D14.

Project title: Multi-level theoretical approach to determining the thermal properties of Si and ZnO nanostructures

 

 

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 is required, and in particular accurate information about the phonon and electronic properties of nanodevices, as these play a determining role in thermal transport. Recent efforts to synthesize and process nanostructured materials and nanoscale devices have created the demand for a better scientific understanding of their thermal properties, such as the specific heats, thermal expansions, and thermal conductivities. In current developments in theory and computation, classical molecular dynamics simulations have proved a popular approach to studying the thermal properties of nanoscale systems. Quantum-mechanical theories have also been used, 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 the proposed 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), namely, semiempirical tight-binding based (DFTB) theory, first-principles DFT, and perturbation DFT (DFPT). DFT and DFPT calculations of ultra small-sized nanostructures will be used to validate the results with the efficient DFTB method, which allows the study of nanostructures of a large size (up to 3 nanometers) and provides qualitative trends that are 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 used to develop computer codes. Our approach will cover the role of electron-phonon and phonon-phonon couplings in the thermal properties of nanostructures based on calculations of the phonon dispersion, as it is expected that the lattice contribution contributor to the total thermal conductivity will be dominant in most cases, whereas the electron contribution will be greater in materials with defects, narrow surfaces, and very thin nanostructures. The theoretically obtained properties will be verified by and fed back to experiments that are actively being pursued at the CityU, and will provide guidance as to how these materials might be exploited in advanced applications.

 



Supervisor:
Prof R Q Zhang (aprqz@cityu.edu.hk)

Suitable for
: M.Phil./Ph. D