427 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
Electron transfer through a single barrier inside a molecule: from strong to weak coupling
In all theoretical treatments of electron transport through single molecules
between two metal electrodes, a clear distinction has to be made between a
coherent transport regime with a strong coupling throughout the junction and a
Coulomb blockade regime in which the molecule is only weakly coupled to both
leads. The former case where the tunnelling barrier is considered to be
delocalized across the system can be well described with common mean-field
techniques based on density functional theory (DFT), while the latter case with
its two distinct barriers localized at the interfaces usually requires a
multideterminant description. There is a third scenario with just one barrier
localized inside the molecule which we investigate here using a variety of
quantum-chemical methods by studying partial charge shifts in biphenyl radical
ions induced by an electric field at different angles to modulate the coupling
and thereby the barrier within the -system. We find steps rounded off at
the edges in the charge versus field curves for weak and intermediate coupling,
whose accurate description requires a correct treatment of both exchange and
dynamical correlation effects is essential. We establish that DFT standard
functionals fail to reproduce this feature, while a long range corrected hybrid
functional fares much better, which makes it a reasonable choice for a proper
DFT-based transport description of such single barrier systemsComment: 8 pages, 4 figures; J. Chem. Phys., in print (2012
Molecular Mechanisms and Biomarkers of Skin Photocarcinogenesis
Skin cancer is the most commonly diagnosed malignancy in the United States and worldwide. While melanoma is the deadliest form of skin cancer, non-melanoma skin cancers, which include basal cell carcinoma and squamous cell carcinoma, are responsible for significant morbidity in millions of Americans each year. While numerous attempts have been made to reduce skin cancer risk factors related to ultraviolet radiation exposure, skin cancer incidence continues to rise. Improved understanding of the molecular pathways underlying skin cancer pathogenesis has led to the investigation of new approaches to skin cancer prevention. In particular, the search for ultraviolet radiation associated biomarkers of skin cancer has become a rapidly expanding and promising area of research. Advances in genetic sequencing have facilitated the discovery of novel biomarkers, which have the potential to profoundly improve patient care. Here we will review the molecular genetics of skin cancer and analyze the existing literature of proposed biomarkers for potential use in skin cancer diagnosis and prevention
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
Charge transport through image charged stabilized states in a single molecule single electron transistor device
The present paper gives an elaborate theoretical description of a new
molecular charge transport mechanism applying to a single molecule trapped
between two macroscopic electrodes in a solid state device. It is shown by a
Hubbard type model of the electronic and electrostatic interactions, that the
close proximity of metal electrodes may allow electrons to tunnel from the
electrode directly into a very localized image charge stabilized states on the
molecule. Due to this mechanism, an exceptionally large number of redox states
may be visited within an energy scale which would normally not allow the
molecular HOMO-LUMO gap to be transversed. With a reasonable set of parameters,
a good fit to recent experimental values may be obtained. The theoretical model
is furthermore used to search for the physical boundaries of this effect, and
it is found that a rather narrow geometrical space is available for the new
mechanism to be effective: In the specific case of oligophenylenevinylene
molecules recently explored in such devices several atoms in the terminal
benzene rings need to be at van der Waal's distance to the electrode in order
for the mechanism to be effective. The model predicts, that chemisorption of
the terminal benzene rings too gold electrodes will impede the image charge
effect very significantly because the molecule is pushed away from the
electrode by the covalent thiol-gold bond.Comment: 9 pages, 5 figue
Distinct human stem cell populations in small and large intestine
The intestine is composed of an epithelial layer containing rapidly proliferating cells that mature into two regions, the small and the large intestine. Although previous studies have identified stem cells as the cell-of-origin for intestinal epithelial cells, no studies have directly compared stem cells derived from these anatomically distinct regions. Here, we examine intrinsic differences between primary epithelial cells isolated from human fetal small and large intestine, after in vitro expansion, using the Wnt agonist R-spondin 2.We utilized flow cytometry, fluorescence-activated cell sorting, gene expression analysis and a three-dimensional in vitro differentiation assay to characterize their stemcell properties. We identified stem cell markers that separate subpopulations of colony-forming cells in the small and large intestine and revealed important differences in differentiation, proliferation and disease pathways using gene expression analysis. Single cells from small and large intestine cultures formed organoids that reflect the distinct cellular hierarchy found in vivo and respond differently to identical exogenous cues. Our characterization identified numerous differences between small and large intestine epithelial stem cells suggesting possible connections to intestinal disease
APPLICATION OF THE PRINCIPLE OF MINIMUM OF ENTROPY PRODUCTION TO THE ANALYSIS OF THE EUTECTIC SOLIDIFICATION
ABSTRACT The principle of minimum entropy production is applied to the analysis of the formation of a solid structure in the course of the eutectic solidification. It is assumed that the liquid and solid phases are divided by the plane boundary and the solidification rate is constant. At these conditions solidification results in the development of the lamellar structure in the solid and the process is defined by the lamellar spacing and the rate of the motion of the liquid-solid interface. The variational equation describing system evolution is modified to describe the process in question. A solution of this equation determines the relationship between the solidification rate and the lamellar spacing. The obtained results complies with experimental data
(Z,E,Z)-1,6-Di-1-naphthylhexa-1,3,5-triene
The title compound, C26H20, lies about an inversion centre. The naphthalene unit and the hexatriene chain are each approximately planar (maximum deviations of 0.0143 and 0.0042 Å, respectively), and are inclined to one another at a dihedral angle of 49.20 (4)°. The dihedral angle between the two naphthalene ring systems of neighboring molecules is 85.71 (4)°
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