Multi-level Theoretical Approach to Determining the Thermal Properties of Si and ZnO Nanostructures

Description

Among the many challenges facing nanodevices, the thermal management of the constituent nanostructures is particularly crucial. Although nanoscale computer processors and semiconductor lasers need a high thermal conductivity to dissipate the heat that they generate, other components of nanoscale devices, such as thermal barriers, must have a low thermal conductivity. To achieve precision thermal control, knowledge of the lattice dynamical behavior of nanomaterials, in particular accurate information about the phonon and electronic properties of nanodevices, is required as these properties play the determining role in thermal transport. Recent efforts to synthesize and process nanostructured materials and nanoscale devices have created the demand for better understanding of their thermal properties, such as the specific heats, thermal expansions, and thermal conductivities. In current developments in theory and computation that complement the challenging experiments, classical molecular dynamics simulations have shown to be a popular approach to studying the thermal properties of nanoscale systems. Quantum-mechanical methods have also been applied, but are limited to very small model systems. However, there is still a strong need to describe the thermal properties of semiconducting nanomaterials based on quantum mechanical theories, as the quantum effect is indispensable.The objective of this project is to uncover the accurate thermal properties of Si and ZnO nanowires, two representative nanostructured materials, using three levels of density functional theory (DFT), semiempirical tight-binding DFT (DFTB), first-principles DFT (DFT), and perturbation DFT (DFPT). DFT and DFPT calculations of ultra small-sized nanostructures will be used to validate the results from the efficient DFTB method, which allows the study of nanostructures of a large size (up to 3 nanometers) and provides qualitative trends useful for experiments. The basic theory and mathematical approach to studying thermal properties such as the thermal expansion coefficient, specific heat, and thermal conductivity will be further verified and then employed in computer code development. The researcher's approach will cover the electron-phonon and phonon-phonon couplings in the thermal properties of nanostructures based on calculations of the phonon dispersion, as the lattice contribution to the total thermal conductivity is expected to be dominant in most cases, whereas the electron contribution will be larger in materials with defects, narrow surfaces, and very thin nanostructures. The theoretical results will be verified by and fed back to experiments that are actively being pursued at the City University of Hong Kong, and will provide guidance as to how these materials might be exploited in advanced applications.