Theoretical Study On The Electronic And Optical Properties Of Two-dimensional Layered Transition Metal Chalcogenides

The rapid development of two-dimensional materials with excellent photoelectric properties,such as graphene,Mo S₂ and WSe₂,has aroused researchers'extensive attention.The larger specific surface area of two-dimensional layered materials is capable of absorbing more photons.Their thinner atomic layer thickness imporves the carrier transport rate and restrains electron-vacancy recombination.At present,it is known that the band gap of two-dimensional semiconductors can be effectively adjusted by constructing a heterojunction.Theoretical study of the influence of this modification method on the photoelectric properties of two-dimensional materials can not only provide in-depth understanding and exploration of electronic and optical properties on the atomic scale,but also provide theoretical support for related experimental research.In this thesis,the photoelectric properties of two-dimensional layered transition metal sulfide compounds are investigated mainly by the First-Principles calculation.The author explored the electronic structure and applications in the field of photocatalysis of binary monolayer MX(M=transition metal,X=S,Se,Te)compounds and ternary monolayer K_nMX(n=0.5,1)compounds.Based on the GGA-PBE and HSE method,we found that the conduction band minimum and valence band maximum of the five semiconductors in the binary monolayer MX compounds and the three semiconductors in the ternary monolayer K_nMX compounds all appear at the?point in the Brillouin zone.As a result,they are all direct band gap semiconductor and easier to produce light-induced electrons and light-induced vacancies.The absorbance of single-layer MX compounds for visible light are generally low,but the absorbance of KAg S,KAg Se,and KAg Te for visible light have a great improvement compared to the corresponding binary compound,which makes it have great potential in the field of photoelectric materials.In addition,the electron and optical properties of the two-layer heterostructure constructed by stable single-layer MX were studied by density functional theory.In the heterostructure formed by a single-layer semiconductor and a single-layer semiconductor,we found that the transfer of charge significantly occured between the Zn Se layer and the Zn Te layer in the two-dimensional heterostructure Zn Se/Zn Te.As for the Zn Se/Zn Te double-layer heterostructure,stronger visible light absorption was observed compared with the single-layer semiconductor that formed it.In the double-layer heterostructure formed by a single-layer conductor and a single-layer semiconductor,there is a built-in electric field and the flow of free electrons at the interface.The position of the band edge of the heterostructure has increased or decreased compared with the position of the band edge of the monolayer semiconductor constituting the heterojunction.The Schottky barrier generated by the band bending can promote the separation of electrons and holes and improve the photocatalytic efficiency.As a result,the application range of tetragonal two-dimensional heterostructure materials is wider.