47 research outputs found
Range separated hybrid exchange-correlation functional analyses of W and/or N(S) (co)doped anatase TiO_2
Electronic properties and atomic structures of W, N, S, W/N, and W/S dopings
of anatase TiO_2 have been systematically investigated using the density
functional theory (DFT). The exchange and correlation effects have been treated
with Heyd, Scuseria and Ernzerhof (HSE) hybrid functional. Mixing traditional
semi-local and non-local screened Hartree-Fock (HF) exchange energies, the HSE
method corrects the band gap and also improves the description of anion/cation
derived gap states. Enhanced charge carrier dynamics, observed for W/N codoped
titania, can partly be explained by the passivative modifications of N 2p and W
5d states on its electronic structure. Following this trend we have found an
apparent band gap narrowing of 1.03 eV for W/S codoping. This is due to the
large dispersion of S 3p states at the valance band (VB) top extending its edge
to higher energies and Ti--S--W hybridized states appearing at the bottom of
the conduction band (CB). W/S-TiO_2 might show strong visible light response
comparable to W/N codoped anatase catalysts.Comment: 8 pages, 5 figures and 3 table
A Density Functional Theory Investigation of Carboranethiol Self-Assembled Monolayer on Au(111)
Isolated and full monolayer adsorption of various carboranethiol
(CBHS) isomers on gold (111) surface have been investigated
using both the standard and van der Waals density functional theoretical
calculations. The effect of differing molecular dipole moment orientations on
the low energy adlayer geometries, the binding characteristics and the
electronic properties of the self-assembled monolayers of these isomers have
been studied. Specifically, the binding energy and work function changes
associated with different molecules show a correlation with their dipole
moments. The adsorption is favored for the isomers with dipole moments parallel
to the surface. Of the two possible unit cell structures, the (55) was
found to be more stable than the ()R23.4 one.Comment: 22 pages, 6 figures and 3 table
Re-assigning (1x2) reconstruction of rutile TiO_2(110) from DFT+U calculations
Physically reasonable electronic structures of reconstructed rutile
TiO_2(110)-(1x2) surfaces were studied using density functional theory (DFT)
supplemented with Hubbard U on-site Coulomb repulsion acting on the d
electrons, so called as the DFT+U approach. Two leading reconstruction models
proposed by Onishi--Iwasawa and Park et al. were compared in terms of their
thermodynamic stabilities.Comment: 4 pages, 4 figures, 2 table
Dispersion Corrected DFT Study of Pentacene Thin Films on Flat and Vicinal Au Surfaces
Here we a present a density functional theory study of pentacene ultra-thin
films on flat [(111)] and vicinal [(455)] Au surfaces. We have performed
crystal and electronic structure calculations by using PBE and optB86b-vdW
functionals and investigated the effects of long range Van der Waals
interactions for different coverages starting from a single isolated molecule
up to 4 monolayers of coverage. For an isolated molecule both functionals yield
the hollow site as the most stable one with bridge-60 site being very close in
energy in case of optB86b-vdW. Binding strength of an isolated pentacene on the
step edge was found to be much larger than that on the terrace sites. Different
experimentally reported monolayer structures were compared and the (6 x 3) unit
cell was found to be energetically more stable than the (2 x 2 ) and
(2 x ) ones. For one monolayer films while dispersion corrected
calculations favored flat pentacene molecules on terraces, standard (PBE)
calculations either found tilted and flat configurations to be energetically
similar (on (111) surface) or favored the tilted configuration (on (455)
surface). PDOS calculations performed with optB86b-vdW functional showed larger
dispersion of molecular orbitals over the Au states for the (455) surface when
compared with the (111) surface, indicating an enhanced charge carrier
transport at the pentacene-gold interface in favor of the vicinal surface.
Starting with the second monolayer, both functionals favored tilted
configurations for both surfaces. Our results underline the importance of the
dispersion corrections for the loosely bound systems like pentacene on gold and
the role played by step edges in determining the multilayer film structure and
charge transfer at the organic molecule-metal interface.Comment: 8 Figures, 4 Table
Range separated hybrid density functional study of organic dye sensitizers on anatase TiO nanowires
The adsorption of organic molecules coumarin and the donor--acceptor
type tetrahydroquinoline (C2-1) on anatase (101) and (001) nanowires have been
investigated using screened Coulomb hybrid density functional theory
calculations. While coumarin forms single bond with the nanowire surface, C2-1
additionally exhibits bidentate mode giving rise to much stronger adsorption
energies. Nonlinear solvation effects on the binding characteristics of the dye
chromophores on the nanowire facets have also been examined. These two dye
sensitizers show different electronic charge distributions for the highest
occupied and the lowest unoccupied molecular states. We studied the electronic
structures in terms of the positions of the band edges and adsorbate related
band gap states and their effect on the absorption spectra of the dye-nanowire
combined systems. These findings were interpreted and discussed from the view
point of better light harvesting and charge separation as well as in relation
to more efficient charge carrier injection into the semiconductor nanowire.Comment: 8 pages, 4 figures, and 1 tabl