Electrical Properties Study Of Sn-based Janus Two-Dimensional Materials

In recent years,due to the successful preparation of graphene,researchers have begun to engage in the study of two-dimensional(2D)materials.The zero band gap structure of graphene has led to its limitations in practical applications,so researchers began to look for new 2D materials with unique structures and excellent electrical properties.Among them,the single-layer TMDs structure stands out due to its extremely high carrier mobility,limited direct band gap,large on-off ratio and strong photoelectric response.This structure has three atomic layers stacked on top of each other.By keeping the middle layer as a transition metal element,the outer layer atoms become two different chalcogen elements to construct a brand new lattice structure called Janus structure.For example,in 2017,researchers successfully synthesized a Janus single-layer Mo SSe through chemical vapor deposition for the first time.This structure destroyed the symmetry of the upper and lower layers of the 2D TMDs structure,causing the material to produce interesting electrical properties.Inspired by the Janus Mo SSe,we studied the electrical properties of Sn-based Janus 2D materials and their heterostructures based on first-principles calculations based on density functional theory.At the same time,we know that the energy band regulation of 2D materials and their heterojunctions is very important for their applications in the field of optoelectronics.Therefore,this paper also studies the effects of stress,electric field and the number of layers on Janus 2D materials and their heterojunctions.In order to realize the regulation of its electrical properties,so that it has a wider range of applications.First,we calculated the electrical properties of the Janus 2D material Sn XY(X,Y=O,S,Se,and Te,and X?Y).The study found that the matrix configurationSn X₂(X=O,S,Se and Te)of the Janus 2D material is an indirect band gap semiconductor or metal.However,the Janus single-layer Sn XY derived from Sn X₂not only behaves as indirect bandgap semiconductors and metals,but also SnOS and SnOSe behave as direct bandgap semiconductors.This is due to the difference in electronegativity between atoms due to the out-of-plane asymmetry,which causes different orbitals of the elements to have different effects on the energy band,and finally causes the essential change in the energy band structure of the material.In addition,studies have found that using Te atoms to replace S and Se atoms inSnS₂and SnSe₂can induce the material to transform from indirect bandgap semiconductors to direct bandgap semiconductors,and then to metals by adjusting the doping concentration.Secondly,the Janus 2D materials SnOS,SnSSe and SnSTe are controlled by stress and electric field.Studies have shown that for direct band gap semiconductor SnOS,stress can transform it into indirect band gap or metal;for indirect band gap semiconductor SnSSe,stress can transform it into metal;for metal SnSTe,stress can transform it into indirect band gap Semiconductors,and finally realize the transition from indirect to direct bandgap semiconductors.In addition,applying an electric field to a Janus single-layer material can also effectively control its energy band,but more interestingly,because the upper and lower layers of the Janus structure are made of different chalcogen elements,the out-of-plane symmetry is broken and its production is induced.The built-in electric field results in a different response to the external positive and negative electric fields,which shows the broad application prospects of double-sided materials.Finally,we constructed a 2D heterojunction with a Sn-based single-layer Janus material.Studies have shown that heterojunctions can greatly control the energy band structure of materials.For example,the SnOS/SnOSe heterojunction composed of two direct band gap semiconductors,SnOS and SnOSe,exhibits metallic properties.In addition,according to the different arrangement of atoms in the heterojunction,such as the heterojunction composed of SnOSe and SnSSe,the closest atoms between the layers can be divided into O-S,O-Se,Se-S and Se-Se.This results in four different types of heterostructures.Due to the destruction of the symmetry between the planes of the Janus structure,the distance between the layers of the different configurations is slightly different,which ultimately leads to different band gap values.Finally,we thoroughly studied the regulation of the energy band structure of the heterojunction by the applied stress and electric field,and found that most of the transition of the energy band structure of the heterojunction is very similar to the monolayer that constitutes the heterojunction.The difference is that the SnOSe/SnSSe heterojunction can realize the mutual transformation from semiconductor to metal,direct band gap to indirect band gap under the action of different electric fields,which is not available in single-layer SnOSe and SnSSe.Therefore,the different Mass junction has greater application prospects in the field of optoelectronics.