568 research outputs found
Intrinsic Oxygen Vacancy and Extrinsic Aluminium Dopant Interplay: A Route to the Restoration of Defective TiO
Density functional theory (DFT) and DFT corrected for on-site Coulomb
interactions (DFT+U) calculations are presented on Aluminium doping in bulk
TiO and the anatase (101) surface. Particular attention is paid to the
mobility of oxygen vacancies throughout the doped TiO lattice, as a means
by which charge compensation of trivalent dopants can occur. The effect that Al
doping of TiO electrodes has in dye sensitised solar cells is explained as
a result of this mobility and charge compensation. Substitutional defects in
which one Al3+ replaces one Ti4+ are found to introduce valence band holes,
while intrinsic oxygen vacancies are found to introduce states in the band-gap.
Coupling two of these substitutional defects with an oxygen vacancy results in
exothermic defect formation which maintain charge neutrality. Nudged elastic
band calculations have been performed to investigate the formation of these
clustered defects in the (101) surface by oxygen vacancy diffusion, with the
resulting potential energy surface suggesting energetic gains with small
diffusion barriers. Efficiency in- creases observed in dye sensitised solar
cells as a result of aluminium doping of TiO electrodes are investigated by
adsorbing the tetrahydroquinoline C2-1 chromophore on the defective surfaces.
Adsorption on the clustered extrinsic Al3+ and intrinsic oxygen vacancy defects
are found to behave as if adsorbed on a clean surface, with vacancy states not
present, while adsorption on the oxygen vacancy results in a down shift of the
dye localised states within the band-gap and defect states being present below
the conduction band edge. Aluminium doping therefore acts as a benign dopant
for 'cleaning' TiO through oxygen vacancy diffusion.Comment: 32 pages, 15 figures, accepted for publication by J. Phys. Chem.
DSSC Anchoring Groups: A Surface Dependent Decision
Electrodes in dye sensitised solar cells (DSSCs) are typically
nanocrystalline anatase TiO2 with a majority (101) surface exposed. Generally
the sensitising dye employs a carboxylic anchoring moiety through which it
adheres to the TiO2 surface. Recent interest in exploiting the properties of
differing TiO2 electrode morphologies, such as rutile nanorods exposing the
(110) surface and anatase electrodes with high percentages of the (001) surface
exposed, begs the question of whether this anchoring strategy is best,
irrespective of the majority surface exposed. Here we address this question by
presenting density functional theory calculations contrasting the binding
properties of two promising anchoring groups, phosphonic acid and boronic acid,
to that of carboxylic acid. Anchor-electrode interactions are studied for the
pro- totypical anatase (101) surface, along with the anatase (001) and rutile
(110) surfaces. Finally the effect of using these alternative anchoring groups
to bind a typical coumarin dye (NKX- 2311) to these TiO2 substrates is
examined. Significant differences in the binding properties are found depending
on both the anchor and surface, illustrating that the choice of anchor is
necessarily dependent upon the surface exposed in the electrode. In particular
the boronic acid is found to show the potential to be an excellent anchor
choice for electrodes exposing the anatase (001) surface.Comment: 44 pages, 15 figures, accepted by J. Phys.:Condens. Matter.
Coordinates for structures available via figshar
Linear Scaling Density Matrix Real Time TDDFT: Propagator Unitarity \& Matrix Truncation
Real time, density matrix based, time dependent density functional theory
proceeds through the propagation of the density matrix, as opposed to the
Kohn-Sham orbitals. It is possible to reduce the computational workload by
imposing spatial cut-off radii on sparse matrices, and the propagation of the
density matrix in this manner provides direct access to the optical response of
very large systems, which would be otherwise impractical to obtain using the
standard formulations of TDDFT. Following a brief summary of our
implementation, along with several benchmark tests illustrating the validity of
the method, we present an exploration of the factors affecting the accuracy of
the approach. In particular we investigate the effect of basis set size and
matrix truncation, the key approximation used in achieving linear scaling, on
the propagator unitarity and optical spectra. Finally we illustrate that, with
an appropriate density matrix truncation range applied, the computational load
scales linearly with the system size and discuss the limitations of the
approach.Comment: Accepted for publication in J. Chem. Phy
Reaction paths of alane dissociation on the Si(001) surface
Building on our earlier study, we examine the kinetic barriers to
decomposition of alane, AlH, on the Si(001) surface, using the nudged
elastic band (NEB) approach within DFT. We find that the initial decomposition
to AlH with two H atoms on the surface proceeds without a significant barrier.
There are several pathways available to lose the final hydrogen, though these
present barriers of up to 1 eV. Incorporation is more challenging, with the
initial structures less stable in several cases than the starting structures,
just as was found for phosphorus. We identify a stable route for Al
incorporation following selective surface hydrogen desorption (e.g. by STM
tip). The overall process parallels PH, and indicates that atomically
precise acceptor doping should be possible.Comment: 19 pages, 8 figures, submitted to J. Physics.: Condens. Matte
Alane adsorption and dissociation on the Si(001) surface
We used DFT to study the energetics of the decomposition of alane, AlH3, on
the Si(001) surface, as the acceptor complement to PH3. Alane forms a dative
bond with the raised atoms of silicon surface dimers, via the Si atom lone
pair. We calculated the energies of various structures along the pathway of
successive dehydrogenation events following adsorption: AlH2, AlH and Al,
finding a gradual, significant decrease in energy. For each stage, we analyse
the structure and bonding, and present simulated STM images of the lowest
energy structures. Finally, we find that the energy of Al atoms incorporated
into the surface, ejecting a Si atom, is comparable to Al adatoms. These
findings show that Al incorporation is likely to be as precisely controlled as
P incorporation, if slightly less easy to achieve.Comment: Submitted to J. Phys.: Condens. Matte
Stable and Efficient Linear Scaling First-Principles Molecular Dynamics for 10,000+ atoms
The recent progress of linear-scaling or O(N) methods in the density
functional theory (DFT) is remarkable. We expect that first-principles
molecular dynamics (FPMD) simulations based on DFT can now treat more realistic
and complex systems using the O(N) technique. However, very few examples of
O(N) FPMD simulations exist so far and the information for the accuracy or
reliability of the simulations is very limited. In this paper, we show that
efficient and robust O(N) FPMD simulations are now possible by the combination
of the extended Lagrangian Born-Oppenheimer molecular dynamics method, which
was recently proposed by Niklasson et al (Phys. Rev. Lett. 100, 123004 (2008)),
and the density matrix method as an O(N) technique. Using our linear-scaling
DFT code Conquest, we investigate the reliable calculation conditions for the
accurate O(N) FPMD and demonstrate that we are now able to do actual and
reliable self-consistent FPMD simulation of a very large system containing
32,768 atoms.Comment: 26 pages, 10 figures, accepted by J. Chem. Theory Compu
Hydrogen adsorption and diffusion around Si(001)/Si(110) corners in nanostructures
While the diffusion of hydrogen on silicon surfaces has been relatively well
characterised both experimentally and theoretically, the diffusion around
corners between surfaces, as will be found on nanowires and nanostructures, has
not been studied. Motivated by nanostructure fabrication by Patterned Atomic
Layer Epitaxy (PALE), we present a density functional theory (DFT) study of the
diffusion of hydrogen around the edge formed by the orthogonal (001) and (110)
surfaces in silicon. We find that the barrier from (001) to (110) is
approximately 0.3 eV lower than from (110) to (001), and that it is comparable
to diffusion between rows on the clean surface, with no significant effect on
the hydrogen patterns at the growth temperatures used.Comment: 11 pages, 4 figure
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