3,466 research outputs found
Oxygen reduction activity on perovskite oxide surfaces: a comparative first-principle study of LaMnO, LaFeO and LaCrO
The understanding of oxygen reduction reaction (ORR) activity on perovskite
oxide surfaces is essential for promising future fuel cell applications. We
report a comparative study of ORR mechanisms on LaO (=Mn, Fe, Cr)
surfaces by first-principles calculations based on density functional theory
(DFT). Results obtained from varied DFT methods such as generalized gradient
approximation(GGA), GGA+ and the hybrid Hartree-Fock density functional
method are reported for comparative purposes. We find that the results
calculated from hybrid-functional method suggest that the order of ORR activity
is LaMnO LaCrO LaFeO, which is in better agreement with
recent experimental results (Suntivich \textit{et al.}, Nature Chemistry 3, 546
(2011)) than those using the GGA or GGA+ method.Comment: submitte
Does Silicene on Ag(111) Have a Dirac Cone?
We investigate the currently debated issue of the existence of the Dirac cone
in silicene on an Ag(111) surface, using first-principles calculations based on
density functional theory to obtain the band structure. By unfolding the band
structure in the Brillouin zone of a supercell to that of a primitive cell,
followed by projecting onto Ag and silicene subsystems, we demonstrate that the
Dirac cone in silicene on Ag(111) is destroyed. Our results clearly indicate
that the linear dispersions observed in both angular-resolved photoemission
spectroscopy (ARPES) [P. Vogt et al, Phys. Rev. Lett. 108, 155501 (2012)] and
scanning tunneling spectroscopy (STS) [L. Chen et al, Phys. Rev. Lett. 109,
056804 (2012)] come from the Ag substrate and not from silicene.Comment: 5 pages, 3 figure
First-principles Simulations of a Graphene Based Field-Effect Transistor
We improvise a novel approach to carry out first-principles simulations of
graphene-based vertical field effect tunneling transistors that consist of a
graphene{\it h}-BNgraphene multilayer structure. Within the density
functional theory framework, we exploit the effective screening medium (ESM)
method to properly treat boundary conditions for electrostatic potentials and
investigate the effect of gate voltage. The distribution of free carriers and
the band structure of both top and bottom graphene layers are calculated
self-consistently. The dielectric properties of {\it h}-BN thin films
sandwiched between graphene layers are computed layer-by-layer following the
theory of microscopic permittivity. We find that the permittivities of BN
layers are very close to that of crystalline {\it h}-BN. The effect of
interface with graphene on the dielectric properties of {\it h}-BN is weak,
according to an analysis on the interface charge redistribution.Comment: 6 pages, 6 figure
Electron Transport Through Ag-Silicene-Ag Junctions
For several years the electronic structure properties of the novel
two-dimensional system silicene have been studied extensively. Electron
transport across metal-silicence junctions, however, remains relatively
unexplored. To address this issue, we developed and implemented a theoretical
framework that utilizes the tight-binding Fisher-Lee relation to span
non-equilibrium Green's function (NEGF) techniques, the scattering method, and
semiclassical Boltzmann transport theory. Within this hybrid quantum-classical,
two-scale framework, we calculated transmission and reflection coefficients of
monolayer and bilayer Ag-silicene-Ag junctions using the NEGF method in
conjunction with density functional theory; derived and calculated the group
velocities; and computed resistance using the semi-classical Boltzmann
equation. We found that resistances of these junctions are {} 0.08 \fom
for monolayer silicene junctions and {} 0.3 \fom for bilayer ones,
factors of 8 and 2, respectively, smaller than Sharvin resistances
estimated via the Landauer formalism.Comment: 5 pages, 4 figure
Using Light-Switching Molecules to Modulate Charge Mobility in a Quantum Dot Array
We have studied the electron hopping in a two-CdSe quantum dot system linked
by an azobenzene-derived light-switching molecule. This system can be
considered as a prototype of a QD supercrystal. Following the computational
strategies given in our recent work [Chu et al. J. Phys. Chem. C 115, 21409
(2011)], we have investigated the effects of molecular attachment, molecular
isomer (trans and cis) and QD size on electron hopping rate using Marcus
theory. Our results indicate that molecular attachment has a large impact on
the system for both isomers. In the most energetically favorable attachment,
the cis isomer provides significantly greater coupling between the two QDs and
hence the electron hopping rate is greater compared to the trans isomer. As a
result, the carrier mobility of the QD array in the low carrier density, weak
external electric field regime is several orders of magnitude higher in the cis
compared to the trans configuration. This is the first demonstration of
mobility modulation using QDs and azobenzene that could lead to a new type of
switching device.Comment: 8 pages, 3 figure
Dynamics of social contagions with memory of non-redundant information
A key ingredient in social contagion dynamics is reinforcement, as adopting a
certain social behavior requires verification of its credibility and
legitimacy. Memory of non-redundant information plays an important role in
reinforcement, which so far has eluded theoretical analysis. We first propose a
general social contagion model with reinforcement derived from non-redundant
information memory. Then, we develop a unified edge-based compartmental theory
to analyze this model, and a remarkable agreement with numerics is obtained on
some specific models. Using a spreading threshold model as a specific example
to understand the memory effect, in which each individual adopts a social
behavior only when the cumulative pieces of information that the individual
received from his/her neighbors exceeds an adoption threshold. Through analysis
and numerical simulations, we find that the memory characteristic markedly
affects the dynamics as quantified by the final adoption size. Strikingly, we
uncover a transition phenomenon in which the dependence of the final adoption
size on some key parameters, such as the transmission probability, can change
from being discontinuous to being continuous. The transition can be triggered
by proper parameters and structural perturbations to the system, such as
decreasing individuals' adoption threshold, increasing initial seed size, or
enhancing the network heterogeneity.Comment: 13 pages, 9 figure
Electronic and transport properties of azobenzene monolayer junctions as molecular switches
We investigate from first-principles the change in transport properties of a
two-dimensional azobenzene monolayer sandwiched between two Au electrodes that
undergoes molecular switching. We focus on transport differences between a
chemisorbed and physisorbed top monolayer-electrode contact. The conductance of
the monolayer junction with a chemisorbed top contact is higher in trans
configuration, in agreement with the previous theoretical predictions of
one-dimensional single molecule junctions. However, with a physisorbed top
contact, the "ON" state with larger conductance is associated with the cis
configuration due to a reduced effective tunneling pathway by switching from
trans to cis, which successfully explains recently experimental measurements of
azobenzene monolayer junctions. A simple model is developed to explain electron
transmission across subsystems in the molecular junction. We also discuss the
effects of monolayer packing density, molecule tilt angle, and contact geometry
on the calculated transmission functions. In particular, we find that a
tip-like contact with chemisorption significantly affects the electric current
through the cis monolayer, leading to highly asymmetric current-voltage
characteristics as well as large negative differential resistance behavior.Comment: 10 pages, 11 figures, publishe
Self-Consistent Effective Hamiltonian Theory for Fermionic Many Body Systems
Using a separable many-body variational wavefunction, we formulate a
self-consistent effective Hamiltonian theory for fermionic many-body system.
The theory is applied to the two-dimensional Hubbard model as an example to
demonstrate its capability and computational effectiveness. Most remarkably for
the Hubbard model in 2-d, a highly unconventional quadruple-fermion
non-Cooper-pair order parameter is discovered
Electronic resistances of multilayered two-dimensional crystal junctions
We carry out a layer-by-layer investigation to understand electron transport
across metal-insulator-metal junctions. Interfacial structures of junctions
were studied and characterized using first-principles density functional theory
within the generalized gradient approximation. We found that as a function of
the number of crystal layers the calculated transmission coefficients of
multilayer silicene junctions decay much slower than for BN-based junctions We
revisited the semiclassical Boltzmann theory of electronic transport and
applied to multilayer silicene and BN-based junctions. The calculated
resistance in the high-transmission regime is smaller than that provided by the
Landauer formula. As the thickness of the barrier increases, results from the
Boltzmann and the Landauer formulae converge. We provide a upper limit in the
transmission coefficient below which, the Landauer method becomes valid.
Quantitatively, when the transmission coefficient is lower than
per channel, the error introduced by the Landauer formula for calculating the
resistance is negligible. In addition, we found that the resistance of a
junction is not entirely determined by the averaged transmission, but also by
the distribution of the transmission over the first Brillouin zone.Comment: 11 pages, 7 figure
Tunneling Field-Effect Junctions with WS barrier
Transition metal dichalcogenides (TMDCs), with their two-dimensional
structures and sizable bandgaps, are good candidates for barrier materials in
tunneling field-effect transistor (TFET) formed from atomic precision vertical
stacks of graphene and insulating crystals of a few atomic layers in thickness.
We report first-principles study of the electronic properties of the
Graphene/WS/Graphene sandwich structure revealing strong interface effects
on dielectric properties and predicting a high ON/OFF ratio with an appropriate
WS thickness and a suitable range of the gate voltage. Both the band
spin-orbit coupling splitting and the dielectric constant of the WS layer
depend on its thickness when in contact with the graphene electrodes,
indicating strong influence from graphene across the interfaces. The dielectric
constant is significantly reduced from the bulk WS value. The effective
barrier height varies with WS thickness and can be tuned by a gate voltage.
These results are critical for future nanoelectronic device designs.Comment: 18 pages, 5 figure
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