Oxidation Mechanism Of Two-dimensional Group-Ⅵ Compounds And Photoelectronic Properties Study From First Principles

Two-dimensional materials have received widespread attention due to their unique physical and chemical properties induced by decreased dimension.Compared with three-dimensional bulk materials,two-dimensional materials possess many advantages:on the one hand,their electronic structure and band gap can be tuned by the film thickness,their carrier mobility is greatly improved and their larger specific surface area provides more reactive sites.On the other hand,due to weakened Coulomb screening effect among electrons after dimension reduction,two-dimensional materials exhibits stronger exciton binding energy that is about an order of magnitude higher than that of three-dimensional materials,which ensures their superior application prospects in the fields of light absorption and optoelectronic devices.As we all know,graphene has good stability,high carrier mobility and massless Dirac fermions,together with the superconductivity found in double-layer graphene in the past two years,these peculiar properties indicate that graphene has huge potential applications in nanodevices,superconductivity and other fields.Transition metal sulfides（TMDs）have high stability and a suitable band gap,making them ideal two-dimensional functional materials.In recent years,the experimentally synthesized Group-V single-layer black phosphorous materials show suitable direct band gap and ultra-high carrier mobility and are expected to be the most influential next-generation optoelectronic materials.In addition,II-Ⅵ,III-V and IV-Ⅵ two-dimensional layered materials have been theoretically predicted and experimentally synthesized.These two-dimensional functional materials all exhibit excellent mechanical,optical and electrical properties,which further enrich the family of two-dimensional functional materials and provide theoretical basis for the practical application of two-dimensional functional materials.However,there are some flaws that are hard to be avoided in the existing two-dimensional functional materials,such as the small tunable bandgap of graphene,the low carrier mobility which is only between 200-500 cm²v^-1s^-1 of transition metal sulfide,and the easy oxidated and degraded properties of the black phosphorus as well as its compounds,etc.Special attention should be paid to the last point as the redox reaction has become one of the key factors hindering the application of two-dimensional functional materials.Because of the oxidation,defects,and their synergistic interactions,the structural changes of the two-dimensional functional materials will occur and then affect their physical and chemical properties.Thus,an in-depth understanding of the redox mechanism is crucial for our theoretical design of two-dimensional functional materials with high performance and strong oxidation resistance.Furthermore,two-dimensional intrinsic magnetic functional materials have attracted extensive attention in recent years due to their potential applications in spintronics,spintronic devices,magnetic storage,magnetic sensors,and so on.In particular,this kind of material has become a hot research issue after the successful synthesis of the two-dimensional Cr₂Ge₂Te₆ and Cr I₃ magnetic semiconductors.But the application of two-dimensional intrinsic magnetic functional materials is far away from being reached because the available two-dimensional intrinsic ferromagnetic semiconductors with high Curie temperature and a good oxide resist are scarce under current experimental conditions.Therefore,the theoretical design of two-dimensional intrinsic magnetic functional materials with strong oxidation resistance and high Curie temperature is of great significance for further understanding of spintronics and promoting the development of spintronics.The main research contents in this article are as follows:1.We have studied the effects of surface adsorption（O₂ and H₂O molecule）and vacancy defects on the structural stability and electronic structure of group-Ⅵ monolayer tellurene.Our studies show that H₂O molecules are physically adsorbed on the tellurene surface,while O₂ molecules are chemically adsorbed.The adsorption energies of H₂O and O₂ molecules will increase with the presence of vacancy defects.H₂O molecules can promote the decomposition of O₂ molecules on tellurene surface,while the presence of vacancy defects drastically reduce the decomposition barrier of O₂molecules on tellurene surface,leading to rapid oxidative degradation of tellurene.In addition,H₂O and O₂ molecule adsorption has little effect on the electronic structure of tellurene,while vacancy defects have a great impact on its electronic structure.Our studies show that tellurene band gap is very sensitive to the concentration of vacancy defects:the greater the concentration,the smaller the band gap.When the concentration increases to 5.3%,an indirect to direct band gap transition will occur.We also studied the adsorption behavior of H₂O and O₂ molecules on the surface of monolayer topological materials Zr Te₅ in ambient conditions.The results show that H₂O is physically adsorbed,while O₂ behaves chemically with the decomposition barrier 0.87e V.Importantly,it was found that Zr Te₅ can still maintain the original topological properties at low O₂ coverage,even when the coverage reaches 33%.The reason could be understood by the analysis of the energy band.The energy level near the Fermi level is mainly contributed by the 2p orbital of the Te atom,both before and after decomposition,while the energy level of O introduced by adsorption is located at the lower energy level in the energy band,and this results in the topological properties invariance.Our study provides an important theoretical basis for the experimental synthesis of high-quality 2D monolayer Tellurene and Zr Te₅ topological materials.2.A series of two-dimensional group-Ⅵ monolayer ABC（a=Na,K,Rb;b=Cu,Ag,Au;C=S,Se,Te）chalcogenide optoelectronic materials have been proposed,which may be stripped from bulk materials.The calculated decomposition barrier of O₂molecule on the surface of monolayer KAg Se exceeds 1.50 e V,suggesting that O₂molecule is not easily oxidized on its surface.High-accuracy G₀W₀ method has been performed to calculate their electronic structures and the results show that single layer KCu Te,KAg S,KAg Se,KAu Te,Rb Cu Te,Rb Ag Se,and Rb Ag Te are direct band gap semiconductors.The electron mobility of KCu Te,Rb Cu Te,and Rb Ag Te is as high as10⁴ cm²v^-1s^-1.Importantly,there is a linear relationship between exciton energy and quasi-particle band gap（QP）with a slope of 1/3.In addition,the highest of solar energy conversion efficiency among the KAg Se,KAg S,Rb Ag Se,and Rb Ag Te is up to 21.5%.By performing advanced hybrid functional and G₀W₀ calculations,we systematically study the origin of the giant anisotropic optoelectronics in layers Pb Sn S（Se）₂ found recently in experiments.We find that the highly anisotropic optoelectronics originates from the giant different lattice responses under uniaxial strain in x and y direction.Such large difference in lattice responses leads to anisotropy of electronic structures and optical properties with fascinating visible light absorption range in single-and bilayer Pb Sn S（Se）₂.Moreover,uniaxial strain in both x and y direction can induce an indirect-to-direct band gap transition with a change of the slope between quasiparticle band gaps and strain,while the quasiparticle indirect band gap presents excellent linear scaling with biaxial strain in monolayer Pb Sn S（Se）₂.We also demonstrate ultrahigh anisotropic mobilities of electrons（μ_y>μ_x）and holes（μ_x>μ_y）in both single-and bilayer Pb Sn S（Se）₂,and significantly reduced exiton binding energies and band gaps with spin orbital coupling（SOC）effects and increased number of layers.Finally,we show that the strong layer-dependence of band structure will weaken with increased film thickness（more than 4 layer）in Pb Sn S（Se）₂.Our results provide a fundamental understanding of highly anisotropic layered Pb Sn S₂ and show two potential candidates for application of photoelectric materials.3.Screening monolayer photocatalysts is motivated with not only the fundamental interest but also the practical applications for clean energy.In this work,we have systematically studied a new family of two-dimensional ternary compounds Li XY₂（X=Al,Ga,In;Y=S,Se,Te）by the G₀W₀ methods.Particularly,the Li Al Se₂and Li Al Te₂ were proposed may be exfoliated from the bulk counterpart in experiments.Besides,all of them have intrinsic polarization and with a direct band gaps,indicating that they exhibit excellent optical absorption under the visible light from to ultraviolet light.In addition,all of them hold great potential and advantages in overall water splitting due to their extremely high electrons mobility(～10³ cm²V^-1s^-1).Especially,for Li Al S₂ and Li Ga S₂ could achieve overall water-splitting without the limitation of band gap because of their larger difference of electrostatic potential between top and bottom surfaces.Our results provide promising avenues to realize the nanoelectronic and photocatalytic applications of 2D ternary compounds.4.We offered a kind of idea to the realization of two-dimensional intrinsic ferromagnetic semiconductors（XCr Y₂,X=Li,Na;Y=S,Se,and Te）,which is based on the separation from corresponding magnetic bulks materials.Evidence from previous experiments indicated that the（001）surface of Na Cr S₂ can be easily exfoliated from its bulk materials.First-principles calculations show that XCr Y₂ belongs to a ferromagnetic ground state and has a high Curie temperature（up to 285 K）.The high-accuracy Heyd-Scuseria-Erzenhof（HSE06）method is used to calculate the electronic structure.It is found that SOC effect has a significant effect on the electronic structure and band gap.SOC effect can also reduce the effective mass of holes and improve the mobility of holes.Moreover,the material itself has a built-in electric field,which has a good potential application in the photocatalytic decomposition of water.Our theoretical study provides a important suggestions for the experiment to obtain high quality 2D function material.We provides a series of high-performance 2D photoelectric,photocatalysic and ferromagnetic semiconductor candidate materials which can been synthesized in experiments.It provides theoretical support for the realization of efficient conversion device and spin electronics.