First-principle Study On The Electronic Properties Of Phosphonene-like Materials And Their Heterojuctions

Due to the excellent characteristics such as high carrier mobility,moderate switching ratio as well as layer-dependent electronic properties,phosphene,as a new representive of low dimensional nanomaterial,is emerging quietly.Meanwhile,phosphonene analogues,which are isostructural to the phosphiene,have been attracted much attention as they were proved to have broad application prospects in the field of solar cells,field-effect transistors and photoelectric converters.In this thesis,through first principles calculations,we first focus on intrinsic doping and chemical doping of SnSe,with the aim of investigation the effect on the electronic properties.Then,we use GeS and phosphonene to form van der Walls heterostructures,the electronic properties and property modifications were systematically explored.The thesis is organized with the following six chapters:In the first chapter,we generally introduce the research status of low dimensional nanomaterials and phosphonene-like materials,as well as the research significance and main contents of this thesis.In the second chapter,theory of the first principles and the related concepts and theorems as well as the VASP software package are briefly introduced.In the third chapter,the formation energy,electronic and magnetic properties of intrinsic defects in bulk SnSe are systematically studied.Three types of defects are simulated,i.e.,vacancies(Vsn/Vse),interstitials(Sni/Sei)and antisite(Snse/Sesn).The results show that different preparation conditions have different effects on the formation energies:Vse,Sni and Snse have lower formation energies under Sn-rich condition,but under.Se-rich condition,the formation energies of Vsn,Sei and Sesn become lower instead.Further through calculations of the formation energy and thermodynamic transition energy level,it is found that Vsn can act as an effective source for p-type conduction due to having a lower formation energy as well as a desirable ultra-shallow transition energy level.Moreover,spin-polarized calculations indicate that no magnetism is found for the intrinsic defects in SnSe.In the fourth chapter,the electronic structures,magnetics and optical properties of SnSe monolayer by chemical doping are systematically investigated.On the basis of the substitution at anion site,it is difficult for F,Cl and Br to realize n-type conduction in single layer SnSe due to their ultra-deep transition energy levels.For p-type doping(N,P,As),because of the lowest formation energy and a desirable ultra-shallow transition energy level,a effective p-type conduction is likely to be realized by P doping.On the other hand,the cavity doping at cation site indicates that the substitution of Mg is easier to be achieved than that of Li and A1 because of lower doping energy.The Li-and Mg-doped systems are nonmagnetic ground state,and the impurity atom A1 not only introduces the magnetic properties,but also improves the absorption of the SnSe nanoscale in the visible region.In the fifth chapter,the electronic properties of vdW heterostructure composed of phosphorene(BP)and GeS monolayer which possesses Phosphonene-like structure are systematically explored.By constructing lattice match model(Mat)and lattice mismatch model(Mis),we demonstrate the carriers are not separated for both heterostructures.For Mat,it is found changing monolayer of GeS to bilayer can enlarge the energy difference of valence band offsets between GeS and BP,thus realizing electron-hole separation.For Mis,altering the layer distance can transform the heterostructure into a typical type-I alignment,but applying the electric-field or doping with F4TCNQ can make it display a perfect desirable type-II alignment.Further research shows that holes migration and electrons transfer are revealed to respectively account for the phenomenon of carrier separation.The last chapter summarizes the research work of this paper,the future research direction and application prospect are introduced.