Energy Valley Structure And Regulation Of Transition-metal Dichalcogenides MX₂ Heterostructures

With the development of thin-film-preparation technology and high-throughput calculations,an increasing number of layered two-dimensional(2D)materials have been discovered.2D materials such as graphene,transition-metal dichalcogenides(TMDs)and layered magnetic semiconductors have shown broad application prospects in the fields of nanoscale optoelectronics,thermoelectricity,magnetoelectricity,and valleytronics devices.These layered materials also provide a rich choice of materials for constructing various kinds of van der Waals(vd W)heterostructures with unique physical properties and device functions.Utilizing and manipulating the valley degree of freedom of electrons as an information carrier has attracted increasing interest recently.The combination of direct bandgap,a large spin-orbit coupling(SOC),and the locked degrees of freedom between the spin and valley makes monolayer TMDs the best candidate materials for studies of valleytronics.And by combining TMDs with other two-dimensional materials to form different types of heterostructures to optimize their electronic and energy valley properties has also attracted widespread attention.In this thesis,the first-principles calculations based on density functional theory(DFT)are used to predict and regulate the electron and energy valley properties of heterostructures based on TMDs.The main contents and results are as follows:First,we found that the band gap of MoS2/WSe2 heterostructure can be significantly tuned by thickness engineering,perpendicular electric fields,forming spin-valley coupling Dirac cones at the K and K'valleys.The intrinsic band structure of the MoS2/WSe2 heterobilayer is found to be a direct bandgap,in which the conduction band minimum is located at the MoS2 layer,but the valence band maximum lies in the WSe2 layer,forming a type-II band alignment,which can be changed easily into type-I band alignment by applying perpendicular electric fields.The special dispersion relation like the Dirac cone and each of these band alignments have particular applications in enabling different varieties of devices.Second,we conducted a comparative study on a series of TMDs/2D-magnetic-material vd W heterostructures.It is found that the magnitude of the valley splitting in TMDs-based vd W heterostructures is correlated with the strength of the magnetic proximity effect,which is positively related to the interlayer charge transfer and Coulomb interaction.As a result,for the same stacking,large valley splittings can be obtained only when the vd W heterostructure has a type-III,instead of a type-I or type-II,band alignment.We demonstrate this finding in TMDs/Ni Y₂(Y=Cl,Br,I)vd W heterostructures in detail based on first-principles calculations,and predict several heterostructures with large valley splittings.This discovery can help to distinguish whether a magnetic material can enable TMDs to produce large valley splittings,and guide experiments for technological applications.Next,we have predicted the vd W heterostructure which with room-temperature ferromagnetism and can make the monolayer TMDs produce large valley splitting by the first-principles calculations.The results showed that in the MnSe₂/MoTe₂heterobilayer,the total valley splitting in the monolayer Mo Te₂ can reach 69 meV at the equilibrium layered distance,equivalents to the Zeeman splitting in a 350 T external magnetic field.Moreover,the substantial splitting can be continuously tuned to as large as more than 130 me V by the external vertical pressure.We expect the unprecedented large valley splitting achieved in the 2D/2D vd W heterostructure which with room-temperature ferromagnetism would advance the practical application of valleytronics devices.Finally,by constructing a series of TMDs-based sandwich vd W heterostructures with different symmetries,we found that the spin splitting and valley splitting in the TMDs layer has been greatly improved.In the MoTe₂/MoSe₂/MoTe₂ sandwich heterojunction which with mirror symmetry and time inversion symmetry,the double spin splitting effect in the MoTe₂ layers can be achieved.And in the case of TMDs/magnetic-material/TMDs heterostructures,both in the MoTe₂/NiCl₂/Mo Te₂without mirror symmetry,or in the MoS₂/h-VN/MoS₂ and MoTe₂/TaSe₂/MoTe₂ with mirror symmetry,the large valley splitting in the TMDs layer is realized.Moreover,by comparing the corresponding magnetic/TMDs heterobilayer,it was found that the valley splitting in the sandwich heterostructure was significantly improved.This finding provides a new way to enhance the spin splitting efficiency and magnitude of valley splitting in TMDs.