First Principles Study On The Effects Of Vacancy Defects On The Properties Of Janus MoSSe And Its Heterostructure

In the preparation of experiments,it is easy to introduce miscellaneous items and defects that often have impacts on the physical properties of materials.In this work,we investigate the effect of vacancy defects on the electronic properties of monolayer Janus MoSSe and Janus MoSSe/Graphene heterostructures using the first-principles calculations method based on density functional theory.The Mo,S,Se,and S-Se vacancy defects in the Janus MoSSe monolayer all result in a reduction of the band gap and a corresponding change in the static potential difference.We found that MoSSe＿V_Seis more stable in all the four cases studied.For chalcogen vacancies,both S and Se vacancy defects are more stable than S-Se vacancy defects,while the vacancy defects in Mo atoms have highest vacancy defect formation energy and tend to be more difficult to form in Janus MoSSe monolayer.With the introduction of a Mo vacancy in MoSSe,the spin-up and spin-down channels in the band structure are not aligned,which produces a magnetic moment of 0.75μ_B.Calculations of the vacancy formation energies shows that S and Se single-vacancy defects are more likely to form in the monolayer MoSSe.On this basis,we further investigated S and Se defect structures in Janus MoSSe/Graphene and compared the effect of two stacking modes,SMoSe/Graphene and Se MoS/Graphene,on the electronic properties.The intrinsic SMoSe/Graphene heterostructure results in the formation of an n-type Schottky barrier of 0.63 e V.After the introduction of the defects,the n-type Schottky barrier drops to 0.07 and 0.03 e V for S and Se vacancy defects,respectively.Therefore,vacancy defects are undesirable for the design of MoSSe/Graphene-based Schottky devices because they always deteriorate the performance and reliability of the device.The Se MoS/Graphene heterostructures form a very small barrier height,almost forming an Ohmic contacts.The generation of vacancy defects makes the Dirac cone undergo a significant upward shift to 0.35 and0.39 e V over the Fermi level,which implies that charge-carrier doping occurs on the graphene surface and a p-type doping is formed.The charge-carrier densities of graphene with S and Se vacancy defects in Se MoS/Graphene are 0.900×10¹³cm^-2and 1.117×10¹³cm^-2,respectively.In this stacking mode,the build-in interface electric field is the same direction as the interlayer electron movement,which greatly promotes the transfer of interlayer electrons,and the Se vacancy defect will have a greater impact on the heterostructure.In addition,the differences between the two stacking modes give rise to interface dipoles in opposite directions.The biaxial strain and the applied electric field in the z-direction can effectively modulate the doping charge-carrier densities of graphene.During the application of biaxial strain,the stability is significantly reduced when the lattice vector changes by6%,and the heterostructures are more stable under compressive strain.Under biaxial strain,it is able to modulate the density but cannot change the type of the graphene doping charge-carrier.The highest carrier density of graphene can reach 2.02×10¹³cm^-2for the Se MoS＿V_S/Graphene stacking mode after a strain of-6%is applied.The applied electric field also enables modulate the doping charge-carrier densities,change its type,as well as suppress the doping altogether.Our results could provide some tending guidance for the use of two-dimensional Janus MoSSe/Graphene heterostructure materials in devices with promising applications in tunable nanoelectronics devices.