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Theoretical Studies Of Low-dimensional Metal Oxide Nanomaterials: Atomic Structures And Electronic Properties

Posted on:2009-04-29Degree:DoctorType:Dissertation
Country:ChinaCandidate:L J LiFull Text:PDF
GTID:1101360245994966Subject:Condensed matter physics
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Accompanying with the rapid progress of nanotechnology,the study of nanomaterials has attracted considerable attention and been one of the most active areas,owing to their unique size-dependent chemical and physical properties.Among them,low-dimensional metal oxide nano-tubular materials exhibit unique quasi one-dimensional structure characteristics,and unique mechanical,optical,electric and electronic properties.So they have wide and great potential in many areas of nanotechnological applications.The research on the structure and the properties of nanotubular materials is essential for better understanding the forming mechanism of the materials and exploring their potential applications.The research subject of this dissertation is on single-walled imogolite nanotubes(nanometric tubular fibre, imogolite),and facted wurtzite nanowires and nanotubes of ZnO and ZnS.Following the rapid development of computer science and technology, computational simulations have become one of the most important methods in the studies of nanostructure materials,besides the experimental and the traditionally theoretical methods.It provides a bridge between experiments and theories in searching for the micromechanism of material syntheses and in predicting the properties of materials,especially for the nanomaterials.In general there are two kinds of simulation methods:one is the so-called ab initio calculations(or first-principle calculation),such as density functional theory(DFT);the other is empirical or semi-empirical methods,e.g.molecular mechanics method and molecular dynamics simulation.Ab initio calculation starts with first-principle quantum chemistry and solves Schrodinger equation self-consistently by iteration,while empirical method calculates some properties of materials through analytic potential functions.In this dissertation,DFT calculations and MM/MD simulations are used in the studies of the structures and the properties of single-walled imogolite nanotubes and faceted wurtzite ZnO/ZnS nanomaterials.There are two parts included in this dissertation.The first part introduces the theoretical fundamentals that we used in our research works.The second part introduces the author's main work done during my Ph.D.degree studies.The following gives a brief outline of the main contents of this dissertation.1.The Theoretical Fundamentals Used in Our Research WorkIn chapterâ…¡,the basic theories of DFT calculation and MM/MD simulations are introduced.At present,DFT calculation is one of the most important investigation methods that can describe the structural mechanical,electronic,magnetic,and optic properties of small systems which contain no more than hundreds of atoms.As for the larger systems,we can only rely on the empirical methods.Classical MM/MD simulation can deal with a system containing ten thousands of atoms,but only the conformational,mechanical,and thermodynamic properties of the system can be obtained.So we are trying to combine the two methods together to obtain as much as information as possible concerning the system under study.2.Theoretical studies of single-walled imogolite nanotube:atomic structure and mechanics propertiesSince the first discovery of single-walled carbon nanotubes(SWNT)in 1991, they have attracted considerable attention because of their unique structures and properties,and prompting the studies of other inorganic nanotubes(e.g.BN,WS2, MoS2).But up to now,how to control over nanotube diameter or monodispersity has remained a challenging task,while single-walled aluminosilicate nanotubes (nanometric tubular fibres "imogolite")have the monodispersity in diameter,for both natural and artificial tubes.In chapterâ…¢we studied the structures and energetics of imogolite nanotubes with different component index(Nv),and confirmed the monodispersity of diameters for this material.We find that the monodispersity is due to the functionalization of the inner surface with silanol groups and consequently the change in bond energies.This result can be considered as an inspiration for diameter control of carbon nanotubes and other inorganic oxide nanotubes.We have also evaluated the mechanical properties of this material and reported it's Young's modulus and Poisson ratio values. 3.Theoretical studies of single-walled imogolite nanotube:surface electric charge and electronic propertiesSeveral experimental results have proved that the imogolite can absorb both cations and anions.So the functionalization of the imogolite nanotube can be easily achieved by absorbing proper atoms,molecules or radicals on the wall,and there have been some studies on its functionalization.In chapterâ…£,we have studied in detail the surface "structural" electric charge distribution and the corresponding electronic structure of the imogolite nanotube with Nv=12,which is predicted to be the most possible index for the synthesized imogolite nanotube.Our results show that the deformation of this material leads to structural electric charges on the tube wall and existence of electric field in the radial direction.The outer wall of the imogolite nanotube was negatively charged while the electric charges distribution on the inner wall is site-dependent.This can give a reasonable explanation for the imogolite's broad-spectrum adsorbability.From the DOS and PDOS we can see that the electric field cause the energy shifts and make the energy band gap narrower.And PDOS also indicate that the most reactive sites are located on the tube wall.Functionalization of the single-walled imogolite nanotube is a new subject worthy of further studies.Our results provide the convincing theoretical prediction for this subject.4.Theoretical studies of zince oxide and zince sulfide nanomaterials:atomic structure and electronic propertiesZince oxide(ZnO)nanomaterials have become one of the major focuses in the research field of semiconductor short-wavelength devices,owing to their uique optical and photonic properties.So far diverse kinds of ZnO nanostructures have been fabricated by various synthetic approaches,such as nanoparticles,nanowires, nanotubes,nanobelts and nanosheets,and their photoluminescence spectrums have also been measured.In chapterâ…¤,the structures,energetics and electronic properties of faceted wurtzite zince oxide nanowires and nanotubes with various morphologies and sizes have been studied.Among them the hexagonal prisms are found to be most stable which agrees with the experiments results.The formation energy of the nanowires with respect to wurtzite ZnO crystal decreases monotonously with the increase of wire radius,whereas that of the multi-walled nanotubes decreases with the increasing wall thickness,irrespective of the tube radius.If the wall thickness of the multi-walled nanotubes is dose to the radius of the nanowires,they will have close Eform values.Their energetic evolution as a function of wire radius or wall thickness, which is attributed to dangling bonds on the surface,is understandable in terms of a simple model based on surface energy.We have also studied their electronic structures and discovered that the blue shift in the photoluminescence spectrum may be relevant to the surface states.Their surface relaxation and the possibility of synthesizing ZnS-SWNTs are also briefly discussed.The surface relaxation also arises from dangling bonds on the surface.The multi-walled ZnO nanotubes prefer hexagonal-faceted wurtzite morphology and the growth of the single-walled ZnO nanotubes is very complicated.ZnS nanomaterials with analogous morphology have very similar results as ZnO ones.These results can provide the guide-line for building nanoscale optical and optoelectronic devices.
Keywords/Search Tags:metal oxide, low-dimensional nanomaterials, geometric structure, electric charge distribution, electronic structure, density functional theory, molecular mechanics method, molecular dynamics simulation
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