First-Principles Study On Thermoelectric Property Modulation Of Two-Dimensional Transition Metal Dichalcogenides And Borophene

Nowadays,environmental problems have attracted people’s growing attention,especially the environmental problems caused by energy urge human beings to explore green energy and new energy technology.Therein,the thermoelectric conversion technology rightly corresponds to the demand for new energy technology in today’s world.Thermoelectric technology can realize not only thermoelectric refrigeration,but also direct conversion from heat energy to electric energy.However,there are many challenges in the research of thermoelectric materials,which impede the development of thermoelectric technology.Among them the most important problems are high manufacturing cost and the low conversion efficiency.However,in recent years,based on the breakthrough of new theory and the progress of experimental method,new materials and controlling approaches continuously appears.For example,enhancing the power factor by band engineering or reducing the lattice thermal conductivity by low-dimensions,along with the discovery of intrinsic high-performance materials,etc.,indicating that the thermoelectric materials have attracted widespread attention again.In this paper,we focused on exploring and controlling the thermoelectric performance of two-dimensional materials.Based on first-principles and Boltzmann transport equations,we studied the thermoelectric performance of monolayer WS₂ and tried to improve the performance by forming WS₂/WSe₂ lateral superlattices.Besides,we also predicted the promising thermoelectric performance in light semiconducting boron monolayer.Firstly,we investigated the thermoelectric properties of WS₂monolayer and WS₂/WSe₂ lateral superlattices.The results shown that the WS₂/WSe₂ lateral superlattice has a low lattice thermal conductivity as compared to monolayer WS₂,and there is obvious anisotropy along the armchair（xx）and zigzag（yy）directions,which are 16.9W/m K and 4.5 W/m K,respectively.The minimized lattice thermal conductivity arises from the enhanced anharmonic scattering as related to the enhanced coupling between optical and acoustic branches after the WS₂/WSe₂ superlattice is constructed.The anisotropy of thermal transport in WS₂/WSe₂ superlattice is mainly attributed to the different group velocities of the acoustic ZA,TA and LA branches along xx and yy directions.At 400 K,the thermoelectric z T exceeds 1,which indicates that the thermoelectric performance of two-dimension materials can be improved by forming such lateral superlattice crystal.Secondly,we also studied the thermoelectric properties of semiconducting boron monolayer.Heavy elements is a mark for high-performance thermoelectric materials since it usually guarantees low lattice thermal conductivity.Our calculation indicates that the lattice thermal conductivity of boron monolayer at room temperature is only about20W/m K,as compared to graphene or black phosphorus,which is ascribed to the complex hexagonal vacancies.Meanwhile,the highly degenerate band and low effective mass lead to the superior electronic performance.The optimal thermoelectric z T at 300K is as high as 0.96,which indicates that light element materials also can have good thermoelectric properties.