Pressure-induced structural transformations in nanomaterials : towards high accuracy large length- and time-scale simulations

• Main Author: Corsini, Niccolo
• Other Authors: Haynes, Peter
• Published: Imperial College London 2015
• Subjects: 620
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.695521

The study of pressure-induced structural transformations in nanomaterials is both of fundamental and technological importance. Accurate simulations of these transformations are challenging because large length- and time-scales have to be simulated to make contact with experiments whilst retaining the atomic detail for a faithful description. In this thesis, both classical and quantum mechanical techniques are used to model pressure-induced structural transformations in realistic Si, Ge and CdS nanocrystals and comparison made to experiment where possible. We implement an electronic enthalpy method within the linear-scaling density-functional theory ONETEP code and, after introducing an approach for calibrating the volume definition, investigate the size-dependent pressure-induced amorphisation and polyamorphic transformations in hydrogenated Si and Ge nanocrystals. For the latter, we elucidate the surface-induced amorphisation and the new high-density amorphous metallic Ge phase observed experimentally. We combine this method with the projector-augmented wave and time-dependent density-functional theory methods to study the size and ligand dependence of deformation and optoelectronic properties of CdS nanocrystals with pressure. We develop a novel classical parametrisation for the simulation of bare and ligated CdS nanocrystals immersed in a pressure-transmitting medium and investigate their transformation under pressure using classical molecular dynamics and the metadynamics method for accelerating rare events. The resulting polymorphic transformation and pressure-induced amorphisation are analysed in detail.