First-principles Study Of The Modulation Of Electronic Structures Of Two-dimensional Transition Metal Dichalcogenides And Their Heterostructures

Since the successful preparation of graphene,Two-dimensional(2D)materials have attracted much attention since they exhibit unique mechanical,thermal,optical and electrical properties with potential applications in optoelectronic devices,sensors,field effect transistors,biocatalysis,and so on.In the rich family of 2D materials,transition metal dichalcogenides(TMDs)become the focus of fundamental research and technological applications owing to their unique crystal structures and a variety of physical properties.However,the explosive popularity of TMDs does not only rely on the intrinsic material properties themselves,but highly depends on the tunable electronic properties.Due to the anisotropy and unique crystal structure,we can effectively tune the electronic properties of 2D TMDs via different methodologies including strain,high pressure,external electric field,adsorption and doping,etc.Compared with other tuning methods,the intrinsic properties of 2D materials can not be damaged by the external electric field,and the external electric field is reversible.So far the modulation of the electronic properties of 2D TMDs via the external electric field rarely are reported,the exploration of the electronic properties tuning is worth further developing.In this paper,the first-principles calculations be used to systematically investigate the modulation of electronic structures of two-dimensional transition metal dichalcogenides and their heterostructures.We obtained some meaningful results.It is expected that the research conclusions can explain some experimental phenomena and their physical meaning,and in order to provide a theoretical guidance for the practical application of TMDs.1.The electronic structures modulation of of the multilayer TMDs by the external electric field.The results show that the external electric fields can effectively tune bandgaps of multilayer TMDs.The bandgaps decrease linearly with the increasing external electric field and eventually achieving a semiconductor–metal transition.The critical electric fields,at which the semiconductor-to-metal transition occurs,depend on the number of layers.This gap tuning effect yields a robust relationship,which is essentially characterized by the giant Stark effect(GSE)coefficient S,for the rate of change of bandgap with the applied external electric field.The GSE coefficient S is proportional to the number of layers and it can be expressed as(N-1)c/2,where N is the number of layers and c is the interlayer distance.2.The electronic structures engineering of different stacking bilayer TMDs under the external electric field.It shows that for all cases,the most stable stacking order is the AB conformation,followed by the AA? stacking fault.The bandgaps of both AB and AA? configurations decrease linearly with the increasing external electric field except for the external electric field along-z direction in the AB conformation,the bandgap first increases to a maximum,but then,it decreases linearly.Both applying the external electric field along z direction and-z direction can make AB and AA? configurations achieve a semiconductor–metal transition.The critical electric fields,at which the semiconductor-metal transition occurs,depending on different stacked conformations and the external electric field direction.Applying the external electric field along z direction and-z direction have different effects on the bandgap of AB conformation,while it has the same effect on the AA? configuration.The different effects are caused by the spontaneous electrical polarization existing between the two monolayers of AB conformation.3.Electric field modulation of the electronic structures in van der Waals(vdW)heterostructures based on TMDs.It is demonstrated that vdW heterostructures based on TMDs are type-II vdW heterostructures,the conduction band minimum(CBM)and valence band maximum(VBM)are contributed by different monolayers in the heterostructure.Type-II heterostructures can facilitate the separation of electron–hole pairs and enhance the lifetime of photo-induced carrier,which is suitable for optoelectronics and solar energy conversion.The bandgaps decrease linearly with the increasing external electric field,eventually a transition from semiconductor to metal occurs.The positive and negative external electric fields have different effects on the bandgap due to the spontaneous electric polarization in the heterostructure.The CBM and VBM of respective monolayers can be significantly modulated by external electric fields and change linearly with external electric fields.Furthermore,the vdW heterostructure experiences transitions from type-II to type-I and then to type-II under various external electric fields.The external electric field can control not only the amount of charge transfer but also the direction of charge transfer at the interface.4.Tuning the Schottky barrier in the graphene/WS2 vdW heterostructure by external electric field.We find that graphene interacts weakly with WS2 via weak vdW interactions and both the intrinsic electronic structures of graphene and WS2 are quite well preserved at the equilibrium interlayer distance.The n-type Schottky contacts with the significantly small Schottky barrier are formed in the graphene/WS2 heterostructure and p-type(hole)doping in graphene occurs during the formation of graphene/WS2 heterostructure.when the heterobilayer is subjected to the positive external electric field,a transition from n-type to p-type Schottky contacts occurs at 1.35 V/nm,then to Ohmic contacts at 3 V/nm.The graphene/WS2 heterostructure remains the n-type Schottky contact under the negative Eext less than-0.55 V/nm.When the negative external electric field exceeds-0.55 V/nm,the Ohmic contact is obtained again.The external electric field is effective to tune the Schottky contacts,which can transform the n-type into p-type,then to Ohmic contact.P-type(hole)doping in graphene is enhanced under the negative external electric field and the large positive external electric field is required to achieve n-type(electron)doping in graphene.The external electric field can control not only the amount of charge transfer but also the direction of charge transfer at the graphene/WS2 interface.