The First-principles Investigations Of Lattice Defects And Electronic Properties In Low-Dimensional Functional Materials

Low-dimensional functional materials have attracted particular attentions due to their outstanding properties and widespread applications.As an important branch of nanomaterials,transition metal oxides(TMOs)play a vital role in many fields.While the material properties are closely related with the defect structures,the structure-property relationships in low-dimensional TMOs are far from well-understood.Besides,two-dimensional(2D)layered nanomaterials have shown promising potentials in future technological applications.Since the isolation of graphene,numerous 2D materials with extraordinary properties have been discovered.Understanding their environmental stability and defect properties is a prerequisite for their applications.The thesis is consisted of two major parts:(1)investigating the static defect structures;(2)understanding the dynamic defect behaviors.(1)Investigating the static defect structuresThe atomistic strutures of(11(?))and(114)twinning boundaries(TBs)and(002)/(223)grain boundary(GB)in Cu O nanosheets are investigated.Unlike the low-energy stoichiometric(11(?))TB,both experimental and theoretical investigations have revealed a loose structure and a nonstoichiometric property at(114)TB,which may facilitate ionic transportation and provide space for elemental segregation.More importantly,according to the calculated electronic structures,(11(?))TB keeps the semiconductor property of the bulk,whereas both(114)TB and(002)/(223)GB are metallic and show spin polarized band structure,leading to magnetic ordered GBs.These findings could contribute to the race of property-directing structural design by GB engineering.(2)Understanding the dynamic defect behaviorsa.V₂O₅ is a typical layered TMO,which is a promising candidate for electrode materials.A variety of methods have been proposed to improve its electrochemical function.Here,we have studied the band structure of Mo doped V₂O₅,((Mo_0.3V_0.7)₂O₅),and found a great improvement on its conductivity,i.e.changing from the semiconductor to a metallic property.Moreover,(Mo_0.3V_0.7)₂O₅ maintains comparable diffusion barriers of multi-valent ions to that of V₂O₅.The improved conductivity and doping induced active sites will make its electrochemical functions better,consistent with the experimental results.b.Besides the diffusion dynamics,we have further studied the dynamic behavior of defects in TMOs under e-beam irradiation.The classical molecular dynamics simulation package(LAMMPS)have been applied to simulate the e-beam bombardment on Zn O nanomaterials in real-time,which predicts an increasing Zn/O ratio at electron incident surface.Unlike{0001},the lower plane atomic density and the double-element truncation feature of{1(?)2 0}make its residual Zn atoms disordered,which is in good agreement with the surface dependent Zn cluster nucleation observed in our experiment.In addition,a Zn/Zn O metastable orientation relationship is found by considering the interface energy,in accordance with our experimental observations.c.The environmental stability and oxygen defect properties of 2D Ge P(space group:C2/m)have been studied.Firstly,its HSE band structures show an adjustable indirect band gap,and the redox potential ofO₂/O₂^-lies just in the bandgap of thin Ge P.With light assistant,the chemical absorption barrier of O₂ on thin Ge P lowers to0.07 e V,and the adsorptions are exothermic reactions.Unlike black phosphorus,the strong covalent bonds Ge-Ge and Ge-P prevent Ge P from degeneration under hydrogen bond interactions between H₂O and adsorbed O,leading to a high environmental stability of Ge P.Secondly,the oxidation of single layered Ge P can reduce its bandgap without introducing midgap states and the oxygen substitutions are deep-acceptor(O_Ge)or deep-donor(O_P)defects,which can act as recombination center of carriers,affecting the lifetime of minority carriers.