Theoretical Investigations On Electronic Structure And Thermal Transport Properties Of Two-dimensional Materials

Two-dimensional materials have attracted enormous attention due to their unique mechanical,thermal,optical and electrical properties.With the progress of experimental technology,their potential applications have been extended from integrated circuits,field-effect transistors,photovoltaic cells,to thermoelectric,ferroelectric and ferromagnetic devices.Therefore,it is essential to understand the physical principles behind their unique properties,whichmay pave the way for the molecular design,device assembly and development of new functionalities.In this dissertation,with the help of state-of-the-art theoretical calculations,we gained the deep insight of the unique thickness and phase dependent electronic structures and thermal transport properties of two-dimensional(2D)materials.In the first part,we studied the layer-number and phase-related electronic properties of transition metal dichalcogenides(TMDCs): 1.We investigated the thickness-dependent electronic band structure of TMDCs.We found that the indirect-to-direct band gap transition of 2D TMDCs can be ascribed to the competition between spin-orbit coupling and interlayer coupling interactions.The stronger spin-orbit coupling tends to favor the transition at the larger number of layers.2.We studied charge-mediated semiconductor-semimetal phase transition of Janus TMDCs.Based on first-principles method we calculated the electronic structure ofsix Janus TMDCs in the H and T' phases respectively.We found that the Janus WSe Te monolayer out of six is thermodynamically stable in the semiconducting H phase and it can be transformed into the metastable semimetallic T' phase by charge injection.Such structural phase transition can be modulated reversibly via gating in field-effect transistors.Importantly,the phase transition in Janus WSe Te monolayer exhibits even lower energy barrier and faster kinetics than Mo Te2,an established 2D phase transition material in the catagory.In the second part of this dissertation,we investigated thermal transport properties of puckered group VA monolayer and their binary analogues.1.Within the framework of Boltzmann transport theory and relaxation time approximation,we predicted the lattice thermal conductivity of puckered arsenene,which is anisotropic and lower than black phosphorene(BP)due tothe larger atomic mass of As.Its electrical and thermal conduction directions are orthogonal to each other,which leads to a high thermoelectric figure of merit(ZT)value of 1.6 at the room temperature.2.In order to achieve even higher ZT value,we designed As P binary monolayers with three different atomic structures and showed that the binary monolayers have lower lattice thermal conductivities than their monoatomic counterparts.At the same time,the different bonding forms of three As P binary monolayers result in different optical phonon behaviors,making their thermal transport properties distinguishable from each other even if the chemical composition is the same.Finally,we studied the effect of phonon-electron interaction on the thermal transport property of Sn Se monolayer based on the density functional perturbation theory and Wannier interpolation approach.We demonstrated that the lattice thermal conductivity of two-dimensional materials could be severely suppressed by the phonon-electron coupling effect with the increase of carrier concentration.