Investigation Of Preparation And Properties Of Low-dimensional SiC/SiO₂ Core-shell Structure Materials

As an important third-generation semiconductor, silicon carbide, with wide bandgap, high breakdown voltage, high thermal conductivity, and high saturation drift velocity, appears to be a promising material for UV detector, short-wave light-emitting diodes and electronic devices that can be operated at high power, high frequency, high temperature and in harsh environment. Zero-dimensional and one-dimensional SiC nanostructure materials, as in the nano-scale, have some new properties compared to the bulk-SiC material. By now, the zero-dimensional and one-dimensional SiC nanostructure materials display huge potential application in photocatalytic, optoelectronic devices and field emission display etc. The investigation of SiC zero-dimensional and one-dimensional nanostructure materials is an attractive and important work. At present, the research of SiC zero-dimensional and one-dimensional nano-materials has been focused on two aspects: preparation of SiC nanomaterials with different methods, and characterization of optical and field emission properties. However, these preparation methods have some disadvantages, such as, low yield, high impurity, high preparation temperature, high cost and so on. The SiC nanomaterials prepared by these methods usually have a large number of surface states, resulting in lower luminous efficiency. In particular, the application of SiC one-dimensional nanomaterials on field emission display needs high turn-on field. Fabricating SiC/SiO2 core-shell zero-dimensional and one-dimensional nanomaterials by coating a SiO2 protection layer can reduce the surface states and improve the luminous efficiency of SiC material. At the same time, a thin SiO2 layer can enhance the field emission characteristics of SiC one-dimensional nanomaterials. Thus, creating some simple and effect methods to prepare SiC/SiO2 core-shell structure materials and study their optical and field emission characteristics have important physical meaning and practical value. Major contents of study in this thesis were summarized as follows:1). First we designed a two-step approach to synthesize the SiC/SiO2 growth of core-shell nanoparticles. By thermal chemical vapor deposition (T-CVD) and thermal annealing process, we successfully synthesize zero-dimensional 6H-SiC/SiO2 core-shell nanoparticles. The scanning electron microscopies (SEM), transmission electron microscopy (TEM), energy dispersive x-ray detector spectroscopy (EDX), X-ray diffraction (XRD) were used to characterize the nano-particles. The effect factors of fabrication of nano-particles were also analysized.2).'Normal-shaped','coralloid'and'aloetic-shaped'β-SiC/SiO2 one-dimensional nanowires were synthesized by using simple synthetic method. The growth mechanism of nanowires is'vapor-liquid-solid (VLS)'with a'tip growth model'. The Fe nanoparticles play a key role in the formation of nanowires and the catalyst morphology is the key factor for synthesizing different shapes of SiC nanowires.3). Photoluminescence (PL) spectrum was used to characterize the optical properties of zero-dimensional and one-dimensional SiC core-shell structure nanomaterials. The results showed that, before annealing in oxygen atmosphere, luminescence of SiC nano-particles were mainly from SiC surface defect emission, emission from surface defects disappeared and the intrinsic emission intensity of SiC obviously enhanced after annealing. This indicated that the luminous efficiency of nano-SiC coated with SiO2 materials has been improved. Room temperature and variable temperature photoluminescence of the'normal shaped'β-SiC/SiO2 one-dimensional core-shell structure nanowires were also investigated. It was found that the PL boradband of nanowires can be decomposed as two emission peaks, with maximum peak at 2.3 eV and 2.1eV, respectively. It was proved that he 2.3 eV luminescence from SiC band edge emission and 2.1eV luminescence from surface defect states.4). Field emission properties of the'normal-shaped','coralloid'and'aloetic-shaped'β-SiC/SiO2 one-dimensional nanowires were investigated detailedly. The results show that all nanowires have a low turn-on field and high emission current density. So the three nanowires are good field emission cathode materials. The reasons of the SiC core-shell nanowires with good field emission characteristics can be summarized as follows: first, core-shell nanowires have a large number of effective emitting point; Second, core-shell nanowires have a great aspect ratio, makes it easy to form strong local electric fields can be easily formed on the tips. This electric field will make the electron overcome the surface barrier to the air with ease and then greatly enhance the electron emission; Third, the thin SiO2 shell with a small electron affinity. By coating with 10nm SiO2 shell, the field emission properties of SiC nanowire emitters can be enhanced.