Theoretical Investigation On Photocatalytic And Spin Polarized Properties Of Novel Two-Dimensional Materials Based On Sn And In

Two dimensional(2D)nanomaterials,represented by monolayer graphene,have attracted much attention due to their rich physical and chemical properties and wide applications in various fields.Recently,the new achievements of 2D materials emerge one by one,including graphene,silicene,germanene,stanene,phosphorene and metal chalcogenides.These 2D materials rich the low-dimensional nanomaterials system,and their excellent properties have shown important application potentials in the field of condescend matter physics,chemistry,material science and nanotechnology.Therefore,the study of 2D nanomaterials is leading the orientation of scientific research.For example,in the field of band engineering and relevant application,usually,2D semiconductor nanomaterials have high carrier mobility,ultrahigh specific surface area,and controllable band structures and band edge positions with an external field,which provide a new way to fabricate new nanoelectronic devices and efficient photocatalysts.In the spintronic applications,2D magnetic systems have stable spin polarization properties,which can meet the requirements of super-high speed and super-large capacity for information storage and processing,as well as minimized device size.It promotes the development of the new generation high-performance spin nanodevices.In this dissertation,we systematically studied the structures,electronic properties,carrer mobility,photocatalytic water splitting and spin polarization of several 2D nanomaterials by first-principles calculations based on density functional theory(DFT).We also discussed the effects of surface modification,external strain,doping,adsorption and other factors on the properties of these systems,and further revealed their micro-physical mechanisms and application potentials.The study on the photocatalytic and spin polarization properties of 2D nanomaterials provides a systematic theoretical guidance for energy conversion and energy consumption reduction.The dissertation contains six chapters.The first chapter is the introduction,which introduces the background and progress of two-dimensional nanomaterials,and briefly outlines the main research contents of this thesis.The second chapter is the theoretical method,which focuses on the density functional theory.In the third chapter,we introduce the structure and electronic properties of the novel material penta-stanene,as well as its application potentials in photocatalytic water splitting and nanoelectronic devices.In the fourth chapter,we investigate the electronic structures of tin monochalcogenides and dichalcogenides as well as the vertical heterojunctions(VHT)constructed by them,and then explore their applications as photocatalysts.In the fifth chapter,we reveal that hole doping can give rise to spin magnetic moment in In2Se3 monolayers and show their applications in spin nanodevices.Also,we discuss the effects of small molecules in the air on the properties of these materials.The sixth chapter summarizes the research contents and innovations of this dissertation,and further provide an outlook for future exploration of new 2D nano-functional materials.The main contents and results are listed as follows:(1)Based on stanene,we propose two stable novel materials,penta-SnH(p-SnH)and penta-SnHF(p-SnHF).The formation energies of the two configurations are lower than the freestanding stanene that has been grown on the Bi2Te3(111)substrate.And we also give the suitable substrate SiC(100)surface,which demonstrates the possibility of preparation in experiments.p-SnH and p-SnHF monolayers are formed by some pentagonal Sn rings in which all of Sn atoms are sp3 hybridized to the four nearest atoms.Results show that the p-SnH and p-SnHF monolayers are indirect and direct band-gap semiconductors with band gaps of 2.02 and 1.88 eV,respectively.The suitable band gaps make them have significant prospects for high-efficiency light harvesting.Both of them have high carrier mobility,could up to 769 and 2520 cm2V-1s-1 for p-SnH and p-SnHF,respectively.Moreover,the electron and hole mobility are quite different,which reduces the probability of carrier recombination.Remarkably,the band edge of p-SnHF is properly aligned that enables the band gap to cover the redox potential of H/H2 and O2/H2O for water splitting.Meanwhile,the band edge of p-SnH can be moved down to the desired level when a small tensile strain(<5%)is applied.These results indicate that p-SnH and p-SnHF monolayers have potential applications in energy materials and nanoelectronic devices.(2)We perform a systematic theoretical investigation on the electronic structures of single-layered tin monochalcogenides(SnX,X = S,Se)and dichalcogenides SnX2 as well as the vertical heterostructure fabricated by SnX and SnX2.And then explored their application potentials in photocatalytic water splitting.The formation energies of them are much smaller than that of the MoS2 which could be synthesized by mechanical exfoliation.SnS and SnSe monolayers are indirect and direct band-gap semiconductors with band gaps of 1.98 and 1.40 eV,respectively;Both SnS2 and SnSe2 monolayers are indirect band-gap semiconductors with band gaps of 2.34 and 1.41 eV,respectively.Therefore,tin chalcogenides are narrow band-gap semiconductors,which are promising materials for harvesting the visible lights.The anisotropic carrier mobility is up to 2486.93 cm2 V-1 s-1 for SnSe and 2181.96 cm2 V-1 s-1 for SnS2.The low photo-generated exciton binding energies indicate that the electron-hole can separate easily,and may lead to remarkable efficiencies on the photocatalysis.By applying small tensile strain,the band edge of SnX can be moved down to the desired level for water splitting.Promisingly,it is possible to stack SnS and SnS2 to fabricate vertical heterostructure.According to band analysis,we find there is a large band offset between SnS and SnS2,causing a small band gap of 0.08 eV.And the valence and conduction bands near the Fermi level are from SnX and SnX2,respectively.Interestingly,their band gaps in the two monolayers are only slightly narrowed compared to those in the corresponding original single layers.Therefore,this VHT can meet the two primary conditions of a photocatalyst for water splitting to generate H2 in SnX and O2 in SnX2.The strong electronegative difference between the two layers develops a significant effective potential gradient in the interval between SnS and SnS2 layers and evokes an effective electric field between them.It is of benefit for the quick charge separation and inter-layer charge transfer.This can realize a high-efficiency of light harvesting and then improve photocatalytic efficiency.(3)We explore the electronic properties and tunable magnetism of 2D ?/?-In2Se3 aswell as their applications in spin electronic devices.?/?-In2Se3 monolayers consist of five atoms planes,and ?-In2Se3 monolayer is a polar 2D material for the broken structural symmetry in vertical direction.Results show that hole doping can induce tunable ferromagnetism in ?/?-In2Se3 monolayers.Especially,the arsenic(As)doping at selenium site can enhance the ferromagnetism and drive a-In2Se3 to a robust half-metal while ?-In2Se3 to a bipolar magnetic semiconductor.As known,van der Waals In2Se3 possesses room temperature in-plane ferroelectricity.Therefore,we predict the As-doped In2Se3(In2Se3-As)may be multiferroic material.Based on these fascinating properties,we propose two types of spin nanodevices,which can achieve 100%spin polarnzation by controlling of gate voltage.In addition,adsorbing O2/H2O has a notable influence on the carrier mobility of ?/?-In2Se3 monolayers.The carrier mobility of a-In2Se3 and ?-In2Se3 are up to 1.04 ×103 and 1.39 × 103 cm2 V-1 s-1,respectively.When adsorbing O2,the electrons mobility of ?-In2Se3 will severely reduce.This is because the partial charge density of the conduction band bottom(CBM)obviously reduces after adsorbing O2.The holes mobility of ?/?-In2Se3 decreases in x direction but increases in y direction whether adsorbing O2 or H2O.Because there is coulomb repulsion between?/?-In2Se3 and adsorbed molecule,which can drive the partial charge density of the valence band maximum(VBM)turn around in xy plane from x direction to y direction.