Tunning Of The Electronic Band Properties Of Several Typical Two-Dimensional Heterostructures

Two-dimensional van der Waals heterostructures constructed by monolayer materials not only maintain the excellent characteristics of monolayer material,more importantly,can produce new properties which is absence in the corresponding monolayer materials.Therefore,it is extremely important to study the basic properties of heterostructures,such as electronic states,and then regulate their electronic states?relatively isolated monolayers?.This paper mainly studies the electronic band characteristics of typical transition metal sulfide?TMDs?heterostructures and the Dirac electronic state of graphene-based heterotrilayer.The main results are as follows:?1?Based on the double-layer Mo S₂/Mo Se₂ van der Waals heterostructure,Mo atoms are replaced with different W compositions to construct Mo S₂/Mo_1-xW_xSe₂and Mo_1-xW_xS₂/Mo Se₂ heterostructures,respectively.Using the first-principles method,calculate the corresponding change trend of their electronic band structure with the change of the W compositions.The calculation results indicate that alloying is also a feasible strategy to realize the reversible direct-to indirect-gap transition in Mo_1-xW_xS₂/Mo Se₂ heterostructure,which is absence in Mo S₂/Mo_1-xW_xSe₂heterostructure.Especially,when the W compositions x>0.57 in Mo_1-xW_xS₂/Mo Se₂,it exhibits remarkable and reversible direct-to indirect-gap transition.This is because for Mo_1-xW_xS₂/Mo Se₂,the valence band maximum located at the K point originates from dominant Mo Se₂,while the competing?state stems from the hybridization of both Mo_1-xW_xS₂ and Mo Se₂,which is extremely sensitive to the interlayer coupling.Consequently,alloying in Mo S₂ layer induces direct-to indirect-gap transition and gap increase due to the weakened p-d coupling.Besides,whether initial alloying in Mo S₂ or Mo Se₂,the?_Mo-?_W poor condition should always be used.These findings on the synthesis of heterostructures are generally applicable to multi-layer structures and are very useful for energy band engineering applications that enrich TMDs heterostructures.?2?Research on the regulation of the Dirac electronic band characteristics of graphene-based heterotrilayers:first,construct the Graphene/Phosphorene/Graphene heterotrilayer,which has not only the relatively large band gap opening as usual,but also possess both distinctly spatially and energetically resolved property.The induced resolved Dirac rings have different Dirac features such as Fermi velocity and asymmetric factor.Such unique features are absence in Phosphorene/Graphene bilayer heterostructure and the counterpart crystal.These findings provide new insights into researching the Dirac electric properties.Second,replacing the middle layer in the Graphene/Phosphorene/Graphene heterostructure with monolayer Cr As₂?1H phase?coupled with line symmetry and antiferromagnetic properties,constructing a new Graphene/Cr As₂/Graphene heterostructure to study the evolution process of the Dirac electron band characteristics.The results clearly identify the common vertical shift of Dirac bands induced by the exchange field for the spin up channel.While for the spin down channel,surprisingly observe the remarkable Rashba-like splitting Dirac cones coupled with available magnetism,even in the absence of spin-orbital coupling.Such unique Dirac states are absent in its nonmagnetic or ferromagnetic counterpart,ferromagnetic system with glide symmetry,and Graphene/Cr As₂ heterobilayer.These findings would provide a new insight into the correlation between Dirac cones and geometric symmetry coupled with magnetism in a graphene-based heterotrilayer.