First-principles Studies On The Electronic Structures And Control Of Two-dimensional Heterostructures

The successful fabrication of graphene has stimulated researchers from various fields to focus on two-dimensional（2D）nanomaterials.2D materials have been widely proposed to have great potential in electronics,catalysis,energy storage and biological medicine due to their unique physical and chemical properties compared with their bulk counterparts.Graphene and molybdenum disulfide（MoS₂）are two most widely explored 2D structures.Up to now,more and more novel 2D materials have been successfully prepared.In this paper,first-principles studies based on density functional theory are adopted to study the influence of interfacial interaction on electronic structures of 2D heterostructures made from graphene and MoS₂.We explored their potential applications in spintronic devices and field effect transistors.Our results show that the electronic structures of graphene change significantly when it is in contact with nickel（Ni）substrate.The interfacial interaction induces magnetism in graphene,making it half-metallic from a semi-metallic material（pristine graphene）.This finding could lead to possible applications in spintronic devices.The contact characteristics between2D MoS₂ and the recently synthesized MXenes,such as Ti₃C₂T_x（T=O,OH,F）,are total different with different ligands in MXenes.Remarkably,by applying biaxial strain to such 2D van der Waals（vdW）heterostructures,we found that the stability,contact characteristics and height of Schottky barrier can effectively tuned.This may have important implications for field-effect transistors（FETs）.The main contents of this paper are as follows,（1）We systematically studied the electronic structures of heterojunctions formed by graphene adsorption on Ni（111）surface.Results show that graphene exhibits half-metallic properties when it is in contact with Ni（111）with top-fcc（TF）and hollow-top（HT）configuration.This will benefit the application of graphene in spintronics.In order to understand this behavior,we analyzed the electronic structures and the charge transfer.We further explored the adsorption of transition metal atoms,including Fe,Co,Ni and V on graphene/Ni（111）and compared it with the adsorption on free-standing graphene.Interestingly,we found that with Ni（111）,magnetic moments of Fe,Co,Ni atoms increases and V atom decreases.The electronic and magnetic properties can be traced back to the crystal orbital splitting and spin density distribution.We found that Fe,Co,Ni and V have 2,1,0 and 3 lone pair electrons in the 3d electron orbitals respectively,corresponding to their magnetic moments on graphene/Ni（111）.（2）Second,we explored the heterostructures made from MoS₂ and Ti₃C₂T_x（T=OH,F,O）,to study the stability and electronic properties of 2D vdW heterostructures.It was found that three heterostructures with different ligands of Ti₃C₂T_x exhibit different contact characteristics,i.e.ohmic contact,n-type Schottky barrier contact,and p-type Schottky barrier contact for T=OH,F,O respectively.In FETs,we expect electrons to move freely between electrode and channel,so the lower the Schottky barrier height is,the better performance FET will be.We further employed tensile strain to engineer the heterostructures by calculating the stability and electronic structures with different tensile strains,ranging from 0 to 10%at an interval of 2%.Remarkably,it was found that all contacts can be changed to ohmic contact with increasing tensile strain.We analyzed the charge transfer,interlayer spacing,and binding energies,and found that with the increase of tensile strain,more electrons are transferred from Ti₃C₂T_x to MoS₂.The interfacial spacing between Ti₃C₂O₂/MoS₂ and Ti₃C₂F₂/MoS₂ decreases with increase of strain,while the interfacial spacing in Ti₃C₂（OH）₂/MoS₂ basically remains unchanged.We show that with increased tensile strain,the binding energies of three heterojunctions become larger,which suggest higher stability of MoS₂/Ti₃C₂T_x（T=OH,F,O）heterojunctions.Therefore,we believe findings presented in this thesis may shed new light on the applications 2D heterostructures for electronic and spintronic devices.