374 research outputs found
Self-passivating edge reconstructions of graphene
Planar reconstruction patterns at the zigzag and armchair edges of graphene
were investigated with density functional theory. It was unexpectedly found
that the zigzag edge is metastable and a planar reconstruction spontaneously
takes place at room temperature. The reconstruction changes electronic
structure and self-passivates the edge with respect to adsorption of atomic
hydrogen from molecular atmosphere.Comment: 4 pages, 4 figures, 1 tabl
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
Efficient approach for simulating distorted materials
The operation principles of nanoscale devices are based upon both electronic
and mechanical properties of materials. Because these properties can be
coupled, they need to be investigated simultaneously. At this moment, however,
the electronic structure calculations with custom-made long-range mechanical
distortions are impossible, or expensive at best. Here we present a unified
formalism to solve exactly the electronic structures of nanomaterials with
versatile distortions. We illustrate the formalism by investigating twisted
armchair graphene nanoribbons with the least possible number of atoms. Apart
from enabling versatile material distortions, the formalism is capable of
reducing computational costs orders of magnitude in various areas of science
and engineering.Comment: 4 pages, 2 figures, 2 table
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
Point Group Symmetry Analysis of the Electronic Structure of Bare and Protected Metal Nanocrystals
The electronic structures of a variety of experimentally identified gold and
silver nanoclusters from 20 to 246 atoms, either unprotected or protected by
several types of ligands, are characterized by using point group specific
symmetry analysis. The delocalized electron states around the HOMO-LUMO energy
gap, originating from the metal s-electrons in the cluster core, show symmetry
characteristics according to the point group that describes best the atomic
arrangement of the core. This indicates strong effects of the lattice structure
and overall shape of the metal core to the electronic structure, which cannot
be captured by the conventional analysis based on identification of spherical
angular momentum shells in the superatom model. The symmetry analysis discussed
in this paper is free from any restrictions regarding shape or structure of the
metal core, and is shown to be superior to the conventional spherical harmonics
analysis for any symmetry that is lower than Ih. As an immediate application,
we also demonstrate that it is possible to reach considerable savings in
computational time by using the symmetry information inside a conventional
linear-response calculation for the optical absorption spectrum of the Ag55
cluster anion, without any loss in accuracy of the computed spectrum. Our work
demonstrates an efficient way to analyze the electronic structure of
non-spherical, but atomically ordered nanocrystals and ligand-protected
clusters with nanocrystal metal cores and it can be viewed as the
generalization of the superatom model demonstrated for spherical shapes ten
years ago (Walter et al., PNAS 2008, 105, 9157)
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
Effect of the disorder in graphene grain boundaries: A wave packet dynamics study
Chemical vapor deposition (CVD) on Cu foil is one of the most promising methods to produce graphene samples despite of introducing numerous grain boundaries into the perfect graphene lattice. A rich variety of GB structures can be realized experimentally by controlling the parameters in the CVD method. Grain boundaries contain non-hexagonal carbon rings (4, 5, 7, 8 membered rings) and vacancies in various ratios and arrangements. Using wave packet dynamic (WPD) simulations and tight-binding electronic structure calculations, we have studied the effect of the structure of GBs on the transport properties. Three model GBs with increasing disorder were created in the computer: a periodic 5-7 GB, a "serpentine" GB, and a disordered GB containing 4, 8 membered rings and vacancies. It was found that for small energies (E = EF ± 1 eV) the transmission decreases with increasing disorder. Four membered rings and vacancies are identified as the principal scattering centers. Revealing the connection between the properties of GBs and the CVD growth method may open new opportunities in the graphene based nanoelectronics. © 2013 Elsevier B.V. All rights reserved
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