Study On Synthesis And Optical Properties Of Semiconductor Nanomaterials

Owing to their unique optical, electronic, and magnetic properties, semiconductor nanomaterials have been attracted much attention in recent years. In this thesis, we selected three kinds of important semiconductive materials, SnO₂, ZnO, and ZnS, and described the synthesis of SnO₂ nanocrystals, ZnO nanocrystals and spherical ZnS nanostructures via sol-gel, combustion, and solution chemical method, respectively, and the exploration of their novel optical properties has also been employed intensively. Furthermore, by introducing dopant ions into the hosts, the optical properties of doped SnO₂ and ZnO nanocrystals have been studied.Semiconductive nanomaterials may exhibit lots of terrific properties deriving from their novel effects such as quantum size, small size, and surface, which give rise to their potential use in the fields of nonlinear optics, magnetics, catalysis, medicine, and functional materials, as well as other areas. And the development of life science and information technology even the basic research in material area can also been greatly influenced simultaneously. In Chapter One, we briefly introduced the history and recent work in the field of semiconductive materials. And several kinds of methods for synthesis of semiconductive nanomaterials, such as sol-gel, vapor-phase, aqueous-phase, and template-assisted method, have also been introduced clearly. In addition, the luminescence theory for semiconductors is included for further discussion in the following chapters.In Chapter Two, a simple sol-gel method has been used for the synthesis of well-dispersive SnO₂ nanocrystals. The optical characteristics of the SnO₂ nanocrystals have been investigated by means of absorption and photoluminescence spectra and the luminescence mechanism for nanocrystalline SnO₂ has beensystematically studied for the first time. The experimental results show the particle size of the SnO2 samples is rather small, and the average particle size is only 2.8,4.2, and 8.8nm after calcining at 400, 500, and 600°C, respectively, exhibiting quantum size effect distinctly. In order to study the luminescence mechanism of nanocrystalline SnC>2, we deliberately created defects in the hosts by introducing Ce3+ and Mn2+ ions for the first time. Blue emission at 400nm can be observed in all emission spectra, and Vo has been proposed to be the recombination center. Upon photoexcitation of SnC>2nanocrystals, v" center can be formed when a hole is trapped at a Vo* center. Then recombination of a yo" center with a conduction band electron gives rise to the blueemission. Besides the size effect, surface effects can strongly influence the optical properties of SnC>2 nanocrystals. As the calcining temperature increases, the particle size of the SnC>2 nanocrystal becomes larger, and the crystallinity becomes better. The significant decrease of both the surface defects and concentration of oxygen vacancies of the SnC>2 nanocrystals will result in the obvious decrease of luminescence intensity. Furthermore, SnC>2 thin films have been prepared by a sol-gel spin-coating method and the luminescence mechanism is also studied. The bandgap is 4.38eV with respect to the SnO2 thin films calcined at 400°C, exhibiting distinct quantum size effect. Upon photoexcitation of SnC>2 thin films, besides the blue emission at 400nm, the other emission at 430nm can also be observed in emission spectra, which has been assigned to the contribution of interstitials or dangling in the SnC>2 thin films. Furthermore, we first introduced rare earth ions such as Eu3+, Dy3*, into the SnC>2 nanocrystals exhibiting quantum size effect and studied their optical properties. Following band gap excitation, energy can be transferred from the host to the doped ions. By inctroducing Eu3+ or Dy3+ ions into SnC>2 nanocrystals, only a minor fraction of the total amount of Eu3+ or Dy3+ substitutes for the tin positions, and most of the rare earth ions may well located at the surfaces of the particles to yield optimum strain relief.In Charpter Three, ZnO nanocrystals have been prepared by a combustion method, and the optical properties of the ZnO nanocrystals codoped with rare earth ions andlithium ions as well as the effect of doped ions on the morphology of the samples have been systematically studied for the first time. The samples have been characterized by XRD, TEM, HRTEM, FTIR, absorption and photoluminescence spectra. The experimental results show the morphology of the samples will not be affected by doping rare earth ions such as Eu3+, Dy3+. Following the band gap excitation, the energy may be transferred from ZnO host to the doped rare earth ions, exhibiting the characteristic emissions of rare earth ions. The crystallinity of the ZnO nanocrystals becomes better after doping lithium ions. Besides ZnO nanoparticles, a large amount of one-dimensional ZnO nanorods can also be found after doping lithium ions into ZnO. Lithium chloride in the precursor solution will be vaporized into liquid droplets during the combustion process, which will provide the energetically favored site for the precipitation of ZnO vapor and the following growth into the rod structure. The process is well coincided with the VLS growth mechanism. It is surely that the initial dimension of nanorod would be determined by the size of alloy droplet. When lithium nitrate is used instead of lithium chloride, no ZnO nanorods can be formed due to the absence of liquid droplets during the combustion process. Following the band gap excitation of ZnO samples codoped with rare earth and lithium ions, the luminescence intensity of rare earth ions can be enhanced greatly, for example, with respect to the ZnO:Dy3+, Li+ sample (CDy=2% and Cu=10%), the luminescence intensity of Dy3* ions has been enhanced 10 times after Li+ codoping. The great enhancement of luminescence intensity may be ascribed to the increased energy transfer from the host to the doped ions and the decrease of quenching centers due to the better crystallinity of ZnO samples after Li+ codoping. In addition, MgxZni.xO nanocrystals have been successfully prepared via the combustion method. By tuning the x value, the MgxZni.xO nanocrystals with cubic or hexagonal structure can be prepared.In Chapter Four, we tentatively fabricated well-dispersive and defined spherical ZnS nanostructures: hollow and solid spheres, via a very simple solution chemical route for the first time. And the optical properties and formation mechanism of the ZnS nanostructures have been investigated systematically. It has been proved that theZnS hollow spheres are formed by the aggregation of ZnS nanocrystals about 20nm around H2S gas bubbles formed during the reaction. By changing the experimental parameters, the diameters of the hollow spheres can be tuned in the range of 90-170nm, as well as the wall thickness in the range of 25-55nm. Intrinsic absorption peak at 340nm can be observed in the absorption spectrum, the absorption edge doesn't exhibit blue-shifted. Upon photoexcitation of the sample, emission peaking at 474nm can be observed deriving from the zinc vacancies. On the other hand, the solid spheres are formed by oriented aggregation of ZnS nanocrystals about 8nm. By changing the experimental parameters, the size can also be tuned in the range of 170-350nm. In addition, through a simple ultrasonocation process, the as-prepared ZnS hollow spheres have been successfully coated by SiO2 layer. TEM images clearly show the core-shell structure of the products. The SiC>2 coating will result in the enhancement of the chemical stability of ZnS and the decrease of the formation of defects. The thickness of the S1O2 layer can be tuned easily by changing the experimental parameters. The luminescence intensity has been enhanced one time after the SiC>2 coating.In Chapter Five, we summarized our work.In brief, the synthesis and study of inorganic nanomaterials, especially, semiconductive nanomaterials, has become a major interdisciplinary research area in recent years. My research involves the fabrication and properties (optical) characterization of several important semiconductor nanomaterials such as SnC>2, ZnO, and ZnS, especially, the exploration of novel optical materials by doping or surface modification. We believe it is meaningful for both basic research and application research by seeking novel methods for the synthesis of semiconductive nanomaterials, investigating their optical properties, summarize the luminescence rule and discussing the luminescence mechanism.