Thermal Transport Properties Of Two-dimensional Silicon Nanomaterials And Their Manipulation

Recently, a two-dimensional(2D) silicon structure(silicene) which is analogy to graphene has been widely studied both in the theoretical predictions and experimental synthesis. Unlike graphene which has a flat hexagonal structure, silicene has a low buckling of structure. It is known that stacked multilayer graphene forms the graphite structure with weak van der Waals interlayer interactions, and there is no surface reconstruction. While, there are covalent interlayer interactions in the stacked silicene structures, which have significant surface reconstructions. The two-dimension silicon nanomaterials have excellent physical properties, and have great application potentials in microelectronics, new energy and other fields and so on. The thermal transport is an important property of two-dimensional silicon nanomaterials. Due to the limitation of experimental conditions, as well as the theoretical simulation is still in its infancy, the understanding of the heat conduction mechanism of silicon-based nanoscale still not clear and people knows little on the ways to effectively manipulate its thermal transport property. Therefore, by using the molecular simulation method calculation, in the present thesis we systematically investigate the thermal conduction in the 2D silicon nanomaterials and explore the ways on their thermal conductivity manipulation. This thesis is structured as following:1. The classification and development processes of silicon nanomaterials are firstly introduced. Then the significance of investigating thermal transport properties of 2D silicon nanometer materials are specifically elaborated.2. The basic concept and the development of molecular dynamics(MD), the MD computation flow chart, as well as the selection of potential and two kinds of typical non-equilibrium MD(NEMD) methods on the thermal conductivity calculations are introduced. A direct-measured method for the phonon spectrum calculation in use of MD is also introduced.3. We have performed the external temperature gradient NEMD calculations on the length dependence of thermal conductivity of silicene both supported on and sandwiched between smooth surfaces of h-BN. It is found that the thermal conductivity ? of silicene follows a power law L?? ? with ? distinctly increasing with the effect of h-BN interface coupling, which shows an enhancement of the ballistic thermal transport property. we have found that the increase of ? for silicene supported on h-BN mainly comes from the diversification of ZA acoustic phonon mode as well as the upshift of optical phonon mode. As for the silicene in the sandwich structure, the enhancement of ballistic thermal transport is attributed to the increase of velocity of all the acoustic phonon modes with the interlayer interaction increasing.4. Based on the NEMD simulations, we have studied the thermal conductivity of multilayer silicon, explored the surface reconstruction and thickness effects on the thermal conductivity of 2D silicon. Firstly, we have explored the effect of surface reconstruction on thermal conductivity of 2D silicon. It is found that the thermal conductivity of two-layer silicon structures shows significant anisotropy, and it can be either high or low depending on the reconstructed surface structures. Then, we have explored the thickness effect on thermal conductivity of 2D silicon with the same surface structure. It is found that the thermal conductivity difference among the multilayer silicon structures is reduced with their thickness increasing. Also, the thermal conductivity anisotropy is much smaller in the thicker multilayer silicon. Especially, the thermal conductivity anisotropic is nearly disappeared when the thickness increases to 10 silicon layers.