Synthesis And Thermal Transport Properties Of Two-dimensional Materials

Graphene(Gr),an atomic layer honeycolmb structure with sp²-hybridzed carbon atoms,has attracted wide attention in scientific community,due to its extraordinary physical/chemical properties and huge potential for application in fundamental research and technological applications.The discovery of Gr has also raised an intensive research interests in the family of two-dimensional materials(2DM),such as hexagonal boron nitride(h-BN),black phosphorus(BP)and transition metal chalcogenides(MoS₂,WS₂),etc.Among 2DM,h-BN is the thinnest wide band gap insulator available,with high mechanical strength,chemical stability,and excellent thermal conductivity due to its strong in-plane covalent sp2 bonds.Thin h-BN layer has an atomically flat surface,free of dangling bonds and charge traps,which makes h-BN an ideal substrate for a variety of devices based on 2DM.Besides,h-BN has a wide range of applications in ultraviolet-light emitter and detector due to its wide band gap.Recent studies show that the Gr and h-BN vertical heterostructure(Gr/h-BN)with tunable inter-layer rotation could induce Moire Superlattice,which deeply affected the band structure of Gr and therefore exhibits many unique physical properties.Gr/h-BN heterostructure can be applied as an ideal research platform for novel physical phenomena,thus broadening the application of the two materials.In this thesis,h-BN,Gr and the Gr/h-BN heterostructure were synthesized by chemical vapor deposition(CVD).Ni foam assisted synthesis of high-quality hexagonal boron nitride with large domain size and controllable thickness were reported for the first time on Pt catalytic substrate via low pressure CVD(LPCVD).The effect of metal foam and oxide on reducing the nucleation density of h-BN on Cu foil were also studied to further increase the domain size of h-BN single crystal.Utilizing the catalytic activity of cobalt nano-powders,layer-by-layer synthesis of bilayer and multilayer Gr on Cu foil were achieved which broken the self-limiting effect.Moreover,in-situ synthesize of the Gr/h-BN heterostructure were performed using metal foam as an additional catalyst,which effectively improve the growth rate as well as enlarging the Gr domain size.On the other hand,we report the experimental study of the thermal conductivity of monolayer h-BN using an optothermal Raman technique.The influence of the average grain sizes on the thermal conductivity of polycrystalline h-BN films at various temperatures was studied.By performing equilibrium molecular dynamics(EMD)simulations,we indicate the grain boundaries can increase phonon scattering and reduce the phonon mean free path,thus leading to the reduction of thermal conductivity.