2 is fully covered, the signature PL
was not fully quenched, suggesting that the interlayer coupling is leaving the excitons
significantly intact. This indicates that either the interlayer coupling is weak, or it does not affect
the monolayer MoS2. The height of the Sb2Se3 was verified with AFM measurements, which are
not provided in this Chapter.
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Figure 9.5 Optical images of monolayer MoS2 prior to growth of Sb2Se3. (a) and (b) show MoS2 triangles, where the blue
spots in the middle are the nucleation sites. (c) and (d) show thin films, where rods or tubes of sulfurized MoO2 crystal
grew around (note the blue lines).
Figure 9.5 shows optical images of monolayer MoS2 prior to growth of Sb2Se3. The PL and Raman
measurements are signature of monolayer MoS2, suggesting it is of sufficient quality (Figure 9.7).
MoO2 rods and/or tubes that have been sulfurized can be seen, a consequence of the chemical
vapor deposition growth. Figure 9.6 are optical images after Sb2Se3 was grown on top, where the
perceived color changed from purple to teal. AFM scans showed that several nanometers of Sb2Se3
grew on top indicating that there was full coverage. It can be seen that the Sb2Se3 appear to prefer
to grow on the MoS2 substrate, vice the SiO2, suggesting that an interlayer coupling makes the
growth more energetically favorable. Figure 9.6d shows an area where Sb2Se3 grew on SiO2, and
then was exposed to a laser in air. The white lines are due to the laser exposure, suggesting that
it is affecting the Sb2Se3, possibly also due to oxygen in the air. This same effect was observed in
Bi2Se3 that was grown on SiO2.
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Figure 9.6 Optical images of heterostructures where Sb2Se3 was grown on top of monolayer MoS2. The growth modifies
the perceived color from purple (see Figure 9.5) to a teal color. Additionally, it appears as though the Sb2Se3 prefers to
grow on the MoS2, suggesting there is an interlayer interaction that makes this substrate more favorable for growth. (a)
and (b) are optical images of the heterostructures. (c) is a close-up of the same location in (a) and (b). (d) shows an area
where Sb2Se3 grew on bare SiO2. The patterns with the color white were “drawn” using a 488nm laser, suggesting that
the Sb2Se3 is sensitive to laser exposure in air. This is in agreement with the sensitivity of Bi2Se3 to oxygen and laser
exposure. Further, these same lines and patterns can be drawn into Bi2Se3 that is grown on SiO2.
Figure 9.7 Photoluminescence and Raman spectroscopy from Sb2Se3/MoS2 heterostructures. (a) PL of monolayer MoS2
and after Sb2Se3 was grown on that area. Interestingly, the PL shifts to a higher power, suggesting that the ratio of
excitons, or the binding energy, have been modified by the Sb2Se3. Despite the fact that several nanometers of Sb2Se3
were grown on top, the PL has only been reduced to ~25% of its pre-growth value, suggesting that the interlayer
interaction is not significantly modifying the exciton dynamics. (b) PL spectra have consecutive laser exposure in air,
where shifts in the peak energy and PL intensity are observed. Although the changes are not as pronounced as in the
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Bi2Se3/MoS2 2D heterostructures, the changes are relatively large, suggesting that alterations in the interlayer coupling
are happening. Such wild changes are not expected for laser exposure of a monolayer MoS2 crystal. (c) Raman spectra
of a Sb2Se3/MoS2 heterostructure before and after laser exposure. The only change detected is that the MoS2 peaks got
stronger, suggesting the interlayer coupling was weakened, in agreement with the Bi2Se3/MoS2 results.
Figure 9.7 show photoluminescence (PL) and Raman spectra of monolayer MoS2 and the
Sb2Se3/MoS2 heterostructure. Figure 9.7a shows the PL spectra of monolayer MoS2 and then the
Sb2Se3/MoS2 heterostructure, where a clear decrease in PL intensity is observed, but not nearly
the same quenching observed for Bi2Se3/MoS2 2D heterostructures. Interestingly, the PL peak
position is shifted higher, suggesting that either lower energy excitons (such as the trion) are
mitigated, or the binding energy of the excitons has increased. The shift in the PL peak suggests
that an influential interlayer coupling is present. Figure 9.7b the same Sb2Se3/MoS2
heterostructure after exposure to a laser in air. Although the same PL recovery is not observed,
significant changes in the PL peak position are, along with moderate changes in the PL intensity.
These changes are not observed in MoS2 when exposed to a laser in air, suggesting that changes
to the heterostructure are being affected. Figure 9.7c shows the PL spectra of as-grown and laser
exposed Sb2Se3/MoS2 heterostructure, where the Sb2Se3 peaks cannot be seen. Additionally, the
Raman spectra of MoS2 appears to increase in intensity with laser exposure, suggesting that the
interlayer coupling has been diminished, in agreement with the Bi2Se3/MoS2 2D heterostructure.
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Figure 9.8 TEM SAED images and TEM images of Sb2se3/MoS2 heterostructures. (a) and (b) are SAED images, where
the crystal structure of MoS2 can be clearly identified, but the Sb2Se3 crystal structure cannot be identified, suggesting
that it did not grow as a crystal. Further, the bright diffuse halo surrounding the beam blocker is indicative of
amorphous materials, which normally have periodicity that induces such patterns. (b) shows numerous MoS2 crystal
grains, where the spots are varying brightness. This is either due to folding, as shown in (d), or due to a polycrystalline
section, as shown in Figure 9.5. (c) and (d) are TEM images, where breaking and folding due to the transfer process can
be seen. The granulated nature of the heterostructure, indicates that Sb2Se3 is on top.
Figure 9.8 shows transmission electron microscope (TEM) select area electron diffraction (SAED)
images and TEM images. Figure 9.8a and Figure 9.8b show SAED images, where the MoS2 spots
can be clearly identified, but the Sb2Se3 spots are noticeably absent, despite the fact that several
nanometers of Sb2Se3 grew on top, suggesting it grew amorphous. Additionally, the bright diffuse
ring around the center is indicative of amorphous materials, which have semi periodicity that
produces such rings. Sb2Se3 is known to have a different crystal structure from either MoS2 or
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Bi2Se3, which is possibly why it cannot be identified and/or grew amorphous. Figure 9.8c and
Figure 9.8d show TEM images that have folding and tearing of the heterostructure from the
transfer process.
9.1.4 MoS2/Bi2Te3 Heterostructures
Bismuth telluride (Bi2Te3) was grown on top of monolayer MoS2 and then characterized using
optical microscopy, photoluminescence spectroscopy, and Raman spectroscopy. The results
indicate that Bi2Te3 and monolayer MoS2 likely form a very weak or near-zero interlayer coupling,
in contrast to the strong interlayer coupling observed between Bi2Se3 and MoS2.
Figure 9.9 Optical images of monolayer MoS2 and Bi2Te3 grown on monolayer MoS2. (a) and (b) are monolayer MoS2.
The triangular shapes suggest they are well-formed crystals. (c) Bi2Te3 was grown on top on the MoS2, and it grew as
gold bars/rods/tubes, in contrast to the Bi2Se3 or Sb2Se3 results, which grew continuous and smooth.
As shown in Figure 9.9, Bi2Te3 grew in a rod/tubular structure on the MoS2, in contrast to both
Bi2Se3 and Sb2Se3 which grew continuous and smooth on MoS2, suggesting that epitaxial growth
is not present. Further, the growth of Bi2Te3 appears to be the same whether on SiO2 or MoS2,
suggesting the interlayer coupling is not strong enough to modify or alter the growth dynamics.
Figure 9.9a and Figure 9.9b show optical images of as-grown monolayer and bulk MoS2. The
monolayer MoS2 is a light purple color, while the blue and bright spots are multilayer or bulk
MoS2. The triangular structure suggests that the MoS2 is crystalline. Figure 9.9c shows the Bi2Te3
grown on top, where the tubular/rod structure can be seen. The tubes appear to have grown
several microns in length.
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Figure 9.10 Photoluminescence and Raman spectra of Bi2Te3/MoS2 heterostructures. (a) PL spectra from monolayer
MoS2 and Bi2Te3 heterostructures, where a clear decrease in PL intensity is observed after Bi2Te3 was grown on top, but
not nearly to the same extent (or quenching) as with Bi2Se3/MoS2 2D heterostructures. The presence of a bright PL
spectra suggests that the interlayer coupling is weaker than that between Bi2Se3 and MoS2. There does not appear to be
a significant shift in the PL peak, suggesting the exciton ratio is approximately the same as in monolayer MoS2. (b)
Raman spectra of pre and post Bi2Te3 growth on monolayer MoS2, where no significant changes were detected. Bi2Te3
modes have demonstrated a weak intensity and appear at low Raman shift values, suggesting that the equipment may
not measure to sufficiently low values.
Figure 9.10 shows the PL and Raman spectra of monolayer MoS2 and a Bi2Te3/MoS2
heterostructure. Figure 9.10a shows the PL spectra, where a clear decrease in PL intensity is
observed to approximately a 15% of the original value. Although this is a significant decrease, it
is not close to the quenching observed in Bi2Se3/MoS2 2D heterostructures. The PL decrease is
possibly not due to an influential interlayer coupling, but by how much light is transmitted
through the upper Bi2Te3 material. The decrease in PL could also be due to a weak interlayer
coupling that facilitates partial charge transfer. Interestingly, no significant changes to the Raman
spectrum were recorded between pre and post Bi2Te3 growth, suggesting that the MoS2 modes
are unaffected. Further, there does not appear to be any new modes that might correspond to
Bi2Te3. This is not surprising as Bi2Te3 Raman modes have been documented to have a low
intensity and are at a low Raman shift of approximately 100 1/cm, suggesting that our equipment
may not be sensitive to detect them.288
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9.2 Programming Code
Below are codes for two programs that were especially important for the work presented in this
dissertation. Both codes are written in the Matlab language with comments kept to provide
amplifying information.
9.2.1 Fitting Lorentzian Functions to Exciton Photoluminescence Spectra
% Versuche zwei L Formeln mit meine nummer anzupassen
% % Define the Lorentzian % dualLorenz = @(x, a1, b1, a2, b2)lorenz(x, a1, b1)+lorenz(x, a2, b2); % % Parameterise the % objFn = @(p, x, y) sum( (y - dualLorenz(x, p(1), p(2), p(3), p(4))) .^ 2 ); % % Use fsolve to % p0 = [100 0.05 0.05 100]; % p = fsolve( @(p) objFn(p, X, Y), p0 );
%Import all the file names files=dir('*.csv');
%Sort by name. Make sure all the files are in the correct order before %running program [~,ind]=sort({files.name}); files=files(ind);
% Matrix for the final values % File name/ a/ b/ Constant/ gamma1/ gamma2/ Slope/ x01/x02/ Max1/ Max2/
MaxTot/ MaxeV/
values=zeros(length(files),11); valuesC = cell(length(files)+1,11); vgof2=zeros(length(files),2);
% % Make Column headers % valuesC(1,1) = {'File name'}; valuesC(1,2) = {'a'}; valuesC(1,3) = {'b'}; % valuesC(1,5) = {'gamma1'}; valuesC(1,6) = {'gamma2'}; valuesC(1,8) =
{'x01'}; % valuesC(1,9) = {'x02'}; valuesC(1,10) = {'Max1'}; valuesC(1,11) = {'Max2'}; % valuesC(1,12) = {'MaxTot'}; valuesC(1,13) = {'MaxeV'}; % valuesC(1,7) = {'linear Slope'}; valuesC(1,4) = {'linear constant'};
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% Make Column headers valuesC(1,1) = {'File name'}; valuesC(1,2) = {'a'}; valuesC(1,3) = {'b'}; valuesC(1,4) = {'gamma1'}; valuesC(1,5) = {'gamma2'}; valuesC(1,6) = {'x01'}; valuesC(1,7) = {'x02'}; valuesC(1,8) = {'Max1'}; valuesC(1,9) = {'Max2'}; valuesC(1,10) = {'MaxTot'}; valuesC(1,11) = {'MaxeV'};
% Array to store SSE values SSE = zeros(length(files),2);
% Script to loop through the files, find a fit, and then output the values for cnt=1:length(files) tmpFileVals = dlmread(files(cnt).name); % Store vals to a temp array valuesC(cnt+1,1)={files(cnt).name};
% Start Fitting algo x = tmpFileVals(:,1); % input these files into algorthm y = tmpFileVals(:,2);
% Initial parameters a = 100; b = 200; gamma1 = 0.05; gamma2 = 0.04; x01 = 1.82; x02 = 1.79; za = 10; zb = 90; start = [a, b, gamma1, gamma2, x01, x02, za, zb];
% Upper and lower bounds low = [0 0 0 0 1.7 1.7 -1000 -2000]; upp = [3000 3000 .15 .15 2.0 2.0 1000 2000];
% 'Robust', 'LAR', opcjedopasowania = fitoptions('Method', 'NonlinearLeastSquares',
'Lower',low, ... 'Startpoint', start, 'Upper', upp, 'MaxIter', 10000); typdopasowania = fittype('a*0.5*gamma1/(pi()*((x-
x01)^2+0.25*gamma1*gamma1))+b*0.5*gamma2/(pi()*((x-
x02)^2+0.25*gamma2*gamma2))+za*x+zb', 'options', opcjedopasowania); [fitobject,gof] =fit(x,y,typdopasowania); tmpCoefVal=coeffvalues(fitobject);
vgof2(cnt,1)=getfield(gof,'sse'); vgof2(cnt,2)=getfield(gof,'rsquare');
% Loop through all coeff values; store to larger value array for cnt2 = 1:6 values(cnt,cnt2)=tmpCoefVal(cnt2); valuesC(cnt+1,cnt2+1)={tmpCoefVal(cnt2)}; end
a=tmpCoefVal(1); b=tmpCoefVal(2); gamma1=tmpCoefVal(3); gamma2=tmpCoefVal(4); x01=tmpCoefVal(5); x02=tmpCoefVal(6); za = tmpCoefVal(7); zb = tmpCoefVal(8);
% Compute the Lorentzian Max values values(cnt,7)=a*2/(pi()*gamma1); values(cnt,8)=b*2/(pi()*gamma2);
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valuesC(cnt+1,8)={a*2/(pi()*gamma1)};
valuesC(cnt+1,9)={b*2/(pi()*gamma2)};
% Compute the overall Max value; anpassung is the fit anpassung1 = a.*0.5.*gamma1./(pi().*((x-x01).^2+0.25.*gamma1.*gamma1)); anpassung2 = b.*0.5.*gamma2./(pi().*((x-x02).^2+0.25.*gamma2.*gamma2)); anpassung = anpassung1+anpassung2+za*x+zb; [M,I] = max(anpassung); values(cnt,9)=M; values(cnt,10)=x(I); valuesC(cnt+1,10)={M}; valuesC(cnt+1,11)={x(I)};
% Put all trions and excitons in the correct columns if values(cnt,6) < values(cnt,5) %if values(cnt,7) < values(cnt,6) % switch first three values for cnt3 = 1:2:7 tmpVal=values(cnt,cnt3); values(cnt,cnt3)=values(cnt,cnt3+1); values(cnt,cnt3+1)=tmpVal;
tmpValC=valuesC(cnt+1,cnt3+1); valuesC(cnt+1,cnt3+1)=valuesC(cnt+1,cnt3+2); valuesC(cnt+1,cnt3+2)=tmpValC; end
% tmpVal=values(cnt,1); % values(cnt,1)=values(cnt,1+1); % values(cnt,1+1)=tmpVal; % tmpValC=valuesC(cnt+1,1+1); % valuesC(cnt+1,1+1)=valuesC(cnt+1,1+2); % valuesC(cnt+1,1+2)=tmpValC; % % tmpVal=values(cnt,4); % values(cnt,4)=values(cnt,4+1); % values(cnt,4+1)=tmpVal; % tmpValC=valuesC(cnt+1,4+1); % valuesC(cnt+1,4+1)=valuesC(cnt+1,4+2); % valuesC(cnt+1,4+2)=tmpValC; % % tmpVal=values(cnt,7); % values(cnt,7)=values(cnt,7+1); % values(cnt,7+1)=tmpVal; % tmpValC=valuesC(cnt+1,7+1); % valuesC(cnt+1,7+1)=valuesC(cnt+1,7+2); % valuesC(cnt+1,7+2)=tmpValC; % % tmpVal=values(cnt,9); % values(cnt,9)=values(cnt,9+1); % values(cnt,9+1)=tmpVal; % tmpValC=valuesC(cnt+1,9+1); % valuesC(cnt+1,9+1)=valuesC(cnt+1,9+2); % valuesC(cnt+1,9+2)=tmpValC;
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% tmpVal=values(cnt,1); % values(cnt,1)=values(cnt,1+1); % values(cnt,1+1)=tmpVal; % tmpValC=valuesC(cnt+1,1+1); % valuesC(cnt+1,1+1)=valuesC(cnt+1,1+2); % valuesC(cnt+1,1+2)=tmpValC; % for cnt3=4:2:8 % tmpVal=values(cnt,cnt3); % values(cnt,cnt3)=values(cnt,cnt3+1); % values(cnt,cnt3+1)=tmpVal; % tmpValC=valuesC(cnt+1,cnt3+1); % valuesC(cnt+1,cnt3+1)=valuesC(cnt+1,cnt3+2); % valuesC(cnt+1,cnt3+2)=tmpValC; % end
% % switch first three values % for cnt4 = 7:7 % tmpVal=values(cnt,cnt4); % values(cnt,cnt4)=values(cnt,cnt4+1); % values(cnt,cnt4+1)=tmpVal; % % tmpValC=valuesC(cnt+1,cnt4+1); % valuesC(cnt+1,cnt4+1)=valuesC(cnt,cnt4+2); % valuesC(cnt+1,cnt4+2)=tmpValC; % end
% Re-Compute the anpassung for making the graphs anpassung2 = a.*0.5.*gamma1./(pi().*((x-x01).^2+0.25.*gamma1.*gamma1)); anpassung1 = b.*0.5.*gamma2./(pi().*((x-x02).^2+0.25.*gamma2.*gamma2)); end
anpassung=anpassung1+anpassung2+za*x+zb;
% % Create jpeg files of all the figures % cntStr = num2str(cnt); % %fig=figure();set(gfc,'Visible', 'off');% suppresses the figure from
popping up % fig=figure();set(fig,'DefaultFigureVisible', 'off');% suppresses the
figure from popping up % plot(x,y,x,anpassung1,x,anpassung2,x,anpassung); % saveas(fig,cntStr,'jpeg'); % %figure();set(gcf,'Visible', 'on');% Allows the figure to appear again % %fig=figure();set(fig,'DefaultFigureVisible', 'on');% suppresses the
figure from popping up
% % Test the goodness of fit % SSE_m = sqrt((anpassung-y).^2); % SSE(cnt,1) = sum(SSE_m)/length(SSE_m);
% % Initial parameters % ap = Anpassp(cnt,1); bp = Anpassp(cnt,4); % gamma1p = Anpassp(cnt,2); gamma2p = Anpassp(cnt,5);
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% x01p = Anpassp(cnt,3); x02p = Anpassp(cnt,6); % % % Compute the overall Max value; anpassung is the fit % anpassung1p = ap.*0.5.*gamma1p./(pi().*((x-
x01p).^2+0.25.*gamma1p.*gamma1p)); % anpassung2p = bp.*0.5.*gamma2p./(pi().*((x-
x02p).^2+0.25.*gamma2p.*gamma2p)); % anpassungp = anpassung1p+anpassung2p; % % SSE_p = sqrt((anpassungp-y).^2); % SSE(cnt,2) = sum(SSE_p)/length(SSE_p); % % Create jpeg files of all the figures cntStr = num2str(cnt); %fig=figure();set(gfc,'Visible', 'off');% suppresses the figure from
popping up fig=figure();set(fig,'DefaultFigureVisible', 'off');% suppresses the
figure from popping up plot(x,y,x,anpassung1,x,anpassung2,x,za*x+zb,x,anpassung); saveas(fig,cntStr,'jpeg'); %figure();set(gcf,'Visible', 'on');% Allows the figure to appear again %fig=figure();set(fig,'DefaultFigureVisible', 'on');% suppresses the
figure from popping up
end
9.2.2 Raw Search for Commensurate Unit Cells and Moiré Patterns
Daten = zeros(7,11);
n_c=1; mx = 10; Xme_fin = 1.28; Xme_int = 1.23;
%1.27 and 1.34
for m = 0:1:mx for n = 0:1:mx for r = 0:1:mx for s = 0:1:mx diph_eq = (r*r+s*s-r*s)/(m*m+n*n-m*n);
% put the Xme and Xme_fin equations here if diph_eq > Xme_int*Xme_int && diph_eq< Xme_fin*Xme_fin &&
r>=s && m>=n Daten(n_c,1) = m; Daten(n_c,2) = n; Daten(n_c,3) = r; Daten(n_c,4) = s; Daten(n_c,5) = diph_eq; Daten(n_c,6) = 180*acos((m-n/2)/sqrt(m*m+n*n-m*n))/pi(); Daten(n_c,7) = abs(180*(acos((m-n/2)/sqrt(m*m+n*n-m*n))-
acos((r-s/2)/sqrt(r*r+s*s-r*s)))/pi());
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if Daten(n_c,7)>30 && Daten(n_c,7)<=60 Daten(n_c,7) = 60-Daten(n_c,7); elseif Daten(n_c,7)>60 Daten(n_c,7) = Daten(n_c,7)-60; end
Daten(n_c,8) = sqrt(m*m+n*n-m*n)*.418; %*.276 *.344
Daten(n_c,10) = m+n+m*n; Daten(n_c,11) = r+s+r*s;
n_c=n_c+1; end end end end end
% histogram(Daten(:,7),300)
% Plot % Xme_fin = 1.364; Xme_int = 1.252394; Xstep = 0.0001; theta_fin = 0.5310; theta_int = -0.0100; theta_step = 0.0001; theta_axis = theta_int:theta_step:theta_fin; theta_axis = theta_axis*180/pi(); x_me_axis = Xme_int:Xstep:Xme_fin;
% Create empty arrays x_tot_cnt = round((Xme_fin-Xme_int)/Xstep); theta_tot_cnt = round((theta_fin-theta_int)/theta_step); %moire_calc = zeros(x_tot_cnt, theta_tot_cnt);
%imagesc(theta_axis, x_me_axis, moire_calc) %colormap(gray)
hold on; Daten(:,5)=sqrt(Daten(:,5)); Daten(:,7)=Daten(:,7); y = transpose(Daten(:,5)); x = transpose(Daten(:,7));
sz = 700; % Assume that there is 1% error on either side c = ['k'];
scatter(x,y,sz,c)
title('') xlabel('') ylabel('') set(gca,'xticklabel',[],'yticklabel',[],'ytick',[1.26 1.28 1.30 1.32 1.34
1.36]) % % % title('Hws Moire Pattern - 3rd order - 2.1% Strain for Both') % % xlabel('Relative Crystal Rotation (Deg)')
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% % ylabel('x = Bi2Se3/TMD')
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