29 research outputs found
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
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
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.
crystal-torture: A Crystal Tortuosity Module
crystal-torture is a Python, Fortran, and OpenMP module for the analysis of diffusionnetworks in crystal structures. Ionic diffusion through crystalline solids depends not only onthe dynamics of ions within the crystal, but also the connectivity of the transport network.Understanding how the connectivity of diffusion pathways in crystal structures is affected bychanges in chemistry is necessary for understanding how chemical modifications change ionicconductivities, for example the doping of solid electrolytes. crystal-torture provides aPython API for interrogating network connectivity and diffusion pathways in partially blockedcrystal structures. It can be used as a tool for materials scientists to quickly build up network connectivity statistics to determine the viability of potential ionic conductors, and howchemical modification affects network connectivit
crystal-torture: A Crystal Tortuosity Module
crystal_torture: A crystal structure analysis code, allowing site tortuosity to be calculated.This work was funded by EPSRC Grant No. EP/N004302/1, and was supported with fund-
ing from the Faraday Institution (faraday.ac.uk; EP/S003053/1), grant number FIRG003.
B.J.M. acknowledges support from the Royal Society (UF130329)
Anion-polarisation--directed short-range-order in antiperovskite LiFeSO
Short-range ordering in cation-disordered cathodes can have a significant
effect on their electrochemical properties. Here, we characterise the cation
short-range order in the antiperovskite cathode material LiFeSO, using
density functional theory, Monte Carlo simulations, and synchrotron X-ray
pair-distribution-function data. We predict partial short-range
cation-ordering, characterised by favourable OLiFe oxygen coordination
with a preference for polar cis-OLiFe over non-polar
trans-OLiFe configurations. This preference for polar cation
configurations produces long-range disorder, in agreement with experimental
data. The predicted short-range-order preference contrasts with that for a
simple point-charge model, which instead predicts preferential
trans-OLiFe oxygen coordination and corresponding long-range
crystallographic order. The absence of long-range order in LiFeSO can
therefore be attributed to the relative stability of cis-OLiFe and
other non-OLiFe oxygen-coordination motifs. We show that this effect is
associated with the polarisation of oxide and sulfide anions in polar
coordination environments, which stabilises these polar short-range cation
orderings. We propose similar anion-polarisation-directed short-range-ordering
may be present in other heterocationic materials that contain cations with
different formal charges. Our analysis also illustrates the limitations of
using simple point-charge models to predict the structure of cation-disordered
materials, where other factors, such as anion polarisation, may play a critical
role in directing both short- and long-range structural correlations
Multiple novel prostate cancer susceptibility signals identified by fine-mapping of known risk loci among Europeans
Genome-wide association studies (GWAS) have identified numerous common prostate cancer (PrCa) susceptibility loci. We have
fine-mapped 64 GWAS regions known at the conclusion of the iCOGS study using large-scale genotyping and imputation in
25 723 PrCa cases and 26 274 controls of European ancestry. We detected evidence for multiple independent signals at 16
regions, 12 of which contained additional newly identified significant associations. A single signal comprising a spectrum of
correlated variation was observed at 39 regions; 35 of which are now described by a novel more significantly associated lead SNP,
while the originally reported variant remained as the lead SNP only in 4 regions. We also confirmed two association signals in
Europeans that had been previously reported only in East-Asian GWAS. Based on statistical evidence and linkage disequilibrium
(LD) structure, we have curated and narrowed down the list of the most likely candidate causal variants for each region.
Functional annotation using data from ENCODE filtered for PrCa cell lines and eQTL analysis demonstrated significant
enrichment for overlap with bio-features within this set. By incorporating the novel risk variants identified here alongside the
refined data for existing association signals, we estimate that these loci now explain ∼38.9% of the familial relative risk of PrCa,
an 8.9% improvement over the previously reported GWAS tag SNPs. This suggests that a significant fraction of the heritability of
PrCa may have been hidden during the discovery phase of GWAS, in particular due to the presence of multiple independent
signals within the same regio
DFT Dataset for "Interfacial Strain Effects on Lithium Diffusion Pathways in the Spinel Solid Electrolyte Li-Doped MgAl2O4"
This dataset contains inputs and outputs for the VASP calculations described in the manuscript "Interfacial Strain Effects on Lithium Diffusion Pathways in the Spinel Solid Electrolyte Li-Doped MgAl2O4", and scripts for extracting the appropriate resulting data into .csv files, for further analysis.
For more information see: C. O’Rourke and B. J. Morgan, "Interfacial Strain Effects on Lithium Diffusion Pathways in the Spinel Solid Electrolyte Li-Doped MgAl2O4"
The repository consists of
1. Input and output files for a series of VASP calculations.
2. A series of Python command-line scripts for extracting relevant data from these DFT calculations, and collating them in .csv files
Interfacial strain effects on lithium diffusion pathways in the spinel solid electrolyte Li-doped MgAl<sub>2</sub>O<sub>4</sub>
The (Li,Al)-codoped magnesium spinel (LixMg1-2xAl2+xO4) is a solid lithium-ion electrolyte with potential use in all-solid-state lithium-ion batteries. The spinel structure means that interfaces with spinel electrodes, such as LiyMn2O4 and Li4+3zTi5O12, may be lattice matched, with potentially low interfacial resistances. Small lattice parameter differences across a lattice-matched interface are unavoidable, causing residual epitaxial strain. This strain potentially modifies lithium diffusion near the electrolyte-electrode interface, contributing to interfacial resistance. Here, we report a density functional theory study of strain effects on lithium diffusion pathways for (Li,Al)-codoped magnesium spinel, for xLi=0.25 and xLi=0.5. We have calculated diffusion profiles for the unstrained materials, and for isotropic and biaxial tensile strains of up to 6%, corresponding to {100} epitaxial interfaces with LiyMn2O4 and Li4+3zTi5O12. We find that isotropic tensile strain reduces lithium diffusion barriers by as much as 0.32eV, with typical barriers reduced by ∼ 0.1eV. This effect is associated with increased volumes of transitional octahedral sites, and broadly follows qualitative changes in local electrostatic potentials. For biaxial (epitaxial) strain, which more closely approximates strain at a lattice-matched electrolyte-electrode interface, changes in octahedral site volumes and in lithium diffusion barriers are much smaller than under isotropic strain. Typical barriers are reduced by only ∼ 0.05eV. Individual effects, however, depend on the pathway considered and the relative strain orientation. These results predict that isotropic strain strongly affects ionic conductivities in (Li,Al)-codoped magnesium spinel electrolytes, and that tensile strain is a potential route to enhanced lithium transport. For a lattice-matched interface with candidate spinel-structured electrodes, however, epitaxial strain has a small, but complex, effect on lithium diffusion barriers
Adsorption of thiophene-conjugated sensitizers on TiO<sub>2</sub>natase (101)
Density functional theory calculations on the adsorption of the donor−π−acceptor, tetrahydroquinoline (C2-1 and C2-2) and carbazole (JK-24 and JK-25), dyes on the anatase (101) titanium dioxide surface are presented. Increased dye surface coverage is shown to have a much more pronounced effect on the electronic structure than extending the thiophene conjugation length, which has been shown to result from dipole−dipole interactions of the chromophores when forming a monolayer. Thiophene linker moieties are shown to be effective for the control of intramolecular charge separation and optical properties. Increased thiophene conjugation is found to positively shift the highest occupied molecular orbital and can introduce extra occupied states into the band gap