New modelling capability for nano-confined phase change materials

点击翻译按钮获取中文摘要

The global demand for smaller and more energy efficient devices has been sustained by a steady decrease in the scale on which silicon microelectronics can be manufactured, from 65nm processes in the mid 2000s to 14nm in the very latest Intel processors. To continue this trend beyond the mid 2020s devices with dimensions of just 1-2nm will be required, likely using alternatives to silicon.In this regime, the cross section of a wire might be no more than 2x2 or 3x3 atoms across, where the relevant materials physics is dominated by surface and confinement effects leading to dramatically different structural and electronic properties to the corresponding bulk material. Such wires can be formed by crystallisation of a molten salt within carbon nanotubes (CNTs) of "Buckytubes", leading to the smallest cross section nano crystals possible, sometimes referred to as Feynman crystals. Research into the fundamental materials physics of these CNT-encapsulated structures is still in its infancy, with UK experimentalists leading the way. Particularly exciting recent work by one of the applicants (Sloan) has demonstrated the possibility of these wires undergoing transitions between nano-crystalline structures with markedly different properties, in response to bending strain in the CNT. These "phase change" properties open the way for nanoscale electromechanical switches and non-volatile memory, as well as providing a playground for fundamental studies of phase changes at the smallest length scale possible in a material.Our aim with the current project, inspired by these results, is to develop a computational modelling capability to aid in interpretation of experiments, understand the origin of the phase change behaviour, and guide our experimental colleagues toward compounds with potentially advantageous properties. Counterintuitively, due to a reduction in symmetry, the computational expense of simulating nanowires can be more demanding when compared to bulk crystals. We will address the limitations of currently available modelling tools when applied to these systems. This will involve significant modifications to existing software and a rigorous study of the various approximations one might employ to increase the tractability of simulations.We will apply cutting-edge methods in structure prediction to these systems, a non-trivial exercise due to the possibility wires with non-crystalline (e.g. helical) symmetry, and connect directly to relevant experiments by computing spectra related to the encapsulated wire's electronic and vibrational properties. Finally, we will study the thermodynamics and kinetics of nano-crystalline phase change, developing an understanding of when and how rapidly structural changes are affected to assess the utility of this mechanism for device applications.