Thermal Transport In Graphene/Boron Nitride And Molybdenum Disulfide/Tungsten Disulfide Heterojuncts

Since the discovery of graphene,two-dimensional(2D)materials have attracted extensive attention due to their excellent properties in thermal,electrical,mechanical and other aspects.Nowadays,the electronic chip is developing towards miniaturization,which requires heat conduction.Compared with single layer of 2D materials,the two-dimensional heterojunction has attracted extensive attention due to its potential applications in thermoelectric and optoelectronic devices.Since monolayer MoS2 and WS2,Graphene and boron nitride(h-BN)have similar crystal structures and very close lattice constants,it is very easy to construct the heterojunction,however,the research on thermodynamic properties and thermal transport has not been perfected.In this work,we investigate thermal transport in two kinds of 2D heterojunctions:MoS2/WS2,Graphene/h-BN.The main work of this thesis is as follows:1.Combined with first principles calculation and Boltzmann transport theory(BTE),thermal transport properties of WS2 are systematically studied.The results show that at T=300K,the thermal conductivity of the monolayer MoS2 is 114W/mK,the thermal conductivity of the monolayer WS2 is 240W/mK,and the thermal conductivity of the MoS2/WS2 heterojunction is 167W/mK.The thermal conductivity of the heterojunction fallsbetween the thermal conductivity of MoS2 and that of WS2 is slightly lower than the average thermal conductivity of monolayer MoS2 and WS2.According to the phonon dispersion curve,gruneisen parameters,phonon group velocity,phonon lifetime and other calculated results,the lattice thermal conductivity of MoS2/WS2 heterojunction is affected by the weak van der waals action between layers compared with independent monolayer MoS2 and WS2.The reason for the decrease of thermal conductivity is that the coupling between the interfaces of heterojunction blocks the phonon transport in the plane direction.2.The first-principles method was used to explore the connection between the interaction and heat transport in the middle layer of Graphene/h-BN heterostructures.Compared with the high thermal conductivity of the bilayer-layer graphene(BLG),the calculation shows that the thermal conductivity of the graphene/h-BN heterostructure is significantly reduced by more than an order of magnitude.Our analysis of phonon modes confirms that the highly inhibited phonon lifetime is the cause of the significant decrease in thermal conductivity of the heterostructures.Our results show that the significantly shortened phonon lifetime is mainly due to the contribution of lattice anharmonic.The calculated results in this paper explain the decrease in thermal conductiyity of the heterojunction and provide an inherent physical insight into the potential mechanism of phonon scattering in the heterojunction.The results of this study are of great significance for the further study of heat transport of van der Waals materials and the exploration of thermal management and thermoelectric devices.This provides some valuable insights for the regulation of thermal conductivity of devices and the exploration of new thermoelectric devices.