Effects Of Many-body Interaction On Energy Bands And Optical Spectra Of Tellurenes

Two-dimensional?2D?materials have excellent properties and broad application prospects,and have become the focus of research in recent years.After the discovery of graphene,novel 2D materials have been discovered,such as silicene,black phosphorenes,hexagonal boron nitride,and transition metal dichalcogenides?TMDs?.Recently,a new type of 2D elemental material composed of Group VI elements—tellurene has been theoretical predicted and experimentally synthesized.The?-and?-phase tellurenes have considerable band gaps,higher carrier mobilities than Mo S₂,and good optical properties,which provides tellurene good prospects in the fields of nanoelectronic devices,optical devices,etc.Therefore,tellurenes have received more and more attention in scientific research.In this thesis,the semiconducting?-and?-phase tellurenes are studied by first-principles calculations to address the electronic and optical properties,as well as the effects of spin-orbit coupling and many-body interaction.The main work is as follows:Based on the DFT-PBE method,we first optimized the geometric structure of?-and?-phase tellurenes,and calculated the energy band structures with and without spin-orbit coupling?SOC?.The bands without considering SOC of?-phase and?-phase tellurenes have indirect band gaps.Since it is a heavy element,tellurium has a strong spin-orbit coupling effect.With the SOC turned on,the band gaps of?-phase and?-phase tellurenes decrease,and the indirect gaps become near-direct gaps.In order to calculate the energy band structure more accurately,we further adopted the GW method to calculate the energy bands of?-and?-phase tellurenes.It is found that the band gaps increase significantly,indicating that there is a strong many-body interaction in tellurene.Because the electrons excited to the conduction band and the holes formed in the valence band are combined into excitons by the attractive Coulomb force,an exciton has lower energy than the non-interacting electron-hole pair.Therefore,the photon energy produced by the recombination the of electron and hole in the exciton is lower than the direct gap.Based on the correct band structures calculated by the GW method,we solved the Bethe-Salpeter equation?BSE?,considering the electron-hole interaction,and obtained the optical spectrum of tellurene.The calculations using k-grids of different k-point densities show that the energy of the first exciton peak in the light absorption spectra of?-and?-phase tellurenes are apparently lower than the direct band gap of the energy band,indicating that the electron-hole interaction is quite strong in tellurene,forming exciton with large binding energy.When calculating the BSE spectrum,we considered the spin-orbit coupling and compared the effects of k-grids of different densities.The calculation found that the k-grid needs to be close to the k point where the smallest direct band gap locates to get the correct spectrum.By comparing the energy value of the minimum direct band gap and the first exciton peak,we estimated the exciton binding energy.The exciton binding energy of?-phase tellurene is larger than that of?-phase tellurene regardless of the density of the k-grids.