996 research outputs found
Adsorption of Thiophene-Conjugated Sensitizers on TiO<sub>2</sub> Anatase (101)
No full textDensity 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
Linear scaling density matrix real time TDDFT: Propagator unitarity and matrix truncation
No full textReal time, density matrix based, time dependent density functional theory (TDDFT) 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 cutoff 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
Workshop report. Linear-Scaling Ab Initio Calculations: Applications and Future Directions
No full textThe study of properties and of processes in materials, frequently hinges upon understanding phenomena which originate at the atomic level. In such cases the accurate description of the interactions between large numbers of atoms is critical and in turn requires the accurate description of the electrons which play a crucial role in the bonding of atoms into molecules, surfaces and solids. This can only be achieved by solving the equations of quantum mechanics. These
equations are too complicated to solve exactly; however their solutions can be approximated by computational techniques. The most accurate ? but also most computationally demanding ? are the “ab initio” techniques which do not use any empirical adjustable parameters. Amongst them, the Density Functional Theory (DFT) formulation of quantum mechanics stands out as an excellent compromise between accuracy and computational efficiency. However, the applicability of ab initio techniques is severely limited by poor scaling: the computational effort needed to perform an ab initio calculation increases with (at least) the third power of the number of atoms, N. This cubic-scaling bottleneck limits the number of atoms we can study to a few hundred at most, even on parallel supercomputers. To overcome this length-scale limitation, a number of researchers worldwide have been pioneering the development of a novel class of ab initio methods with linear-scaling or “Order N” (O(N)) computational cost which nevertheless retain the same high level of accuracy as the conventional approaches. While physically motivated, such methods have proved particularly hard to develop as they introduce highly non-trivial localisation constraints. Nevertheless, many major obstacles have been overcome and a number of O(N)
methods (SIESTA, CONQUEST, ONETEP, etc.) for ground state DFT calculations on systems with a gap (e.g. molecules, semiconductors and insulators) are now available and have reached a state of maturity that allows them to be used to study ”real” materials. The particular focus of this workshop is therefore to look forward to what can be achieved in the next few years. Our aim is twofold: (1) As O(N) methods are currently extending the applicability of DFT calculations
to problems involving biomolecules and nanostructures they are leading to completely new levels of understanding of these systems. This CECAM meeting will give us the opportunity to make an appraisal of such large-scale simulations and their potential to connect more directly to experiments. (2) We also want to examine the options for extending linear-scaling to problems that cannot be treated by ground-state DFT but require other, more complex approaches. These include methods for treating metallic systems, excited states and wavefunction-based theories for including electronic correlation. Finding ways to transform these methods to linear-scaling
cost, and hence extent their applicability to the nano-scale, is the next big challenge that the community of developers of large-scale electronic structure methods is beginning to face. We
hope that this workshop will stimulate these major new O(N) methodological developments by bringing together the leading groups in the development of O(N) DFT methods with the leading
groups in the development of metal and excited-state or wavefunction-based methods. Strong emphasis during the workshop will be given to discussion in order to promote the exchange
of ideas between different communities (Physics, Chemistry, Materials Science, Biochemistry) which are all interested in large-scale applications with ab initio accuracy but are approaching
them from different perspectives
Intrinsic Oxygen Vacancy and Extrinsic Aluminum Dopant Interplay: A Route to the Restoration of Defective TiO2
No full textDensity functional theory (DFT) and DFT corrected for on-site Coulomb interactions (DFT+U) calculations are presented on aluminum doping in bulk TiO2 and the anatase (101) surface. Particular attention is paid to the mobility of oxygen vacancies throughout the doped TiO2 lattice, as a means by which charge compensation of trivalent dopants can occur. The effect that Al doping of TiO2 electrodes has in dye-sensitized 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 increases observed in dye sensitized solar cells as a result of aluminum doping of TiO2 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 localized states within the band gap and defect states being present below the conduction band edge. Aluminum doping therefore acts as a benign dopant for “cleaning” TiO2 through oxygen vacancy diffusion
Response model of superconductive, pulsed eddy-current probes for detection of deep-lying flaws
No full textMaintenance of aging aircraft requires methods of nondestructive evaluation sensitive enough to detect small cracks and corrosion embedded in multi-layered structures of airframes. Combining pulsed eddy-current technology with the unrivalled magnetic-field sensitivity of superconductive probes can meet this need. It is shown experimentally and theoretically that this combination has an outstanding capability for detecting deep-lying flaws. Results from validation of the theoretical model are presented here. Experimental data and model predictions are in close agreement.This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Bowler, N., J. R. Bowler, and W. Podney. "Response model of superconductive, pulsed eddy-current probes for detection of deep-lying flaws." In AIP Conference Proceedings, vol. 557, no. 1, pp. 941-948. American Institute of Physics, 2001, and may be found at DOI: 10.1063/1.1373857. Copyright 2001 American Institute of Physics. Posted with permission
Eddy current measurements on case hardened steel
No full textThe case-hardening process modifies the near-surface permeability and conductivity of steel, as can be observed through changes in eddy current probe signals measured over a range of frequency. In this work, experiments have been performed using normal absolute probe coils on flat steel specimens and coils encircling case-hardened steel rods. By fitting model results to the experimental data, estimates of electrical material properties are found. The approach also allows an assessment of the sensitivity of the measurements to the case depth.This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing.
This article appeared in Sun, H., J. R. Bowler, N. Bowler, and M. J. Johnson. "Eddy current measurements on case hardened steel." In AIP Conference Proceedings, vol. 615, no. 1, pp. 1561-1568. American Institute of Physics, 2002, and may be found at
DOI: 10.1063/1.1472979.
Copyright 2002 American Institute of Physics.
Posted with permission
Monitoring the Effect of Relative Humidity During Curing on Dielectric Properties of Composites at Microwave Frequencies
No full textThe electromagnetic parameters of a composite material can depend on the environment in which the material is cured. Nondestructive monitoring of composite materials during curing offers a means of assessing whether or not the final product will function as specified. Microwave measurements of complex permittivity and permeability have been made on a polyurea/polyurethane hybrid containing ferromagnetic filler particles. It was observed that the permittivity of the samples is strongly affected by environmental relative humidity whereas the effect on the permeability is less significant.This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing.
This article appeared in Bowler, N., and E. R. Abram. "Monitoring the Effect of Relative Humidity During Curing on Dielectric Properties of Composites at Microwave Frequencies." In AIP Conference Proceedings, vol. 820, no. 1, pp. 469-476. American Institute of Physics, 2006, and may be found at
DOI: 10.1063/1.2184565.
Copyright 2006 American Institute of Physics.
Posted with permission
Structural properties of Silicon-Germanium and Germanium-Silicon Core-Shell Nanowires
No full textCore-shell nanowires made of Si and Ge can be grown experimentally with excellent control for different sizes of both core and shell. We have studied the structural properties of Si/Ge and Ge/Si core-shell nanowires aligned along the [110] direction, with diameters up to 10.2 nm and varying core to shell ratios, using linear scaling Density Functional Theory (DFT). We show that Vegard's law, which is often used to predict the axial lattice constant, can lead to an error of up to 1%, underlining the need for a detailed ab initio atomistic treatment of the nanowire structure. We analyse the character of the intrinsic strain distribution and show that, regardless of the composition or bond direction, the Si core or shell always expands. In contrast, the strain patterns in the Ge shell or core are highly sensitive to the location, composition and bond direction. The highest strains are found at heterojunction interfaces and the surfaces of the nanowires. This detailed understanding of the atomistic structure and strain paves the way for studies of the electronic properties of core-shell nanowires and investigations of doping and structure defects
Raoult's law revisited: accurately predicting equilibrium relative humidity points for humidity control experiments
No full textThe humidity surrounding a sample is an important variable in scientific experiments. Biological samples in particular require not just a humid atmosphere but often a relative humidity (RH) that is in equilibrium with a
stabilizing solution required to maintain the sample in the same state during measurements. The controlled dehydration of macromolecular crystals can lead
to significant increases in crystal order, leading to higher diffraction quality. Devices that can accurately control the humidity surrounding crystals while
monitoring diffraction have led to this technique being increasingly adopted, as the experiments become easier and more reproducible. Matching the RH to the
mother liquor is the first step in allowing the stable mounting of a crystal. In previous work [Wheeler, Russi, Bowler & Bowler (2012). Acta Cryst. F68, 111–
114], the equilibrium RHs were measured for a range of concentrations of the most commonly used precipitants in macromolecular crystallography and it was shown how these related to Raoult’s law for the equilibrium vapour pressure of water above a solution. However, a discrepancy between the measured values and those predicted by theory could not be explained. Here, a more precise humidity control device has been used to determine equilibrium RH points. The new results are in agreement with Raoult’s law. A simple argument in statistical mechanics is also presented, demonstrating that the equilibrium vapour pressure of a solvent is proportional to its mole fraction in an ideal solution: Raoult’s law. The same argument can be extended to the case where the solvent and solute molecules are of different sizes, as is the case with polymers. The results provide a framework for the correct maintenance of the RH surrounding a sample
Anatase (101) Surface
No full textWe have investigated the adsorption stability of ruthenium N749 dye [black dye (BD)], a highly efficient dye for dye-sensitized solar cells (DSCs), through protonated and deprotonated carboxyl group anchors on a TiO2 anatase (101) surface by using first-principles calculations. Geometry optimizations of the surface system with a supercell and the UV–visible spectrum calculation of the optimized dye structure were carried out. Among the configurations with one and two anchors, the BD adsorption anchored with one protonated carboxyl group was found to be the most stable, in contrast to most previous reports. Hydrogen bonding between the proton retained in BD and the surface oxygen is responsible for the stability of the protonated anchor. We confirmed that the calculated UV–visible spectrum of the most stable dye structure shows the best consistency with the experimental data. It is also demonstrated that the electronic density of states largely depends on the proton position. This novel aspect of adsorption via a protonated carboxyl anchor gives a new perspective for interfacial electronic processes of DSCs
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