Study On Rare Earth Element-doped Transition Metal Dichalcogenides And Heterostructures

With the development of integrated circuit technology,the feature size of device decreased,two-dimensional materials represented by graphene have attracted great attention.Since graphene was stripped successfully,the research of two-dimensional semiconductor materials reaches a new stage,but since the intrinsic graphene has a zero band gap,the switching current ratio is small,in order to explore it’s excellent characteristics,it is necessary to adjust the band gap,but this would weaken other characteristics,and is not easy to achieve.After that,graphene-like two-dimensional materials typified by transition metal dichalcogenides,black phosphorus,hexagonal boron nitride have attracted attention and were widely used in various devices Among them,the ultra-thin monolayer of transition metal dichalcogenide can be obtained by strip as graphene,as the number of layers decreases,the band gap value gradually increases,and the band gap value of the monolayer structure is between 1 eV and 2 eV,which make it very suitable for use in photovoltaic materialsIn this paper,with the generalized gradient approximation based on density functional theory,the first-principles plane wave pseudopotential method was used to study the electrical and optical properties of two-dimensional transition metal chalcogenides（MoSe₂ and WSe₂）,and the influence of rare earth element doping on the MoSe₂ and WSe₂ properties were discussed.Three heterostructures of MoSe₂-WSe₂,MoSe₂-CrSe₂ and WSe₂-CrSe₂ were established,the effects of heterojunction structure and materials on its electronic structure,electrical properties and optical properties are analyzed.The main conclusions are as follows:（1）Firstly we calculated and analyzed the lattice parameters of MoSe₂ and WSe₂ before and after La,Nd,and Ce doping,we found that after doping,the bond length which around the doping atoms increased,the bond angle decreased,and the degree of change is proportional to the radius difference between the doped rare earth element atoms and the atoms which were substituted.Secondly,band structure and density of state are calculated,the materials exhibit the characteristic of P-type semiconductor,which means that the rare earth elements were acceptor dopants,the band gap value decreased significantly after rare earth element doping,the conductivity of the material enhanced.From the aspect of optical properties,the absorption coefficient increased and reflectivity decreased in the visible and near infrared regions after doping,which strengthen the applications.（2）By calculating the formation energies of three heterostructures with different stack structures,the most stable structures were applied in the following studies.Then the band structure,density of states of the three heterojunction structures were calculated,and we found that the band gap value decreased significantly,but the material still maintains the direct band gap,which is favorable to the transition of electron,the conductivity increased.Through overlap population,we found that internal electric field was established in the three heterojunction structures,this can enhance the separation of photogenerated electron-hole pairs.Finally,by calculating the optical properties of the heterostructures,it is found that both the absorption coefficient and the photoconductivity increased and red-shifted,which promotes the application of the heterostructures in the visible and near-infrared regions.