Surfaces And Interfaces Of Two-Dimensional Materials:Electronic Structure Regulation And Related Mechanisms

Unique layered structure endows two-dimensional(2D)materials with considerable specific surface area,which is pivotal to their applications in some fields that are highly dependent on surface or interface properties,such as gas sensing,field effect transistors(FETs),and 2D substrates or buffer layers.In this dissertation,modulation methods of surface and interface electronic structure of several 2D non-metal compounds(carbide: Ti2C;nitride: h-Al N;sulfide: GeS,SnSSe)have been investigated theoretically.Meanwhile,some issues such as high performance gas sensing materials,interactions between 2D substrate and epitaxy layer,metal for electrode of 2D FETs and Janus modification were explored,their underlying mechanisms were also explained from an atomic scale.The main contents are list as follows:1.Based on density functional theory(DFT)method,the intrinsic and doping electronic structure of 2D GeS were calculated,and a kind of high sensitive and high selective GeSbased room-temperature volatile organic compounds(VOCs)sensing material was proposed,known as Al-GeS.The O-contained VOCs molecules(methanol,ethanol,formaldehyde,and acetone)converted from physical to chemical adsorption after Al doping,however,the adsorption type of other molecules(trichloroethylene and benzene)remain undisturbed.Especially,O-contained VOCs-Al-GeS adsorption systems are dynamically stable at 300 K,while other molecules will desorb from Al-GeS immediately.That is to say,Al-GeS can identify O-contained VOCs molecules effectively with desired sensitivity at room temperature.In addition,functional groups in O-contained molecules(carbonyl or oxhydryl)can be clarified by studying the electronic and magnetic properties,which may further enhance the selectivity of sensor.This work manifests Al-GeS nanosheet as a practical candidate to room-temperature VOCs sensor with high sensitivity and selectivity.2.Based on DFT method,the stability and related mechanism of honeycomb borophene on 2D Ti2 C substrate was studied,and a step-related kinetic process occurs at the edge of boron cluster was proposed as the epitaxy mode.Our results indicated that honeycomb borophene is energetically favorable on Ti2 C with desirable kinetic and thermodynamic stablity.The dominant mechanism of it may attribute to the strong interface interactions,including charge redistribution,orbital hybridization and bond formation.Limited by the potential energy suface of Ti2 C substrate,boron atoms are tend to migrate along the hexagonal network composed by hollow sites,and a six-membered boron ring is formed as basic epitaxy unit.Successive boron atoms preferred to neutralize the unsaturated bonds at the atomic edge of boron islands,so as to realize the expand and coalesce of islands,and finally form the borophene monolayer.Our results indicate that 2D Ti2 C is a potential substrate or buffer layer to stabilize honeycomb borophene.3.The regulation of intrinsic polarization field in Janus SnSSe to the electronic structures on different terminal surface was investigated by adopting two typical application scenarios,namely molecule adsorption and metal contact.In molecule adsorption part,polar molecule NH3 was found as weak chemisorption among other physisorbed molecules,and its molecule polarization field is succumb to SnSSe.After NH3 adsorbed on SnSSe sensor,its current along x direction enlarged 217.30% for S layer and 203.33% for Se layer at 1.8 V bias,nearly ten times larger than other molecules,and also a huge step up from other 2D NH3 sensors.Thus SnSSe behaves desirable sensitivity and selectivity to NH3 molecule,which can be further regulated by external fields.In metal contact part,SnSSe has small lattice mismatch with common metal electrode(Au,Ag,Cu,Pt,and Pd),and stronger interactions are found in Pt/Pd-SnSSe interface than Au/Ag.The contact type of Au/Ag/Cu-SnSSe is Ohmic,while for Pt/Pd-SnSSe,a n-type Schottky barrier is occured.In particular,carrier’s tunneling probability for Se SnS/Cu(111)and SSnSe/Cu(111)can reach 100% and 97.44%,respectively,indicating high transporting efficiency.Additionally,FLP effect on S layer is found weaker than Se layer.These results provide theoretically reference for the application of SnSSe in NH3 sensors and FETs.4.Based on first principles calculation,the electronic structure regulation of 2D h-Al N hydrogen or halogen(-F,-Cl,-Br)radicals was systematically studied,and the stability of relevent structures were also validated from energetical,dynamical and thermodynamical perspectives.For X-Al N-X(X=H、F、Cl、Br)structures,N-side atoms locate at top site,as for Al-side atom,-H at top site and halogen atom tends to hollow site.These asymmetric structures also induce intrinsic dipole moment,and X-Al bond is more ionic than X-N.The band-gap values for X-Al-X vary from 0.80-2.45 e V,all of them exhibit indirect-to-direct band-gap conversion except H-Al N-H.In addition,H-Al N-H is found to be kinetic and thermodynamic stable among four X-Al N-X structures with a wide photonic band-gap about40 THz.For X-Al N-Y(X,Y=H、F、Cl、Br and X≠Y),N-side atoms remain at top sites along with-F on Al-side,while other atoms on Al-side are adsorbed on hollow sites.Considerable dipole moment is also induced by asymmetric structures in X-Al N-Y,while their bond type is similar to that of X-Al N-X.X-Al N-Y exhibit direct band-gap varying from0.42-3.08 e V.Moreover,Cl-Al N-H and Br-Al N-H are kinetic and thermodynamic stable with ultra-wide photonic band-gaps near 70 THz.This work provide a deeper understanding to the surface electronic structure regulation of h-Al N,and also paves way to their application in microelectronic,optoelectronic and acoustic areas.