arXiv subjects

David R. Bowler

Publications and source records attributed to David R. Bowler.

43 records · Page 3Linked to original sources

arXiv2014

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.

cond-mat.mtrl-sci↗

arXiv2014

Intrinsic Oxygen Vacancy and Extrinsic Aluminium Dopant Interplay: A Route to the Restoration of Defective TiO$_2$

Density functional theory (DFT) and DFT corrected for on-site Coulomb interactions (DFT+U) calculations are presented on Aluminium doping in bulk TiO$_2$ and the anatase (101) surface. Particular attention is paid to the mobility of oxygen vacancies throughout the doped TiO$_2$ lattice, as a means by which charge compensation of trivalent dopants can occur. The effect that Al doping of TiO$_2$ 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$_2$ 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$_2$ through oxygen vacancy diffusion.

cond-mat.mtrl-sci↗

arXiv2014

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.

cond-mat.mtrl-sci↗

arXiv2013

Understanding the role of ions and water molecules in the NaCl dissolution process

The dissolution of NaCl in water is one of the most common everyday processes, yet it remains poorly understood at the molecular level. Here we report the results of an extensive density functional theory study in which the initial stages of NaCl dissolution have been examined at low water coverages. Our specific approach is to study how the energetic cost of moving an ion or a pair of ions to a less coordinated site at the surface of various NaCl crystals varies with the number of water molecules adsorbed on the surface. This "microsolvation" approach allows us to study the dependence of the defect energies on the number of water molecules in the cluster and thus to establish when and where dissolution becomes favorable. Moreover, this approach allows us to understand the roles of the individual ions and water molecules in the dissolution process. Consistent with previous work we identify a clear preference for dissolution of Cl ions over Na ions. However, the detailed information obtained here leads to the conclusion that the process is governed by the higher affinity of the water molecules to Na ions than to Cl ions. The Cl ions are released first as this exposes more Na ions at the surface creating favorable adsorption sites for water. We discuss how this mechanism is likely to be effective for other alkali halides.

physics.chem-ph↗

arXiv2013

Investigating individual arsenic dopant atoms in silicon using low-temperature scanning tunnelling microscopy

We study sub-surface arsenic dopants in a hydrogen terminated Si(001) sample at 77 K, using scanning tunnelling microscopy and spectroscopy. We observe a number of different dopant related features that fall into two classes, which we call As1 and As2. When imaged in occupied states the As1 features appear as anisotropic protrusions superimposed on the silicon surface topography, and have maximum intensities lying along particular crystallographic orientations. In empty-state images the features all exhibit long-range circular protrusions. The images are consistent with buried dopants that are in the electrically neutral (D0) charge state when imaged in filled states, but become positively charged (D+) through electrostatic ionisation when imaged under empty state conditions, similar to previous observations of acceptors in GaAs. Density functional theory (DFT) calculations predict that As dopants in the third layer of the sample induce two states lying just below the conduction band edge, which hybridize with the surface structure creating features with the surface symmetry consistent with our STM images. The As2 features have the surprising characteristic of appearing as a protrusion in filled state images and an isotropic depression in empty state images, suggesting they are negatively charged at all biases. We discuss the possible origins of this feature.

cond-mat.mes-hall↗

arXiv2012

Linear-scaling implementation of exact exchange using localized numerical orbitals and contraction reduction integrals

We present enhancements to the computational efficiency of exact exchange calculations using the density matrix and local support functions. We introduce a numerical method which avoids the explicit calculation the four-center two-electron repulsion integrals and reduces the prefactor scaling by a factor N, where N is the number of atoms within the range of the exact exchange Hamiltonian. This approach is based on a contraction-reduction scheme, and takes advantage of the discretization space which enables the direct summation over the support functions in a localized space. Using the sparsity property of the density matrix, the scaling of the prefactor can be further reduced to reach asymptotically O(N).

physics.chem-ph↗

arXiv2011

Van der Waals density functionals applied to solids

The van der Waals density functional (vdW-DF) of Dion et al. [Phys. Rev. Lett. 92, 246401 (2004)] is a promising approach for including dispersion in approximate density functional theory exchange-correlation functionals. Indeed, an improved description of systems held by dispersion forces has been demonstrated in the literature. However, despite many applications, standard general tests on a broad range of materials are lacking. Here we calculate the lattice constants, bulk moduli, and atomization energies for a range of solids using the original vdW-DF and several of its offspring. We find that the original vdW-DF overestimates lattice constants in a similar manner to how it overestimates binding distances for gas phase dimers. However, some of the modified vdW functionals lead to average errors which are similar to those of PBE or better. Likewise, atomization energies that are slightly better than from PBE are obtained from the modified vdW-DFs. Although the tests reported here are for "hard" solids, not normally materials for which dispersion forces are thought to be important, we find a systematic improvement in cohesive properties for the alkali metals and alkali halides when non-local correlations are accounted for.

cond-mat.mtrl-sci↗