Study On Transport And Photogalvanic Properties Of New Nanomaterial WTe₂

Transition metal chalcogenides(TMDCs)has attracted extensive research interests due to their novel physical phenomena in photovoltaics,thermals,and mechanics.TMDCs may have great application prospects in devices because of their good transport properties.WTe₂mainly exists semiconductor 2H phase and semi-metallic Td phase,so it has different properties from other TMDCs.WTe₂is the first predicted type II Weyl semi-metallic material.The biggest feature of this new Weyl semimetal is that it has a severely inclined Weyl cone.Due to the unique nature of the energy band,many theoretical studies predict that it has many differences.The peculiar properties of the first type of Weyl semimetal:Klein tunneling in momentum space,anomalous magnetic field selectivity and inherent anomalous Hall effect,etc.Our work is mainly focused on the study of the semiconductor phase(2H)and semi-metal(Td)phase of WTe₂.The main research contents are as follows:(1)we study the electronic structure and phonon spectrum of WTe₂.The phonon spectrum has a strong interaction between the acoustic and optical branches in the range of0-10 me V,and the relative vibration is strengthened.Therefore,it is predicted that WTe₂has good heat transport properties.Meanwhile,WTe₂is doped with C,H,S,and Se,and the influence of different doping elements on the electronic structure of WTe₂is studied.(2)we study the photogalvanic effect(PGE)of Td-WTe₂under different photon energies in the small bias ranges.The study found that Td-WTe₂can generate a larger photocurrent under all small bias voltages,which indicates the broadband optical response of WTe₂.Meanwhile,we found that at 2.8 e V,a larger photocurrent can be generated under all bias voltages.We combined the electronic energy band structure to analyze the microscopic origin of the larger photocurrent,and found that the main reason for the larger photocurrent comes from electronic transition at point S in the first Brillouin zone.(3)we study the impact of defects on the PGE of WTe₂.With the help of the lattice defects introduced by Ga⁺implantation in the experiment,we constructed several types of lattice defects.The PGE of WTe₂under this defect states and intrinsic states are calculated respectively.The results show that this defect structures will increase the photocurrent of WTe₂,and further analyze the regulation mechanism of this defect structures on the PGE.(4)we study the PGE of WTe₂/MoTe₂heterostructure under zero bias voltage,Linearly and elliptical polarized light are used to irradiate the intrinsic WTe₂and WTe₂/MoTe₂heterostructure devices,respectively,and the photon energies ranges from 0.8-3.0 e V.The entire device is calculated under zero bias voltage,so the dark current can be effectively avoided.Our calculation results show that the WTe₂/MoTe₂heterojunction is a good candidate material as a photodetector because it can generate a large photocurrent and has a high polarization sensitivity.