This project investigates, at the atomic level, the structural and electronic properties of nanostructures by means of a low temperature scanning tunnelling microscopy/spectroscopy in ultrahigh-vacuum environments (UHV-LTSTM). This allowed the study of these properties in a local way with atomic precision and ultimate energy resolution and also to modify the systems in a controlled way by direct manipulation using the STM tip. The main results achieved are described in the following: Impact of point defects in graphene Systems -1,5 -1,0 -0 ,5 0,0 0 ,5 1,0 1,5 0,2 0,4 0,6 V acancy -1,5 -1,0 -0 ,5 0,0 0 ,5 1,0 1,5 0,2 0,4 0,6 dI/dV (a.u ) V oltage (V ) G raphene/P t(111) -200 -100 0 100 200 0.5 1.0 LD O S vacancy d I/d V (a.u .) V (m V) LD O S grap h ite a) HOPG G/Pt(111) b) c) (a) Art illustration of the generation and STM analysis of individual vacancies in graphene layers. (b-c) 4K STM data showing atomically resolved images of C vacancies in graphene layers growth of graphite and Pt(111) and its impact in the electronic density of states of the graphene layers. How does the presence of single atomic defects modify the properties of materials? Such a general and fundamental question was addressed in this project for atomic vacancies in graphene systems, where the presence of such defects is expected to have a dramatic impact in its properties due to graphene's pure bidimensionality. Introducing vacancies in graphene-like systems by irradiation has been shown to be an efficient method to vary its mechanical behavior, tune its electronic properties and even to induce magnetism in otherwise non- magnetic samples. While the role played by these vacancies as single entities has been extensively addressed by theory, experimental data available refer to statistical properties of the whole heterogeneous collection of vacancies generated in the irradiation process. In this project we have overcome this limitation: we first created perfectly characterized single vacancies on graphene layers by Ar+ ion irradiation and then, using low temperature scanning tunneling microscopy (LT-STM), we individually investigated the impact of each of such vacancies in the electronic, structural and magnetic
Transcript
This project investigates, at the atomic level, the structural and electronic properties of nanostructures by means of a low temperature scanning tunnelling microscopy/spectroscopy in ultrahigh-vacuum environments (UHV-LTSTM). This allowed the study of these properties in a local way with atomic precision and ultimate energy resolution and also to modify the systems in a controlled way by direct manipulation using the STM tip.
The main results achieved are described in the following:
Impact of point defects in graphene Systems
-1,5 -1,0 -0,5 0,0 0,5 1,0 1,5
0,2
0,4
0,6
Vacancy
-1,5 -1,0 -0,5 0,0 0,5 1,0 1,5
0,2
0,4
0,6
dI/d
V (a
.u)
Voltage (V)
Graphene/Pt(111)
-200 -100 0 100 200
0.5
1.0
LDOS vacancy
dI/d
V (a
.u.)
V (mV)
LDOS graphitea) HOPG
G/Pt(111)
b)
c)
(a) Art illustration of the generation and STM analysis of individual vacancies in graphene layers. (b-c) 4K STM data showing atomically resolved images of C vacancies in graphene layers growth of graphite and Pt(111) and its impact in the electronic density of states of the graphene layers.
How does the presence of single atomic defects modify the properties of materials? Such a general and fundamental question was addressed in this project for atomic vacancies in graphene systems, where the presence of such defects is expected to have a dramatic impact in its properties due to graphene's pure bidimensionality. Introducing vacancies in graphene-like systems by irradiation has been shown to be an efficient method to vary its mechanical behavior, tune its electronic properties and even to induce magnetism in otherwise non-magnetic samples. While the role played by these vacancies as single entities has been extensively addressed by theory, experimental data available refer to statistical properties of the whole heterogeneous collection of vacancies generated in the irradiation process. In this project we have overcome this limitation: we first created perfectly characterized single vacancies on graphene layers by Ar+ ion irradiation and then, using low temperature scanning tunneling microscopy (LT-STM), we individually investigated the impact of each of such vacancies in the electronic, structural and magnetic properties of several graphene systems. Our work demonstrated that vacancies lead to a dramatic reduction of the electronic mobility and confirmed the creation of magnetic moments associated to the vacancies in this pure carbon material, indicating a suitable route to the creation of non-metallic, cheaper, lighter, and bio-compatible magnets.Thanks to this absolutely pioneering works, planned and developed since the beginning of the present ERG project, our group has become a world-leading reference in the field. Our first work, which was published in 2010 in Physical Review Letters, has received already more than hundred citations, being the 10th (out of 3512) most cited paper in PRL this year. Research performed in this field since then has continued with this successful tendency as reflected in the new publications in Physical Review Letters and Physical Review B and on paper in revision in Science. The works have been highlighted in Physical Review Focus, PhysOrg.com or Physics.
Epitaxial growth of graphene in inert substrates
Graphene/Au(111)Método
Schematic representation of the novel growth method together with STM images showing the formation of graphene on A(111) surfaces.
We reported a novel technique for growing graphene on relatively inert metals, consisting in the thermal decomposition of low energy ethylene ions irradiated on hot metal surfaces in ultrahigh vacuum. By this route, we have grown graphene monolayers on Cu(111) and, for the first time, on Au(111) surfaces. For both noble metal substrates, but particularly for Au(111), our scanning tunneling microscopy and spectroscopy measurements provide sound evidences of a very weak graphene–metal interaction.Published in Nano Letters and highlighted by popular media as El Mundo or 20 Minutos, two of the most read Spanish newspapers.
Electronic properties of epitaxial graphene in silicon carbide.q = 9.6�
a = 9.6� a = 6.4� a = 3.5� a = 1.4�
5 nm
-1.2 -0.8 -0.4 0.0 0.4 0.8 1.20.0
0.2
0.4
0.6
0.8
1.0 1.4°(max) 1.4°(min) 3.5° 6.4° 9.6°
dI/d
V (a
u)
Sample bias (V)
0 2 4 6 8 100.0
0.5
1.0
1.5
2.0
VHs
split
ting
(eV)
Rotation angle q (°)
3.514 7 2.4 1.7 1.4Moiré size
DE vHs
K1
DOS
K2
DK
q = 9.6�G
K1
K2
DK
a) b)
c)
d) e)
d)
-1.2 -0.8 -0.4 0.0 0.4 0.8 1.20.0
0.2
0.4
0.6
0.8
1.0
1.4°(max) 1.4°(min)
3.5° 6.4° 9.6°
dI/d
V (a
u)
Sample bias (V)a-b) Illustration of a moiré pattern arising from a rotation angle of 9.6° and the emergence of van Hove singularities as a consequence of the reciprocal space rotation. c) STM images of several MP with different rotation angle. d)Measure of the of van Hove singularities on the real graphene rotated layers.
Understanding the coupling of graphene with its local environment is absolutely critical to be able to integrate it in tomorrow's electronic devices. In this project, we show how the presence of the silicon carbide substrate affects the properties graphene layers.
In particular, we focused on the following graphene properties:Graphene pseudospin.Published in Physical Review B.Twisted Bilayers.Published in Physical Review Letters.
Superconducting properties of single nanoparticles electronically decoupled from the substrate.
We have shown how the superconducting energy gap can be enhanced by ~60% from its bulk value in superconducting nanoparticles of Sn which are at the limit of superconductivity for a zero dimensional system. Though the occurrence of these ‘shell effects’ which originates from quantum confinement had been predicted theoretically, our experiments on single, isolated Sn nanoparticles by a scanning tunneling microscope show for the first time in a real system that the superconducting energy gap is very sensitive on the particle size and shape and very small changes can cause large oscillations in its value, leading to huge enhancements. In addition, similar measurements on Pb nanoparticles where this effect is highly suppressed establishes the role of the superconducting coherence length on the quantum phenomena of shell effects. Our results have been also confirmed by theoretical calculations.The importance of the enhancement of the superconducting energy gap lies in the field of Material Science where the biggest technological challenge is to devise ways to enhance the superconducting transition temperature. Our results demonstrate that this mechanism may prove useful to boost superconductivity in conventional superconductors like the fullerides or the hexaborides which are known to show much higher Tc in bulk compared to the elemental superconductors.
Published in Nature Materials and highlighted in Nature NanotechnologyPublished in Physical Review B and highlighted as Editor’s Suggestion.
