First Principle Studies Of 2D Spin-valley Polarized Systems

In many fields of physics and material science,the key to scientific and technological progress is to understand and manipulate the properties of materials at the atomic and molecular levels.The first principle is a very successful method,which can accurately solve the basic equation describing the quantum properties of atoms and molecules-Schrodinger equation,and then calculate the required physical quantities.As a combination of mathematics,physics and computer science,physical theory and mathematical algorithms ensure the scientific accuracy of the calculation,and computer science provides an ideal platform for scientific computing with its flexible operability and high efficiency.Due to its unique optical,electrical and magnetic properties,van der Waals two-dimensional materials are very suitable for small size and high-performance optoelectronic nanodevices.Among which,transition metal dichalcogenides MX₂（M=Mo/W,X=S/Se）has a honeycomb hexagonal lattice.In single-layer limit,the monolayer MX₂ shows a direct band gap in the visible frequency range due to the breaking of the spatial inversion symmetry and the strong spin-orbit coupling.In addition,two nonequivalent spin-valley polarization conduction band minimum and valence band maximum are displayed at the neighboring K points in the hexagonal Brillouin zones of these monolayer systems.Such valley pseudospin can be used to encode information in electronic devices,which is of great significance in the explorations and applications of optoelectronic devices.With Intel and GNU compilers,we first compile and install the DFT and related softwares under the Linux system and studied the electronic structures of spin-valley polarized systems mentioned above.The main achievements are listed as follows:1.In spin-valley polarized MoSe₂-MoS₂ heterobilayers,stacking type and out-plane magnetic moment play significant roles in band gap regulation.The effect of bandgap tunning of the out-plane magnetic moment is very dependent on the stacking type.Calculation results of the phonon spectrum also demonstrate the stabilities of the systems.In WSe₂-Mo S₂ heterostructures,thicknesses of 2H-Mo S₂ adjust the band gap while maintaining the valley polarization of the system.Similar to Mo Se₂-Mo S₂heterobilayers,both thickness of 2H-Mo S₂ and out-plane magnetic moment regulations are very depend on the stacking type.The qualitative interpretation of the out of plane magnetic field by the k·p model verifies some DFT calculations.Compared with former studies,our method is relatively easier to operate and has wider bandgap regulation range.2.Due to the fact that there is no spin-valley property in the bilayer WSe₂ systems,we found that the rotation angle（around 30 degrees）can be used to introduce spin-valley polarization,band gap and strain with the aid of DFT.The effect of in-plane magnetic moment given by the k·p model and DFT can also adjust the split of conduction band minimum（valence band maximum）and band gap regulation at K/K1points.In addition,the similarities between the studied in-plane magnetic moment and twisted angle are analyzed.Our result provides new idea for experimental researches and development of photoelectric devices.3.Scanning tunneling microscope（STM）studies of monolayer transition metal dichalcogenides have very broad prospects in theoretical study and experimental innovation.In our study,the electron/hole doping effect of single layer WSe₂ on different surfaces of Au substrate has been studied by first principle and STM experiments.Electronic structures of Au/WSe₂ systems are calculated after geometric structures approximately treated.Calculation results show different surfaces of Au substrate can be used in electron/hole doping regulation of the adsorption layer,which agree well with the experimental results.This work demonstrates the possibility of achieving atomic size p-n junctions,and is very important for the development,design and manufacturing of high-performance 2D TMD devices.