The Properties Of ?-? Layered Chalcogenide Semiconductors And Their Heterojunctions:First Principle Study

Because of the novel chemical and physical properties and the excellent potential applications in many fields,low-dimensional materials become the frontier and focus for the research of nanomaterials,which have more and more profound impact in the fields of energy sources and information.In the aspects of microscopic design and performance prediction for the nanomaterials,density functional theory(DFT)based on first-principles has been proved as a wide and effective means.In this dissertation,firstly,we systematically investigated the stabilities,electronic and optical properties as well as their property modulations of SnSe2 and SnS2 solid solution heterostructure.Secondly,we laid our emphasis on the properties of MXs(M=Sn,Ge;X= Se,S),especially the layer-dependent electronic properties.Finally,according to the band edge alignment calculations,we discussed the possibility of forming the type-II heterostructures through mutual coupling among MXs,MXs and few-layer phosphorene,and we proved that the type-I to II or type-II to I transition can be realized by changing the number of layers.The dissertation is divided into six chapters altogether.The first chapter mainly introduces the research status of the materials,the research background and the goal of this dissertation.In the second chapter,we briefly explain the density functional theory and introduce the first-principles software VASP package,which is used in this dissertation.In the third chapter,we systematically investigated the stabilities,electronic and optical properties of SnSe2(1-x)S2x alloys.We found that a “random” alloy is formed when the solubility temperature reaches 702 K,only guaranteeing that the sulfur atoms are evenly distributed in the upper and lower layers.With the increase of the concentration of sulfur atoms,the band gap value decreases non-linearly and the optical properties exhibit anisotropic behaviors in which the absorption peaks are obviously enhanced in the visible region.In the fourth chapter,the layer-dependent properties,i.e.,stabilities,electronic structures,optical properties and effective masses of MXs(M = Sn,Ge;X = Se,S)are systematically explored.It was found that with the layer number increasing,MXs become more stable but the band gap value takes a decrease tendency.The band gap values exhibit an excellent linear relationship with the reciprocal of the layer numbers.Optical properties investigations show that MXs are anisotropic,especially for the SnS and SnSe,they display a strong absorption character in the visible region.On the basis of band alignment calculations for the mono-and few-layer MXs and phosphorene,in the fifth chapter,we mainly focused on the possibilities of forming type-II van der Waals heterostructures,and the layer-dependent properties are further investigated.By constructing the Moiré pattern for the heterostructures,the transition from type-I to II or type-II to I is confirmed to be achieved by changing the number of layers.The last chapter is the summary and prospects of this dissertation.