First-Principles Study On The Electronic Properties Modulations Of The Van Der Waals Heterobilayer Built By Pb I₂

Since monolayer graphite was successfully isolated via mechanical exfoliation,great attentions has been paid to the two dimensional(2D)research field,and as a result,lots of other atomically thin 2D materials have been widely studied due to their extraordinary fundamental physical properties.Such 2D materials have been theoretically and experimentally confirmed to possess novel properties which are different from even batter than their bulk counterparts.Therefore,the materials have attracted intensive research efforts due to their remarkable electronic or optoelectronic properties and potential applications in energy,photonic and nanoelectronic drives.However,the practical application based on the above 2D materials have serious band gap hurdles,i.e.the lack of obvious gap in graphene and too large gap in boron nitride.Single layer metal di-chalcogenides such as MoS2 possess an appropriate band gap,but are strongly influenced by metal contacts,interface traps,charged impurities,dielectric environment,and structural defects.Therefore the search for new types of 2D structures with proper band gaps is of paramount importance for next generation nano-device fabrication.PbI2 is a wide bandgap(about 2.5 eV)lamellar semiconductor with a high density of 6.16g/cm3 which has been found to successfully growth freestanding single crystals in experiment.As an emerging research area,to assemble the isolated atomic configurations layer by layer into heterostructures(often called “Van der Waals heterostructures”)in a precisely controlled sequence is an exciting research field.The heterostructure has attracted intensive research efforts due to their remarkable electronic or optoelectronic properties and potential applications as lasers,light-emitting diodes,solar cells,and high-electron-mobility transistors,and so on.In this paper,we performed a theoretical simulation to investigate the electronic structures of PbI2 monolayer and the effect of external perpendicular electric field(Efield)on the electronic properties in van der Waals heterobilayer built by PbI2.Our results show that the PbI2 based vdW heterostructure with a well-Efield-controlled manner can be used as nano-electronic and advantage optoelectronics devices.The main work and innovation points in this study are summarized as follows,1.Effects of the Spin Orbit Coupling as well as strain on the electronic properties of single layer lead iodide.Because there is no studies on the electronic properties by theoretical simulation of PbI2 monolayer,the basic electronic structure of PbI2 is unclear.Therefore,we firstly study the stability of the T-and H-phase for PbI2 films,and that the T phase is more stable was proved by the analysis of energy and phonon spectrum.Taking into account the number of extranuclear electrons in Pb atom,the electronic structure of monolayer PbI2 was studied with the SOC effects as well as PBE and HSE06 methods and a prefect electronic band structure was obtain by HSE06+SOC approach.Additionally,the smallest gap of PbI2 monolayer appears between the conduction band minimum(CBM)at ? and the valance band maximum(VBM)medially lies between K and ? which indicates a indirect band gap.The electronic properties vary obviously with biaxial strain indicating that the properties of monolayer PbI2 are intensely sensitive to in-plane strains.2.Band structure engineering in MoS2/PbI2 van der Waals heterostructure via an external electric field.Band structures engineering in MoS2/PbI2 van der Waals(vdW)heterostructure with an external electric fields(Efield)are investigated by density functional theory(DFT).It is demonstrated that the Mo S2/PbI2 vdW heterostructure is a type-? heterojunction with a direct bandgap,and thus the lowest energy electron-hole pairs are spatially separated.Meanwhile,the band structure could be effectively modulated under an Efield and the bandgap shows linear variations with Efield,indicating a giant Stark effect.It gets further support from the band edges of MoS2 and PbI2 in the heterostructure.Moreover,the Mo S2/PbI2 vdW heterostructure experiences transitions from type-? to type-I and then to type-? with various Efield.Our calculated results pave the way for experimental research and provide a new perspective for the application of vdW heterostructure in electronic and optoelectronic devices.3.Effect of external electric field on electronic properties of SnS2/PbI2 van der Waals heterostructure.Future development of optoelectronic devices will require an advanced control technology in electronic properties,for example by an external Efield.Here we demonstrate an approach that the heterostructure based on vdW heterobilayer built by monolayer Sn S2 and Pb I2 has a well-controlled electronic properties with Efield.A type-? staggered-gap band alignment is achieved from the SnS2/PbI2 vdW heterostructure with which SnS2 dominated the lowest energy holes as well as the lowest energy electrons are separated in PbI2.The charge redistribution with an Efield is mainly on the surface of SnS2 layer and PbI2 and the numbers of polarized electrons on the monolayers display a linear evaluation with external Efield.The band structure under different Efield experiences not only a transition from semiconductor to metal but also conversions between type-?straddling-band alignment and type-?staggered-gap,which results in different spatial distribution of the lowest energy electrons and holes.Moreover,when the Efield is between-0.06 V/? and-0.34 V/?,the material manifests a varied direct bandgap which is more favor to optoelectronics and solar cell.Consequently,this vdW heterobilayer with well-controlled manner shows expectation for huge potential in optics and electronics.4.Modulation of interfacial electronic properties in PbI2 and BN van der Waals heterobilayer via external electric fieldThe interfacial electronic properties of PbI2 and BN van der Waals(vdW)heterobilayer are explored by using density functional theory(DFT)method.An intrinsic type-? heterostructure with a wide bandgap is demonstrated.The spatial separation of the lowest energy electron-hole pairs can be actualised and make PbI2/BN heterostructure as a good candidate for applications in optoelectronics and solar cell.A simulation of Efield is actualized to modify its electronic properties.Band alignment converts from type-? to type-? heterostructure separated by a forward voltage with the value of about 0.07 V/?.Three regions implying different Efield-sensitive properties are obtained from the variations of bandgap with Efield.The charge redistribution with an Efield is mainly on the surface of PbI2 and BN layers as well as the amount of electrons depends on the strength of Efield.In addition,the PbI2/BN heterobilayer exhibits more outstanding optical conductivity capability.Our results could bring forward a new perspective on sensor and shed light on the design of novel nano-and optoelectronics based on the PbI2/BN vdW heterostructure.5.Tunable Schottky contacts in the graphene/PbI2 van der Waals heterostructures.The contact between semiconductor and metal is a very important issue in the fabrication of nanoelectronic and optoelectronic devices.In the last chapter,in order to know more about the contraction between PbI2 and metal,the graphene was used to stack with PbI2 to discuss the modulation of the external electric field on the Schottky barrier of graphene/PbI2 heterostructure.The results show that the graphene/PbI2 heterobilayer is composed of weak van der Waals interactions,and retains its independent electronic structure to a great extent.The interface of heterosyructure shows the p-type Ohmic contact and leads to the p-type doping of graphene.The content can be effectively modulated by external electric field and the transitions between n-type and p-type Schottky barrier.The negative electric field greatly enhanced the p-type doping of the graphene layer but the n-type doping required a large positive electric field.