First-principles Study Of Structural And Electronic Properties Of Two-dimensional Materials And Their Hybrid Heterostructures

Two dimensional materials,represented by graphene,hexagonal boron nitride and metal dichalcogenides,have been of prominent interest due to a number of exceptional properties and potential applications in several technological fields.In particular,the ultrathin 2D nanomaterials possess strong quantum confinement effect,ultrahigh specific surface area and maximum mechanical flexibility,in contrast to the counterparts in bulk form.Having the powerful theoretical tools at the level of density functional theory,the researchers now are able to deeply understand the unique properties of the 2D materials at atomic scales.Meanwhile,computer-aided material designs and the testing of material properties not only can lead to discovery of new materials but also provide useful guidance for experimental explorations.In this thesis,using first principles calculations within the framework of density functional theory,we have systematically studied the structural,electronic and chemical properties of a series of 2D materials,their derivatives and related hybrid heterostructures,which include MoS₂/graphene,MoS₂/BN/graphene,MoS₂/Si C composites,Zn O/VS₂ vertical heterostructures.We have comprehensively examined the modification in structures,electronic properties and optical properties of these structures upon applying external electric field,engineering strain and chemical doping.The major findings are summarized as follows:?1?Phase crossover in transition metal dichalcogenide nanoclusters.We perform first-principles analyses on the stability of MX₂ nanoclusters.The MX₂?M = Mo,W;X = S?clusters thermodynamically show a high level of phase variability,i.e.varying from 2H phase,which is the ground state of two-dimensional MX₂,to 1T phase with decreasing cluster size or chemical potential of X.In addition,lower chemical potential of X endows the clusters with a stronger propensity of shaping in hexagons,instead of commonly observed triangles,consistent with recent experiments.Based on numerical analyses,we further express the energy of different types of clusters in terms of chemical potential and cluster size,and map out a structural phase diagram.These findings call for a revisit of the lattice structures of MX₂ clusters and may also rationalize the frequent observation of meta-stable 1T domains embedded in otherwise perfect 2H MX₂ monolayers.?2?Effect of interface structures on photocatalytic properties of the MoS₂/graphene heterostructures.The photocatalytic behavior and mechanism of MoS₂/graphene,MoS₂/nitrogen doped graphene,MoS₂/graphene oxide and niobium doped MoS₂/ nitrogen doped graphene composites are studied using first-principles calculations.For Nb doped MoS₂/ nitrogen doped graphene composite,Nb doping gives rise to an acceptor impurity for the MoS₂ layer,which is apt to accept more charges from the N-doped graphene,and the increased degree of charge transfer enhances the stability of the heterostructures.The reason for the enhanced photocatalytic performance of MoS₂/ nitrogen doped graphene and Nb doped MoS₂/ nitrogen doped graphene composites results can be twofold: i)the favorable electron transmission?the N-doped graphene sheets were electron-deficient?,ii)the obviously reduced recombination of electrons and holes and relatively strong light absorption within the visible light region.?3?Electronic properties of MoS₂/Si C heterostructure.We reveal by density functional theory calculations that the MoS₂/single-layer Si C heterostructure is a direct band-gap semiconductor with type II alignment,which promises the applications in photocatalysis.The heterostructure of MoS₂/bilayer Si C possesses a direct bandgap of 0.66 e V.In contrast,the MoS₂ monolayer on a Si-terminated Si C substrate shows a metallic behavior,whereas the MoS₂ monolayer on a C-terminated Si C substrate has an indirect bandgap of 0.8 e V.?4?Electric-field and strain-tunable electronic properties of MoS₂/h-BN/graphene vertical heterostructures.Our comprehensive first-principles analyses show that an applied electric field not only enhances the interlayer coupling but also linearly controls the charge transfer between graphene and MoS₂ layers,ending up the electric-field tunable doping in graphene.Specially,the Schottky barrier height in the heterostructure can be widely adjusted from zero to ⁰.6 e V by applying a moderate electric field.In contrast,the applied strain has an opposite effect on stabilizing the whole structures and has little effect on these electronic parameters.The tunable electronic properties by strain and electric fields are immune to the presence of sulfur vacancies,the most common defect in MoS₂.?5?Field emission properties of a Zn O cluster chemisorbed on a 2D VS₂ monolayer.Our density functional theory calculations show that the stability of composite structures of Zn O clusters on the VS₂ monolayer can be enhanced by applying external electric fields.Under the electric field,the S atoms of VS₂ layer tend to amass negative charges which can contribute to the field-emission of the VS₂ layer.The work functions and ionization potentials for the VS₂–Zn O composites linearly decrease with increasing the field strength.A S-substituted Zn O cluster adsorbed on the VS₂ monolayer may possess even better performance of field emission.