Study On The Electronic Structures And Transport Properties Of Two-dimensional Group-? Monochalcogenides

Group-? monochalcogenides MX?M=Sn/Ge,X=S/Se?have phosphorene-like puckered honeycomb structure,which are also called phosphorene analogues.The chemical stability of MX materials is higher than phosphorene,and have excellent electronic,thermoelectric,piezoelectric,ferroelectric properties,which are promising candidates for next-generation nanodevices.The study on the properties of MX materials have drawn increasing attention.In this work,the electronic structures and transport properties of monolayer GeSe and SnSe,SnS-SnSe nanoribbon and GeSe/BP lateral heterostructure are investigated by using density functional theory combined with nonequilibrium Green's function?NEGF?.We found some physical phenomena,such as negative differential resistance effect and type-?band alignment,which have potential applications in oscillators,field-effect transistors,solar cells and so on.Their inherent physical mechanisms are explained through Landauer theory and band structure theory,which may provide more theoretical guidance in the design of nanodevices based on group-IV monochalcogenides.The main contents of this thesis are concluded as follows:1.The influences of strain and Au electrode contact on the electronic structures and transport properties of GeSe and SnSe were investigated.Monolayer GeSe and SnSe are semiconductors with indirect band gaps,and the strain along zigzag direction can effectively tune their band gaps.When coupled with Au electrode,the band structures of GeSe and SnSe undergo hybridization.The study of transport properties demonstrated that the effects of strain exhibit strong anisotropy.The strain along armchair direction enhances transport properties and the strain along zigzag direction inhibits transport properties.GeSe and SnSe form p-type Schottky contact with Au electrode,and the peak current is enhanced under the applied voltage.These findings provide guidance for the study on the interface properties of metal-MX systems and the design of devices based on monolayer MX.2.The effects of width and edge on the electronic structures and transport properties of SnS-SnSe nanoribbons were studied.It is demonstrated that SnS and SnSe nanoribbons with armchair edges are semiconductors.Their band gaps vary with the ribbon width and have indirect-to-direct transition.In contrast,SnS and SnSe nanoribbons with zigzag edges are metals.The calculations of transport properties revealed that armchair SnS-SnSe nanoribbons exhibit semiconductor-like transport characteristics,and are independent of the ribbon width.The I-V curves of zigzag SnS-SnSe nanoribbons exhibit the negative differential resistive?NDR?effect and are also independent of the ribbon width,but the peak-to-valley ratio?PVR?of current decreases with the increase of width.The study demonstrated that zigzag SnS-SnSe nanoribbon have potential applications in NDR devices,which provide an important reference for the design of nanoribbon devices based on group-IV monochalcogenides.3.The electronic structures and transport properties of GeSe/BP lateral heterostructure were investigated,and their properties were tuned by applied strain.The calculations of electronic structures demonstrated that?GeSe?₈/?BP?₈ lateral heterostructure is direct band gap semiconductor with type-?band alignment,in which CBM and VBM are contributed by GeSe and BP,respectively.Their band gaps fluctuate with the ribbon width and the band alignment conversion from type-?to quasi type-?.Further studied found that the strain can significantly tune the band gap,but it cannot modify the band alignment.The study of transport properties found that the?GeSe?₈/?BP?₈ lateral heterostructure exhibited semiconductor characteristics,and strain can enhance its transport properties.Furthermore,the?GeSe?₈/?BP?₈ lateral heterostructure exhibited negative differential resistance effect when the applied strain is 5%.These studies provide a theoretical basis for the design of GeSe/BP lateral heterostructure nanodevices,and also provide corresponding guidance for future research based on other MX lateral heterostructures.