Computational Modelling of Nanomaterials - PhD Projects Available

I would like to hear from candidates interested in embarking upon a PhD in Materials Simulation at the Cavendish Laboratory in 2013. I have funding secured for one project, and there are several further routes available which I would be happy to discuss. Here is a brief overview and some specific topics I am interested in:

Overview
Nanomaterials offer us exciting new ways to control material properties, by varying material attributes why are not available in bulk systems. For example, in nanocrystals, growth conditions can be tuned to vary particle size, shape, surface terminations, composition and defect structure, and in an aggregate of nanocrystals, alignment, mutual interactions and interations with a solvent come into play. Nanomaterials are thus the key to making a success of many emerging technologies, in particular Photovoltaics and Photocatalysis, both means of turning light from the sun into other useful forms of energy. However, these variable factors result in a vast phase space to explore in order to design optimal materials for a given purpose. First-principles computational simulation can be used to explore this space, enabling computational "experiments" to disaggregate competing factors influencing a property, in a way which is impossible in real-world tests. For example, by modelling TiO2 nanocrystals, an important component in many photoactive devices, we can understand how to expose high-energy crystalline facets to maximise their potential for photocatalysis.

1) Methodological Development for Nanomaterials Simulation
Along with other academics at Imperial, Cambridge and Southampton, I am an author of the ONETEP Linear-Scaling Density Functional Theory (DFT) Code . The computational effort involved in traditional approaches to DFT scales cubically with system size, placing a barrier to calculations of more than a few thousand atoms. Linear-Scaling approaches to DFT, of which ONETEP is one of only a few practical examples worldwide, use methods based on local orbitals to avoid the cubic scaling steps and thus scale linearly with system size, allowing first-principles simulation of systems of many thousands of atoms.However, DFT itself is far from perfect as a simulation method, due to the approximations of available density functionals. DFT underestimates the bandgap of crystalline materials, and cannot be regarded as quantitatively accurate for quasiparticle spectra. Methods beyond DFT, such as the so-called GW approximation, are currently finding considerable success in smaller systems but do not scale well to nanostructures. Once again, the local orbitals methods used by ONETEP may be the key to overcoming this limitation.
A highly-motivated student with strong computational and mathematical skills is sought, to embark on a project to develop methodology and code for GW quasiparticle calculations with ONETEP. This would allow a range of exciting new types of calculation, principally relating to the crucial topic of quasiparticle alignment at interfaces between semiconductor nanocrystals and organic crystals (see below).

2) Computational Simulation of interfaces of II/VI semiconductor nanocrystals and organic molecules for photovoltaics
Singlet Fission is a promising means to increase the efficiency of photovoltaic devices by enabling each photon absorbed by the system to generate more than one excited electron/hole pair. It refers to a process in which a chromophore (part of a molecule which absorbs light) absorbs a photon to create a singlet excited state, which then relaxes to form a pair of triplet excitations with opposite spin. This has been shown to occur in several polycrystalline organic materials such as pentacene: however, the process currently operates with a relatively low efficiency. Such processes can be modelled theoretically, for example using high-level quantum chemistry calculations, but these are only usable for very small systems, and do not scale well to larger, more realistic models.
Real-world materials demonstrating singlet fission that can be harnessed for solar energy comprise complex interfaces between semiconductor nanocrystals such as PbSe and layered organic crystals such as pentacene. To be able to contribute to the understanding of carrier dynamics for these applications, it is vital to use methods able to treat realistically-sized model systems.
A PhD project is proposed investigating the possibility of using advanced techniques in electronic structure simulation, involving some combination of Linear-scaling Density Functional Theory with GW, BSE, TDDFT or Constrained DFT to investigate multi-exciton generation processes. We will try to estimate energies of singlet and triplet states in realistic systems and study their dynamics, by parameterising higher-level models of energy and exciton transport. We would hope, thereby, to find strategies to optimise the enhancement of photocurrent. This project would benefit from experimental collaboration with researchers from the Optoelectronics group at the Cavendish (in particular Neil Greenham).

3) Computational Simulation of TiO2 Nanocrystals
TiO2 is at the heart of a wide range of photoactive devices: dye-sensitized solar cells, water-splitting photocatalysts, and many more. However, in its usual bulk crystalline forms, TiO2 absorbs only a small fraction of the solar spectrum. Nanostructuring, ie creating materials with structure on the nanoscale, via the very wide range of options available in processing and treatment, can result in materials with very different (and considerably enhanced) properties compared to the bulk. For example, nanocrystalline TiO2 generally occurs in the anatase (rather than rutile) phase, rather than the usual rutile. Simulation of whole realistic nanocrystals using ONETEP opens up many new vistas for understanding and controlling the properties of TiO2-based nanomaterials. A PhD project is proposed to investigate this, requiring a student with a strong interest in computational simulation and solar energy production. Examples of topics to be studied would include "black" TiO2, the influence of water on the surface bandstructure of TiO2, and photocatalytic reactions on the surfaces of TiO2 nanostructures.