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

Supervisors: Dr Peter Haynes, Dr Carla Molteni and Dr Nicholas Hine

Project description:

Semiconductor nanomaterials, including nanocrystals, nanorods and tetrapods, display a number of peculiar and tunable properties that distinguish them from their bulk counterparts and make them versatile materials for use as e.g. effective optical probes in medical diagnostics or photovoltaic devices. Of particular interest is their response to applied pressure, as they transform from one crystalline or amorphous structure to another. Accurate simulations are important for understanding finite size effects in the atomistic mechanisms of phase transformations (difficult to observe clearly in macroscopic experiments), for the opportunity to uncover novel metastable phases stabilized in finite systems, and for potentially innovative applications of nanomaterials e.g. as stress sensors.  Some progress has been achieved in recent years with the development of methods for constant pressure molecular dynamics of finite nanoscale systems. First-principles methods are essential to accurately describe the bond breaking/making in phase transformations and the realistic description of surfaces (often covered by complex surfactants). However the computational cost limits both the length- and time-scales attainable, the latter in particular being too short to allow the observation of phase transformations at experimental pressures. In this project we will combine an order-N density functional theory code for large systems, a constant pressure molecular dynamics method for finite systems and the metadynamics technique for rare events to model with quantum mechanical precision processes induced by pressure in nanomaterials (including their surfaces) under realistic conditions. The focus will be CdSe, CdS and ZnO nanocrystals that are currently favoured for technological applications.