The Prediction Of Two Dimensional Structures Of High-k Materials And The Investigation Of Their Physical Properties

The low dimension nanomaterials with high specific surface to volume atio and significant quantum size effect have become the focus of intense research due to their unique physical and chemical properties.The discovery of graphene and its astonishing properties have given birth to a new class of materials known as 2D materials.2D materials not only represent the ultimate scaling in the vertical direction of potential bull materials but also show a rich variety of novel and useful electronic,optical,mechanical,or piezoelectric properties that differ from their three dimensional counterparts,which are highly desired for potential electronic and energy applicaitons.In experiment,various attempts have made to realize 2D materilas such as liquid exfoliation,chemical vapor deposition（CVD）,Van der Waal epitaxial growth on substrates,or hydrothermal synthesis.Due to reduced bonding coordinations and surface polaration effect,the materials in 2D form might take very different structures compared with their corresponding bulks,especially for oxides.The dominant bonding character in most oxide mateials is ionic.The dangling bonds and surface polarization will occur when they are reduced to monolayer limit,thus strong structure relaxation or reconstruction is required to minize these effects.This gives us difficulty in predicting the structures of oxide 2D materials.First-principle calculations based on density functional theory have been proved to be an effective and reliable method to predict the structurural properties of materials,as well as their electronic,optical,and magnetic properties.In this thesis,via first-principles calculations,we systematically present our results in predicting stable oxide 2D structures and studying their electronic,optical and catalytic properties.The main findings are briefly concluded below:1.The exfoliation of graphene triggered dramatic interest to explore other two-dimensional materials for functionalizing future Nano electronic devices.In this study,via first-principles calculations,we predict a stable planar Y₂O₃（111）monolayer with a direct band gap of 3.96 eV.This oxide monolayer can be further stabilized by a graphene substrate.We show that for the interface between graphene and h-Y₂O₃,the interaction is weak and driven by the hybridization between C pz and O pz,as well as C pz and Y4 d orbitals,while for the interface between unreconstructed Y₂O₃（111）monolayer and graphene,interfacial charge transfer is much larger and strong C-O covalent bonds are formed at the interface.In addition,We also find that with an increase of Y₂O₃（111）thickness,the interaction becomes much weaker and thus results in decreased band gaps.Our results indicate that a high-k dielectric monolayer can be more easily formed on a substrate with weak interfacial interaction via a physical deposition process,and this result sheds light on engineering extremely thin high-k dielectrics on graphene-based electronics with desired properties.2.The miniaturization of future electronic devices requires the knowledge of interfacial properties between two-dimensional channel materials and high-κ dielectrics in the limit of one atomic layer thickness.With a combination study of particle-swarm optimization（PSO）method and first-principles calculations,we present a detailed study of structural,electronic,mechanical,and dielectric properties of Al₂O₃ monolayer.We predict that planar Al₂O₃ monolayer is globally stable with a direct band gap of 5.99 eV and thermal stability up to 1100 K.The stability of this high-κ oxide monolayer can be enhanced by substrates such as graphene,for which the interfacial interaction is found to be weak.The band offsets between the Al₂O₃ monolayer and graphene are large enough for electronic applications.Our results not only predict a stable high-κ oxide monolayer,but also improve the understanding of interfacial properties between a high-κ dielectric monolayer and two-dimensional material.3.Due to reduced bonding coordination and the suppress of surface polarization,the structures of oxides in two-dimensional limit are much different with the corresponding bulks which brings additional diffculty in the growth or prediction of 2D oxide structures.Through PSO method and first-principle calculations,we predict a stable two-dimensional TiO₂ crystal with hexagon structure and an indirect band gap about 5.2 eV.The Ti atom is located at the center of O tetrahedron.The five degenerate d electrons are divided into three parts: a trigonal bipyramid and regular triangle.The gap of 2D-TiO₂ decreases with the increase of stress.For multilayer 2D-TiO₂,the interaction between layers is found to be weak for small formation energy and electronic structure has no relationship with layer stacking.We also find that oxygen vacancies can be formed easily in this TiO₂ two-dimensional structure due to their relatively low formation energy,which result in mid-gap states that reduce the band gap to 1.9 eV,within the visible light range of photocatalytic activity.Our results suggest a stable TiO₂ two-dimensional structure that refines recent experimental observation,and also provide an improved understanding of photocatalytic properties of materials at nanoscale.