Investigations On Electronic Structures Of Graphene Based Heterostructures And The Structural Growth Of Bismuthene

Since the successful exfoliation of graphene,its novel properties have attracted extensive attention to explore new two dimensional(2D)materials.To date,many other 2D materials have been discovered,such as silicene,germanene,phosphorene,hexagonal boron nitrides and transition metal dichalcogenides.The 2D materials possess quantum confinement effect,high specific surface area and maximum mechanical flexibility,which makes them exhibit fascinating properties different from that in bulk materials.Therefore,they are promising candidates for fabricating new generation electronic and optical devices.However,the individual 2D materials have certain intrinsic shortcoming that they cannot overcome on their own.Take grapheme as an example,the unique Dirac cone band structure gives it incredible ultrahigh carrier mobility,which is considered an alternative to metals,but the zero band gap results in that transistors will not switch off,restraining it applications in graphene based transistors.So band gap engineering for graphene has always been a key problem.Besides,the growth of 2D materials is also important.With more 2D materials discovered,how to grow high quality 2D materials is a key step to their future applications.So it is of great significance to elucidate the growth mechanism of 2D materials theoretically.Theoretical calculation is an effective avenue to assist experiments.By using theoretical calculation,people can not only simulate the properties of materials,but also deeply understand the physical mechanism from the perspective of atoms.In this thesis,using the first principles calculations within the framework of density function theory,we firstly study the graphene based heterostructures.The effect of in-plane uniaxial strain and spin orbit couple(SOC)effect on electronic structures of graphene/blue phosphorene(Gr/BPh)and grapheme/GaSb(Gr/GaSb)heterostructures are investigated in detail.Secondly,we study the growth rules of bismuthene(Bi)on different substrates and predict the growth possibility of Bi on different substrates The main results are summarized as below:I.The effect of in-plane uniaxial strain on the electronic structure of Gr/BPh is investigated.We propose an effective way to tune the electronic properties of Gr/BPh.The results show that the van der Waals interaction has little influence on the electronic structure of graphene and blue phosphorene layers.Upon the application of in-plane uniaxial strain,the band gap of the Gr/BPh bilayer can be easily opened.The value of band gap increases with increasing the strain value.A maximum gap value～240 meV is obtained around ±4%strain.By analyzing the variation of lattice structure and charge density differences,we reveal that the variation of electronic structures is mainly attributed to the asymmetry of lattice structure and charge redistribution in Gr/BPh bilayer.Further investigation on variation of interlayer binding energies shows the strain cannot cause the separation of Gr/BPh bilayer.By calculating the carrier mobility,we suggest that the suitable strains should be in the range of±4%,meanwhile,the transport properties can still be keep well.The results provide feasible avenue to fabricate graphene based devicesⅡ.The electronic properties of Gr/GaSb are investigated,we provide an effective way to design graphene based nanoscale devices.We study the electronic structure of Gr/GaSb,GaSb/Gr/GaSb heterostructures.The results show that the band gap of graphene can be opened in Gr/GaSb.After inclusion of SOC,the Gr/GaSb exhibits band inversion accompanied with the nontrivial band gap opened Nevertheless,there is no opened nontrivial gap observed in GaSb/Gr/GaSb due to the emergent surface states.Further investigation shows that hydrogen passivation is feasible to eliminate the surface states.By calculating nanoribbon band structure and topological invariant,we predict that the Gr/GaSb possesses quantum spin Hall effect.Normal strain can further accelerate the increment of band gap and the strength of band inversion.Using non-equilibrium Green’s functions,we calculate the transport properties of heterostructures and find that the armchair nanoribbon(with graphene boundary as reference)exhibits better transport performance.III.The growth rules of Bi(110)/(111)on Cu(111)and MoTe2 substrates are analyzed.We predict that the growth possibility of Bi(110)/(111)on different substrates.On Cu(111),we study the growth rules of Bi atoms.By analyzing the most stable structures of Bin(n=1～8)clusters,we find that Bi atoms prefer to form periodic zigzag chains,and eventually form a Bi(110)layer.Flat surface a-bismuthene(Fa-Bi)can be formed by depositing two Bi(110)layers on Cu(111)and maintains its stability below 60 K.When Fa-Bi stays away from Cu(111),the distorted surface a-bismuthene(Da-Bi)will be formed and maintains its stability up to 450 K.In multilayer Bi(110),we find that the Bi(110)shows more stable with even layers than that with odd layers.On MoTe2,We study the nucleation and formation energy variation.The results show that,when Bi atoms are few,they tend to form small polyhedral clusters.The differences of formation energy between Bin clusters and Bin sheet show that Bi atoms begin to form stable layered structures only when the number of Bi atoms reaches a critical value.Furthermore,by using ab initio molecular dynamics simulations,we predict that Bi(111)can directly grow on MoTe2.The Bi(111)/MoTe2 forms "type Ⅱ" band structure,accelerating the separation of electrons and holes,which shows great potential in photoelectronic devices.