617 research outputs found
Screening effects in a density functional theory based description of molecular junctions in the Coulomb blockade regime
We recently introduced a method based on density functional theory (DFT) and
non-equilibrium Green's function techniques (NEGF) for calculating the addition
energies of single molecule nano-junctions in the Coulomb blockade (CB) regime.
Here we apply this approach to benzene molecules lying parallel and at various
distances from two aluminum fcc (111) surfaces, and discuss the distance
dependence in our calculations in terms of electrostatic screening effects. The
addition energies near the surface are reduced by about a factor of two, which
is comparable to previously reported calculations employing a computationally
far more demanding quasi-particle description
Towards a theoretical description of molecular junctions in the Coulomb blockade regime based on density functional theory
Non-equilibrium Greens function techniques (NEGF) combined with Density
Functional Theory (DFT) calculations have become a standard tool for the
description of electron transport through single molecule nano-junctions in the
coherent tunneling regime. However, the applicability of these methods for
transport in the Coulomb blockade (CB) regime is still under debate. We present
here NEGF-DFT calculations performed on simple model systems in the presence of
an effective gate potential. The results show that: i) the CB addition energies
can be predicted with such an approach with reasonable accuracy; ii) neither
the magnitude of the Kohn-Sham gap nor the lack of a derivative discontinuity
in the exchange-correlation functional represent a problem for this purpose
A multideterminant assessment of mean field methods for the description of electron transfer in the weak coupling regime
Multideterminant calculations have been performed on model systems to
emphasize the role of many-body effects in the general description of charge
quantization experiments. We show numerically and derive analytically that a
closed-shell ansatz, the usual ingredient of mean-field methods, does not
properly describe the step-like electron transfer characteristic in weakly
coupled systems. With the multideterminant results as a benchmark, we have
evaluated the performance of common ab initio mean field techniques, such as
Hartree Fock (HF) and Density Functional Theory (DFT) with local and hybrid
exchange correlation functionals, with a special focus on spin-polarization
effects. For HF and hybrid DFT, a qualitatively correct open-shell solution
with distinct steps in the electron transfer behaviour can be obtained with a
spin-unrestricted (i.e., spin-polarized) ansatz though this solution differs
quantitatively from the multideterminant reference. We also discuss the
relationship between the electronic eigenvalue gap and the onset of charge
transfer for both HF and DFT and relate our findings to recently proposed
practical schemes for calculating the addition energies in the Coulomb blockade
regime for single molecule junctions from closed-shell DFT within the local
density approximation
Electric-Field-Induced Mott Insulating States in Organic Field-Effect Transistors
We consider the possibility that the electrons injected into organic
field-effect transistors are strongly correlated. A single layer of acenes can
be modelled by a Hubbard Hamiltonian similar to that used for the
kappa-(BEDT-TTF)(2)X family of organic superconductors. The injected electrons
do not necessarily undergo a transition to a Mott insulator state as they would
in bulk crystals when the system is half-filled. We calculate the fillings
needed for obtaining insulating states in the framework of the slave-boson
theory and in the limit of large Hubbard repulsion, U. We also suggest that
these Mott states are unstable above some critical interlayer coupling or
long-range Coulomb interaction.Comment: 9 pages, 7 figure
Tuning of the Photovoltaic Parameters of Molecular Donors by Covalent Bridging
The synthesis of donor-acceptor molecules involving triarylamines and dicyanovinyl blocks is described. Optical and electrochemical results show that rigidification of the acceptor part of the molecule by a covalent bridge leads to a ca. 0.20 eV increase of the band gap due to a parallel increase of the lowest unoccupied molecular orbital level. A preliminary evaluation of these compounds as donor materials in organic solar cells shows that although this structural modification reduces the light-harvesting properties of the donor molecule, it nevertheless induces an increase of the efficiency of the resulting solar cells due to a simultaneous improvement of the open-circuit voltage and fill factor
Conjugation-Length Dependence of Spin-Dependent Exciton Formation Rates in Pi-Conjugated Oligomers and Polymers
We have measured the ratio, r = of the formation cross
section, of singlet () and triplet () excitons
from oppositely charged polarons in a large variety of -conjugated
oligomer and polymer films, using the photoinduced absorption and optically
detected magnetic resonance spectroscopies. The ratio r is directly related to
the singlet exciton yield, which in turn determines the maximum
electroluminescence quantum efficiency in organic light emitting diodes (OLED).
We discovered that r increases with the conjugation length, CL; in fact a
universal dependence exists in which depends linearly on ,
irrespective of the chain backbone structure. These results indicate that
-conjugated polymers have a clear advantage over small molecules in OLED
applications.Comment: 5 pages, 4 figure
Switching the Electronic Properties of ZnO Surfaces with Negative T Type Photochromic Pyridyl dihydropyrene Layers and Impact of Fermi Level Pinning
Electronic polarization in pentacene crystals and thin films
Electronic polarization is evaluated in pentacene crystals and in thin films
on a metallic substrate using a self-consistent method for computing charge
redistribution in non-overlapping molecules. The optical dielectric constant
and its principal axes are reported for a neutral crystal. The polarization
energies P+ and P- of a cation and anion at infinite separation are found for
both molecules in the crystal's unit cell in the bulk, at the surface, and at
the organic-metal interface of a film of N molecular layers. We find that a
single pentacene layer with herring-bone packing provides a screening
environment approaching the bulk. The polarization contribution to the
transport gap P=(P+)+(P-), which is 2.01 eV in the bulk, decreases and
increases by only ~ 10% at surfaces and interfaces, respectively. We also
compute the polarization energy of charge-transfer (CT) states with fixed
separation between anion and cation, and compare to electroabsorption data and
to submolecular calculations. Electronic polarization of ~ 1 eV per charge has
a major role for transport in organic molecular systems with limited overlap.Comment: 10 revtex pages, 6 PS figures embedde
Optical absorption in boron clusters B and B : A first principles configuration interaction approach
The linear optical absorption spectra in neutral boron cluster B and
cationic B are calculated using a first principles correlated
electron approach. The geometries of several low-lying isomers of these
clusters were optimized at the coupled-cluster singles doubles (CCSD) level of
theory. With these optimized ground-state geometries, excited states of
different isomers were computed using the singles configuration-interaction
(SCI) approach. The many body wavefunctions of various excited states have been
analysed and the nature of optical excitation involved are found to be of
collective, plasmonic type.Comment: 22 pages, 38 figures. An invited article submitted to European
Physical Journal D. This work was presented in the International Symposium on
Small Particles and Inorganic Clusters - XVI, held in Leuven, Belgiu
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