A Theoretical Study For Electronic And Transport Properties Of Covalent Functionalized MoS₂ Monolayer

Monolayer two-dimensional?2D?nanomaterial with atomic or molecular thickness has become a focus in the field of materials research.It can be used to build the high quality nanoelectronic devices which benefit from its unique structure and excellent properties.Graphene is a typical monolayer 2D nanomaterial.Following its research,scientific workers threw enthusiasm into exploring the new 2D layered materials.MoS₂ monolayer is a new inorganic 2D nanomaterial,whose emergence actives monolayer 2D nanomaterial efforts and implements the transition of organic to inorganic 2D material,at the same time it possesses excellent semiconductor properties that is different from graphene,so monolayer 2D nanomaterials have a richer properties and more extensive application.While,experiment method to obtain the properties of the 2D nanomaterial sometimes is very hard on account for the atomic size and preparation process,with the development of the related theory of quantum mechanics,the theoretical calculation has become the powerful means to research nanomaterials,and theoretical results can give theoretical support to the nanomaterial research and application.In this paper,the electronic and electron transport properties of MoS₂ monolayer and a series of covalent functionalized MoS₂ monolayers were investigated.The content includes the following two aspects:For 2D periodic systems,the calculations of geometry optimizations,band structures,density of states?DOS?,and the wave functions are performed using density-functional theory?DFT?methods implemented in the SIESTA package.-C3H7,-Si3H7,-C6H5,-C6H4NO2 and-C6H4OCH3 are chosen as electron-donating groups to covalently functionalize to MoS₂ monolayer.The electronic properties of the functionalized systems were investigated,and the influences of different grafted functional groups are studied.The results show that the C or Si atom on these groups covalently bond to the S atom of MoS₂ monolayer and the formation energies of all functionalized systems are relatively large.Covalent functionalization make a transformation from typical semiconducting properties of pristine MoS₂ monolayer to the metallicity for appearence of the grafting group induced midgap level that is a couple band across the Fermi level.The covalent functionalized effects of different grafting functional groups for MoS₂ monolayer shows that-C6H5,-C6H4NO2 and-C6H4OCH3 with strong electron-donating make larger effect on the whole of the conduction band relative to the n-propyl or n-trisilicyl functionalization.In addition,compared with d-bulk and f-bulk,e-bulk forms a bigger ?-conjugation by the introduction of the nitro to phenyl para-position,so it produces a stronger effect on system and shows trap states,which indicates the different substituent groups at the para-position of phenyl has an effect for electronic properties of functionalized MoS₂ monolayer.Taking phenyl functionalized structures as exmaple,we observe the effects of grafting density through the comparative study of functionalized 3×3×1 MoS₂ and functionalized 2×2×1 MoS₂,and find that the bigger grafting density leads to wider bandwidth of midgap level across the Fermi level,and the functional groups make a greater contribution to the midgap level.For the two-probe devices,the transport calculations about I-V curve,transmission spectrum,Molecular Projected Self-Consistent Hamiltonian?MPSH?spectrum and potential distribution are carried out using DFT in combination with the non-equilibrium Green's function?NEGF?methods in ATK package.Then,we study the influence of the different grafting functional groups or different grafting density on electrical conductivity.The results show that covalent functionalized systems show the metallicity and electrical conductivity increases,and phenyl functionalization gives the most obvious effect for the conductivity of MoS₂ monolayer.And the current response of the bigger grafting density is obvious larger,leading to better conductivity.At the same time,all I-V curves of functionalized systems show NDR effect,and MPSH states and potential distribution not only give explaination for the NDR effect,but also show that the grafting functional groups contribute to the major transmission path.Therefore,electronic and transport properties of MoS₂ monolayer can be tuned by covalent functionalization and then expand its potential applications as low dimensional electronic materials.