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Understanding materials and devices at the nanoscale using atomistic simulations

Final Report Summary - UMDNAS (Understanding materials and devices at the nanoscale using atomistic simulations)

Project objectives

Paul Tangney (PT) took up his position at Imperial College in September 2007 as a lecturer in the Departments of Physics and Materials. Since then he has been engaged in research and teaching undergraduate and postgraduate students. In October 2010 he successfully completed his probationary period and his position became permanent. He has recruited six doctoral (PhD) students and two postdoctoral researchers and secured research funding from both industry (NSG Group), government agencies (e.g. EPSRC) and charitable organisations like the Leverhulme Trust.

Work performed

In this project, substantial progress has been made in the area of force field development. A software package known as ASAP (Atomistic simulations with accurate potentials) has been substantially rewritten to make it more user-friendly and to increase its functionality. A major and unique piece of functionality that has been added is the ability to model ions whose charge and polarisation state can simultaneously vary. ASAP is distributed by a server at Imperial College, which is currently being accessed by approximately 20 developers in eight countries. Its development is being coordinated by PT and is being prepared for widespread release via a GNU public licence.

Using ASAP, force fields have been developed for a number of oxide materials, including alumina (Al2O3), zirconia (ZrO2), germania (GeO2), titania (TiO2), magnesium silicate perovskite (MgSiO3), and barium titanate (BaTiO3), as well as a linear-scaling force-field for silica (SiO2), which is suitable for very large-scale simulations of, for example, fractures. All of the force fields perform very well. The works on TiO2 and SiO2 have been published, and papers are being prepared on MgSiO3, Al2O3, and BaTiO3.

The distribution of charge in polar nanocrystals and nanorods has been investigated with the help of density functional theory (DFT) calculations. The reason why the asymmetry of the wurtzite lattice structure plays a subordinate role to the effects of adsorbed atoms and molecules is explained theoretically, with supporting evidence from DFT simulations. Three papers have been published on this work and a fourth is being prepared for submission.

Silicate glasses were studied in a project funded by glass manufacturers NSG Group and the UK Knowledge Transfer Network. A PhD student combined continuum modelling (based on reaction-diffusion equations) with secondary ion mass spectrometry (SIMS) data. The inverse problem was partially solved: experimental data on the diffusion of corroding impurities, such as tin in window glass, was used to develop a mathematical model of the corrosion mechanism, and to predict the relevant diffusing species and their diffusivities and reaction rates. A paper on this work is currently being prepared.

Two students have recently begun working on nanostructured ferroelectric materials (barium strontium titanate), using DFT and ASAP, in collaboration with experimental groups at Imperial College and Ohio State University. Early results of this work are very promising and will soon be published.

Main results

The outcomes of this project can be split into two types: those associated with applications to real materials and the development of a new methodology. A major outcome will be the free availability of the ASAP software package, which promises to have a major impact on science as it will allow reliable simulation of bulk and nanostructured materials comprising many thousands of atoms on nanosecond timescales. This will allow simulations to play a central role in understanding materials, predicting their properties and guiding their synthesis.

The second type of outcome will be associated with the application of quantum mechanical simulations and classical simulations to real-world problems. For example, our simulations of nanorods will help experimentalists to tune their properties by varying surface chemistry, and our simulations of glasses will help NSG Group to improve their manufacturing process.