1,712 research outputs found
Influence of Functional Groups on Charge Transport in Molecular Junctions
Using density functional theory (DFT), we analyze the influence of five
classes of functional groups, as exemplified by NO2, OCH3, CH3, CCl3, and I, on
the transport properties of a 1,4-benzenedithiolate (BDT) and
1,4-benzenediamine (BDA) molecular junction with gold electrodes. Our analysis
demonstrates how ideas from functional group chemistry may be used to engineer
a molecule's transport properties, as was shown experimentally and using a
semiempirical model for BDA [Nano Lett. 7, 502 (2007)]. In particular, we show
that the qualitative change in conductance due to a given functional group can
be predicted from its known electronic effect (whether it is pi/sigma
donating/withdrawing). However, the influence of functional groups on a
molecule's conductance is very weak, as was also found in the BDA experiments.
The calculated DFT conductances for the BDA species are five times larger than
the experimental values, but good agreement is obtained after correcting for
self-interaction and image charge effects.Comment: 6 pages, 3 figures, J. Chem. Phys (in press
Influence of O2 and N2 on the conductivity of carbon nanotube networks
We have performed experiments on single-wall carbon nanotube (SWNT) networks
and compared with density-functional theory (DFT) calculations to identify the
microscopic origin of the observed sensitivity of the network conductivity to
physisorbed O2 and N2. Previous DFT calculations of the transmission function
for isolated pristine SWNTs have found physisorbed molecules have little
influence on their conductivity. However, by calculating the four-terminal
transmission function of crossed SWNT junctions, we show that physisorbed O2
and N2 do affect the junction's conductance. This may be understood as an
increase in tunneling probability due to hopping via molecular orbitals. We
find the effect is substantially larger for O2 than for N2, and for
semiconducting rather than metallic SWNTs junctions, in agreement with
experiment.Comment: 6 pages, 5 figures, 1 tabl
Linear density response function in the projector-augmented wave method: Applications to solids, surfaces, and interfaces
We present an implementation of the linear density response function within
the projector-augmented wave (PAW) method with applications to the linear
optical and dielectric properties of both solids, surfaces, and interfaces. The
response function is represented in plane waves while the single-particle
eigenstates can be expanded on a real space grid or in atomic orbital basis for
increased efficiency. The exchange-correlation kernel is treated at the level
of the adiabatic local density approximation (ALDA) and crystal local field
effects are included. The calculated static and dynamical dielectric functions
of Si, C, SiC, AlP and GaAs compare well with previous calculations. While
optical properties of semiconductors, in particular excitonic effects, are
generally not well described by ALDA, we obtain excellent agreement with
experiments for the surface loss function of the Mg(0001) surface with plasmon
energies deviating by less than 0.2 eV. Finally, we apply the method to study
the influence of substrates on the plasmon excitations in graphene. On
SiC(0001), the long wavelength plasmons are significantly damped although
their energies remain almost unaltered. On Al(111) the plasmon is
completely quenched due to the coupling to the metal surface plasmon.Comment: 11 pages, 8 figures, articl
Renormalization of Optical Excitations in Molecules near a Metal Surface
The lowest electronic excitations of benzene and a set of donor-acceptor
molecular complexes are calculated for the gas phase and on the Al(111) surface
using the many-body Bethe-Salpeter equation (BSE). The energy of the
charge-transfer excitations obtained for the gas phase complexes are found to
be around 10% lower than the experimental values. When the molecules are placed
outside the surface, the enhanced screening from the metal reduces the exciton
binding energies by several eVs and the transition energies by up to 1 eV
depending on the size of the transition-generated dipole. As a striking
consequence we find that close to the metal surface the optical gap of benzene
can exceed its quasiparticle gap. A classical image charge model for the
screened Coulomb interaction can account for all these effects which, on the
other hand, are completely missed by standard time-dependent density functional
theory.Comment: 4 pages, 3 figures; revised versio
Unraveling the acoustic electron-phonon interaction in graphene
Using a first-principles approach we calculate the acoustic electron-phonon
couplings in graphene for the transverse (TA) and longitudinal (LA) acoustic
phonons. Analytic forms of the coupling matrix elements valid in the
long-wavelength limit are found to give an almost quantitative description of
the first-principles based matrix elements even at shorter wavelengths. Using
the analytic forms of the coupling matrix elements, we study the acoustic
phonon-limited carrier mobility for temperatures 0-200 K and high carrier
densities of 10^{12}-10^{13} cm^{-2}. We find that the intrinsic effective
acoustic deformation potential of graphene is \Xi_eff = 6.8 eV and that the
temperature dependence of the mobility \mu ~ T^{-\alpha} increases beyond an
\alpha = 4 dependence even in the absence of screening when the full coupling
matrix elements are considered. The large disagreement between our calculated
deformation potential and those extracted from experimental measurements (18-29
eV) indicates that additional or modified acoustic phonon-scattering mechanisms
are at play in experimental situations.Comment: 7 pages, 3 figure
Graphene on metals: a Van der Waals density functional study
We use density functional theory (DFT) with a recently developed van der
Waals density functional (vdW-DF) to study the adsorption of graphene on Al,
Cu, Ag, Au, Pt, Pd, Co and Ni(111) surfaces. In constrast to the local density
approximation (LDA) which predicts relatively strong binding for Ni,Co and Pd,
the vdW-DF predicts weak binding for all metals and metal-graphene distances in
the range 3.40-3.72 \AA. At these distances the graphene bandstructure as
calculated with DFT and the many-body GW method is basically unaffected
by the substrate, in particular there is no opening of a band gap at the
-point.Comment: 4 pages, 3 figure
Dispersive and Covalent Interactions Between Graphene and Metal Surfaces from the Random Phase Approximation
We calculate the potential energy surfaces for graphene adsorbed on Cu(111),
Ni(111), and Co(0001) using density functional theory and the Random Phase
Approximation (RPA). For these adsorption systems covalent and dispersive
interactions are equally important and while commonly used approximations for
exchange-correlation functionals give inadequate descriptions of either van der
Waals or chemical bonds, RPA accounts accurately for both. It is found that the
adsorption is a delicate competition between a weak chemisorption minimum close
to the surface and a physisorption minimum further from the surface
Computational design of chemical nanosensors: Transition metal doped single-walled carbon nanotubes
We present a general approach to the computational design of nanostructured
chemical sensors. The scheme is based on identification and calculation of
microscopic descriptors (design parameters) which are used as input to a
thermodynamic model to obtain the relevant macroscopic properties. In
particular, we consider the functionalization of a (6,6) metallic armchair
single-walled carbon nanotube (SWNT) by nine different 3d transition metal (TM)
atoms occupying three types of vacancies. For six gas molecules (N_{2}, O_{2},
H_{2}O, CO, NH_{3}, H_{2}S) we calculate the binding energy and change in
conductance due to adsorption on each of the 27 TM sites. For a given type of
TM functionalization, this allows us to obtain the equilibrium coverage and
change in conductance as a function of the partial pressure of the "target"
molecule in a background of atmospheric air. Specifically, we show how Ni and
Cu doped metallic (6,6) SWNTs may work as effective multifunctional sensors for
both CO and NH_{3}. In this way, the scheme presented allows one to obtain
macroscopic device characteristics and performance data for nanoscale (in this
case SWNT) based devices.Comment: Chapter 7 in "Chemical Sensors: Simulation and Modeling", Ghenadii
Korotcenkov (ed.), 47 pages, 22 figures, 10 table
An investigation of the formation and line properties of MgH in 3D hydrodynamical model stellar atmospheres
Studies of the isotopic composition of magnesium in cool stars have so far
relied upon the use of one-dimensional (1D) model atmospheres. Since the
isotopic ratios derived are based on asymmetries of optical MgH lines, it is
important to test the impact from other effects affecting line asymmetries,
like stellar convection. Here, we present a theoretical investigation of the
effects of including self-consistent modeling of convection. Using spectral
syntheses based on 3D hydrodynamical COBOLD models of dwarfs
(4000K, log(g),
) and giants (K,
log(g), ), we perform a detailed
analysis comparing 3D and 1D syntheses.
We describe the impact on the formation and behavior of MgH lines from using
3D models, and perform a qualitative assessment of the systematics introduced
by the use of 1D syntheses.
Using 3D model atmospheres significantly affect the strength of the MgH
lines, especially in dwarfs, with 1D syntheses requiring an abundance
correction of up to +0.69 dex largest for our 5000K models. The corrections are
correlated with and are also affected by the metallicity. The
shape of the strong MgH component in the 3D syntheses is poorly
reproduced in 1D. This results in 1D syntheses underestimating MgH by up
to percentage points and overestimating MgH by a similar amount
for dwarfs. This discrepancy increases with decreasing metallicity. MgH
is recovered relatively well, with the largest difference being
percentage points. The use of 3D for giants has less impact, due to smaller
differences in the atmospheric structure and a better reproduction of the line
shape in 1D.Comment: 20 pages, 15 figures, accepted for publication in Ap
Computational Design of Chemical Nanosensors: Metal Doped Carbon Nanotubes
We use computational screening to systematically investigate the use of
transition metal doped carbon nanotubes for chemical gas sensing. For a set of
relevant target molecules (CO, NH3, H2S) and the main components of air (N2,
O2, H2O), we calculate the binding energy and change in conductance upon
adsorption on a metal atom occupying a vacancy of a (6,6) carbon nanotube.
Based on these descriptors, we identify the most promising dopant candidates
for detection of a given target molecule. From the fractional coverage of the
metal sites in thermal equilibrium with air, we estimate the change in the
nanotube resistance per doping site as a function of the target molecule
concentration assuming charge transport in the diffusive regime. Our analysis
points to Ni-doped nanotubes as candidates for CO sensors working under typical
atmospheric conditions
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