SCANNING TUNNELING MICROSCOPY

CHARACTERIZATION AND METALLIC NANOCONTACTS

FOR ATOMICALLY PRECISE GRAPHENE NANORIBBONS

BY

XIMENG LIU

DISSERTATION

Submitted in partial fulfillment of the requirements

for the degree of Doctor of Philosophy in Electrical and Computer Engineering

in the Graduate College of the

University of Illinois at Urbana-Champaign, 2018

Urbana, Illinois

Doctoral Committee:

Professor Joseph W. Lyding, Chair

Professor Gregory S. Girolami

Professor Xiuling Li

Assistant Professor Wenjuan Zhu

ii

ABSTRACT

As a potential candidate for replacing silicon (Si) as a next-generation semiconducting

material, atomically precise graphene nanoribbons (GNRs) have been predicted to show very

interesting electronic properties based on their geometries and their underlying substrates. Once

the ribbons are synthesized, confirmation of their geometries and investigating their electronic

properties are essential for further implementation in devices.

This dissertation addresses investigations of three different solution-synthesized

atomically precise GNRs by scanning tunneling microscopy (STM) and scanning tunneling

spectroscopy (STS). A dry contact transfer (DCT) technique was implemented for depositing

GNRs onto various semiconducting substrates. Detailed STM and STS measurements of

doublewide GNRs on InAs(110) and InSb(110) confirmed their geometries and revealed a 2 eV

bandgap as well as the 3-D distribution of the local density of states. Computational modeling of

the ribbon´s electronic structure showed good agreement with our experimental results,

indicating a weak coupling between the InAs substrate and the GNR. STM studies of two

additional types of GNRs, the extended chevron GNRs and the nitrogen-doped GNRs on InAs,

demonstrate how structural modifications affect the properties of the ribbons including their

bandgaps and interactions with the substrate.

We also proposed a scheme of writing metallic hafnium diboride nanocontacts onto

isolated GNRs using STM tip-assisted deposition for conducting transport measurements. In

order to perform transport measurement in situ through sample biasing, we prefabricated an array

of large metallic electrodes on Si and loaded it into the STM system. The material chosen,

iii

structural design and e-beam fabrication process are described in detail. The effect on thermal

treatment to the formation of metal-silicide compounds was explored.

iv

To my parents, for their unconditional love and support

v

ACKNOWLEDGMENTS

It would not have been possible to complete this dissertation without the help and support

of the kind people around me, to only some of whom it is possible to give particular mention

here.

My deepest gratitude goes first and foremost to my advisor, Professor Joseph Lyding, for

his guidance, assistance, expertise and patience throughout the process of my graduate study. It

was he who was always open to questions and discussions, providing valuable suggestions and

ideas that taught me how to think critically and prospectively as a scientist. He often worked late

with me in the lab on solving problems with the STM, and showed me how important passion

and persistence are for being successful in one´s career. He encouraged me to try new and

different things and to never be mad and disappointed when I fail. I thank him for always

promoting my work when necessary and sending me to conferences for developing my

professional skills. I have continually learned from him, not only as a distinguished scholar, but

also as a human.

I am extremely thankful for becoming a member of the Lyding group and working with

all those talented and hardworking group members. Dr. Kevin He, who was my first student

mentor and collaborator on building the low-temperature STM, taught me how to operate an

STM from scratch and triggered my great interest in STM for conducting my later work. Dr.

Scott Schmucker, Dr. Justin Koepke and Dr. Pamela Martin have been greatly helpful for STM-

related questions. Dr. Jaewon Do and Dr. Josh Wood were always willing to share their expertise

on device fabrication. Dr. Adrian Radocea has been a great resource and helper for random

vi

questions and issues related to STM. I had a great time collaborating with him on one of the

graphene nanoribbon projects, and I thank him for sharing his Matlab code with me for

generating vivid STS maps and current imaging tunneling spectroscopy images. I also enjoyed

my time talking with Yaofeng Chen, Sartaj Grewal, Kaitlyn Parsons, Faraz Arastu, Gang Wang,

Yingjie Zhang and Huy Nguyen. They often provide bright and interesting insight into my

research and are always supportive.

I also feel obligated to sincerely thank all my collaborators: Professor Aluru and his

student Tao Sun for conducting computational modeling for the doublewide graphene

nanoribbons, which is invaluable for understanding the experimental observations; Professor

Alexander Sinitskii from the University of Nebraska and his student Mohammad Mehdi for

sharing many graphene naoribbon samples to us for STM characterization; and Meng Liu from

the Chinese Academy of Science for providing her insight and help on fabricating the large

metallic electrodes.

In addition, I would like to express my gratitude to many staff members at the Beckman

Institute and the Micro and Nanotechnology Laboratory, including but not limited to Scott

Robinson, Cate Wallace and Edmond Chow for their high-quality training on the instruments

that I frequently used throughout my research. The people in the ECE machine shop, Scott A.

McDonald and David Switzer, also provided tremendous help on making the parts that were

required for construction and modification of the STM system. I would like to thank Dr. Jinju

Lee from TipTekTM for continuously providing extremely sharp STM probes that played an

essential role in collecting high-quality STM data.

Lastly, nobody has been more important to me in this process of pursuing my Ph.D.

degree than my family members. My dearest parents, who have sacrificed much to support my

vii

study here in the United States and always showed their faith in me, have and will be my

emotional support forever. My husband, Yi Sun, who I feel exceedingly fortunate to be married

to, is always respectful and supportive and by my side. Just three months ago we were blessed

with the arrival of a baby girl, Skylar, and I have come to find that her smiles simply lighten my

life, even if it is unintentional.

viii

TABLE OF CONTENTS

CHAPTER 1 INTRODUCTION ....................................................................................... 1

1.1 Background ............................................................................................................... 1

1.2 Scanning Tunneling Microscopy and Spectroscopy ................................................. 2

1.3 Graphene Nanoribbons ............................................................................................. 3

1.4 STM of Graphene Nanoribbons ................................................................................ 6

1.5 Motivation ................................................................................................................. 8

1.6 Thesis Statement and Dissertation Organization ...................................................... 9

1.7 References ............................................................................................................... 10

CHAPTER 2 CROSS-SECTIONAL STM OF SOLUTION-SYNTHESIZED

DOUBLEWIDE GRAPHENE NANORIBBONS (WGNRS) ON INAS(110) AND

INSB(110)……………………………………………………………………………..16

2.1 Introduction and Motivation ................................................................................... 16

2.2 Cross-sectional STM Imaging of wGNRs on InAs(110) ........................................ 18

2.3 STS Measurement of wGNRs on InAs(110) .......................................................... 19

2.4 WGNR-InAs Interaction ......................................................................................... 21

2.5 WGNRs on InSb(110)............................................................................................. 24

2.6 Conclusion .............................................................................................................. 26

2.7 Methods................................................................................................................... 27

2.8 Figures..................................................................................................................... 31

2.9 References ............................................................................................................... 44

CHAPTER 3 CROSS-SECTIONAL STM OF EXTENDED-CHEVRON AND

NITROGEN-DOPED GNRS ON INAS(110) .............................................................. 48

3.1 Introduction and Motivation ................................................................................... 48

3.2 Extended-chevron GNRs ........................................................................................ 49

3.3 Nitrogen-doped GNRs ............................................................................................ 51

ix

3.4 Conclusion .............................................................................................................. 52

3.5 Figures..................................................................................................................... 54

3.6 References ............................................................................................................... 58

CHAPTER 4 FABRICATION OF MICROMETER-SIZED METAL ELECTRODE ARRAYS

FOR STM TIP-ASSISTED NANOCONTACTS WRITING FOR GNRS ............................... 60

4.1 Introduction and Motivation ................................................................................... 60

4.2 Substrate and Electrode Materials .......................................................................... 62

4.3 Structural Design and Fabrication Process ............................................................. 63

4.4 Low-Temperature Sample Preparations and the Annealing Effect on Evaporated

Metal Surfaces ........................................................................................................ 65

4.5 Conclusion .............................................................................................................. 68

4.6 Methods................................................................................................................... 68

4.7 Figures..................................................................................................................... 70

4.8 References ............................................................................................................... 78

CHAPTER 5 SUMMARY AND FUTURE WORK ....................................................... 81

5.1 References ............................................................................................................... 83

APPENDIX A: DESIGN OF A LOW-TEMPERATURE STM SYSTEM...................... 84

A.1 System Design........................................................................................................ 84

A.2 Figures .................................................................................................................... 86

A.3 References .............................................................................................................. 89

1

CHAPTER 1

INTRODUCTION

1.1 Background

Moore´s Law, which has been the guideline for the development of the semiconducting

industry for over half a century, is possibly reaching the end of its practical path. With the

demand of producing smaller and faster devices, the wonder material, silicon, has been pushed to

its absolute limit at 10 nm. To hit 7 nm and beyond, new materials and structures, even novel

operational concepts, are required to overcome the current issues with Si such as low electron

hole mobility, current leakage through SiO2 layers and poor heat dissipation. In the short term,

without abandoning all the current manufacturing technologies for keeping the production costs

low, companies like Intel and IBM have shown that implementing a hybrid approach, which

blends silicon with III-V material1,2 and germanium,3 can solve the problem to some extent.

However, challenges still remain in terms of lattice mismatch for growing high-mobility

materials and high-K dielectrics on Si, which has a large effect on the device´s quality and

reliability. In the long term, devices with new architectures such as ultra-thin body multi-gate

MOSFETs with lightly doped channels,4,5 gate-all-around and vertical nanowires devices6,7 will

be needed. In addition, devices with innovative operational mechanisms such as tunneling FETs8

and spintronics9,10 have also been predicted to be promising. Meanwhile, 2-D materials such as

graphene nanoribbons, carbon nanotubes and MoS2, with their small sizes and extraordinary

properties, are expected to overcome the scaling limit of bulk transistors.

2

1.2 Scanning Tunneling Microscopy and Spectroscopy

Since the invention of the first scanning tunneling microscope (STM) in 1981,11 followed

by the demonstration of a real-space topograph of a Si(111) surface at atomic resolution,12 STM

has played an important role in the field of surface science. The operational principle of an STM

is based on quantum tunneling. When a sharp metallic tip is brought very close to a conducting

surface with a bias applied between the two, electrons can tunnel either from the filled states of

the tip to the empty states of the substrate or the other way around. The resulting tunneling

current, I, has an exponential dependency on the distance between the tip and the substrate and

thus by keeping the current constant, sub-angstrom-scale corrugations at the surface can be

captured. Since the tunneling current also depends on the electron local density of states (LDOS)

of the substrate, while moving the very sharp tip across the surface, a real-space 3-D surface-

state topography with atomic resolution is generated.

In addition to showing the atomic topography of a surface, STM also shows local

electronic structures of the material by scanning tunneling spectroscopy (STS).13 By measuring

the variation of the tunneling current as a function of voltage at a fixed tip-sample separation,

one can obtain the I-V spectra at any location of the surface. The I-V spectra can be further

calculated to get the normalized tunneling conductance spectra, (dI/dV)/(I/V) as a function of

applied bias V, which reflects the allowed energy of the LDOS of the substrate. STS is

exceptionally useful for determining the bandgap, doping level, and LDOS of a material with

high spatial resolution.

Another technique, called current imaging tunneling spectroscopy (CITS),14 combines the

spectroscopic capability and the scanning ability of an STM. By taking a measurement of the I-V

spectrum at each scan point of a topographic image, one can get an image that reflects the

3

tunneling current or calculated LDOS over the scanned area at different biases. In other words,

such an image can provide direct observation on the distribution of the LDOS over the scanning

area.

In addition to performing STM and STS, tunneling current from the tip can be utilized to

modify surface features, such as breaking certain bonds between atoms for nanolithography, as

well as directly manipulating atoms and molecules on the surface. Examples include hydrogen

de-passivation of the H-Si surface,15 cutting of graphene into graphene nanoribbons,16 and

moving Fe atoms on a Cu surface to create a ˝quantum corral˝.17

With its numerous capabilities as stated above, STM, as a surface characterization

instrument, is also preferably applicable for characterizing 2-D and 1-D nanomaterials. Not only

have investigations been conducted on conductive and semiconducting materials such as

graphene,18,19 2-D transition metal dichalcogenide,20,21 carbon nanotubes22,23 and quantum

dots,24,25 STM has also been applied on insulating thin films such as boron nitride26 for the

purpose of confirming the morphology, determining the electronic structure, manipulating the

surface as well as investigating the material-substrate interaction.

1.3 Graphene Nanoribbons

Graphene nanoribbons (GNRs) are narrow strips of graphene. With widths ranging from

one to tens of nanometers, quantum confinement opens an electrical bandgap, which is predicted

to be inversely proportional to the GNR width.27,28 A simple way of understanding this is as

follows: with the quantum confinement in the x-direction, the resulting wave function becomes

𝜓(𝑥) ∝ sin(𝑘𝑥𝑥) with 𝑘𝑥 = 𝑛𝜋/𝑊

4

where 𝑛 is an integer, 𝑊 is the ribbon´s width and kx is the wave vector. Assuming graphene´s

linear dispersion relation near the Fermi level is

𝐸 ≈ ℎ𝑣𝑓√𝑘𝑥2 + 𝑘𝑦

2

then the allowed energy levels are shown to be

𝐸𝑛 = 𝑛ℎ𝑣𝑓𝜋

𝑊

and therefore the bandgap is

𝐸𝑔 =ℎ𝑣𝑓𝜋

𝑊

In addition, not only does the bandgap have a dependency on the ribbon width, but also on the

edge structure such as crystallographic orientation and chemical edge termination. There are

some early theoretical calculations29,27 of GNR electronic structures showing that zigzag ribbons

and armchair ribbons of the family N = 3m + 2 where N is the number of carbon atoms across

the width and m is an integer are metallic, and armchair ribbons of other two families N = 3m

and N = 3m + 1 are semiconducting. Later on, however, another theoretical work on zigzag

GNRs30 showed an opening in the bandgap due to degenerative states near the Fermi level

corresponding to localized edge states, and the existence of the edge states has been proven by

some experimental work,31,32 too. Furthermore, work that takes an ab initio approach in their

local density approximation calculations showed that armchair ribbons in all three families have

a non-zero bandgap.33 Despite some discrepancies in the calculated results of the electronic

structure of GNRs, the tunable bandgap, along with its exceptional graphene-like properties such

as high carrier mobility34,35 and high thermal conductivity,2 makes GNRs a promising candidate

for replacing Si as the future wonder material for the semiconductor industry.

5

With properties strongly relying on geometric structure, a way of fabricating atomically

precise GNRs is desired. Early work of fabricating GNRs from top-down approaches such as

oxygen plasma etching,36,37 probe lithographic etching38,32 and graphene cutting with catalytic

particles16,39 resulted in ribbons with poor control over width and edge chirality, and led to

bandgap variations along the ribbon.

Bottom-up synthesis approaches, such as CVD growth with a metallic template,40,41 have

geometry limitations due to the inability to create proper templates; epitaxial growth of GNRs on

SiC sidewalls42,43 also lacks atomic control over the edge. A scheme towards linking deposited

precursor monomers like DBBA (10,10´-dibromo-9,9´-biantyryl) on Au(111) surfaces, followed

by a higher temperature annealing for dehydrogenation of the polymer chains resulting in

graphene nanoribbons with atomic precision, was first demonstrated in 2010.44 Following that, a

lot of work has shown the success of synthesizing GNRs with different geometries using the

metallically catalytic on-surface synthesis approach. For example, armchair GNRs with width of

N = 5,45 9,46 13,47 1448 and 15,49 zigzag GNRs with N = 9,31 GNR hetero-structures,50,51 cove-

edged GNRs,52 as well as GNRs with various dopants53,54,55,56,57 or defects58 have been realized.

This method enables efficient, clean and massive production of atomically precise GNRs but

lacks controls over the ribbon´s length, position and orientation. Only a very recent paper on

epitaxial growth of atomically precise chevron GNRs on Cu(111) showed that GNRs tended to

form along the <112> crystallographic directions of the Cu(111), which suggests the possibility

of directional growth of GNRs for device fabrication.59 Measurements on the electronic structure

of these ribbons confirmed the 1/ 𝑊 bandgap dependency, but the measured bandgaps were often

far from the theoretical results due to the strong screening effect from the metal surfaces.60,54 So

far, other than introducing etching solution and polymer supports to transfer the GNRs,61,62

6

which leaves residues on the surface, only one etchant-free method of transferring GNRs from

Au to insulating substrates by mechanical delamination has been demonstrated.63 Although many

efforts have been attempted to get rid of the influence from the metal substrate, including

synthesizing GNRs on Ge semiconducting substrates64 and intervening a layer of NaCl65 and Si66

between the ribbon and the substrate, very few cases of success were achieved in finding the

intrinsic electronic structure of the GNRs.

Besides the on-surface synthesis method, several research groups have demonstrated a

way of chemically synthesizing armchair GNRs from solution.67–70 The wet-chemical method

includes Yamamoto coupling of precursor molecules followed by oxidative

cyclodehydrogenation of the resulted polymer chains from Scholl reactions,67 which produces

GNRs with atomic precision and has high yields. However, this method also lacks a way of

cleanly depositing those solution-synthesized GNRs onto a substrate.69 Only one STM study that

implemented the DCT method for depositing chevron GNRs on an H-Si(100) surface has been

demonstrated.71 Such a method introduced minimal amounts of residue to the surface and thus

enabled characterization of GNRs on the semiconducting Si substrate. But for fabricating GNR

devices, this method provides no control over the positioning and alignment of GNRs and thus

makes accurately depositing contacts on them challenging.

1.4 STM of Graphene Nanoribbons

STM is often used for monitoring the synthesis process of the GNRs that are synthesized

in situ on the metal substrate. From the precursor molecules to the polymer chains and then to the

final product of GNRs, STM conveniently achieves atomic resolution imaging that clearly shows

7

the chirality and edge structure of the GNRs; and the height profiles extracted from the

topograph also provide direct comparison between the measured data and the theoretical

values.31,44,46,54,55,57,60,66,72 One can even push the resolution limit to observing a single chemical

bond of GNR heterojunctions by implementing a low-temperature bond-resolved STM.50

Following STM, STS has also been used to obtain the electronic structure of the GNR.

Local I-V spectra yield the bandgap and LDOS of the ribbon. Normalized dI/dV maps provide a

direct view of the changes in the band edges of a GNR. For GNR heterojunctions that include

dopant substitution, conventional STM topography cannot show the difference since it is not

chemically sensitive. However on a STS dI/dV map, a sharp transition along the heterojunction

due to the doping effect will prove the existence of the heterojunctions.60 In addition, the

distribution of LDOS inside the ribbon at different energies can be obtained by conducting CITS,

and localized edge states of the zigzag ribbons73 as well as the quantum-well behavior of a 7-13

GNR heterojunction were therefore confirmed.51

The ribbon-substrate interaction can also be probed by STM and STS. One work on

GNRs exfoliated onto H-Si(100)71 observed a tunable transparency effect of the ribbon on the

underlying substrate and showed that once the hydrogen atoms were depassivated, those ribbons

would covalently bond to the Si substrate and became metallic. Even though no covalent bond

exists between the ribbon and the metal substrate, STS can capture the substrate´s screening

effect, which modifies the bandgap of the GNR by tenths of an eV.42,48

Recently, several research groups have demonstrated the possibility of STM tip-induced

on-surface synthesis of GNRs. For example, Ma et al.74 have shown that holes injected from the

STM tip can lower the cyclodegydrogenation barrier in the formation of GNRs sitting on top of

another layer of GNR polymers, and Radocea demonstrated the tip-induced polymerization of

8

DBBA molecules on Au(111).75 These demonstrations have pointed out a new route towards

positional control over synthesis of GNRs using STM.

1.5 Motivation

Once the GNRs have been synthesized, further characterization, including confirming the

geometry and measuring their electronic properties (such as bandgap), is essential. Amongst

various techniques for 2-D nanomaterial characterizations, STM is uniquely suitable due to its

ability to achieve atomic resolution imaging and high spatial-resolution electronic measurements.

However, so far most STM studies on GNRs were done for on-surface synthesized GNRs on

metal surfaces, for which spectroscopic measurements have been limited because this one-atom

thick material is greatly influenced by its underlying substrate.76, 77 Due to the requirement of a

metal surface as a catalyst for the on-surface synthesis method78 and the lack of clean transfer

methods,67 a lot of work has shown discrepancies between the experimental data collected and

their computational simulations, and that the reason comes down to the presence of the

substrate´s screening effect.60, 54, 79, 80 In addition, to further integrate GNRs into devices and

measure their electrical conductivity, nanoscale metallic contacts for the GNRs are needed. Due

to a lack of control in precisely placing GNRs at desired locations, depositing the contacts for an

individual GNR by conventional lithography techniques is very challenging.

9

1.6 Thesis Statement and Dissertation Organization

This dissertation address the challenges stated above by exfoliating solution-synthesized

atomically-precise GNRs onto atomically clean III-V semiconducting surfaces by a DCT

technique.23 The DCT process is done under ultrahigh-vacuum (UHV) to avoid issues with

contamination and metallic substrates; and the use of semiconducting substrates for GNRs is an

important step in the direction of making GNR devices. We studied three different GNRs

including doublewide GNRs, extended chevron GNRs and nitrogen-doped GNRs on various

semiconducting substrates including InAs(110) and InSb(110). Not only was the geometry

confirmed by high-resolution STM images, but STS also revealed the ribbon´s electronic

structure. In addition, I also investigated the ribbon-substrate interactions. This dissertation

proves the effect on the ribbon´s electronic properties by modifying its geometry and

demonstrates how substrates can affect the measurement of the band structure. In addition, I

demonstrated a way of depositing nanocontacts on isolated GNRs, which implements the STM

tip-assisted chemical vapor deposition of hafnium diboride technique for depositing contacts that

connect GNRs to prefabricated large electrodes. This makes in situ transport measurements on

isolated GNRs possible.

The dissertation is organized as follows. Chapter 2 covers STM characterization of

doublewide GNRs exfoliated on InAs(110) and InSb(110) substrates. Chapter 3 explores the

extended chevron and nitrogen-doped GNRs on InAs (110) substrate. Chapter 4 discusses the

fabrication of metal electrode arrays for tip-assisted STM deposition of nanocontacts on GNRs.

Chapter 5 summarizes the dissertation and gives directions of future work.

10

1.7 References

1. Hopkinson, M., Martin, T. & Smowton, P. III–V semiconductor devices integrated with

silicon. Semicond. Sci. Technol. 28, 090301 (2013).

2. Pop, E., Varshney, V. & Roy, A. K. Thermal properties of graphene: Fundamentals and

applications. MRS Bull. 37, 1273–1281 (2012).

3. Armasu, L. IBM beats Intel to 7 nm process thanks to silicon-germanium transistors, EUV

lithography. Tom´s Hardware (2015). Available at:

https://www.tomshardware.com/news/ibm-7 nm-silicon-germanium-

transistors,29546.html. (Accessed: 5th June 2018)

4. Xiong, W. (Wade). Multigate MOSFET Technology. In FinFETs and Other Multi-Gate

Transistors 49–111 (Springer US, 2008). doi:10.1007/978-0-387-71752-4_2

5. Knoblinger, G. et al. Multi-gate MOSFET design. ESSDERC 2006 - Proc. 36th Eur.

Solid-State Device Res. Conf. January, 65–68 (2006).

6. Colinge, J. P. et al. Junctionless nanowire transistor (JNT): Properties and design

guidelines. Solid. State. Electron. 65–66, 33–37 (2011).

7. Larrieu, G. & Han, X.-L. Vertical nanowire array-based field effect transistors for ultimate

scaling. Nanoscale 5, 2437 (2013).

8. Ionescu, A. M. & Riel, H. Tunnel field-effect transistors as energy-efficient electronic

switches. Nature 479, 329–337 (2011).

9. Žutić, I., Fabian, J. & Das Sarma, S. Spintronics: Fundamentals and applications. Rev.

Mod. Phys. 76, 323–410 (2004).

10. Linder, J. & Robinson, J. W. A. Superconducting spintronics. Nat. Phys. 11, 307–315

(2015).

11. Binnig, G., Rohrer, H., Gerber, C. & Weibel, E. Surface studies by scanning tunneling

microscopy. Phys. Rev. Lett. 49, 57–61 (1982).

12. Binnig, G., Rohrer, H., Gerber, C. & Weibel, E. 7 × 7 reconstruction on Si(111) resolved

in real space. Phys. Rev. Lett. 50, 120–123 (1983).

13. Zandvliet, H. J. W. & van Houselt, A. Scanning tunneling spectroscopy. Annu. Rev. Anal.

Chem. 2, 37–55 (2009).

14. Hamers, R. J., Tromp, R. M. & Demuth, J. E. Surface electronic structure of Si(111)-(7 ×

7) resolved in real space. Phys. Rev. Lett. 56, 1972–1975 (1986).

11

15. Lyding, J. W., Shen, T. C., Hubacek, J. S., Tucker, J. R. & Abeln, G. C. Nanoscale

patterning and oxidation of H-passivated Si(100)-2 × 1 surfaces with an ultrahigh vacuum

scanning tunneling microscope. Appl. Phys. Lett. 64, 2010–2012 (1994).

16. Ci, L. et al. Controlled nanocutting of graphene. Nano Res. 1, 116–122 (2008).

17. Crommie, M. F., Lutz, C. P. & Eigler, D. M. Confinement of electrons to quantum corrals

on a metal surface. Science. 262, 218–220 (1993).

18. Ritter, K. A. & Lyding, J. W. Characterization of nanometer-sized, mechanically

exfoliated graphene on the H-passivated Si(100) surface using scanning tunneling

microscopy. Nanotechnology 19, 015704 (2008).

19. Schmucker, S. Scanning tunneling microscopy studies of fluorinated graphene films and

field-directed sputter sharpening. Ph.D dissertation, University of Illinois at Urbana-

Champaign, Urbana, IL (2012).

20. Lu, C., Li, G., Mao, J., Wang, L. & Andrei, E. Y. Bandgap and doping effects in MoS2

measured by scanning tunneling microscopy and spectroscopy. Nano Lett. 14 (8), pp 4628

(2014). doi:10.1021/nl501659n

21. Zhang, C., Johnson, A., Hsu, C.-L., Li, L.-J. & Shih, C.-K. Direct imaging of band profile

in single layer MoS2 on graphite: Quasiparticle energy gap, metallic edge states, and edge

bandbending. Nano Lett. 14 (5), pp 2443–2447 (2014). doi:10.1021/nl501133c

22. Ruppalt, L. B. & Lyding, J. W. Charge transfer between semiconducting carbon nanotubes

and their doped GaAs(110) and InAs(110) substrates detected by scanning tunnelling

spectroscopy. Nanotechnology 18, 215202 (2007).

23. Albrecht, P. M. & Lyding, J. W. Ultrahigh-vacuum scanning tunneling microscopy and

spectroscopy of single-walled carbon nanotubes on hydrogen-passivated Si(100) surfaces.

Appl. Phys. Lett. 83, 5029 (2003).

24. Cheng, J.-Y., Fisher, B. L., Guisinger, N. P. & Lilley, C. M. Atomically manufactured

nickel–silicon quantum dots displaying robust resonant tunneling and negative differential

resistance. Quantum Mater. 2, 25 (2017).

25. Nguyen, D. et al. Orientation-dependent imaging of electronically excited quantum dots.

J. Chem. Phys. 148, 064701 (2018).

26. Koepke, J. C. et al. Role of pressure and in the growth of hexagonal boron nitride from

ammonia-borane. Chem. Mater. 28 (12), pp 4169–4179 (2016)

27. Nakada, K., Fujita, M., Dresselhaus, G. & Dresselhaus, M. S. Edge state in graphene

ribbons: Nanometer size effect and edge shape dependence. Phys. Rev. B - Condens.

Matter Mater. Phys. 54, 17954–17961 (1996).

12

28. Sigrist, M. Electronic and magnetic properties of nanographite ribbons. Phys. Rev. B -

Condens. Matter Mater. Phys. 59, 8271–8282 (1999).

29. Ezawa, M. Peculiar width dependence of the electronc property of carbon nanoribbons.

Phys. Rev. B 73, 045432 (2006).

30. Son, Y. W., Cohen, M. L. & Louie, S. G. Half-metallic graphene nanoribbons. Nature

444, 347–349 (2006).

31. Ruffieux, P. et al. On-surface synthesis of graphene nanoribbons with zigzag edge

topology. Nature 531, 489–492 (2016).

32. Magda, G. Z. et al. Room-temperature magnetic order on zigzag edges of narrow

graphene nanoribbons. Nature 514, 608–611 (2014).

33. Son, Y. W., Cohen, M. L. & Louie, S. G. Energy gaps in graphene nanoribbons. Phys.

Rev. Lett. 97, 1–4 (2006).

34. Dean, C. R. et al. Boron nitride substrates for high-quality graphene electronics. Nat.

Nanotechnol. 5, 722–726 (2010).

35. Bolotin, K. I. et al. Ultrahigh electron mobility in suspended graphene. Solid State

Commun. 146, 351–355 (2008).

36. Xu, G. et al. Low-noise submicron channel graphene nanoribbons. Appl. Phys. Lett. 97,

95–98 (2010).

37. Xu, G. et al. Linewidth roughness in nanowire-mask-based graphene nanoribbons. Appl.

Phys. Lett. 98, 1–4 (2011).

38. Qi, Z. J. et al. Correlating atomic structure and transport in suspended graphene

nanoribbons. Nano Lett. 14, 4238–4244 (2014).

39. Datta, S. S., Strachan, D. R., Khamis, S. M. & Johnson, A. T. C. Crystallographic etching

of few-layer graphene. Nano Lett. 8, 1912–1915 (2008).

40. Sokolov, A. N. et al. Direct growth of aligned graphitic nanoribbons from a DNA

template by chemical vapour deposition. Nat. Commun. 4, 1–8 (2013).

41. Martin-Fernandez, I., Wang, D. & Zhang, Y. Direct growth of graphene nanoribbons for

large-scale device fabrication. Nano Lett. 12, 6175–6179 (2012).

42. Sprinkle, M. et al. Multilayer epitaxial graphene grown on the SiC (0001̄) surface;

structure and electronic properties. J. Phys. D. Appl. Phys. 43 (2010).

13

43. Baringhaus, J. et al. Exceptional ballistic transport in epitaxial graphene nanoribbons.

Nature 506, 349–354 (2014).

44. Cai, J. et al. Atomically precise bottom-up fabrication of graphene nanoribbons. Nature

466, 470–3 (2010).

45. Kimouche, A. et al. Ultra-narrow metallic armchair graphene nanoribbons. Nat. Commun.

6, 10177 (2015).

46. Talirz, L. et al. On-surface synthesis and characterization of 9-atom wide armchair

graphene nanoribbons. ACS Nano 11, 1380–1388 (2017).

47. Chen, Y. C. et al. Tuning the band gap of graphene nanoribbons synthesized from

molecular precursors. ACS Nano 7, 6123–6128 (2013).

48. Huang, H. et al. Spatially resolved electronic structures of atomically precise armchair

graphene nanoribbons. Sci. Rep. 2, 1–7 (2012).

49. Abdurakhmanova, N. et al. Synthesis of wide atomically precise graphene nanoribbons

from para-oligophenylene based molecular precursor. Carbon. 77, 1187–1190 (2014).

50. Nguyen, G. D. et al. Atomically precise graphene nanoribbon heterojunctions from a

single molecular precursor. Nat. Nanotechnol. (2017). doi:10.1038/nnano.2017.155

51. Chen, Y.-C. et al. Molecular bandgap engineering of bottom-up synthesized graphene

nanoribbon heterojunctions. Nat. Nanotechnol. 10, 156–160 (2015).

52. Liu, J. et al. Toward cove-edged low band gap graphene nanoribbons. J. Am. Chem. Soc.

137, 6097–6103 (2015).

53. Carbonell-Sanromà, E. et al. Doping of graphene nanoribbons via functional group edge

modification. ACS Nano 11, 7355–7361 (2017).

54. Vo, T. H. et al. Nitrogen-doping induced self-assembly of graphene nanoribbon-based

two-dimensional and three-dimensional metamaterials. Nano Lett. 15, 5770–5777 (2015).

55. Kawai, S. et al. Atomically controlled substitutional boron-doping of graphene

nanoribbons. Nat. Commun. 6, 1–6 (2015).

56. Cloke, R. R. et al. Site-specific substitutional boron doping of semiconducting armchair

graphene nanoribbons. J. Am. Chem. Soc. 137, 8872–8875 (2015).

57. Nguyen, G. D. et al. Bottom-up synthesis of N = 13 sulfur-doped graphene nanoribbons.

J. Phys. Chem. C 120, 2684–2687 (2016).

58. Liu, M. et al. Graphene-like nanoribbons periodically embedded with four- and eight-

14

membered rings. Nat. Commun. 8, 1–7 (2017).

59. Teeter, J. D. et al. Epitaxial growth of aligned atomically precise chevron graphene

nanoribbons on Cu(111). Chem. Commun. 53, 8463–8466 (2017).

60. Cai, J. et al. Graphene nanoribbon heterojunctions. Nat. Nanotechnol. 9, 896–900 (2014).

61. Llinas, J. P. et al. Short-channel field-effect transistors with 9-atom and 13-atom wide

graphene nanoribbons. Nat. Commun. 8, 8–13 (2017).

62. Bennett, P. B. et al. Bottom-up graphene nanoribbon field-effect transistors. Appl. Phys.

Lett. 103, 1–5 (2013).

63. Ohtomo, M., Sekine, Y., Hibino, H. & Yamamoto, H. Graphene nanoribbon field-effect

transistors fabricated by etchant-free transfer from Au(788). Appl. Phys. Lett. 112 (2018).

64. Olszowski, P. et al. Aryl halide C-C coupling on Ge(001):H surfaces. J. Phys. Chem. C

119, 27478–27482 (2015).

65. Kharche, N. & Meunier, V. Width and crystal orientation dependent band gap

renormalization in substrate-supported graphene nanoribbons. J. Phys. Chem. Lett. 7,

1526–1533 (2016).

66. Deniz, O. et al. Revealing the electronic structure of silicon intercalated armchair

graphene nanoribbons by scanning tunneling spectroscopy. Nano Lett. 17, 2197–2203

(2017).

67. Vo, T. H. et al. Large-scale solution synthesis of narrow graphene nanoribbons. Nat.

Commun. 5, 1–8 (2014).

68. Khandelwal, M. & Kumar, A. One-step chemically controlled wet synthesis of graphene

nanoribbons from graphene oxide for high performance supercapacitor applications. J.

Mater. Chem. A 3, 22975–22988 (2015).

69. Vo, T. H., Shekhirev, M., Lipatov, A., Korlacki, R. A. & Sinitskii, A. Bulk properties of

solution-synthesized chevron-like graphene nanoribbons. Faraday Discuss. (2014).

doi:10.1039/C4FD00131A

70. Gao, J. et al. Ambipolar transport in solution-synthesized graphene nanoribbons. ACS

Nano 10, 4847–4856 (2016).

71. Radocea, A. et al. Solution-synthesized chevron graphene nanoribbons exfoliated onto H:

Si(100). Nano Lett. 17, 170–178 (2017).

72. Bronner, C. et al. Hierarchical on-surface synthesis of graphene nanoribbon

heterojunctions. ACS Nano 12, 2193–2200 (2018).

15

73. Wang, S. et al. Giant edge state splitting at atomically precise graphene zigzag edges. Nat.

Commun. 7, 11507 (2016).

74. Ma, C. et al. Polymer chains to graphene nanoribbons. Nat. Commun. 8, 1–7 (2017).

75. Radocea, A. Scanning tunneling microscopy investigation of atomically precise graphene

nanoribbons. Ph. D. dissertation, University of Illinois at Urbana-Champaign, Urbana, IL

(2017).

76. Wang, Q. H. et al. Understanding and controlling the substrate effect on graphene

electron-transfer chemistry via reactivity imprint lithography. Nat. Chem. 4, 724–732

(2012).

77. Sforzini, J. et al. Structural and electronic properties of nitrogen-doped graphene. Phys.

Rev. Lett. 116, 1–6 (2016).

78. Shen, Q., Gao, H. Y. & Fuchs, H. Frontiers of on-surface synthesis: From principles to

applications. Nano Today 13, 77–96 (2017).

79. Neaton, J. B., Hybertsen, M. S. & Louie, S. G. Renormalization of molecular electronic

levels at metal-molecule interfaces. Phys. Rev. Lett. 97, 1–4 (2006).

80. Thygesen, K. S. & Rubio, A. Renormalization of molecular quasiparticle levels at metal-

molecule interfaces: Trends across binding regimes. Phys. Rev. Lett. 102, 1–4 (2009).

16

CHAPTER 2

CROSS-SECTIONAL STM OF SOLUTION-

SYNTHESIZED DOUBLEWIDE GRAPHENE

NANORIBBONS (WGNRS) ON INAS(110)

AND INSB(110)

2.1 Introduction and Motivation

The quasi-one-dimensional nature of graphene nanoribbons (GNRs) results in advantages

over traditional bulk semiconducting materials. Not only is the bandgap of the GNR tunable by

its geometry,1,2 but its behavior is also substrate-dependent.3,4 For example, both theoretical and

experimental works have shown that an increase in a ribbon´s width will cause a decrease in its

electronic bandgap.5 So far, the only approach to synthesizing GNRs with atomic precision is

from bottom-up.6,7,8 Starting with different precursor molecules, uniquely desired and even

sophisticated GNR hetero-structures have been realized by either wet-chemical or on-surface

polymerization methods.9,10,11 However, characterization of the GNR´s intrinsic electronic

structure remains a challenge: for GNRs made by wet-chemical methods, ribbons deposited from

a solution suspension leave a lot of residue on the substrate, making measurements

inconsistent;12 and the requirement of a metal surface as a catalyst for the on-surface synthesis

method leads to inaccurate results due to the strong screening effect from the substrate.13,14,15,16,17

We overcome this challenge by implementing the DCT technique to deposit solution-

synthesized GNRs on InAs(110) and InSb(110) surfaces.18 This method enables STM

17

characterization of isolated GNRs on any STM-compatible substrate without the involvement of

chemicals and thus resolves the residue issue. We chose InAs(110) as the substrate because it is a

semiconducting surface with its atomic and electronic structure thoroughly studied by STM and

STS.19,20,21 Its 0.4 eV bandgap is far smaller than that of the GNR´s and thus makes determining

the GNR band edge much easier, and the non-polar nature of InAs(110) should not result in any

significant charge transfer to/from the GNR.19,22 In addition, only a van der Waals (VDW) force

is expected to be present between the GNRs and the surface, and such a weak ribbon-substrate

interaction is expected to cause little effect on the GNR´s electronic structure.23

The GNRs we studied were synthesized by the Sinitskii group at the University of

Nebraska. Figure 2.1a (top) is the schematic showing the geometry of the ribbon: this armchair-

edge ribbon has twice the width of the previously reported chevron GNRs, so we call it

doublewide GNR (wGNR). High-resolution STM topographs not only confirmed the geometry

of the ribbon, but also revealed out-of-plane differences in the energy state´s decay lengths

between the edge and the center of the wGNR. STS measurements showed a bandgap of 2 eV

and CITS unfolded the distribution of LDOS at different energies. Ribbon-substrate interaction

was investigated by means of manipulation, orientation and transparency analysis of the wGNRs.

Our findings are in good agreement with the DFT modeling, indicating that InAs(110) is an ideal

substrate for studying the intrinsic properties of GNRs by STM.

We also characterized wGNR on InSb(110). Ribbons showed similar behavior on

InSb(110) to InAs(110), and their bandgap was measured to be 2 eV as well. The only difference

is that the states from the InSb substrate often interfered with the states of the ribbons when

taking STS, making determining the band edges of the wGNR difficult. The scanning condition

needs to be carefully chosen to get a clear I-V spectrum of the ribbon.

18

2.2 Cross-sectional STM Imaging of WGNRs on InAs(110)

After sample preparation (described in Section 2.7.1) and DCT, we found isolated

wGNRs lying flat on the InAs(110) surface. Through high-resolution STM scans, we were able

to confirm the expected geometry of the wGNR. Figure 2.1a (bottom) is a filled-states STM

topograph showing the wGNR and the underlying substrate with the 0.6 nm-apart arsenate dimer

rows clearly seen (Profile 3 in Figure 2.1b). The height profile extracted across the ribbon

(Profile 1 in Figure 2.1b) shows a ribbon height of 0.3 nm, which is close to the spacing between

graphene layers in a graphite and comparable to previously studied graphene nanoflakes on

InAs,24 indicating a weak VDW force. The measured 1.58 nm width and 1.34 nm elbow period

(Profile 2 in Figure 2.1b) are very close to the theoretical values as indicated in Figure 2.1a (top).

Figure 2.1c shows another GNR scanned at higher current showing intra-ribbon resolution. The

protrusions at the center and edge correspond to benzene rings inside and at the elbow of the

ribbon, respectively. Figures 2.1c, d, e and f are topographs of the same GNR but scanned with

different parameters. Those differences in the intra-ribbon structure do not represent graphene

lattices but a change in the state´s orbital shape at different heights above the ribbon.

We have also noticed that scanning with different parameters resulted in wGNRs with

different topographic appearances. We investigated this phenomenon by taking STM images

with a constant voltage but varying current. What we found was that while scanning with a lower

current, which means at a larger tip-surface distance, the features at the ribbon´s edge appear

brighter and clearer and dominate over the features in the center (as shown in Figures 2.2a and

e); bringing the tip closer to the surface (larger tunneling current) resulted in the disappearance

of those edge features, and the features at the center start to show in greater detail (Figures 2.2c

19

and g). An STM topographic image reflects not only the atomic structure but also the electronic

structure.25 Since the ribbon´s lattice is invariant, it is the intensity of the LDOS that has a tip-

surface distance dependency. That is, the LDOS not only distributes planarly but also extends

outwards in the z-direction. For this wGNR specifically, the intensity of the edge states is weaker

at the surface, and decays slower or possibly increases when extending outwards; on the other

hand, the center state´s intensity is stronger at the surface and decays faster away from the

surface. DFT modeling of the LDOS maps at different tip-surface distances shows the same

trend that we noticed. Figure 2.3 provides the simulated LDOS maps of an isolated infinitely

long wGNR at a distance of 1 through 4 Å above the ribbon. With the energy determined by

STS, the correspondent LDOS maps for Figures 2.3a, c, e and g are shown in Figures 2.3b, d, f

and h, respectively, met in good agreement. A previous study26 on the modeling of regular

chevron GNRs showed the same trend, and here we were able to prove it with the STM

topographic images presented.

2.3 STS Measurement of wGNRs on InAs(110)

We collected point spectroscopy on both the ribbon and the substrate. Figure 2.4a is the

normalized conductance (dI/dV) point spectra taken at the ribbon´s edge (red curve), center

(green curve) and on the InAs substrate (black curve), respectively. A total of six distinct states

located at −2.1 V, −1.8 V, −1.1 V, 0.9 V, 1.3 V and 1.9 V were identified (as States 1 to 6) for

the ribbon, which gives a bandgap of 2 eV. Compared to the 2.8 eV bandgap of the regular

chevron GNRs, this decrease of 0.8 eV could be caused by its increased width. The measured

0.35 eV bandgap of the InAs substrate ensures the accuracy of the data collected. It is worth

20

mentioning that not all six states are present throughout the GNR. For example, the point

spectrum taken at the center of the ribbon did not show any peaks near −1.6 V, which

corresponds to State 2, indicating no such state is localized at the center of the wGNR. The

normalized dI/dV spectra map across the wGNR (as shown in Figure 2.4b) with the positions of

the three point spectra marked by three dashed lines visually shows the variation of the energy

states on the ribbon and the substrate. For example, the band edges for the substrate are marked

by white arrows and the band edges for the wGNR are marked by black arrows; on some sites of

the ribbon, the substrate´s conduction and valence band states also contribute to a small extent

inside the ribbon´s bandgap (as marked by orange arrows), and no states with energy of −1.6 eV

at the center of the ribbon (yellow arrow) are present. To further justify the accuracy of the

collected data, we did DFT modeling on the band structure of the wGNR without the inclusion of

the substrate (Figure 2.4c). Despite the simulated bandgap of the wGNR often being

underestimated (0.99 eV as shown by the red curve in Figure 2.4d), the energy splitting between

other adjacent states (except State 3 and 4) should be accurate.27 Pinning States 3 and 4 at −1.1

eV and 0.9 eV, the simulated energy of States 1, 2, 5 and 6 are −2.17 eV, −1.75 eV, 1.35 eV and

1.85eV, respectively, which is consistent with the experimental STS data.

In order to get a good estimate on the bandgap of the wGNR, we carried out a DFT

simulation with the GW approximation and results gave a 2.35 eV bandgap (blue curve in Figure

2.4d). Compare to the averaged value of 2.03 eV among the bandgaps of 30 wGNRs collected,

the 0.32 eV difference between the measured and simulated bandgap is significantly small

compared to previously reported values for GNRs on other substrates such as H-Si26 and

Au(111).15 We can conclude that InAs(110) has a weaker screening effect and thus better

preserves the intrinsic electronic structure of the wGNR.

21

To fully capture the landscape of the LDOS, 50 × 50-points CITS data were obtained

over an area with a wGNR and substrate. 1024 current measurements were recorded on each

point with the bias sweeping from −2 V to +2 V and the calculated intensity of the LDOS at each

energy was mapped out. Figures 2.5a, c, e, g, and i are the CITS images with energies of −1.6 V,

−1.1 V, 0.8 V, 1.45 V and 2 V, representing States 2 through 6, respectively. Distribution and

intensity of the LDOS at different energies are clearly seen in these CITS images. For example,

at −1.6 V (Figure 2.5a), four brighter stripes spanning the length of the ribbon indicate that State

2 is located at the edges and the space between the center and the edges, but not the center of the

wGNR; whereas for states with an energy of −1.1 V (State 3), they concentrate at the elbows and

the center of the ribbon, but not the space in between; at 0.8 V, the CITS does not show a clear

elbow structure, whereas at 1.45 V, the states at the elbow start to show up. Comparing the

collected CITS with the corresponding simulated data with a tip-sample distance of 3 Å (Figures

2.5b, d, f, h and j) shows great consistency. Such little interference between the substrate states

and the ribbon states enables capturing the true electronic structure of the ribbon.

2.4 wGNR-InAs Interaction

The tunable transparency effect of a GNR on its underlying InAs substrate has been

theoretically predicted and observed experimentally on H-Si.26 We also observed this

phenomenon. Figure 2.6a is an example exhibiting the same GNR that changed its appearance

from non-transparent to transparent by an increase in the tunneling current: when scanning at

−1.5 V 8 pA, the ribbon is non-transparent, whereas increasing the current to 10 pA by bringing

the tip closer to the surface, the ribbon turned partially transparent with the underlying arsenate

22

atoms showing through; further increasing the current to 80 pA turned the ribbon completely

transparent to the substrate, while the geometry of the ribbon disappeared. It seems that with an

increase in the tunneling current, the tip started pushing the ribbon closer to the substrate and

thus picking up the states from the substrate, causing a decrease in the measured apparent height

(from 0.36 nm to 0.18 nm to 0.12 nm), agreeing with previous STM studies on the transparency

effect of graphene nanoflakes on InAs.24 With over 52 STM topographs of the wGNRs collected

under different scanning conditions, we divided the appearance of the ribbons into three groups:

group one represents the ribbons that are transparent to the substrate; group two is the ribbons

that are not transparent with no intra-ribbon structure shown; and group three is the ribbons that

are not transparent but with intra-ribbon structure resolution. Examples displaying topographs of

one ribbon from each group are shown in Figure 2.6b (left) with the calculated average apparent

height displayed in the right chart. The tunable transparency effect of the GNR with an increase

in the current suggests that the InAs states decay faster than those of the wGNR´s in the z-

direction. Choosing a proper scanning condition with the tip positioned just far enough from the

substrate, the weak coupling between the InAs and the ribbon´s states facilitates probing only the

ribbon´s states. Whereas for substrates like Cu(111)28,29 and Au(111),27,30 their surface states

couple strongly with the ribbon and decay slower, thus resulting in inaccurate spectral data.

Figure 2.7 shows some large STM scans displaying multiple wGNRs on InAs with

random orientations. Different from carbon nanotubes exfoliated onto InAs, there is no chemical

force existing between the ribbon and the substrate. The similar orientations in some scans are

most likely caused by the direction of force exerted during DCT. The angle between the

longitudinal axis of the ribbon and the substrate dimer row was measured for 164 wGNRs. The

histogram shown in Figure 2.7d summarizes the number of wGNRs versus their angles with

23

respect to the substrate lattice divided into 10 even intervals between 0° and 90°. The random

distribution confirmed the absence of strong interaction between the wGNR and the InAs. To

further justify this finding, we collected STS data over 30 wGNRs with different orientations,

and the results are summarized in Figure 2.8. Figure 2.8a displays the measured valence band

(blue diamonds) and conduction band (red squares) edges for each wGNR versus their

orientations. The corresponding calculated bandgaps (substrate CB by VB) and Fermi levels

(adding CB and VB and then dividing by 2) are shown in Figure 2.8b. These statistics show very

small variations in the bandgap, which range between 1.9 eV to 2.1 eV with a standard deviation

of 0.05 eV, and the Fermi level, which varies between −0.16 eV to 0.167 eV with a standard

deviation of 0.17 eV. If a strong bond is present between the ribbon and the substrate, a change

in a ribbon´s orientation often causes a change in its electronic structure.31 No obvious changes

observed in our case again proved that no such bonds are present. In Figure 2.8b, we can see that

the Fermi levels of those ribbons lie close to the middle of the bandgap, which suggests no

significant charge transfer between the ribbon and the substrate. Despite the fact that those

ribbons have different lengths, STS shows no bandgap dependency on the length.

The exfoliated wGNRs are immobile during normal scans; however, we were able to

manipulate them by bringing the tip closer while being moved with a higher tunneling current.

Figures 2.9a and b show before and after images of a ribbon that had been rotated approximately

60° clockwise by the tip (with its motion indicated by the arrow). A tip change after the

manipulation caused a poorer resolution in the scan (Figure 2.9b). Figures 2.9c and e are another

example showing a ribbon moved by the tip, and the corresponding dI/dV map shown in Figures

2.9d and f shows no significant change in the band structure after manipulation.

24

2.5 wGNRs on InSb(110)

After the wGNRs have been DCTed onto the InSb(110) substrate, similar to the

InAs(110) substrate, GNRs were not mobile during sequential scans, larger scans (Figure 2.10)

that contain multiple GNRs show no preferable alignment between the GNRs and the substrate.

GNRs laid in different directions could be torn or overlaid with each other during the DCT

process. Figure 2.11a is a high-resolution STM image of a single GNR with intra-ribbon

resolution and indium atoms clearly shown. Height profiles extracted across and along the GNR

gave a width of 0.16 nm, height of 0.27 nm and average elbow periodicity of ~0.3 nm for the

GNR; and, as shown in Figures 2.11b and c, these measured numbers are very close to the

theoretical values.

Sometimes the wGNR also showed the tunable transparency effect. Figure 2.12 shows

the STM images (top) with the corresponding height distributions taken inside the red grid

(below) of the same GNR scanned at the same tunneling current of 8 pA but with different

biases. At −2 V (Figure 2.12a), the GNR is non-transparent to the substrate with an apparent

height of 0.33 nm, and the brighter protrusions at the edge represent its elbow structure. At −1.5

V (Figure 2.12b), the GNR is still non-transparent but its apparent height decreased to 0.23 nm.

Further decreasing the bias to −1 V (Figure 2.12c), the GNR became transparent to the

underlying Sb lattice. The apparent height decreased to 0.17 nm and the protrusions at the edge

disappeared. Different from the GNRs on InAs substrate whose transparencies change with

current, here by keeping the current constant while changing the bias, the transparency is still

tunable. In either case, it seems that the GNR will become transparent when scanned at a small

tip-sample distance, which is the same as the graphene nanoflakes on III-V semiconducting

25

surfaces reported previously.24 It is possible that the stronger interaction between the tip and the

GNR can push the GNR closer to the substrate, making it transparent.

Sometimes the GNRs, rather than changing from non-transparent to transparent, would

remain non-transparent while changing their appearance with different scanning biases. For

example (as shown in Figure 2.13), when decreasing the bias from −2 V to −0.7 V while keeping

the current at 60 pA, the GNR showed a more detailed intra-ribbon structure and less elbow

structure with a smaller bias (smaller tip-surface distance). Similarly, this trend remains when

increasing the current while keeping the bias constant. For example, as shown in Figures 2.13e

and f, changing the current from 60 pA to 110 pA resulted in topographic images of the GNRs

with more detailed resolution at the center and less at the edge. This behavior is consistent with

the difference in the out-of-plane orbital shapes of the wGNRs on InAs as previously discussed

in Section 2.2. In addition, there was not much difference in the topographic images of a GNR

when switching the polarity of the scanning bias as shown in Figures 2.13a and d; for scans on

the same GNR at −2 V and +2 V, both images showed the GNR to be non-transparent with no

intra-ribbon resolution. This indicates that there is little difference between tunneling from the

filled states of GNR and Sb atoms and tunneling to the empty state of GNR and In atoms.

To further explore the influence of the substrate on probing the electronic structure of the

GNRs, STS data were also collected. Figure 2.14 exhibits normalized conductance maps and

point spectra on three different GNRs and the substrate. From the point spectra in Figures 2.14

(a−c, left), a variation from 1.9 eV to 2.5 eV in the GNR bandgap was observed. This variation

arises from the difficulty in determining the GNR band edges due to their overlap with the

substrate electronic states. From the dI/dV maps (middle diagrams of Figure 2.14a−c) generated

on the GNR, we can see that the states from the substrate were also detected by the probe while it

26

was positioned over the GNR (as indicated by the white dashed lines). This can also be seen

from the point spectra (right diagrams of Figures 2.14a−c): when comparing the dI/dV curves of

the GNR (black curve) and the substrate (blue curve), the conduction band (CB) and valence

band (VB) peaks coincide with each other. In addition, the GNR CB is very close to the CB+1 of

the substrate in (a), and the GNR VB is very close to the VB of the substrate in (b). Since the tip

also probed the states from the substrates, the states of the GNRs were probably the mixed states

of the GNR´s and the substrate. Therefore, distinguishing the true states of the GNR is difficult.

Proper scanning parameters should be carefully chosen since the influence of the substrate can

greatly affect the dI/dV spectra. STS spectra showed no doping effect from the undoped

substrate to the GNR where its Fermi level was close to the midgap.

Even though there is a variation in the bandgap, those measured values were not far away

from the 2 eV for the same GNR on InAs. The 1.9 eV bandgap determined from Figure 2.14c is

probably the most accurate value among all since there is little mixture between the states of the

GNR and the substrate. More STS characterizations and DFT simulations of the band structure

of GNRs with the underlying substrate would be helpful for better understanding the influence of

the substrate on the GNRs.

2.6 Conclusion

In conclusion, we carried out detailed STM and STS characterizations of solution-

synthesized doublewide GNRs DCTed onto III-V semiconducting InAs(110) and InSb(110)

substrates. The geometry of the wGNR was confirmed by high-resolution STM images. STS

revealed a decreased bandgap of 2 eV due to the increased GNR width. GNR energy states as

27

well as the 3-D spatial distribution of those states were mapped out experimentally. Our results

showed little or no discrimination compared to the DFT simulation of the isolated GNRs.

Combined with investigations on the GNR´s tunable transparency, orientation and tip

manipulation, we concluded that InAs(110) interacts weakly with the GNRs, making probing the

intrinsic properties of the GNRs possible without the need of intercalating an extra isolating layer

for STM characterization. 32

2.7 Methods

2.7.1 Substrate Preparation for Cross-sectional STM

InAs or InSb wafers were cut into 5 × 8 mm rectangular pieces with a mark lightly

scribed for later in situ cleavage. The sample was mounted vertically (with its cross section

facing out) on a modified sample holder as shown in Figure 2.15. This holder is composed of

two stainless steel blocks and a thin aluminum plate, which can be clamped to one of the blocks

using a setscrew with the sample in between. Indium foil can be added in between the sample

and the block to provide a soft mechanical cushion and better thermal contact. The sample was

then degassed by heating the dipstick with a tungsten filament to 120 °C (below the melting

temperature of the indium foil) for several hours until the chamber´s pressure returned to 5E−11

torr. The sample was then covered with a thin layer of evaporated titanium (Ti) in-situ by

running an 8 A current through a Ti filament for 15 mins; this Ti layer served as a getter for

contaminations on the sample to extend the sample´s imaging lifetime. After the sample was

cooled down to room temperature, it was cleaved in situ (by hitting it with the wobblestick) with

its clean cross section exposed with 1 × 1 surface reconstruction. An optical microscope and an

28

angled mirror were used for providing a better view for aligning the STM probe with the cross-

section of the substrate (as shown in Figure 2.15c) and the cleanliness was confirmed by STM

scans. The InAs and InSb wafers used are Zn-doped p-type InAs with resistivity of ~0.01 Ω·cm

and undoped InSb with resistivity of 0.08 Ω·cm. For a detailed description of DCT applicator

preparation, see Radocea, Section 2.7.4.33

2.7.2 STM/STS Experiments

The STM used for conducting these experiments was a custom-built Lyding system

(Chamber A) that is capable of operating at room temperature under UHV (base pressure of

5E−11 torr).34 STM topography and current imaging were conducted under constant current

mode. Chemically etched platinum-iridium, tungsten tips and iridium-coated field-directed

sputter-sharpened tungsten tips from TiptekTM were used for STM and STS experiments.

Variable-spacing STS spectra were collected by bringing the tip linearly towards the surface by

0.2 nm and back to its original position over the bias sweep. This method helps effectively

reduce the electronic noise floor in the low LDOS regions of the spectra, resulting in more

accurate data. An exponential correction factor was used to convert the I-V data to constant

spacing data. Log-scale I-V as well as normalized tunneling conductance dI/dV/(I/V) were

numerically calculated from the I-V data and used to determine the energy states of the sample.

Current imaging tunneling spectroscopy images were collected by recording I-V spectra on every

pixel of a 50 × 50 grid, with 512 or 1024 biases between −2 V and +2 V and were displayed as

numerically calculated dI/dV images with a grey color scale. Manipulation of the wGNRs was

carried out by moving the tip at 80 Å/s with a sample bias of −2 V and tunneling current of 0.2

nA.

29

2.7.3 Computational Modeling

The parameter settings we used to simulate our wGNRs are similar to a previous study on

chevron GNRs,26 and we chose an infinitely long wGNR as the model. For modeling the LDOS

and projected DOS, we did the calculations with the Quantum Espresso package35 by setting a

supercell that is periodic in the GNR´s longitudinal axis and large enough in the other directions

so that the ribbon can be separated from its images. Norm-conserving pseudopotentials with the

Becke-Lee-Yang-Parr exchange-correlation functional were employed with a plane-wave energy

cutoff of 60 Ry.36 For structural relaxations and electronic property calculations, we used the

Monkhorst-Pack grid of 2 × 1 × 1 and 16 × 1 × 1, respectively, and optimized the structure with

a maximum residual force of smaller than 0.002 eV/nm. We simulated six energy states and the

corresponding LDOS mappings for these states at a distance of 1 to 4 Å above the GNR plane.

We used the XCrysDen37 software for the visualizations of the ribbon´s geometry and LDOS.

For the DFT modeling the wGNR´s band structure, we used the SIESTA software38 and

employed the generalized gradient approximation with the Perdew-Burke-Ernzerhof exchange-

correlation functional.39 For structural relaxation and band structure calculations, we employed

the Monkhorst-Pack grid of 2 × 1 × 1 and 18 × 1 × 1, respectively.

For simulating the GW corrected band structure, we used the VASP package40 with the

Perdew-Burke-Ernzerhof exchange-correlation functional, and the energy cutoff was set to be

400 eV for the projected augmented wave pseudopotentials. The Gamma-point-centered k-grids

of 4 × 1 × 1 were applied. First, we simulated the ground states by DFT calculations and then

calculated the quasi-particle energies with the single-shot G0W0 approximation.41 Due to the

30

large band dispersions of the wGNR, we did the Wannier interpolation for getting the G0W0

bands with the WANNIER90 package.42

31

2.8 Figures

Figure 2.1. STM topographs of wGNR on InAs(110). (a) Top: schematic showing the geometry

of the wGNR; bottom: an STM scan of a wGNR with the underlying InAs(110) substrate lattice

seen. Scanned at −2 V and 10 pA. Scale bar is 3 nm. (b) Top: height profile extracted across the

ribbon as indicated by the blue line in (a) revealed a ribbon height of 0.3 nm and width of 1.58

nm; bottom: height profiles extract along the ribbon´s edge (red) and on arsenate dimer rows

(blue). Ribbon´s elbow period is 1.34 nm and the distance between adjacent arsenate lattice is 0.6

nm, as expected. (c−f) STM topographs of the same wGNR scanning with different parameter

showed different intra-ribbon structures. Scan conditions: (c) 2 V, 30 pA; (d) 2 V, 40 pA; (e) 2 V,

60 pA; (f) −2 V, 30 pA. Scale bars are 5 nm.

1.49nm

1.27nm

0.37nm

0.43nm

1.58nm

0.3nm

576 pm

− 1 1 5

0

100

200

300

400

500

1

2

3

- Profile 2

- Profile 3

0.36nm

0.45nm

a b

c d

1.58nm

0.3nm

576 pm

− 1 1 5

0

100

200

300

400

500

1

2

3

- Profile 2

- Profile 3

e f

32

Figure 2.2. STM topographs of two wGNRs scanning with different parameters show different

appearances and the corresponding DFT simulated LDOS distribution for each case. (a,c) Scans

of a wGNR taken at −1.5 V and 10 pA and 50 pA, respectively; at 10 pA the ribbon´s edge

structure is clear; whereas, at 50 pA, the edge feature disappeared. (b) DFT simulated LDOS

mappings of State 3 at a distance of 3 Å above the ribbon; intensity of the edge states are high. (d)

DFT simulated LDOS mapping of the State 3 at a distance of 1 Å above the ribbon; intensity of

the edge states decreases and is weaker than that of the center states. (e,f) Scans of another

wGNR taken at −2 V and 10 pA and 200 pA, respectively; at 10 pA, edge structure appeared

brighter and at 200 pA, edge structure got dimmer compared to the center. (f) Corresponding

DFT simulated LDOS mapping for a mixture of States 1, 2 and 3 at a distance of 4 Å above the

ribbon; intensities of the edge states are much higher than the center states. (h) Simulated LDOS

at a distance of 2 Å above the ribbon shows that the intensity of the center states got stronger and

became comparable to that of the edge states. Scale bars are 3 nm.

3 nm1

2

3

476 pm

− 112

− 50

0

50

100

150

200

250

300

350

400

3 nm

1

2 3

365 pm

− 57

50

100

150

200

250

300

States 3

3 nm

12

3

446 pm

− 68

0

50

100

150

200

250

300

350

400

3 nm

1 2

3

504 pm

− 22

50

100

150

200

250

300

350

400

450

States 1&2&3

a

c

e

g

b

d

f

h

H=1

H=3

H=4

H=2

b

d

H=3

H=1

33

Figure 2.3. DFT simulated LDOS mapping of six energy states in an infinitely long wGNR.

Mappings of the states are plotted at heights of 4 to 1 Å above the GNR.

34

Figure 2.4. STS spectra and CITS of wGNR. (a) Normalized dI/dV point spectra taken at three

points indicated in the top topographic image: red is at the ribbon´s edge, green is at ribbon´s

center and black is on the InAs substrate. Six energy states (States 1 through 6) were identified at

the ribbon´s edge and five states at center (missing State 2). The energy splitting between States

3 and 4 gave a bandgap of 2 eV. (b) Normalized dI/dV map across the wGNR (indicated by the

blue line). White arrows point to the band edges of the substrate; orange arrows point to the

probing of the substrate´s state through the ribbon; yellow arrow points to the missing State 2 at

the center of the ribbon; black arrows point to the band edges of the wGNR. Three dashed lines

(blue, red and green) represent the position of the corresponding dots in (a). (c) DFT-simulated

dI/dV spectrum of an infinitely long wGNR (no substrate included) with six states predicted. (d)

Simulated band structure with GW correction gives bandgap of 2.3 eV.

-3 -2 -1 0 1 2 3

-1

0

1

2

3

4

5

6

7

8

9

10N

orm

aliz

ed

Co

nd

ucta

nce

V

InAs

GNR edge

GNR center3 nm

1 2

3

468 pm

− 58

50

100

150

200

250

300

350

400

a b

c d

GNR CB

GNR VB

InAs CB

InAs VB

state 2

state 3

-1.6V

-1.1V

1 2 3 4 5 6

1.45V

0.8V

2V

State

4

State

5

State

6

e

c d

35

Figure 2.5. (a) Left: CITS images of a wGNR showing the LDOS distribution with negative

energy at −1.6 V and −1.1 V. Right: the corresponding simulated LDOS maps for states with

energy of −1.6 V and −1.1 V. (b) Left: CITS images of the same wGNR showing the LDOS

distribution with positive energy at 0.8 V, 1.45 V and 2 V. Right: the corresponding simulated

LDOS maps for states at the same energy, respectively. No substrate states appear through the

ribbon. The collected data and the simulation results show great consistency, indicating weak

coupling between the ribbon and the InAs substrate.

-3 -2 -1 0 1 2 3

-1

0

1

2

3

4

5

6

7

8

9

10

No

rma

lize

d

Co

nd

ucta

nce

V

InAs

GNR edge

GNR center3 nm

1 2

3

468 pm

− 58

50

100

150

200

250

300

350

400

a b

c d

GNR CB

GNR VB

InAs CB

InAs VB

state 2

state 3

-1.6V

-1.1V

1 2 3 4 5 6

1.45V

0.8V

2V

State

4

State

5

State

6

e

-3 -2 -1 0 1 2 3

-1

0

1

2

3

4

5

6

7

8

9

10

No

rma

lize

d

Co

nd

ucta

nce

V

InAs

GNR edge

GNR center3 nm

1 2

3

468 pm

− 58

50

100

150

200

250

300

350

400

a b

c d

GNR CB

GNR VB

InAs CB

InAs VB

state 2

state 3

-1.6V

-1.1V

1 2 3 4 5 6

1.45V

0.8V

2V

State

4

State

5

State

6

e a b

c d

e f

g h

i j

36

Figure 2.6. Tunable transparency effect of the wGNR on InAs. (a) STM topographs showing a

ribbon becoming transparent with increasing scanning current from 8 pA to 10 pA and 80 pA

while keeping the bias constant at −1.5 V. The larger the current, the more transparent the ribbon

appears and the lower its apparent height. Scale bars are 5 nm. (b) Left: STM topographs

showing three examples of the ribbon´s appearance. Right: the appearance of the wGNR was

grouped into three categories: the transparent case, which has an average apparent height of 0.21

nm; the non-transparent case with no intra-ribbon structure shown, which has an average height

of 0.27 nm; and the non-transparent case with intra-ribbon resolution, which has an average

height of 0.23 nm.

aCategory Average

Height(nm)

Transparent 0.21

Non-Transparentnointra-ribbonresolu on

0.27

Non-Transparentwithintra-ribbonresolu on

0.23

b

0

5

10

15

20

25

9 18 27 36 45 54 63 72 81 90

c 458 pm

− 87

100

200

300

d

-1.5V

8pA

10p

A

80p

A

# of GNRs

degrees

459 pm

− 109

100

200

300

aCategory Average

Height(nm)

Transparent 0.21

Non-Transparentnointra-ribbonresolu on

0.27

Non-Transparentwithintra-ribbonresolu on

0.23

b

0

5

10

15

20

25

9 18 27 36 45 54 63 72 81 90

c 458 pm

− 87

100

200

300

d

-1.5V

8pA

10p

A

80p

A

# of GNRs

degrees

459 pm

− 109

100

200

300

a b

37

Figure 2.7. (a−c) Larger scans with multiple wGNRs shown. The orientation of the ribbon was

represented by the black lines, which are along the longitudinal axis of the ribbon. (d) The angle

between the ribbon and the substrate lattice was manually measured and summarized as a

histogram: y-axis represents the number of GNRs counted, and the x-axis represents the

measured angle, which is divided into 10 even intervals between 0° and 90°. Such a random

distribution indicates no preferential alignment for the wGNR on InAs.

20 nm

1

2

3

4

5

6

1.75 nm

− 0.37

5nm1

2

3

4

0.9 nm

− 0.2

20 nm

1

23

4

5

6

7

8

9

10

11

12

13

1.29 nm

− 0.70

aCategory Average

Height(nm)

Transparent 0.21

Non-Transparentnointra-ribbonresolu on

0.27

Non-Transparentwithintra-ribbonresolu on

0.23

b

0

5

10

15

20

25

9 18 27 36 45 54 63 72 81 90

c 458 pm

− 87

100

200

300

d

-1.5V

8pA

10p

A

80p

A

# of GNRs

degrees

459 pm

− 109

100

200

300

a b

c d

38

b

Figure 2.8. Statistics showing the STS data collected over 30 wGNRs with their orientations

specified on the x-axis. (a). Conduction (red square) and valence band edge (blue diamonds) of

the 30 wGNRs. (b). Calculated bandgap (red square) and Fermi level (blue diamond) for the

corresponding 30 wGNRs. No significant variations are seen in both the bandgap and the Fermi

level.

-0.3-0.2-0.1

00.10.20.30.40.50.60.70.80.9

11.11.21.31.41.51.61.71.81.9

22.12.22.3

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95

Angle (Degree)

Energy (eV)

Fermi level

Band gap

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95

HOMO LUMOEnergy(V)

Angle(degree)

a

vb cb

39

Figure 2.9. Manipulation of wGNR. (a,b) Before and after images of a wGNR being rotated by

the STM probe. (c,e) Another wGNR being pushed by the STM probe with its dI/dV map shown

in (d) and (f). No significant changes in the electronic structure are seen after manipulation,

which indicates no strong bond existed between the ribbon and substrate. Scale bars are 3 nm.

0.4 nm

− 0.7

0.0 nm

− 1.0

aCategory Average

Height(nm)

Transparent 0.21

Non-Transparentnointra-ribbonresolu on

0.27

Non-Transparentwithintra-ribbonresolu on

0.23

b

0

5

10

15

20

25

9 18 27 36 45 54 63 72 81 90

c 458 pm

− 87

100

200

300

d

-1.5V

8pA

10p

A

80p

A

# of GNRs

degrees

459 pm

− 109

100

200

300

aCategory Average

Height(nm)

Transparent 0.21

Non-Transparentnointra-ribbonresolu on

0.27

Non-Transparentwithintra-ribbonresolu on

0.23

b

0

5

10

15

20

25

9 18 27 36 45 54 63 72 81 90

c 458 pm

− 87

100

200

300

d

-1.5V

8pA

10p

A

80p

A

# of GNRs

degrees

459 pm

− 109

100

200

300

a b

c d

e f

40

Figure 2.10: Larger scans exhibiting GNRs exfoliated onto the InSb(110) surface with randomly

distributed orientations: (a) A scan showing multiple GNRs exfoliated on the surface over a 50 ×

50 nm area: most GNRs have a length of about 10 nm; kinked/bent and overlaid GNRs were

normally seen due to the process of DCT. White dots could be surface contaminants or buckled

GNR debris. Scale bar is 10 nm. (b) A 30 × 30 nm scan showing multiple GNRs with their ends

stacked together. Scale bar is 10 nm. (c) 15 × 15 nm scan exhibiting two GNRs; one is slightly

bent and one is straight. Scan parameters: I = 50 pA, V = −2 V. Scale bar is 5 nm.

Figure 2.11: High-resolution STM of wGNR on InSb(110) after DCT: (a) Topographic image of

a kinked GNR with intra-ribbon resolution. (b) Height profiles extracted across (b) and along the

edge (c) of the GNR (with solid black lines indicating the location where profile was collected)

showed a width of 0.16 nm, height of 0.27 nm for the GNR and an average elbow periodicity of

~0.3 nm. Scan parameters: I = 50 pA, V = −1.5 V. Scale bar is 5 nm.

5 nm

− 983 pm

−166610 nm

1.22 nm

− 0 .1210 nm

0.98 nm

− 0 .28

0.00

0.20

0.40

0.60

0.80

a b c

0.27nm 1.86nm

a

b c 1.38nm 1.29nm

41

Figure 2.12: Tunable transparencies and apparent height distributions of the same wGNR on

InSb(110) under different scanning biases: (a) Scanned at −2 V and 8 pA, the GNR is non-

transparent with no intra-ribbon resolution but elbow structure is showing; the apparent height

distribution collected within the area indicated by the red box gave a 0.33 nm height for the GNR.

(b) Same GNR scanned at −1.5 V and 8 pA, The GNR remains non-transparent; however, the

elbow feature started fading out and the apparent height decreased to 0.22 nm. (c) Scans taken at

−1 V and 8 pA show that the GNR became transparent with the Sb atoms showing through the

GNR. The elbow feature of the GNR was gone and apparent height decreased to 0.17 nm. Scale

bars are 5 nm.

a b c

0.22nm 0.17nm 0.33nm

42

Figure 2.13: Topographic and current images of the same GNR scanned under different

conditions: (a−c) topographic (top) and current images (bottom) of the GNR scanned at 60 pA

and −2 V, −1 V and −0.7 V, respectively. Decreasing the bias did not tune GNR transparency but

rather gave different appearances; when decreasing the scanning bias, GNRs became narrower

with its edges disappearing and intra-ribbon structure starting to show. Those line features inside

the ribbon represent the orbital shapes at a certain distance above the GNR. (e−f) Topographic

images of the same GNR scanning at −1 V and 60 pA and 100 pA, respectively. GNRs showed

the same change in appearance with an increasing current as decreasing the bias. (d) An image

taken at +2 V and 6 pA. Compared to (a), no major changes were seen in the GNR´s appearance

when switching the scanning bias. Scale bars are 5 nm for all images.

5 nm

275 pA

−12 5 nm

275 pA

85 nm

275 pA

0

5 nm

463 pm

− 63

5 nm

540 pm

−42

5 nm

532 pm

−58

a b c

d f e

5 nm

531 pm

−76

200

300

400

5 nm

363 pm

−97

100

200

5 nm

162 pm

−219

−100

− 50

0

50

100

43

Figure 2.14 (a−c): STS data of three GNRs on InSb(110) with the topographic images shown on

the left, and corresponding normalized dI/dV maps (middle) along/across the GNR. STS data

was collected from the bottom to the top of the black dashed lines and shown from left to right in

the maps. White dashed lines indicate the states of the InSb substrate; red dashed lines indicate

the states of the GNR; point spectra (right) collected on the GNR (black curve) and the substrate

(blue curve) with the bandgap of the GNR determined by the energy splitting between the first

two peaks around the Fermi level. (a) Scanned at −1.5 V and 20 pA, the valence band (VB) of

the GNR has not been detected for most of the data; the conduction band (CB) edge of the GNR

is almost mixed with the CB+1 of the substrate; the states representing the CB and VB of the

substrate were also detected when placing the probe over the GNR (the coincidence of first two

states around the Fermi level for both GNR and the substrate shown from the spectra (right) is

also evident). The broadened CB peak of the GNR could be a combination of the CB of the GNR

and the substrate. The missed VB and mixed CB made it hard to determine the exact bandgap: a

2.2 eV bandgap was approximated. (b) Scanned at −2 V and 6 pA, the VB of the GNR has been

detected but seems to be mixed with the VB of the substrate; the CB of the GNR still mixed with

the CB+1 of the substrate; a 2.5 eV bandgap was determined. (c) Scanned at −2 V 8 pA, dI/dV

map shows no mixture between the VBs and CBs of the GNR and the substrate; a 1.9 eV

bandgap was determined. Scale bars are 3 nm for all images.

2.2eV

2.5eV

1.9eV

a

b

c

44

Figure 2.15. Cross-sectional STM sample mounting and tip-sample alignment. (a,b) Front and

side view of the modified sample holder showing the clamping mechanism of the sample mount.

A setscrew was screwed into one of the stainless steel blocks from the side to adjust the distance

between the aluminum plate and the other block for clamping. The sample was clamped between

the aluminum plate and the block. (c) An optical microscopic image exhibiting the probe was

aligned to the cross section of the sample. The bright area on the left is the polished side of the

sample, and the thin dark region is the cross section where the tip should land.

2.9 References

1. Barone, V., Hod, O. & Scuseria, G. E. Electronic structure and stability of semiconducting

graphene nanoribbons. Nano Lett. 6, 2748–2754 (2006).

2. Ruffieux, P. et al. On-surface synthesis of graphene nanoribbons with zigzag edge

topology. Nature 531, 489–492 (2016).

3. Wang, Q. H. et al. Understanding and controlling the substrate effect on graphene

electron-transfer chemistry via reactivity imprint lithography. Nat. Chem. 4, 724–732

(2012).

4. Sforzini, J. et al. Structural and electronic properties of nitrogen-doped graphene. Phys.

Rev. Lett. 116, 1–6 (2016).

5. Palacios, J. J., Fernández-Rossier, J., Brey, L. & Fertig, H. A. Electronic and magnetic

structure of graphene nanoribbons. Semicond. Sci. Technol. 25, 033003 (2010).

6. Narita, A., Feng, X. & Müllen, K. Bottom-up synthesis of chemically precise graphene

nanoribbons. Chem. Rec. 15, 295–309 (2015).

7. Talirz, L., Ruffieux, P. & Fasel, R. On-surface synthesis of atomically precise graphene

a b c

45

nanoribbons. Adv. Mater. 6222–6231 (2016). doi:10.1002/adma.201505738.

8. Talirz, L. et al. Termini of bottom-up fabricated graphene nanoribbons. J. Am. Chem. Soc.

135, 2060–2063 (2013).

9. Nguyen, G. D. et al. Bottom-up synthesis of N = 13 sulfur-doped graphene nanoribbons.

J. Phys. Chem. C 120, 2684–2687 (2016).

10. Schwab, M. G. et al. Bottom-up synthesis of necklace-like graphene nanoribbons. Chem. -

An Asian J. 10, 2134–2138 (2015).

11. Yang, W., Lucotti, A., Tommasini, M. & Chalifoux, W. A. Bottom-up synthesis of soluble

and narrow graphene nanoribbons using alkyne benzannulations. J. Am. Chem. Soc. 138,

9137–9144 (2016).

12. Vo, T. H. et al. Large-scale solution synthesis of narrow graphene nanoribbons. Nat.

Commun. 5, 1–8 (2014).

13. Shen, Q., Gao, H. Y. & Fuchs, H. Frontiers of on-surface synthesis: From principles to

applications. Nano Today 13, 77–96 (2017).

14. Cai, J. et al. Graphene nanoribbon heterojunctions. Nat. Nanotechnol. 9, 896–900 (2014).

15. Vo, T. H. et al. Nitrogen-doping induced self-assembly of graphene nanoribbon-based

two-dimensional and three-dimensional metamaterials. Nano Lett. 15, 5770–5777 (2015).

16. Neaton, J. B., Hybertsen, M. S. & Louie, S. G. Renormalization of molecular electronic

levels at metal-molecule interfaces. Phys. Rev. Lett. 97, 1–4 (2006).

17. Thygesen, K. S. & Rubio, A. Renormalization of molecular quasiparticle levels at metal-

molecule interfaces: Trends across binding regimes. Phys. Rev. Lett. 102, 1–4 (2009).

18. Albrecht, P. M. & Lyding, J. W. Ultrahigh-vacuum scanning tunneling microscopy and

spectroscopy of single-walled carbon nanotubes on hydrogen-passivated Si(100) surfaces.

Appl. Phys. Lett. 83, 5029 (2003).

19. Szamota-Leandersson, K. Electronic structure of clean and adsorbate-covered InAs

surfaces. Ph.D dissertation, Royal Institute of Technology, Stockholm, Sweden (2010).

20. Andersson, C. B. M., Andersen, J. N., Persson, P. E. S, & Karlsson, U. O. Surface

electronic structure of InAs(110). Physical Review B 32, 7830 (1985).

21. Klijn, J. et al. STM measurements on the InAs(110) surface directly compared with

surface electronic structure calculations. Phys. Rev. B 68, 1–10 (2003).

22. Yelgel, C., Srivastava, G. P. & Miwa, R. H. Ab initio investigation of the electronic

46

properties of graphene on InAs(111)A. J. Phys. Condens. Matter 24, 485004 (2012).

23. Andrade, D. P., Miwa, R. H. & Srivastava, G. P. Graphene and graphene nanoribbons on

InAs(110) and Au/InAs(110) surfaces: An ab initio study. Phys. Rev. B - Condens. Matter

Mater. Phys. 84, 1–7 (2011).

24. He, K. T., Koepke, J. C., Barraza-Lopez, S. & Lyding, J. W. Separation-dependent

electronic transparency of monolayer graphene membranes on III-V semiconductor

substrates. Nano Lett. 10, 3446–3452 (2010).

25. Tersoff, J. & Hamann, D. R. Theory of the scanning tunneling microscope. Phys. Rev. B

31, 805–813 (1985).

26. Radocea, A. et al. Solution-synthesized chevron graphene nanoribbons exfoliated onto H:

Si(100). Nano Lett. 17, 170–178 (2017).

27. Liang, L. & Meunier, V. Electronic structure of assembled graphene nanoribbons:

Substrate and many-body effects. Phys. Rev. B - Condens. Matter Mater. Phys. 86, 2–6

(2012).

28. González-Herrero, H. et al. Graphene tunable transparency to tunneling electrons: A direct

tool to measure the local coupling. ACS Nano 10, 5131–5144 (2016).

29. Li, Y., Zhang, W., Morgenstern, M. & Mazzarello, R. Electronic and magnetic properties

of zigzag graphene nanoribbons on the (111) surface of Cu, Ag, and Au. Phys. Rev. Lett.

110, 1–5 (2013).

30. Zhang, H. et al. On-surface synthesis of rylene-type graphene nanoribbons. J. Am. Chem.

Soc. 137, 4022–4025 (2015).

31. Ruppalt, L. B. & Lyding, J. W. Charge transfer between semiconducting carbon nanotubes

and their doped GaAs(110) and InAs(110) substrates detected by scanning tunnelling

spectroscopy. Nanotechnology 18, 215202 (2007).

32. Deniz, O. et al. Revealing the electronic structure of silicon intercalated armchair

graphene nanoribbons by scanning tunneling spectroscopy. Nano Lett. 17, 2197–2203

(2017).

33. Radocea, A. Scanning tunneling microscopy investigation of atomically precise graphene

nanoribbons. Ph. D. dissertation, University of Illinois at Urbana-Champaign, Urbana, IL

(2017).

34. Lyding, J. W., Skala, S., Hubacek, J. S., Brockenbrough, R. & Gammie, G. Variable-

temperature scanning tunneling microscope. Rev. Sci. Instrum. 59, 1897–1902 (1988).

35. Giannozzi, P. et al. Quantum Espresso: A modular and open-source software project for

47

quantum simulations of materials. J. Phys. Condens. Matter 21 (2009).

36. Becke, A. D. Density-functional exchange-energy approximation with correct asymptotic

behavior. Phys. Rev. A 38, 3098–3100 (1988).

37. Kokalj, A. XCrySDen - A new program for displaying crystalline structures and electron

densities. J. Mol. Graph. Model. 17, 176–179, 215–216 (2000).

38. Soler, J. M. et al. The SIESTA method for ab initio order- N materials simulation. J. Phys.

Condens. Matter 14, 2745–2779 (2002).

39. Perdew, J. P., Burke, K. & Ernzerhof, M. Generalized gradient approximation made

simple. Phys. Rev. Lett. 77, 3865–3868 (1996).

40. Kresse, G. & Furthmüller, J. Efficient iterative schemes for ab initio total-energy

calculations using a plane-wave basis set. Phys. Rev. B - Condens. Matter Mater. Phys.

54, 11169–11186 (1996).

41. Shishkin, M. & Kresse, G. Self-consistent GW calculations for semiconductors and

insulators. Phys. Rev. B - Condens. Matter Mater. Phys. 75, 1–9 (2007).

42. Mostofi, A. A. Wannier90: A tool for obtaining maximally-localised Wannier functions.

Comput. Phys. Commun. 178, 685–699 (2008).

48

CHAPTER 3

CROSS-SECTIONAL STM OF EXTENDED-

CHEVRON AND NITROGEN-DOPED GNRS

ON INAS(110)

3.1 Introduction and Motivation

An extraordinary feature of the bottom-up synthesized atomically precise GNRs is that its

properties can be altered by changing the geometry.1 Understanding how to synthesize GNRs

with the desired properties is important for application purposes. Both theoretical and

experimental results have shown that varying the width and edge structure of a GNR can

dramatically affect its electronic band structure.2,3 For GNRs of the same chirality, the bandgap

is inversely proportional to the width.4 By substituting the carbon atoms with other atoms or

functional groups at the edge, electron donors can dope the GNR and thus modify its density of

states.5,6,7

Since InAs(110) is a preferable platform to metal surfaces for investigating the intrinsic

properties of isolated GNRs, we deposited two types of solution-synthesized chevron-family

GNRs on InAs by DCT for STM characterization. The first one is called extended-chevron GNR

(eGNR). Compared to the regular chevron GNR (cGNR) as shown in Figure 3.1a, it has an

additional naphthalene group attached at the elbow site (Figure 3.1b).8 Different from the

previously reported GNRs with their width extended all throughout the edges,9,10 here we have a

partial extension only at certain sites. After depositing the eGNR powders on InAs by DCT, we

49

were able to identify its extended feature, and STS measurement revealed a decrease in the

bandgap, as expected.

The second GNR is called eight-nitrogen-doped GNR (8N-GNR), which has the same

precursor monomer as regular chevron GNR but with four carbon atoms substituted by four

nitrogen atoms resulting for a total of eight nitrogen atoms substitution per unit cell of a planar

ribbon (Figure 3.1c). An interesting phenomenon has been observed on 8N-GNRs deposited on

Au(111) from a solution suspension during which those GNRs tend to align side-by-side and

stack up in layers due to hydrogen bonds between H and N atoms at the edge of the ribbon.11

STM scans on the bonded GNR sheet affirmed a slightly offset alignment along their edge axes

for adjacent ribbons, which confirmed the existence of hydrogen bonding. Furthermore, STS

revealed a 1.9 eV bandgap on those bonded 8N-GNRs. Here we did STM of solution-synthesized

isolated 8N-GNRs exfoliated on InAs. Detailed STS and ribbon-substrate interactions were

investigated, and results showed that nitrogen dopants did not alter the band structure much but

had a large effect on ribbon-ribbon and ribbon-substrate interactions.

3.2 Extended-chevron GNRs

The Sinitskii group at the University of Nebraska-Lincoln synthesized the eGNRs by the

solution method, and the final product is in powder form. After depositing onto a freshly-cleaved

InAs(110) surface by DCT, isolated GNRs were identified easily by STM imaging. Figure 3.2

shows some STM topographic images of eGNRs on InAs. These GNRs showed random

orientation with respect to the underlying substrate lattice, which indicates no strong bonds

existed with the substrate. In contrast to eGNRs on the H-Si(100) surface,12 they also stay

50

immobile during scans, which enabled collecting some high-resolution images. One possible

reason for the immobility of the eGNRs on InAs could be the existence of dangling bonds on the

InAs surface that coupled with the π bonds of GNRs.13 Figure 3.2a (left) is an STM image of a

hook-shaped long (or multiple short) GNR(s). Such a shape is most likely caused by the random

direction of the exerting forces during DCT. Three height profiles extracted from the GNR are

shown in Figure 3.2a (right). Profile 1 taken across the ribbon gives a ribbon width of 2.5 nm;

Profile 2 shows that the distance between the extended elbows is 3.5 nm, and Profile 3 taken

along the ribbon´s edge showed a 1.9 nm elbow periodicity. Compared to cGNR, the width of

the eGNR is 0.3 nm wider due to the lateral extension,14 and the elbow periodicity remains about

the same, as expected. The consistent apparent height of the eGNR (0.3 nm with respect to the

substrate) is also comparable to the cGNR, indicating a complete cyclodehydrogentation. Figure

3.2b is an STM image of a different ribbon with the substrate lattice clearly seen. This ribbon is

shorter in length, but the width, height and elbow periodicity are consistent. Figure 3.2c shows

two ribbons with their ends stacked together. The extended elbow is very clear in this image, and

part of the left ribbon is transparent, revealing the substrate. This scan also provides intra-ribbon

details with the center of the ribbon appearing lower (as shown in Profiles 2 and 3), which relates

to the local density of states but not the atomic structures. Figures 3.2d and 3.2e show more STM

images of eGNRs and a histogram of their length distribution, showing that most ribbons have a

length of about 10 nm, which is much shorter than the 42.5 nm average length measured by size-

exclusion chromatography.8 This could be a result of DCT tearing the long GNRs apart.

STS analysis carried out by Radocea revealed an average bandgap of 2.66 eV.8,12 The

lateral extension on the elbow site reduced the bandgap of the eGNR by 0.1 eV, which is also

consistent with the computational simulation.

51

3.3 Nitrogen-doped GNRs

GNRs with different dopants or functional groups have drawn great interest because of

their ability to alter properties such as electronic structure and ribbon-substrate interactions.

Those properties are essential for synthesizing ribbons and implementing them as electronic

devices such as sensors and photovoltaic devices.15,16 Among various technologies for 2-D

material characterization, STM/STS is uniquely suitable due to its ability to achieve atomic

resolution imaging and spectroscopy. Not only can one ˝see˝ those dopants/functional groups

using STM and measure the band structure, but one can also manipulate the dopants with an

electron beam.17

Solution-synthesized 8N-GNRs were exfoliated onto InAs(110) by DCT. Figure 3.3a

shows an STM topograph of several 8N-GNRs with a length of less than 10 nm scattering on the

surface with random orientations. It is likely that the DCT process broke not only the hydrogen

bonds between GNRs but also the carbon bonds in the ribbon itself. Figure 3.3b shows the height

profiles extracted across and along an isolated 8N-GNR. The ribbon shows an apparent height of

0.3 nm, width of ~3 nm and edge period of 1.9 nm, which are very close to the values of the

regular chevron GNRs. To better understand how nitrogen dopants modify the band structure,

STS measurements were taken. Figure 3.3c shows an STM image of another 8N-GNR with STS

I-V data collected along the blue line. The numerically calculated normalized tunneling

conductance map is shown in Figure 3.3d. The energy difference between the conduction band

and valence band is 2.6 eV, and point spectra taken on the ribbon and substrate (as shown in

Figure 3.3e) also show that the Fermi level of the ribbon is slightly closer to the conduction

band, indicating n-type doping of the GNRs by the nitrogen atoms. The 2.6 eV bandgap is very

52

close to the reported 2.76 eV bandgap of the regular chevron GNR12 but 0.7 eV larger than the

value measured on hydrogen-bonded 8N-GNR arrays. Our measurements are consistent with the

theoretical prediction that although nitrogen serves as an n-type dopant, it does not change the

ribbon´s band structure much.18

We also saw multiple small ribbons bonded side-by-side to form a long row, as shown in

Figure 3.4a. After an attempt to collect STS data on these ribbons (indicated by three blue lines)

by bringing the tip 0.2 nm closer to the surface and sweeping the voltage from -3 to 3 V, the

hydrogen bonds were broken by the tunneling current (as shown in Figure 3.4b). Once the bonds

were broken, these ribbons were separated by a similar distance of ~2 nm and became highly

mobile, which is different from the regular chevron GNRs. Often, we observed movement of

ribbons during normal scans like the ones shown in Figures 3.4c and d; after repeating the scan at

−1.5 V and 5 pA, both ribbons have moved and rotated. Such a high mobility makes collecting

STS data difficult. It is possible that substituting carbon atoms with nitrogen changes the GNR-

InAs interaction. Since the adsorption energy of nitrogen is smaller than that of carbon,19 the

attractive force between the 8N-GNR and the substrate could decrease.

3.4 Conclusion

Once DCTed onto the InAs(110) surface, not only were the geometries of the eGNRs and

8N-GNRs confirmed, we also investigated their electronic structures by taking high-resolution

STM and STS. Compared to the regular chevron GNRs, the lateral extension at the elbow of the

eGNR results in a slight decrease in bandgap. On the other hand, substituting carbon atoms at the

edge with nitrogen atoms will not affect the bandgap much but will reduce the ribbon-substrate

53

interaction. This opens the possibility of positioning ribbons on the substrate to create ribbon

arrays for device applications. Additional low-temperature STM and STS along with

computational modeling may assist in accomplishing further modification to the GNRs by

manipulating the nitrogen atoms using the scanning probe.

54

3.5 Figures

Figure 3.1: The precursors (a) 6,11-dibromo-1,2,3,4-tetraphenyltriphenylene. (b) 2-([1,1´:2´,1˝-

terphenyl]-3´-yl)-6,11-dibromo-1,4-diphenyltriphenylene, and (c) 5,5′-(6,11-dibromo-1,4-

diphenyltriphenylene-2,3-diyl)-dipyrimidine are used to synthesize chevron graphene

nanoribbons (cGNRs), extended-chevron graphene nanoribbons (eGNRs), and 8N-GNRs,

respectively.

Regular Chevron GNR

Extended−Chevron GNR

8N GNR

a

b

c

55

Figure 3.2: (a) An STM topograph of an eGNR on InAs(110) and height profiles across and

along the eGNR, showing an elbow to elbow distance of 3.5 nm, width of 2.6 nm, edge period of

1.9 nm and apparent height of 0.3 nm, as expected. (b) Another STM topograph of eGNR with

height profiles showing the same geometry as in (a). Scale bars are 5 nm. (c) Two eGNRs with

their ends stacked together display clear extended elbow features and intra-ribbon resolution.

Height profiles across two ribbons (red and green) show that the center of the ribbon appears

lower than the edge, which relates to the LDOS of the ribbon. (d) More STM scans of different

ribbons. Scale bars are 10 nm. (e) Histogram showing the length distribution of the eGNRs. Scan

parameters: −2 V, 8 pA.

3.5nm

2.6nm

1.9nm

0.3

nm

a

2.5nm

1.9nm

b

2.5nm

10 nm

1

2

3

439 pm

−65

0

50

100

150

200

250

300

350

400

10 nm

0.36 nm

−0. 8110 nm

0.5 nm

−0. 9

c

d e

56

Figure 3.3: (a) An STM tomographic image of 8N-GNRs on InAs: ribbons are shorter than

expected with an average length of ~8 nm. Scan parameters: 1.5 V, 30 pA. (b) Height profiles

extracted across (black) and along (red) the elbow of the ribbon show a width of 3.1 nm and

elbow period of 1.9 nm. (c) Another short 8N-GNR scanned at −1.5 V, 5 pA. (d) Normalized

conductance map taken along the blue line in (c) showing a ribbon bandgap of 2.6 eV (orange

dashed lines) with its Fermi level closer to the conduction band, indicating an n-type doping of

the GNR; the bandgap of InAs (red dashed lines) is 0.5 eV. (e) Normalized conductance spectra

taken on the substrate (black curve) and on the GNR (blue curve) as indicated by the black and

red points in (c), respectively. A bandgap of ~2.6 eV is measured for the GNR and 0.5 eV for the

InAs substrate.

10 nm

404 pm

− 80

0

50

100

150

200

250

300

350

3.1nm

1.9 nm

5 nm

1776 pm

1124

2.6eV 2.6eV

a b

c d e

57

Figure 3.4: (a) Several short GNRs with their edges bonded together in a long row caused by

hydrogen bonds between H and N atoms; image scanned at 1.5 V, 10 pA. (b) The same area after

doing STS along three lines as indicated in (a). The hydrogen bonded GNRs were separated after

doing STS. STS parameters: −0.2 nm variable spacing with the voltage ramped from −3 V to 3 V

and a setpoint current of 10 pA. (c) Two short GNRs scanned at −2 V, 10 pA. (d) Repeating the

scan at −1.5 V and 5 pA resulted in movements of the ribbons: the left one rotated counter-

clockwise and the right one moved to the right and also rotated. Scale bars are 5 nm.

5 nm

0.7 nm

−0.6

5 nm

1776 pm

1124

5 nm

2.0 nm

0.05 nm

0.95 nm

− 0 .22

a b

c d

58

3.6 References 1. Barone, V., Hod, O. & Scuseria, G. E. Electronic structure and stability of semiconducting

graphene nanoribbons. Nano Lett. 6, 2748–2754 (2006).

2. Palacios, J. J., Fernández-Rossier, J., Brey, L. & Fertig, H. A. Electronic and magnetic

structure of graphene nanoribbons. Semicond. Sci. Technol. 25, 033003 (2010).

3. Ruffieux, P. et al. On-surface synthesis of graphene nanoribbons with zigzag edge

topology. Nature 531, 489–492 (2016).

4. Chen, Y.-C. et al. Tuning the band gap of graphene nanoribbons synthesized from

molecular precursors. ACS Nano 7, 6123–8 (2013).

5. Sforzini, J. et al. Structural and electronic properties of nitrogen-doped graphene. Phys.

Rev. Lett. 116, 1–6 (2016).

6. Carbonell-Sanromà, E. et al. Doping of graphene nanoribbons via functional group edge

modification. ACS Nano 11, 7355–7361 (2017).

7. Durr, R. A. et al. Orbitally matched edge-doping in graphene nanoribbons. J. Am. Chem.

Soc. 140, 807–813 (2018).

8. Mehdi Pour, M. et al. Laterally extended atomically precise graphene nanoribbons with

improved electrical conductivity for efficient gas sensing. Nat. Commun. 8, 820 (2017).

9. Cai, J. et al. Atomically precise bottom-up fabrication of graphene nanoribbons. Nature

466, 470–3 (2010).

10. Chen, Y. C. et al. Tuning the band gap of graphene nanoribbons synthesized from

molecular precursors. ACS Nano 7, 6123–6128 (2013).

11. Vo, T. H. et al. Nitrogen-doping induced self-assembly of graphene nanoribbon-based

two-dimensional and three-dimensional metamaterials. Nano Lett. 15, 5770–5777 (2015).

12. Radocea, A. Scanning tunneling microscopy of atomically precise graphene nanoribbons,

Ph.D. dissertation, University of Illinois at Urbana-Champaign, Urbana, IL (2017).

13. Klijn, J. et al. STM measurements on the InAs(110) surface directly compared with

surface electronic structure calculations. Phys. Rev. B 68, 1–10 (2003).

14. Radocea, A. et al. Solution-synthesized chevron graphene nanoribbons exfoliated onto H:

Si(100). Nano Lett. 17, 170–178 (2017).

15. Huang, J., Wang, W., Li, Q. & Yang, J. Negative differential resistance devices by using

59

N-doped graphene nanoribbons. J. Chem. Phys. 140, 164703 (2014).

16. Bellone, D. Nitrogen doped graphene nanoribbons for organic photovoltaic applications.

M.S. Thesis, University of California, Berkeley, CA, (2016).

17. Kawai, S. et al. Atomically controlled substitutional boron-doping of graphene

nanoribbons. Nat. Commun. 6, 1–6 (2015).

18. Cai, J. et al. Graphene nanoribbon heterojunctions. Nat. Nanotechnol. 9, 896–900 (2014).

19. Shimonishi, T., Nakatani, N., Furuya, K. & Hama, T. Adsorption energies of carbon,

nitrogen, and oxygen atoms on the low-temperature amorphous water ice: A systematic

estimation from quantum chemistry calculations. Astrophys. J. 855, 27 (2018).

60

CHAPTER 4

FABRICATION OF MICROMETER-SIZED

METAL ELECTRODE ARRAYS FOR STM

TIP-ASSISTED NANOCONTACTS WRITING

FOR GNRS

4.1 Introduction and Motivation

Graphene nanoribbons (GNRs) with their extraordinary electrical and thermal

properties1−3 have high potential to replace silicon in the semiconducting industry.4 However,

owing to the nanometer-sized nature of GNRs and their current synthesis methods,5−7 a lack of

precise control over the positioning and alignment of GNRs remains a big challenge for single

GNR device fabrication and application. Transport measurements on isolated GNRs were limited

to those fabricated using the top-down approach with tens of nanometers in GNR width and poor

edge structure, resulting in poor consistency.8−10 Conducting transport measurements on isolated

atomically-precise GNRs remains challenging due to the difficulty of depositing small contacts

onto the GNR with conventional lithography techniques.

As demonstrated in the previous chapters, once we exfoliated GNRs onto the substrate

using the DCT method, not only were we able to find isolated GNRs on the surface, but we could

also manipulate them using the STM tip. Furthermore, previous work carried out by the Lyding

group demonstrated the writing of <5 nm hafnium diboride metallic nanostructures on the

hydrogen-passivated silicon(100) (H-Si) surface, including on a 1-nm-diameter carbon

61

nanotube.11,12 The principle behind this is that the precursor molecules, in this case the carbon-

free Hf(BH4)4, will be decomposed by the electron beam from the STM tip at the tip-sample

junction and therefore a local CVD deposition will occur right beneath the position of the tip. By

moving the tip in a desired path, the dimensions of the structure written can be controlled with

proper writing parameters and dosing pressures: 5 nm metallic nanodots, 4-nm-wide metallic

lines and 0.1-μm-long lines have been realized. In addition, the metallic nature of the hafnium

diboride (HfB2) written was conveniently confirmed by STS, which gave 0 eV in electrical

bandgap.

We want to further implement this STM current-assisted chemical vapor deposition

technique on writing metallic HfB2 nanocontacts for GNRs sitting on the H-Si(100) surface.

These nanocontacts will serve as bridges connecting the nanometer-sized individual GNRs with

pre-fabricated micrometer-sized metal electrodes for in situ transport measurements. After a Si

sample with large prefabricated metallic electrodes arrays is loaded into the UHV-STM chamber

and after sample flashing and hydrogen passivation, GNRs will be DCTed onto the surface. Once

an isolated GNR between the adjacent large electrodes is located, metallic HfB2 lines will be

written over the surface as well as on the GNR in order to connect the GNR to the large

electrodes. Once a well-controlled HfB2 path between the GNR and the large electrodes is

established, transport measurements can be carried out in situ by independently applying biases

to the sample through the large electrodes (a schematic of contacting a GNR and connecting with

large electrodes is shown in Figure 4.1).

This chapter will focus on the fabrication and testing of the micrometer-sized metal

electrodes arrays, including the structure design, the determination of the proper material,

62

fabrication technique and the effect of Joule heating on the deposited metal electrodes during the

sample preparation process.

4.2 Substrate and Electrode Materials

Ideally, for conducting a transport measurement, the GNR should be placed onto an

insulating substrate. However, due to the incompatibility of an insulator with the STM technique

and the success of writing metallic nanostructures on H-Si, the H-Si substrate is used for this

experiment. Opportunely, a Schottky barrier will form at the metal-semiconductor junction (in

our case the deposited metal and the Si substrate) which can stem the current flowing into the

semiconducting substrate.13 Based on this scenario, a n-type doped Si substrate and a metal with

a high work function is preferable. A band diagram illustrating the formed Schottky barriers at

the metal-semiconductor-metal junction is shown in Figure 4.2.

There are certain requirements for the material of the electrodes: first, it needs to be

resistant to oxidation in air so that it remains conductive after deposition for STM; second, the

melting point needs to be high enough to survive the Si flashing process which usually occurs at

around 1200 °C in UHV; and third, its work function should be as high as possible to enhance

the effect of the Schottky barrier. Based on these requirements, platinum (Pt) and titanium (Ti)

were chosen as the electrode materials. Ti serves as an adhesive layer between Pt and Si. They

both are noble metals that are stable at high temperatures (melting point of over 1700 °C for Pt

and 1600 °C for Ti) and have stable electrical properties (work function of 6.35 eV and 4.33 eV;

electrical resistivity of 105 nΩ·m and 420 nΩ·m at 20 °C for Pt and Ti, respectively).14−17 It is

worth noting that during a rapid high-temperature annealing process, metal-silicide compounds

63

can form at the interface.18 The crystallography, morphology and the electrical sheet resistance

of the formed silicide films are dependent on the cleanliness and doping level of the Si as well as

the annealing temperature.19,20 The formed silicide layer can also expand laterally due to atomic

diffusion.21 That spreading layer is thinner than the deposited metal and thus makes it easier to

link to the HfB2 nanostructure. In addition, the work function of the metal silicide does not

change much compared to the metal´s work function; the reported values are ranges from 4.9 to

5.2 eV for PtSi and around 4.5 eV for TiSi.22,23 In addition, there are reports showing that the

sheet resistance of the silicide will improve by nearly an order of magnitude after annealing at

900 °C.24,25

4.3 Structure Design and Fabrication Process

The design for the structure and dimension of the large electrodes fully took into

consideration the compatibility with the STM sample and the convenience of writing HfB2 lines

between GNRs and the electrodes. Figure 4.3 shows the layout of the large prefabricated metallic

electrodes. As shown in Figure 4.3a, three devices will be fabricated at a time on a 12 × 12 mm

Si chip, which can be easily cut into STM sample-sized chips later on. Each device has two large

metal pads on each side, which can be individually biased through the two clamping sites of our

customized STM sample holder. The center (Figure 4.3b zoom-in image) is composed of long

(198 μm) narrow (1 μm) strips connected to the large pads alternating within a 200 × 200 μm

window, which enabled tip positioning with the help of an optical microscope. There are gaps

between the adjacent strips so that the left and right pads are initially electrically insolated.

Figure 4.3c is a further zoomed-in image of the electrode strip pattern. The gap distance between

64

adjacent strips can vary from 250 nm to 1 μm depending on the difficulty of fabrication. The

reasons for choosing these distances are: 1) the gap should be large enough to locate one or

several GNRs after DCT but not so large that we waste time writing significantly long lines

between them; 2) several gaps can be positioned within the STM scan range without a frequent

course offsetting, also saving much time.

Based on the requirement of fabricating such small features, we chose e-beam

lithography and evaporation techniques to minimize the fabrication steps. A process flow is

shown in Figure 4.4. First, 500 nm of photoresist (PMMA) was spin-coated onto Si and patterned

by e-beam lithography. Then a 5 nm layer of Ti, which serves as an adhesive layer, followed by

another layer of Pt (5 to 50 nm) was evaporated onto the Si by e-beam evaporation. Finally, the

unwanted part of the metal was removed along with the photoresist by dissolving it in an acetone

solution, leaving us with the desired structure.

Devices with two different gap sizes (250 nm and 1μm) between adjacent strips were

fabricated using the process described above. Figure 4.5 shows scanning electron microscopy

(SEM) images of the fabricated electrodes with the 250 nm gap. The contrast near the bottom

part of the strips in Figures 4.5a,b indicates incomplete PMMA liftoff. A zoomed-in image of the

incomplete liftoff area (Figure 4.5c) shows that the PMMA tends to peel off from the end of the

strip where the window size is larger. At some areas where the major PMMA layers were

removed, there is still a thin layer of residue left (as shown in Figure 4.5d). Post-acetone

treatment (being dipped in an acetone bath overnight and sonication for 3 hours) could not solve

the issues of incomplete liftoff nor the in-gap residues, as shown in Figures 4.5e and f,

respectively. According to the equation of the writing time needed for the e-beam lithography,

D × A = T × I (where T is the time of exposure, I is the beam current, D is the dosage and A

65

is the area exposed), the reason for the incomplete liftoff for the smaller gap could be insufficient

exposure time during lithography.24 Thus we tried fabricating a device with the 1 μm-sized gaps

with the same fabrication recipe. This time the writing area decreased and the resulting PMMA

was easily and completely dissolved by acetone (as shown in Figure 4.6) with the Pt electrodes

and clean Si substrate exposed as desired. The height profile extracted from the atomic force

microscopic (AFM) topographic images (shown in Figure 4.6b) showed the evaporated metal

with a width of 1.2 μm. The 0.2 μm difference could have resulted from the laterally diffused

silicide, AFM tip convolution effect as well as the errors from the e-beam lithography process.

4.4 Low-Temperature Sample Preparations and the Annealing Effect on

Evaporated Metal Surfaces

As mentioned above, a metal-silicide compound could form upon the heating of Si during

the sample preparation process, which may lead to a change in the Si surface morphology. We

did a series of heating tests at various temperatures on the deposited Pt and Ti on Si samples.

Figure 4.7 shows the before and after SEM images of the evaporated Pt degassed at 600 °C

overnight followed by flashing at 1200 °C for 30 s in UHV. It is obvious that the Pt layer has

been seriously damaged and even peeled off with the central strip´s structure destroyed during

the high-temperature annealing process. Such temperature conditions seemed too harsh for the

thin metal layer.

The reason for flashing the Si at above 1200 °C is to evaporate the carbon contamination

and prevent the formation of SiC islands.24 Figures 4.8a and b are the STM topographic images

of the Si surface after a degassing at 600 °C overnight followed by flashing at a lower

66

temperature of 900 °C for 2 mins. Those 200-to-400 nm2 pyramid islands of SiC that need to be

prevented. There are ways of preparing a carbon-free Si surface at a lower flashing temperature

of 900 °C. One could pretreat the Si sample with acid such as NH4OH, HF and HCL right before

being loaded into the STM chamber.24 The other way is to clean the sample with ozone

generated by ultraviolet (UV) light to oxidize the organic compounds into volatile substances.26

After being treated with UV ozone for 10 mins, the Si surface was free of contamination with

dimer rows clearly seen under STM after a low-temperature flash at 900 °C (shown in Figures

4.8 c and d).

To further explore the effect of annealing on the deposited metals, we took atomic force

microscopic (AFM) images on the fabricated Ti and Pt electrodes after being annealed at various

temperatures. Figure 4.9 shows the before and after AFM images of a 20 nm Ti surface annealed

at 600 °C overnight. Upon evaporation, the surface is fairly flat with an approximate 4 nm

variation in height and surface roughness of Ra (arithmetic average of the absolute values of the

surface height deviations measured from the mean plane) is 0.69 nm and Rms (root mean square

average of the height deviation from the mean plane) is 0.91 nm (Figures 4.9a and b). After

annealing, disordered islands agglomerated on the surface, indicating the formation of silicide

compounds with large vacancies between the islands (Figures 4.9c and d). The surface becomes

rougher with a maximum height variation of over 30 nm with Ra = 4.27 nm and Rms = 5.41 nm.

After a higher annealing of 900 °C for 2 mins, sufficient thermal energy has been provided to the

micro-structured islands to rearrange them into a more ordered form in order to reduce the total

free energy (Figure 4.9e). Those formed islands became larger and taller with a surface

roughness of Ra = 21.8 nm and Rms = 27.9 nm and height variation of ~50 nm. The enlargement

in the islands´ sizes and the disappearance of the vacancies beneath them indicates the growth of

67

grains upon annealing, and therefore the sheet resistivity is expected to decrease. We measured

the ambient resistance of the deposited Ti layer before and after the heat treatment, and the

results showed an order of magnitude of decrease in the resistance after annealing: from 100 to

10 Ω over a 1 mm distance between the two probes. Another surface with 20 nm of Ti plus 50

nm of Pt evaporated was also characterized (as shown in Figure 4.10). Before heating, the

surface is flat with only a ~3 nm variation in height, surface roughness Ra of 0.84 nm and Rms of

1.13 nm (Figure 4.10a). After degassing at 600 °C and flashing at 900 °C, the surface again

became rougher with nano-islands forming (Figures 4.10b, and c). Those PtSi islands are less

prevalent compared to TiSi and have a height variation of ~40 nm and surface roughness Ra =

6.63 nm and Rms = 8.3 nm (as shown in the zoomed-in image of Figure 4.10c). Transport

measurement showed a resistance of 50 Ω after the heating. In comparison, the resistance of the

Si is measured to be 500 Ω, which is larger than both TiSi and PtSi, suggesting that the current

will flow dominantly from the HfB2 to the silicide over the silicon, as expected.

In summary, upon annealing at 600 °C and flashing at 900 °C in UHV, both the deposited

Ti and Pt layers will form silicide compounds with nano-islands arising at the surface. Different

annealing temperatures resulted in different surface morphology, possibly due to the formation

of the silicide with different phases. The island height and surface roughness for TiSi are greater

than those of PtSi, indicating that TiSi requires less energy for the surface reconstruction in order

to reduce the total free energy. Although the surface gets rougher upon heating, there is a

decrease in resistivity due to the grain growth after annealing.

68

4.5 Conclusion

Micrometer-sized Pt and Ti electrodes structures were fabricated by e-beam lithography

and evaporation techniques on Si. Both materials could survive and remained conductive after a

low-temperature annealing of 900 °C. The Si surface remains clean enough for STM with a pre-

treatment of UV ozone cleaning process. With these prefabricated electrodes, conductive paths

can be written to isolated GNRs with HfB2 nanocontacts connected in between for future in situ

transport measurement.

4.6 Methods

4.6.1 Metallic Electrode Fabrication

The e-beam lithography process was carried out by Edmond Chow at the Micro and

Nanotechnology Laboratory at University of Illinois at Urbana-Champaign. A 12 × 12 mm

RCA-cleaned Si wafer (Sb-doped n-Si(110) with sheet resistance 0.01 to 0.05 Ω·cm) was first

spin- coated with PMMA (950 A4) for 30 s, resulting in a 500 nm thickness. Then, lithography

was done by a JEOL JBX-6000FS e-beam lithography system. Then the sample was developed

using methyl isobutyl ketone:IPA (2:1) for 2 mins and the structure was checked with SEM. The

sample was loaded into a CHA SEC-600 e-beam evaporator system. Ti and Pt were evaporated

with a chamber pressure of 1E−7 torr. The source was heated with a voltage of 10 kV, and the

current was adjusted to get a 0.1 nm/s deposition rate. Following that, the sample was placed in

an acetone bath to lift off the PMMA, followed by sonication with water and IPA for 15 mins.

Finally, the sample was loaded into a UV ozone cleaner for 10 mins right before being loaded

into the STM system.

69

4.6.2 Sample Preparation for STM

Once the sample was loaded onto the dipstick in the STM preparation chamber, we ran

current through the sample until the sample reached 600 °C and the chamber pressure reached

1E−11 torr. Then we flashed the sample at 900 °C for 2 mins while keeping the chamber

pressure at 1E−10 torr and slowly cooled it down to room temperature over the course of 1000

s.27 After flashing, the sample´s cleanliness was confirmed by STM imaging. For hydrogen

passivation, the sample was then heated to 377 °C while exposed to 1200 liters of molecular

hydrogen that was cracked into atomic hydrogen with a 1500 °C tungsten filament. STM

experiments were conducted using a custom-built Lyding STM system (chamber D).

70

4.7 Figures

Figure 4.1: Top and side views showing the schematic of HfB2 nanocontact writing for a GNR

deposited on a Si substrate. H-Si surface (light blue) with prefabricated micrometer-sized

metallic electrodes (brown) is introduced into the UHV STM chamber; after locating an isolated

GNR (beige) sitting in between the large electrodes, HfB2 (navy) are written by electron beam-

assisted deposition to connect the GNR to the large electrodes. By applying a bias between the

two large electrodes, transport measurement can be carried out in situ.

V

Pre−fabricated large electrodes

HfB2

nanocontacts GNR

V

Si Large electrodes

HfB

2

GNR

Top view Side view

71

Figure 4.2: Band diagram showing the Schottky barrier at the metal-semiconductor interface. For

Si, its affinity X is 4.15 eV, so the barrier height equals the work function of metal subtracted by

4.15. The larger the work function of the metal, the larger the barrier height. Such an energy

barrier will prevent the current from flowing from one side of the metal to the other regardless of

the bias applied.

Ef

Vacuum Level

φs

φM

φB=φ

M−

X

Ev

Ec

X

Metal Semiconductor

φM

φB=φ

M−

X

Metal

72

Figure 4.3: Layouts of the micrometer-sized metal electrodes. (a) Each 12 × 12 mm Si chip

contains three devices with e-beam-fabricated electrodes structures that will later be cut into

three STM-sized samples. Each device has two large metallic pads with dimensions of 0.5 × 1.5

mm for clamping them to the STM sample holder and biasing; these two pads connect to a center

piece by 2.75 mm long lines separately. (b) Zoomed-in view showing the detailed structure of

the center piece: two 500 × 150 μm pads are connected with the center interdigital electrodes

alternately. The size of the interdigital electrode arrays is 200 × 200 μm and this dimension will

contain hundreds of electrodes while maintaining the feasibility to locate an STM probe. (c) A

further zoomed-in view showing the dimensions of the parallel long electrodes with a length of

198 μm and width of 1 μm. There is a 2 μm gap between one end of the electrode and the large

pad, which ensures no conductive path between adjacent electrodes. The gap size between

adjacent electrodes varies from 0.25 to 1 μm depending on the difficulty of fabrication.

a

b

c

73

Figure 4.4. Process flow showing the e-beam lithography fabrication steps: 1. A layer of

photoresist is spin-coated onto the Si chip. 2. The photoresist is patterned by e-beam lithography.

3. A layer of metal is deposited by e-beam evaporation. 4. Unwanted metal is removed by lifting

off the photoresist in solvent.

1. Resist

4. Remove resist

2. E-beam lithography

3. Metal deposition

74

Figure 4.5. SEM images of the prefabricated large electrodes with the adjacent electrode gap size

of 250 nm: (a) Large scan on the center electrodes: the image contrast between the topmost part

and the small portion at the bottom indicates that only the PMMA at the bottom has been lifted

off. (b and c) Zoomed-in images of the bottom portion showed that the PMMA tends to peel off

at the end of the electrode where the window size is larger compared to the small gap between

the electrodes. (d) Zooming-in more onto those opened gaps shows PMMA residue stuck to one

side of the deposited metal. (e and f) SEM images of the electrodes after further dipping and

sonication in acetone. (e) Large scan shows more PMMA has been dissolved compare to 4.5a but

not completely. (f) Zoomed-in image showing the PMMA residue inside the gap was not

dissolved easily.

a b c

d e f

75

Figure 4.6. SEM and AFM images of the fabricated large electrode with the gap size of 1 μm. (a)

SEM image of the electrodes after e-beam fabrication shows all PMMA has been completely

lifted off and no residue left inside the gap. (b) AFM image shows flat surface topography both

on the deposited metal (5 nm Ti and 10 nm Pt) and the exposed Si. The white dots are surface

contaminants that can be cleaned by solvent. The height profile extracted confirmed the height of

the metal to be 15 to 20 nm and a width of ~1.2 μm.

Figure 4.7. SEM images of the fabricated electrodes before (a) and after (b) the chip being

degassed at 600 °C and followed by flashing at 1200 °C for 30 s: after high-temperature

annealing, the deposited metal was damaged and the surface became very rough, and the

interdigital structure was completely destroyed.

2 µm

38.1 nm

−6.2

a b

a b

76

Figure 4.8. STM images of the bare Si(100) surface after 900 °C flash. (a and b) 50 × 50 nm

STM topographic images of the Si surface after degassing at 600 °C overnight and followed by a

900 °C flash for 2 mins. Large square-shaped SiC islands cover much of the surface, indicating

that the surface was badly contaminated. The surface is very rough and scans larger than 50 nm2

were hard to obtain; the height of the island varies, while some are too tall to be scanned without

a crushing the tip. Scan parameters: I = 0.1 nA, V = −2 V. (c and d) STM topographic images of

the Si surface with a pre-treatment of UV ozone for 10 mins and then degassed at 600 °C

followed by flashing at 900 °C. No SiC islands are found on the surface; Si terraces (c) and

dimer rows (d) can be clearly seen with very minimal surface contamination. Scan parameters: I

= 0.5 nA, V = −2 V.

c

b a

d

77

Figure 4.9. AFM images of 20 nm Ti evaporated onto Si. (a) 15 × 5 μm AFM image of the Ti

surface upon evaporation; surface is fairly flat. (b) Zoomed-in image of (a). (c and d) AFM and

zoomed-in images of 20 nm Ti after degassing at 600 °C in UHV overnight with their height

profiles shown below. The surface became rougher after heating, small islands started to form

and there are vacancies between the islands. (e) AFM image after further annealing in UHV at

900 °C for 2 mins: larger and taller islands with similar sizes formed at the surface with fewer

vacancies in between. The surface got rougher with the height of those islands varying greatly, as

shown in the height profile.

2 µm

13 nm

−4 2 µm

12 nm

−5

a b

c d e

78

Figure 4.10. AFM topographic images of 20 nm Ti plus 50 nm Pt evaporated onto Si. (a) Upon

evaporation the surface is very flat. (b) After degassing at 600 °C and flashing at 900 °C, the

surface became rougher and small islands formed. (c) Zoomed-in image of (b). The size of those

islands varies, and there are vacancies in between. (d) Height profile extracted from (b) showing

that the heights of the islands are similar but the lateral size varies.

4.8 References

1. Bai, J. & Huang, Y. Fabrication and electrical properties of graphene nanoribbons. Mater.

Sci. Eng. R 70, 341–353 (2010).

2. Dutta, S. & Pati, S. K. Novel properties of graphene nanoribbons: A review. J. Mater.

Chem. 20, 8207 (2010).

3. Castro Neto, A. H., Peres, N. M. R., Novoselov, K. S. & Geim, A. K. The electronic

properties of graphene. Rev. Mod. Phys. 81, 109–162 (2009).

4. Marmolejo-Tejada, J. M. & Velasco-Medina, J. Review on graphene nanoribbon devices

for logic applications. Microelectronics J. 48, 18–38 (2016).

1 µm

315 nm

−19

2 µm

41.7 nm

−36 .0

a b

c d

79

5. Talirz, L. et al. On-surface synthesis and characterization of 9-atom wide armchair

graphene nanoribbons. ACS Nano 11, 1380–1388 (2017).

6. Vo, T. H. et al. Large-scale solution synthesis of narrow graphene nanoribbons. Nat.

Commun. 5, 1–8 (2014).

7. Talirz, L., Ruffieux, P. & Fasel, R. On-surface synthesis of atomically precise graphene

nanoribbons. Adv. Mater. 6222–6231 (2016). doi:10.1002/adma.201505738

8. Hamam, A. M. M., Schmidt, M. E., Muruganathan, M., Suzuki, S. & Mizuta, H. Sub-10

nm graphene nano-ribbon tunnel field-effect transistor. Carbon. 126, 588–593 (2018).

9. Hwang, W. S. et al. Graphene nanoribbon field-effect transistors on wafer-scale epitaxial

graphene on SiC substrates. APL Mater. 3, 1 (2015).

10. Hwang, W. S. et al. Transport properties of graphene nanoribbon transistors on chemical-

vapor-deposition grown wafer-scale graphene transport properties of graphene nanoribbon

transistors on chemical-vapor-deposition grown wafer-scale graphene. Appl. Phys. lett.

100, 203107 (2012).

11. Ye, W. et al. Direct writing of sub-5 nm metals on carbon nanotubes and graphene using a

UHV-STM. 8, 825–826 (2009).

12. Ye, W. et al. Direct writing of sub-5 nm hafnium diboride metallic nanostructures. ACS

Nano 4, 6818–6824 (2010).

13. Tung, R. T. The physics and chemistry of the Schottky barrier height. Appl. Phys. Rev. 1,

011304 (2014).

14. McDonald, D. et al. A History of Platinum and its Allied Metals. (Johnson Matthey Plc,

1982).

15. Seymour, R. J. & O´Farrelly, J. I. Platinum-group metals. In Kirk-Othmer Encyclopedia of

Chemical Technology (John Wiley & Sons, Inc., 2001).

doi:10.1002/0471238961.1612012019052513.a01.pub2

16. Lütjering, G. & Williams, J. C. Titanium. (Springer, 2007).

17. National Materials Advisory Board. Titanium: Past, Present, and Future. (National

Academies Press, 1983).

18. CHEN, L.J. Metal silicides: An integral part of microelectronics. TMS 24–31 (2005).

Available at: http://www.tms.org/pubs/journals/jom/0509/chen-0509.html. (Accessed: 18

April 2018)

19. The properties of thin platinum silicide films. Platin. Met. Rev., 20, 9 (1976).

80

20. Stephenson, A. W. & Welland, M. E. Scanning tunneling microscope investigation of the

growth morphology of titanium silicide on Si(111) substrates. J. Appl. Phys. (1995).

doi:10.1063/1.359747

21. Palermo, V., Buchanan, M., Bezinger, A. & Wolkow, R. A. Lateral diffusion of titanium

disilicide as a route to contacting hybrid Si/organic nanostructures. Appl. Phys. Lett. 81,

3636–3638 (2002).

22. Bucher, E. et al. Work function and barrier heights of transition metal silicides. Appl.

Phys. A Solids Surfaces (1986). doi:10.1007/BF00616480

23. Alptekin, E., Ozturk, M. C. & Misra, V. Tuning of the platinum silicide Schottky barrier

height on n-type silicon by sulfur segregation. IEEE Electron Device Lett. 30, 331–333

(2009).

24. Tovr, P. A. et al. Silicide formation correlated with surface resistivity measurements.

Phys. Stat. Sol. 36, 217 (1976).

25. Interconnections: Silicides. Available at:

https://web.stanford.edu/class/ee311/NOTES/Silicides.pdf. (Accessed: 14 June 2018)

26. Ultraviolet-ozone surface treatment. Three Bond Technical News. 3 (1987).

27. Hubacek, J. S. Scanning tunneling microscopy of silicon(100) 2 × 1. Ph. D. dissertation,

University of Illinois at Urbana-Champaign, Urbana, IL (1992).

81

CHAPTER 5

SUMMARY AND FUTURE WORK

To summarize, we have demonstrated that the DCT method is applicable for depositing

solution-synthesized GNRs onto III-V semiconducting substrates, a result that has never been

accomplished before. STM characterization of wGNRs, which have never been previously

studied, exfoliated onto InAs and InSb illustrates how substrate choice plays a role in the

observed GNR electronic structure. STM of two additional types of GNRs elucidated how

structural modification changed their properties and behaviors.

We conducted detailed STM and STS characterizations of wGNRs on InAs(110) and

InSb(110). High-resolution images confirmed the geometry of the ribbons and revealed the 3-D

orbital shapes of the LDOS. Band structure and distribution of the LDOS were determined by

STS, and results showed great consistency with computational simulations. The weak coupling

between the wGNRs and the InAs surface enabled probing of the intrinsic properties of GNRs

using STM. For the InSb substrate, however, a strong coupling between the ribbon and substrate

states was observed. Current tunneling to the substrate had an influence on STS measurements

and thus made distinguishing the electronic states of the GNR from those of the substrate

somewhat difficult. Compared to metal substrates, III-V semiconducting substrates were shown

to have a weak screening effect and therefore are better platforms for discovering the intrinsic

properties of GNRs. Further computational modeling with the substrate included will provide a

82

better understanding on how substrates play a role in affecting the electronic structure of the

GNR.

We have also explored two additional GNR geometries, the extended chevron GNR

(eGNR) and the nitrogen-doped GNR (8N-GNR) DCTed on InAs(110). When a chevron GNR

was modified by adding extra benzene rings at the elbow, STS measurement showed that such a

lateral extension in the eGNR width caused a decrease in its bandgap by 0.1 eV.1 Different from

undoped chevron GNRs, the 8N-GNR is highly mobile on the InAs substrate. Electron beams

from the STM probe could break the hydrogen bonds between adjacent ribbons and thus separate

the bonded ribbons. STS on isolated 8N-GNRs revealed a bandgap of 2.6 eV, which is close to

that of the regular chevron GNR. Changing a ribbon´s geometry is most likely to cause a change

in the bandgap, whereas doping the ribbon by dopant substitution will not alter its band structure

but will change other properties, such as creating active sites for hydrogen bonding.

Since the semiconducting substrates were shown more suitable for GNR characterization,

an experiment that is worth trying could be depositing the ribbon´s precursor molecules onto

those semiconducting substrates and then using a metallic STM probe to facilitate the synthesis

process, thus achieving a precise positioning of the synthesized GNRs. With different molecules

deposited, complex ribbon heterostructures could be realized.

With the feasibility of manipulating the GNRs with the STM probe demonstrated, some

interesting experiments could be conducted. For example, by depositing the nitrogen-doped

GNRs on the H-Si substrate, one could selectively de-passivate the hydrogen atoms and create

dangling bonds at the surface,2 which may create a site that preferentially bonds with the

nitrogen atoms at the ribbon´s edge. With a well-controlled de-passivated pattern, one might

achieve a global positioning and alignment of GNRs.

83

With the ability to locate isolated GNRs on any STM-compatible substrate established,

we described a scheme of writing HfB2 nanocontacts on GNRs by STM tip-assisted deposition3

and fabricated the micrometer-sized metallic electrode arrays. With those prefabricated Pt and Ti

electrodes on Si, transport measurement on GNRs can be realized after connecting them with the

written HfB2 lines.

Future work includes connecting the GNRs with the large electrode using tip-assisted

deposition of HfB2 and conducting the transport measurements. To further eliminate the issue of

current leakage from the GNR to the substrate, a 2-D insulating layer such as boron nitride could

be intervened between the substrate and the electrode. Such a thin insulating layer could hinder

the current flow to the substrate, enhance the GNR´s carrier mobility and still be STM

compatible.4

5.1 References

1. Mehdi Pour, M. et al. Laterally extended atomically precise graphene nanoribbons with

improved electrical conductivity for efficient gas sensing. Nat. Commun. 8, 820 (2017).

2. Lyding, J. W., Shen, T. C., Hubacek, J. S., Tucker, J. R. & Abeln, G. C. Nanoscale

patterning and oxidation of H-passivated Si(100)-2 × 1 surfaces with an ultrahigh vacuum

scanning tunneling microscope. Appl. Phys. Lett. 64, 2010–2012 (1994).

3. Ye, W. et al. Direct writing of sub-5 nm hafnium diboride metallic nanostructures. ACS

Nano 4, 6818–6824 (2010).

4. Dean, C. R. et al. Boron nitride substrates for high-quality graphene electronics. Nat.

Nanotechnol. 5, 722–726 (2010).

84

APPENDIX A

DESIGN OF A LOW-TEMPERATURE STM

SYSTEM

A.1 System Design

This section reports continuing work on designing and constructing a (helium-based)

closed-cycle refrigerator cooled STM capable of operating at temperatures down to 10 kelvin.

The design concept and previous work done were recorded in He1 and Liu2,3.

This low-temperature STM uses a two-box design for thermal isolation. During a sample

walking process, the side windows of the two boxes need to be kept open for gaining a view on

the sample-tip distance. We implemented a dual-magnet design to simplify the opening and

locking mechanism. One small magnet is embedded inside the upper frame of the window and

the other is inside the lower ˝handle.˝ When bringing the handle up towards the frame to open the

window, the magnetic force between the two magnets is strong enough to hold the weight of the

two windows and keeps them open (as shown in Figure A.1a). When closing the windows, just a

slight force is required to separate the ˝handle˝ from the frame, and gravity keeps the window

shut (as shown in Figure A.1b). A wobble stick installed at the front flange of the chamber

opens and closes the window.

We also use the same concept for keeping the front doors of the inner and outer boxes

shut tight during scans. A small magnet is embedded inside a small box in the upper right corner

of the outside door, and the other magnet is on the door´s frame. These two magnets keep the

85

doors closed by magnetic force and can be separated easily during sample loading (as shown in

Figure A.2). The doors are also manipulated by the same wobble stick used to load tips and

samples into the STM.

For locking the scanner and vibrational damping, we implemented a rack and pinion

system consisting of two magnets bringing a heavy weight (which is attached to the scanner) up

and down while providing magnetic damping at the same time. Figure A.3a is the schematic

showing the scanner suspended by a heavy copper weight and attached to a spring. To provide

linear motion to the weight in the y-direction, we put two magnets above the top and below the

bottom of the copper weight, respectively. With these magnets attached to a rack, rotating a

pinion that is in contact with the rack can move them up and down. These magnets will be used

to push the weight down during cooling and sample loading. They can also provide magnetic

damping during scans to eliminate low-frequency noise and minimize the thermal leakage at the

same time since they will not be touching the copper weight. In order to secure the rack so that it

only moves in the y-direction, we designed a guided rail that will be mounted inside the chamber

wall (as shown in Figure A.4). With four UHV-compatible bearings mounted onto a T-shaped

arm attached to the rack, the rack can move up and down smoothly but not in other directions.

Future work includes designing a way to provide good thermal contact between the

bottom of the scanner and the box for better cooling, as well as a method for monitoring the

position of the suspension rod to make sure it doesn´t touch the top side of the boxes.

86

A.2 Figures

Figure A.1: Photos of the side window. (a) Two small magnets are embedded inside the handle

and frame; gravity keeps the window shut. (b) As the window is lifted, the two magnets attract

each other, keeping the window open.

a b

87

Figure A.2: Photos of the front door. (a) Front view showing the small magnet embedded inside

a block that can be attached to the door using a screw. (b) Back view showing the other magnet

embedded inside the frame of the door. Door is kept tightly shut when the two magnets attract

each other.

88

Figure A.3; Schematics showing the vibrational isolation mechanism for the STM´s scanner. (a)

The STM scanner is suspended by a spring with a copper weight. (b) The motion of the weight

along with the scanner will be manipulated by a rack and pinion design; with two magnets

attached to the rack, they can push the copper weight up and down by rotating the pinion; the

magnets also provide additional vibrational damping to the scanner.

Figure A.4: Photos of the pieces designed for securing the rack. (a) Side view of the three

stainless steel pieces before assembly: left and right rails that will be attached to the inside

chamber wall; a T-shaped arm (which will be attached to the rack) has four bearings (which will

slide into the slot of the rails). (b) Top view of the pieces after assembly: there is just enough

space between the bearings and the rail for the arm to move in the y-direction only.

a b

Bearing × 4

T−shaped arm

Rails × 2

a b

89

A.3 References

1. K. T. He, Characterization of graphene-substrate interactions using scanning tunneling

microscopy, Ph.D. dissertation, University of Illinois at Urbana-Champaign, Urbana, IL

(2013).

2. X. Liu, Design of a low-temperature, ultrahigh-vacuum scanning tunneling microscope, B.S.

thesis, University of Illinois at Urbana-Champaign, Urbana, IL (2011).

3. X. Liu, Large-scale graphene transfer in ultra-high vacuum and design of a low-temperature,

ultra-high vacuum scanning tunneling microscope, M.S. thesis, University of Illinois at

Urbana-Champaign, Urbana, IL (2014).