Theoretical Study Of The Electromagnetic Properties Of Two-dimensional Graphene Heterojunctions

With the successful exfoliation of graphene,more and more two-dimensional(2D)materials have been attracting researchers’s wide attention due to their excellent physical and chemical properties,such as molybdenum disulfide(MoS2),h-boronnitride(h-BN),black phosphorus(BP),silicene,borophene etc.However,the limination of many pristine 2D nanomaterials make them insufficient to meet their application in electronic devices.For example,although graphene has ultrahigh carrier mobility,the metal property largely limits its application in logic switching devices.Moreover,as graphene is nonmagnetic,it is still unsuitable in the spin device application.As for molybdenum disulfide,although it has a tunable wide band gap,its carrier mobility is ultra low.Moreover,BP is insulator with tunable band gap and high carrier mobility,but it is found to be unstable in the atmosphere.In recent years,the emergence of vdW(van der Waals)heterostructure by combining two kinds of 2D materials has provided a reliable way to enhance their performance.Taking G/TMDCs(Transition-metal dichalcogenides)heterostructureas as an example,the vdW heterostructure can help open the band gap of graphene,however,the band gap is so small(～several meV)and it will be overcomed at room temperature.More importantly,the vdW heterostructures can not introduce magnetism into the systems and meet the requirements of spintronic devices.In this thesis,by using density functional theory calculation methods,we systematically explored the the electronic and magnetic properties of G/TMDCs with of 3d transition metal(TM)atoms intercalation.Our results show that the embedding of 3d TM atoms further opened the band gap of graphene and introduced the magnetic properties of the system,largely improving the electronic properties.Firstly,the energetics and electronic and magnetic properties of G/MS2 hybrid structures embedded with 3d transition metal atoms,TM@(G/MS2)(G=graphene;M=W,Mo;TM=Sc-Ni),have been systematically studied.TM atoms were found to be covalently bound to two-sided graphene and MS2 layers with sizable binding energies of 4.35-7.13eV.Interestingly,a variety of electronic and magnetic properties were identified for these TM@(G/MS2)systems.Except for TM = Ni,all other systems were ferromagnetic,due to exchange splitting of the TM 3d orbitals.In particular,four TM@(G/MoS2)systems(TM= V,Mn,Fe Co)and three TM@(G/WS2)systems(TM=Mn,Fe,Co)were half-metals or quasi half-metals,while Ni@(G/MoS2)and Ni@(G/WS2)were semiconductors with bandgaps of 33meV and 37meV,respectively.Further quasi-particle scattering theory analysis demonstrated that the origin of semiconducting or half-metallic properties could be well understood from the variation in on-site energy by the transition metal dichalcogenide substrate or the different on-site scattering potential induced by TM atoms.Secondly,various defects,such as single vacancy,double vacancy and one-dimensional grain boundaries,can inevitably appear in graphene or TMDC during the process of CVD,however,impact of the presence of defects on their properties of such TM@(G/TMDC)heterostructures has not been revealed.Therefore,we perform a comprehensive study on the geometries,electronic and magnetic properties of 3d transition metal(TM=Sc-Ni)atoms intercalated G/WSe2 heterostructures,TM@(G/WSe2),and their counterparts with single vacant graphene or WSe2 sublayer,TM@(G_sv/WSe2)s and TM@(G/WSe_svSe)s.Except those Ni intercalated and most of the Se vacant TM@(G/WSe_svSe)systems,the hybridized systems display rich electronic and magnetic properties,where the magnetic ground states are highly sensitive to the nature of sub-graphene layer and sub-WSe2 layer.Interestingly,the magnetic moments of Mn/Co doped systems are almost fixed and not affected by the substrates.Importantly,TM@(G_sv/WSe2)s(TM=Ti,Cr,Fe,Ni)and TM@(G_sv2/WSe2)s(TM=Ti,Cr,Fe,Ni)systems are transformed to be semiconductors with sizable band gaps of-0.09eV-0.51eV.In addition,ferromagnetic half metals are identified for TM@(G/WSe2),TM@(G_sv/WSe2)and TM@(G_sv2/WSe2)s(TM=V,Co).Our findings propose an effective route for manipulating the electronic and magnetic properties of graphene@TMDc heterostructures,allowing their potential application in modern spintronic and electronic devices.