Manipulation Of Structure And Electronic Properties For Novel 2D Materials

In recent years,nanomaterials with excellent physical and chemical properties have been developed rapidly,showing great potential in many fields.Since the successful exfoliation of graphene in 2004,a large number of two-dimensional(2D)materials have been predicted in theory and some of them also have been synthesized in experiments.These 2D nano-materials possessing high carrier mobility,excellent photoelectric performance,and adjustable band gap,are expected to become the next generation of microelectronic device materials replacing traditional silicon-based materials.Field effect transistors(FETs)as one of the basic devices in many microelectronic devices can be used in controller switches,gas sensors and many other devices.Based on 2D nanomaterials,FETs has higher stability and performance.In addition,the performance of 2D nano-materials can be easily regulated by applying suitable substrates,stress or electric field due to the layered structure.Meanwhile,2D materials with intrinsic magnetism have been fabricated successfully in experiment,which further broadens the field of 2D nano-spintronic devices.By making full use of charge and spin freedom,2D nano-spintronic devices have the advantages of small size,fast speed,high storage density and low power consumption.Therefore,they are believed to be developed into the next generation of nano-spintronic devices.Moreover,low-dimensional materials also have important application prospects in the field of photocatalysis.As an important means to solve the issues of environmental pollution and energy shortage,photocatalysis attract great interests.However,the photocatalytic efficiency of existing photocatalytic materials can't satisfy the need of large-scale applications.Therefore,improving the efficiency of photocatalysts is the key point of photocatalytic research.Metals depositing on photocatalyst surface as co-catalysts can reduce the activation energy of reactants and promote the catalytic reaction.With the rapid development of nano-science,a new concept of single-atom catalysis(SACs)has been proposed.When the catalyst is reduced from the nano or sub-nano clusters to the single atom,the enhanced size effect,increased unsaturated coordination and exposed active sites help to maximize the catalytic efficiency.In this dissertation,we study the structure,electronic properties,magnetic properties,gas-sensitive properties and photocatalytic properties of two-dimensional nano-materials by first-principles calculations.Through the systematic research,the regulation of constructing two-dimensional composite system on electronic and magnetic properties,the effect of built-in electric field on gas adsorption behaviors,and the influence of loading single noble metal on photocatalytic performances are revealed.This dissertation is divided into six chapters.In the first chapter,a brief overview of the research background,the current research status and the related applications of two-dimensional functional nano-materials is provided.In the second chapter,the basic theory and software package of first-principles calculations are introduced.In the third chapter,the design of new functional 2D composite system and its application in field-effect transistors and spintronic devices are discussed.In the fourth chapter,the gas adsorption behavior of 2D polarized material MoSSe as a field-effect transistor spintronic device are studies in detail.In the fifth chapter,the enhancement of photocatalytic activity of anatase titanium dioxide by adhering noble metals is verified theoretically.In the sixth chapter,the main research contents and innovations's summary and outlook are provided.The thesis's main research contents and conclusions are as follows:(1)Opening a sizable and tunable band gap in silicene without degrading the carrier mobility is eagerly desired for high-speed switching devices.In the present work,the structural and electronic properties of two-dimensional silicene modulated by AsSb with van der Waals(vdW)interaction are investigated by density functional theory.Notably,there is almost no lattice mismatch introduced in Silicene/AsSb heterointerface,which is quite beneficial in comparison with silicene on other substrates.A sizable band gap(213-563 meV)appears in silicene owing to the breaking of the inversion symmetry due to the interface effects,which reveals a potential in applications in such as field effect transistors(FETs)at room temperature.In addition,the nearly linear band dispersion of silicene,guaranteeing the high carrier mobility,can be preserved in Silicene/AsSb heterostructures considered in this work.Furthermore,the band gap can be effectively tuned by changing the interlayer distance between silicene and AsSb and,interestingly,an indirect-direct band gap transition occurs.Our results provide a potential avenue for experimental fabrication and the applications of silicene-related materials.(2)Recently,the TiFeO3 monolayer with intrinsic magnetism has been experimentally successfully stripped from non-van der Waals layered bulk crystals.Through the first-principles calculation,we found the excellent substrate material Ti2CO2 can reduce the dangling bonds on the surface of 2D TiFeO3,enhancing its stability,and at the same time this substrate could realize the magnetic regulation of TiFeO3 monolayer.We have studied six different TiFeO3/Ti2CO2 stacking configurations,exhibiting different magnetic coupling(FM,AFM and FIM)between neighboring Fe atoms within TiFeO3,owing to the variety of the charge distribution.From the analysis of electronic structure,TiFeO3/Ti2CO2 composite systems exhibit half-metallic(for the configuration d with 100%spin-polarization ratio)and metallic properties.Moreover,the application of an external electric field(-0.4?0.2 V/A)to the TiFeO3/Ti2CO2(configuration d)can change the charge distribution of Fe atoms in TiFeO3 single-layer,further realizing the precise regulation of magnetic coupling mode and strength.In addition,the Curie temperature in configuration d is predicted to 315 K based on the Ising model,which proves that its ferromagnetism can be stably maintained at room temperature.These interesting properties enable TiFeO3/Ti2CO2 configurations to be a promising candidate material for 2D spintronics device applications.Our study offers apromising way to control interfacial magnetic ordering in the future spintronics.(3)Recent fabrication of Janus MoSSe monolayer has raised exciting prospects of the polar two-dimensional(2D)materials that exhibit excellent properties for nanodevice applications.Here,we proposed MoSSe as the superior gas sensing material by investigating the adsorption of CO,CO2,NH3,NO and NO2 on the Janus layer by first-principles calculations.Due to the presence of out-plane polarization originated from the asymmetrical structure,it is found that NH3 adopts distinct opposite orientations when it is adsorbed on Se or S side of the Janus layer.The binding strengths of all the molecules adsorption on Se surface are generally much stronger than these on the S surface.More interestingly,upon the strain deformation,the adsorption strengths of molecules NH3 and NO2 on Se side of MoSSe can be remarkably enhanced,but gradually lowered on S side.We revealed that the strain-dependent adsorption behavior is driven by a significant change of electrostatic potential difference between the Se and S surfaces under tensile strain.Corresponding to the distinct adsorption behaviors on two sides,different electronic variations are also revealed.With the higher gas selectivity,surface and strain selectivity,Janus MoSSe is proposed as an ideal material for constructing ultrahigh-sensitivity nanoscale sensors.(4)The effects of single metal atom(Pt,Pd,Rh and Ru)adsorption on the photocatalytic properties of anatase TiO2 are investigated by means of the first-principles calculations based on density functional theory(DFT).Our results show that the most stable adsorption site for single metal atom on anatase TiO2(101)surface is the bridge site formed by two twofold coordinated oxygen(O2c)atoms at the step edge.Due to the charge transfer from metal atoms to anatase TiO2(101)surface,the work function of adsorbed surface is significantly smaller than the clean one,indicating enhanced surface activity.Fukui functions are highly localized around the isolated metal atoms,indicating that single metal atoms on anatase TiO2(101)surface serve as the active reduction and oxidation sites in the photocatalytic process.Photo-induced electrons in the electronically excited TiO2 photocatalyst can be transferred to target species through the deposited single atoms.The band structures of host TiO2 are almost unchanged upon the adsorption,and the metal induced states are located in the band gap of the host.Remarkably,due to the metal atoms adsorption,the upward shift of conduction band edge will improve the reducing capacity of anatase TiO2.Moreover,when single metal atoms are adsorbed,potential energy of topmost surface Ti atoms turns to get close to the vacuum level,which significantly facilitates the electron transfer for hydrogen evolution.Results in this work provide new insights into improving the photocatalytic performance by single metal atoms adsorption.