Thermal Transport Of Two-Dimensional Chalcogenides And Polycrystalline Diamond

Layered two-dimensional?2D?materials,especially 2D chalcogenides,recently have attracted increasing attention due to their unique electrical,optical,thermal properties and great potential in nanoscale electronic and optoelectronic applications.In contrast to the widely studied optical and electrical properties,little is known about their phonon thermal transport in 2D materials.Particularly,the in-plane phonon thermal transport of 2D materials becomes more scientifically important for understanding the energy transport in micro-and nanoscale structures.Meanwhile,the thermal transport properties of 2D materials and their modulation technologies lay important physical basics for the thermal management and optimization of 2D materials-based devices.In this dissertation,the in-plane thermal conductivities of tin diselenide?SnSe₂?nanofilms with varied thickness and in-plane size are systematically characterized using non-contact optothermal Raman technique;also,their thickness and in-plane size dependences are investigated.An ultralow in-plane thermal conductivities of less than 3 W/mK were revealed in 9.7⁴6.3 nm-thick suspended SnSe₂ nanofilms,making SnSe₂ a promising thermoelectric material.In contrast to the thickness dependence of graphene,the in-plane thermal conductivity of SnSe₂ increases with its thickness,this can be attributed to the weakened phonon boundary scattering with increased thickness;a phonon mean free path of²0 nm for SnSe₂ can be further extracted from the thickness dependence of the in-plane thermal conductivity.The thermal conductivity of 9.7-nm-thick SnSe₂ suspended over holes with diameters ranging from 3 to 6?m is also measured,showing no obvious in-plane size dependence;this is attributed to the far larger hole diameter than the phonon mean free path.In addition to high lattice-symmetry 2D materials e.g.SnSe₂,2D chalcogenides also contains some low lattice-symmetry members,such as Rhenium disulfide?ReS₂?,which usually have anisotropic in-plane physical properties.Here,an experimental method is designed to measure the anisotropic in-plane thermal conductivity of ReS₂ by suspending on a T-slit hole substrate and using a focus line laser.Note that the anisotropic in-plane crystal lattice axis of the single crystal ReS₂nanofilm is determined by angle resolved Raman spectra.In addition,the thermal transport properties of polycrystalline diamond films in high power GaN-on-diamond transistors are studied using finite element thermal simulation.Considering an anisotropic and inhomogeneous thermal conductivity existed in polycrystalline diamond which was induced by its columnar growth nature,the effective thermal conductivity perceived by the device in fact is unknown when such a diamond film is integrated to a GaN transistor.In this dissertation,the effective thermal conductivity of polycrystalline diamond films in transistors is simulated,and the effects of materials'internal?the grain boundary conductance and the grain evolution rate of diamond?as well as external factors?the GaN/diamond thermal boundary resistance and the GaN transistor geometry layout?on device thermal transport are investigated.These studies provide important guidance for the device design and optimization of diamond growth.