Epitaxial Growth And Electronic Structure Modulation Of Tellurium(Te) Films On Graphene

In recent years,elementary two-dimensional(2D)materials,such as Graphene,Silicene,Germanene,Stanene,Antimonene et al.,have attracted tremendous interests for their characteristic electronic structures and exotic properties.Besides the searching for new elementary 2D materials,people attempt to stack these 2D materials into van der Waals(vdWs)hetero-structures,forming lots of novel composites.In addition,based on 2D materials,one can create in-plane p-n junction,which paves the way for the investigation of the exotic phenomena in 1D systems.Because of the lack of appropriate materials,however,the progress of this field is somewhat restricted.In this thesis,by using molecular beam epitaxy(MBE)and scanning tunneling microscopy/spectroscopy(STM/STS),we realized the epitaxial growth of group-VI element Tellurium(Te)on Graphene/6H-SiC(0001),fabricated the vdWs hetero-structure of monolayer Te and graphene,and modulated the electronic structures of monolayer Te films.More important,we created the in-plane p-n junction of monolayer Te film at the boundary of different graphene layers.The main contents are summarized as follows:Firstly,by using MBE/STM,we explored the solid state graphitization of 6H-SiC(0001)surface.In order to get a better control of the thickness of graphene,both flash annealing and longtime annealing methods are carried out with various graphitization conditions to compare the different processes of graphene grown on SiC.Based on longtime annealing method,we can readily obtain monolayer,bilayer,tri-layer,as well as few-layer graphene.For flash annealing method,the situation is different,it is difficult to obtain tri-layer and few-layer graphene.However,with this method,we can delicately control the extent of graphitization and obtain uniform bilayer graphene on SiC substrate.This chapter paves the way for the investigations in the following chapters.Secondly,monolayer and few-layer Te films are obtained by MBE on Graphene/6H-SiC(0001)substrate.Combined with STM and first-principles calculations,we raveled that the Te films are composed of parallel-arranged Te chains flat-lying on graphene substrate,exposing the(101?0)facet of the bulk crystal.The band gap of Te films decreases monotonically with increasing the thickness from 0.92 eV(monolayer)to 0.33(bulk)as measured by STS,covering the spectral range from mid-infrared to near-infrared.Moreover,an upward band bending and the coincident holes accumulation at the boundary of monolayer Te and tri-layer graphene are observed.The topological properties of monolayer Te was explored by spatially resolved dI/dV line scan,and we didn't find robust evidence to support a non-trival state at the boundary of monolayer Te and graphene.In addition,the effect of charge transfer from Graphene/SiC to monolayer Te film was discussed with different thickness of graphene.Based on amount of reliable results,we found that,tri-layer graphene can effectively screen the charge transfer from SiC to Te film and maintain the eigen p-type semi-conductivity of monolayer Te film.However,in the cases of monolayer Te film grown on bi-layer,monolayer graphene and Buffer layer,the effect of charge transfer is significant,which lead to the n-type semi-conductivity of monolayer Te film.By changing the scanning position,we found a monolayer Te film spread across two adjacent graphene terraces(trilayer and monolayer graphene).We demonstrated that,at the boundary,the conductivity of Te film gradually changed from p-type to n-type,signaling the formation of p-n junction,and the electronic transition region across the edge is about 6.2 nm.Namely,by using the charge transfer from Graphene/SiC to monolayer Te film,we realized the fabrication of in-plane hetero-structures.