146 research outputs found
Gold in graphene: in-plane adsorption and diffusion
We study the bonding and diffusion of Au in graphene vacancies using
density-functional theory. Energetics show that Au adsorbs preferably to double
vacancies, steadily in-plane with graphene. All diffusion barriers for the
complex of Au in double vacancy are above 4 eV, whereas the barriers for larger
vacancies are below 2 eV. Our results support the main results of a recent
experiment [Gan et al., Small 4, 587 (2008)], but suggest that the observed
diffusion mechanism is not thermally activated, but radiation-enhanced.Comment: 3 pages, 3 figure
Comparison of Raman spectra and vibrational density of states between graphene nanoribbons with different edges
Vibrational properties of graphene nanoribbons are examined with density
functional based tight-binding method and non-resonant bond polarization
theory. We show that the recently discovered reconstructed zigzag edge can be
identified from the emergence of high-energy vibrational mode due to strong
triple bonds at the edges. This mode is visible also in the Raman spectrum.
Total vibrational density of states of the reconstructed zigzag edge is
observed to resemble the vibrational density of states of armchair, rather than
zigzag, graphene nanoribbon. Edge-related vibrational states increase in energy
which corroborates increased ridigity of the reconstructed zigzag edge.Comment: 4 pages, 4 figure
Contact resistance in graphene-based devices
We report a systematic study of the contact resistance present at the
interface between a metal (Ti) and graphene layers of different, known
thickness. By comparing devices fabricated on 11 graphene flakes we demonstrate
that the contact resistance is quantitatively the same for single-, bi-, and
tri-layer graphene (), and is in all cases
independent of gate voltage and temperature. We argue that the observed
behavior is due to charge transfer from the metal, causing the Fermi level in
the graphene region under the contacts to shift far away from the charge
neutrality point
Cones, pringles, and grain boundary landscapes in graphene topology
A polycrystalline graphene consists of perfect domains tilted at angle
{\alpha} to each other and separated by the grain boundaries (GB). These nearly
one-dimensional regions consist in turn of elementary topological defects,
5-pentagons and 7-heptagons, often paired up into 5-7 dislocations. Energy
G({\alpha}) of GB computed for all range 0<={\alpha}<=Pi/3, shows a slightly
asymmetric behavior, reaching ~5 eV/nm in the middle, where the 5's and 7's
qualitatively reorganize in transition from nearly armchair to zigzag
interfaces. Analysis shows that 2-dimensional nature permits the off-plane
relaxation, unavailable in 3-dimensional materials, qualitatively reducing the
energy of defects on one hand while forming stable 3D-landsapes on the other.
Interestingly, while the GB display small off-plane elevation, the random
distributions of 5's and 7's create roughness which scales inversely with
defect concentration, h ~ n^(-1/2)Comment: 9 pages, 4 figure
Grain Boundaries in Graphene on SiC(000) Substrate
Grain boundaries in epitaxial graphene on the SiC(000) substrate are
studied using scanning tunneling microscopy and spectroscopy. All investigated
small-angle grain boundaries show pronounced out-of-plane buckling induced by
the strain fields of constituent dislocations. The ensemble of observations
allows to determine the critical misorientation angle of buckling transition
. Periodic structures are found among the flat
large-angle grain boundaries. In particular, the observed highly ordered grain boundary is assigned to the previously
proposed lowest formation energy structural motif composed of a continuous
chain of edge-sharing alternating pentagons and heptagons. This periodic grain
boundary defect is predicted to exhibit strong valley filtering of charge
carriers thus promising the practical realization of all-electric valleytronic
devices
Properties of Graphene: A Theoretical Perspective
In this review, we provide an in-depth description of the physics of
monolayer and bilayer graphene from a theorist's perspective. We discuss the
physical properties of graphene in an external magnetic field, reflecting the
chiral nature of the quasiparticles near the Dirac point with a Landau level at
zero energy. We address the unique integer quantum Hall effects, the role of
electron correlations, and the recent observation of the fractional quantum
Hall effect in the monolayer graphene. The quantum Hall effect in bilayer
graphene is fundamentally different from that of a monolayer, reflecting the
unique band structure of this system. The theory of transport in the absence of
an external magnetic field is discussed in detail, along with the role of
disorder studied in various theoretical models. We highlight the differences
and similarities between monolayer and bilayer graphene, and focus on
thermodynamic properties such as the compressibility, the plasmon spectra, the
weak localization correction, quantum Hall effect, and optical properties.
Confinement of electrons in graphene is nontrivial due to Klein tunneling. We
review various theoretical and experimental studies of quantum confined
structures made from graphene. The band structure of graphene nanoribbons and
the role of the sublattice symmetry, edge geometry and the size of the
nanoribbon on the electronic and magnetic properties are very active areas of
research, and a detailed review of these topics is presented. Also, the effects
of substrate interactions, adsorbed atoms, lattice defects and doping on the
band structure of finite-sized graphene systems are discussed. We also include
a brief description of graphane -- gapped material obtained from graphene by
attaching hydrogen atoms to each carbon atom in the lattice.Comment: 189 pages. submitted in Advances in Physic
The Hide-and-Seek of Grain Boundaries from Moire Pattern Fringe of Two-Dimensional Graphene
Grain boundaries (GBs) commonly exist in crystalline materials and affect various properties of materials. The facile identification of GBs is one of the significant requirements for systematical study of polycrystalline materials including recently emerging two-dimensional materials. Previous observations of GBs have been performed by various tools including high resolution transmission electron microscopy. However, a method to easily identify GBs, especially in the case of low-angle GBs, has not yet been well established. In this paper, we choose graphene bilayers with a GB as a model system and investigate the effects of interlayer rotations to the identification of GBs. We provide a critical condition between adjacent moire fringe spacings, which determines the possibility of GB recognition. In addition, for monolayer graphene with a grain boundary, we demonstrate that low-angle GBs can be distinguished easily by inducing moire patterns deliberately with an artificial reference overlayopen0
Electronic Structure and Bonding of Icosahedral Core-Shell Gold-Silver Nanoalloy Clusters Au_(144-x)Ag_x(SR)_60
Atomically precise thiolate-stabilized gold nanoclusters are currently of
interest for many cross-disciplinary applications in chemistry, physics and
molecular biology. Very recently, synthesis and electronic properties of
"nanoalloy" clusters Au_(144-x)Ag_x(SR)_60 were reported. Here, density
functional theory is used for electronic structure and bonding in
Au_(144-x)Ag_x(SR)_60 based on a structural model of the icosahedral
Au_144(SR)_60 that features a 114-atom metal core with 60 symmetry-equivalent
surface sites, and a protecting layer of 30 RSAuSR units. In the optimal
configuration the 60 surface sites of the core are occupied by silver in
Au_84Ag_60(SR)_60. Silver enhances the electron shell structure around the
Fermi level in the metal core, which predicts a structured absorption spectrum
around the onset (about 0.8 eV) of electronic metal-to-metal transitions. The
calculations also imply element-dependent absorption edges for Au(5d)
\rightarrow Au(6sp) and Ag(4d) \rightarrow Ag(5sp) interband transitions in the
"plasmonic" region, with their relative intensities controlled by the Ag/Au
mixing ratio.Comment: 4 figure
sp-Electron Magnetic Clusters with a Large Spin in Graphene
Motivated by recent experimental data (Sepioni, M. et al. Phys. Rev. Lett.
2010, 105, 207205), we have studied the possibility of forming magnetic
clusters with spin S> 1/2 on graphene by adsorption of hydrogen atoms or
hydroxyl groups. Migration of hydrogen atoms and hydroxyl groups on the surface
of graphene during the delamination of HOPG led to the formation of seven-atom
or seven-OH-group clusters with S=5/2 that were of a special interest. The
coincidence of symmetry of the clusters with the graphene lattice strengthens
the stability of the cluster. For (OH)7 clusters that were situated greater
than 3 nm from one another, the reconstruction barrier to a nonmagnetic
configuration was approximately 0.4 eV, whereas for H7 clusters, there was no
barrier and the high-spin state was unstable. Stability of the high-spin
clusters increased if they were formed on top of ripples. Exchange interactions
between the clusters were studied and we have shown that the ferromagnetic
state is improbable. The role of the chemical composition of the solvent used
for the delamination of graphite is discussed.Comment: 22 pages, 1 table, 4 figures. Minor changes, few refs added. Accepted
to ACS Nan
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