First-principles Study On The Valleytronics In Heterostructures Of Two-dimensional Materials

Conventional electronic devices using electrical charge to process information are approaching the thermodynamic and quantum limits.Spintronics is a promising solution to circumvent these fundamental limits by encoding information with the spin of electrons.Recently some materials are found to have opposite Berry curvatures at different energy valleys of the electronic bands,leading to opposite electronic,magnetic and optical properties.Valleytronics uses a new quantum degree of freedom,namely valley or valley pseudospin,to encode information.Currently,there are two issues in valleytronics.One of them is that the research is almost limited to transition metal dichalcogenides（TMDs）.Other valleytronic materials,such as silicene and germanene are far inferior to TMDs in respect of valleytronic properties.The second issue is that the lifetime of the intralayer bright excitons in TMDs carrying valley pseudospin is only about ps,limiting the distance of spatial transport of the exciton,which is unfavorable for valleytronic applications.In this dissertation,the heterostructures of two dimensional materials are studied regarding the above two issues.The main results are as follows:1.The electronic properties and Berry phase related physics in the heterostructure of silicene/Bi（111）.Silicene is a potential valleytronic material with Dirac valleys.However,its spin-orbit coupling（SOC）is very weak and its gap is nearly zero,limiting its valleytronic applications.Bi（111）film has very strong SOC.It is found that by forming the silicene/Bi（111）heterostructure,sizable gap and giant spin splittings are induced at the Dirac valleys of silicene.The splittings are even larger than those in MoS₂.Due to the time reversal symmetry and the breaking of spatial inversion symmetry,Berry curvatures and spin splittings have opposite nonzero values at K and K′valleys,leading to valley-momentum locking.The circular dichroism is also valley contrasting,making possible the selective excitation of the valley carriers by circularly polarized light.Our results illustrate that forming heterostructure with strong SOC materials is an effective approach to significantly enhancing the valleytronic properties of weak SOC materials.2.The valleytronic properties and nontrivial Z₂ topological phase of germanene/SbF.Germanene/SbF is not an ideal valleytronic material due to its tiny SOC and energy gap.Although Sb（111）film has very strong SOC and can produce very large spin splittings in germanene–Sb（111）heterostructure,germanene/Sb is gapless and the Dirac cones of germanene are broken,making the valleytronic properties unrealizable.Orbital analysis shows that the Sb-p_z states contribute considerably in Ge-Sb hybridization,suggesting that F-adsorption on Sb atoms can help to open a gap.The calculations show that germanene/SbF has an appreciable gap at the Driac valleys.The time reversal symmetry and spatial inversion asymmetry dictate that the Berry curvatures and spin splittings are opposite at the inequivalent Dirac valleys.We calculated the interband transition matrix elements of circularly polarized light,which are found to be opposite at the K and K′valleys.The calculated Z₂ topological invariant confirms that germanene/SbF is a topological insulator.Our results show that forming heterostructure can not only convert weak SOC materials into excellent valleytronic materials,but also lead to simultaneous realization of valleytronic properties and nontrivial Z₂ topological phase in one material.3.The radiative transition of the intralayer and interlayer excitons in MoSe₂/WSe₂heterostructure.It is found that the intralayer exciton radiative transition is dependent on the stacking of the two layers in the heterostructure.For AA stacking,the incident circularly polarized light will induce intralayer valley polarization of MoSe₂ and WSe₂ at the same K point.For AB stacking,the intralayer valley polarization of the two layers will occur at inequivalent K points due to the 180 degree relative rotation between their respective Brillouin zone.After forming the intralayer excitons,part of the carriers in one layer will be transferred immediately to the other layer as a result of the type II band alignment of the two layers,leading to the formation of interlayer excitons.It is found that the circularly polarized light transition matrix element of interlayer excitons is one order smaller than that of intralayer excitons.Accordingly,the lifetime of interlayer excitons is longer than that of intralayer excitons by 500 times,in agreement with the experiments.For AA and AB stackings,the emitting light from the recombination of the interlayer excitons has the same circular polarization as the incident circularly polarized light,in accord with the experiments.We also found that the circular polarization of the interlayer exction radiative transition varies with respect to the relative positions of the two layers and band gap and band splitting of MoSe₂/WSe₂ heterostructure can be tuned sides by applying external electric field.