Theoretical Design And Study Of Novel Two-Dimensional Functional Materials

With the advent of graphene,the research on the properties and applications of two-dimensional(2D)materials has opened up a new direction in condensed matter physics.The smooth surface structure,adjustable band gap range,large specific surface area and mature synthesis process have made 2D materials show great application potential in functional device applications(electronic devices,catalysis,and so on).Besides,the new paradigm created by the heterojunction structure based on 2D materials has greatly enriched the research and application of 2D materials.With the development of com-puter science,the first-principles calculations play an important role in the simulation of 2D material properties and functional design,greatly reducing the difficulty and cycle of material development,and providing theoretical basis and guidance for experimental preparation and material synthesis.The main content of this paper is to study the elec-tronic properties of some new 2D materials and heterojunctions through first-principles calculations,and design 2D materials with specific magnetic properties.At the same time,the electronic structures of the interface between TiO2 and H2O are also discussed.This paper contains the following chapters:In the first chapter,we first make an introduction to 2D materials and briefly de-scribe the basic knowledge of quantum chemistry,and then introduce the development process of density functional theory,including the basic framework,theorems,the func-tionals and basis sets used in density functional theory.Finally,we will introduce the relevant calculation software packages used in material calculation and simulation.In Chapter 2,we use first-principles calculations to design a novel one-dimensional(1D)electride monolayer Ca4N2 with ultrahigh conductence.Based on the study of its electronic,transport,mechanical properties and protective measures,we find that:anionic electrons can form a 1D chain array in 1D grooves formed by calcium atoms on the surfaces;Ca4N2 has high carrier concentration(1.14×1015 cm-2)and Fermi velocity(0.46 ×106 m·s-1);the carrer mobility of Ca4N2 at room temperature is as high as 215 cm2·V-1·s-1,which is higher than that of the best conductive metals such as copper and silver;by reducing the temperature to 2K,the carrier mobility increases to 106 cm2·v-1·s-1 and the conductivity of the material is as high as 966 S,which is the highest among the materials currently reported;Ca4N2 has a smaller Young’s modulus and a larger Poisson’s ratio,which can be used for flexible electronic devices;graphane can protect the anionic electrons from the damage of molecular;the 2D monolayer Sr4N2,which has the same structure as the monolayer Ca4N2,is also a 1D electride and shows similar electronic structure to Ca4N2.Based on the fact that 1D anionic electron chains can exist on the surface of 2D materials and exhibit excellent transport properties,we propose a simpler and more controllable method to obtain 1D electrides.The diamond(100)surface undergoes a 1×2 reconstruction,forming a C(100)1×2 surface.The two carbon atoms on the surface are reconstructed to form a dimer.A 1D groove is formed on the surface along the direction perpendicular to the bonding of the dimer.By adsorbing alkali metal calcium atoms in the grooves,we found that the electrons can be localized directly above the calcium atom chain and exhibit the properties of one-dimensional electron gas.In Chapter 3,we design a kind of highly efficient ultra-thin photovoltaic cells based on van der Waals heterojunction composed of transition metal sulfide single-layer Janus structure(JTMDs)and graphene.Using first-principles calculations and non-adiabatic molecular dynamics calculations,we verify that the photo-generated carriers can be effectively separated and transferred.Single-layer JTMDs(MoSSe,MoSeTe)are direct bandgap semiconductors.Both of them have good optical absorption properties in the ultraviolet-visible range.JTMDs have inherent dipoles.After forming a G/JTMDs/G"sandwich" heterojunction,the photogenerated electrons or holes tend to transfer to the graphene by the side with lower or higher potential under the non-adiabatic mechanism or adiabatic mechanism,and the transfer time scale is about 100-200 fs.In Chapter 4,we use graphene quantum dots with inherent magnetic moments to design a series of 2D magnetic materials with specific magnetic properties.The ground state of triangular zigzag graphene quantum dots is a ferromagnetic state,the magnetic moment is mainly localized at the zigzag edge carbon atoms,and the three sides are coupled together in a ferromagnetic alignment.There is antiferromagnetic coupling be-tween the two sublattice carbon atoms in quantum dots.We use small-size graphene quantum dots,magnetic metal atoms,or small conjugated organic molecules as link-ages to connect large-size graphene quantum dots together to form 2D magnetic ma-terials with specific magnetic properties.The linkages and the apex carbon atoms are antipallel coupled thought the direct exchange interaction,and thus the carbon atoms at the edges of the graphene quantum dots are coupled with the linkages in the form of fer-romagnetism.By using different linkages,we can get ferromagnetic semiconductors,ferromagnetic metals or bipolar magnetic semiconductors.In the Appendix,we use first-principles calculations and experiments to study the geometric structure,electronic structure and interaction with water molecules of the anatase TiO2(001)surface.The interaction between the rutile TiO2(110)surface and the water molecules is also discussed at the excited state level.We found that the anatase(001)surface is a mixed structure of ADM and AOM.Using first-principles calcula-tions,we have determined the attribution of the different energy bands measured in the experiment and that the adsorption of water molecules will change the charge transfer between oxygen atoms at different positions,thereby affecting the surface state.Besides that,through time-dependent density functional theory calculations,we found that wa-ter molecules will capture the photo-excited holes on the rutile(110)surface,but not the photo-excited electrons.It shows that the first step of photolysis of water molecules may be a process of capturing light-excited holes.