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Multi-Shapes Of SiC Nanomaterials: Growth, Structure And Properties

Posted on:2010-10-23Degree:DoctorType:Dissertation
Country:ChinaCandidate:R B WuFull Text:PDF
GTID:1101360302481329Subject:Materials science
Abstract/Summary:PDF Full Text Request
Over the past decade, low-dimensional nanostructured semiconductors have stimulated considerable interests owing to their novel physical and chemical properties as well as the potential applications in novel electrical, optical and magnetic nanodevices. Among them, silicon carbide (SiC) is one of the most promising materials due to its high-temperature semi-conductivity, high mechanical strength, high thermal conductivity, excellent chemical inertness caused by covalent bond and wide band-gap. Nanostructured SiC, because of its low dimensionality, quantum confinement and shape effect, possess even more promising mechanical, electronic and optical properties. It is a suitable material both at present and in the future for the fabrication of high performance nanodevices in severe environment applications. The studies on the growth, structure and properties of SiC nanostructures are of great importance to fundamental nanoscience research as well as future nanoscale electronic, optical and mechanical devices.Inspired by above, this dissertation has put emphasis on the synthesis of SiC nanomaterials using relatively simple and economic techniques, mainly the thermal evaporation method. The products have been found to have multi-shapes and various morphologies. By means of X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), energy dispersion spectroscopy (EDS), the morphologies, structures, compositions and growth mechanisms were investigated systematically. The optical properties of SiC nanostructures were also studied by Raman spectroscopy and photoluminescence spectrum. More over, the structure defects and the relationships between the structure and properties have been elucidated based on the above investigations. The research is fundamental and pioneering, and is of great significance from the view point of theory as well as potential applications in the future. The important results achieved in this dissertation are given as below:High quality 3C-SiC nanorods have been synthesized using Si vapor evaporating onto carbon nanotubes (CNTS) situated on an alumina plate with regularly arrayed fine holes by direct heating method. Compared to previous method, it is simple, environment-friendly, and easy to be large-scale. The synthesized 3C-SiC nanorods are all perfect single crystals with [111] direction parallel to the rod direction. The reaction of Si vapor and C may presumably started at the end, the most active part of a CNT to form randomly oriented 3C-SiC nuclei, and only that whose growing direction, i.e. [111] parallel to the tube direction continue to grow by subsequently taking in Si atoms. The CNTS with one-dimensional configuration played the space-confined effect during the SiC nanorods growth process. It was also found that the origin morphology of CNTS have an important influence on the morphology of SiC product.Besides the products from the alumina plate, bicrystalline SiC nanobelts were collected from the surface of sintered silicon at the bottom of the graphite crucible and SiC/SiO2 core-shell heterstructures were obtained from the inner wall of the graphite crucible through the similar heating method. The nanobelts display a unique bicrystalline structure that has growth directions, i.e. [1(2|-)1] and [(1|-)1(1|-)] split along the twin boundary that exists the centerline. The fact no liquid droplets were observed during the growth process suggests that the growth of bicrystalline SiC nanobelts is likely dominated by the vapor-solid (VS) mechanism. For the growth of SiC/SiO2 core-shell heterstructures, the overall process involved the SiC nanowires formation firstly and then SiO2 species deposition on the surface of SiC nanowires during the cooling stage. The different deposition ways lead to the different SiC/SiO2 structures. Compared with the band gap of bulk 3C-SiC, the photoluminescence spectra of bicrystalline SiC nanobelts are obviously blue-shifted, which might result from the quantum confinement effect of nanobelts thickness. For the SiC/SiO2 core-shell nanocables, the unique emission spectra may be due to the defects of twofold coordinated silicon such as -O-Si-O and -O-Si-C-O-formed at the interface.SiC nanoprisms were obtained by a reaction between carbon black and silicon powder through the thermal evaporation method. Various durations of heating gave different growth stages and the morphologies of the product changed from particles, worm-like to regular and even prism-shaped at last. The formation is suggested to be a VS mechanism. Using the similar experiment setup, but heating a milled Si and SiO2 powder mixture in the presence of ZnS, one-dimensional (1-D) hierarchical SiC nanostructures could be synthesized. Structure and morphology characterizations revealed that the 1-D hierarchical SiC nanostructures are single crystalline and consisted of consecutive truncated cones along its length. A vapor-liquid self-assembly mechanism is proposed for the formation of hierarchical SiC nanostructures. ZnS is recognized to act as a SiC nucleation initiator and to regulate the formation of the consecutive truncated cones. The as-synthesized 1-D hierarchical SiC nanostructures exhibit a strong and sharp blue emission at 445.2 nm as well as a broad weak and cyan emission ranging from 475 to 500 nm. Based on the same vapor deposition method, single crystalline SiC nanowires with different morphologies have been successfully controlled through the reaction between elemental silicon and graphite carbon. Within a 6h reaction time, the morphology of the SiC nanowires can be tuned to cylinder, hexagonal prism, and bamboo shape by simply altering the reaction temperature from 1470℃, 1550℃, to 1630℃, respectively. After the thermodynamic calculations, it was found that reaction related to the oxygen participation and high pressures of SiO and CO have been thought responsible for the nucleation and growth of SiC nanowires. The vapor-solid growth mechanisms for the multi-shaped SiC nanowires are proposed by taking into account the possible reactions between intermediate gas phases, the reaction steps, and the surface energy minimization. The photoluminescence spectra were investigated, and they display shape dependence, but the blue-shift relative to bulk 3C-SiC is really attributed to the microstructures and defects within the nanowires rather than the size and morphology.The growth of SiC nanostructures was also studied in more complicated systems such as C-Si-O-Fe, C-Si-O-Ni, and C-Si-O-NiSi. For the C-Si-O-Fe system, with increasing Si content, the yield of SiC nanowires and the ratio of length and diameter enhanced. The SiC nanowires morphology can be tuned by changing the mole ratio between the Fe and Si. It could be even obtained three-dimensional elegant heliotrope-like SiO nanoarchitectures when the mole ratio of Fe and Si was 1:1. It is suggested that the multi-nucleation via a Fe-catalyzed vapor-liquid-solid (VLS) process and then by the self-assembly process of curl SiOx nanowire arrays around a core, which results in the formation of heliotrope-like nanoarchitectures. For the C-Si-O-Ni system, the content of Ni has no obvious influence on the SiC nanowires growth. About C-Si-O-NiSi system, it was an effective and simple solution approach for the growth of high yield single crystal SiC nanowires. The reaction temperature and chamber vacuum were demonstrated to influence the formation and morphology of SiC nanowires. The formation of SiC nanowires is discussed by a combination of the solid-liquid-solid (SLS) for nucleation and the VLS process for nanowire growth. The Raman spectroscopy of the nanowire was investigated by a down shift and asymmetric broadening of the peaks, which illustrated the role of phonon confinement and structural defects.Twinned zigzag 3C-SiC nanoneedles can be synthesized by thermal evaporation method. The characterization results show that the SiC nanoneedles have zigzag surface, which is composedof (111) [11(2|-)] periodic twined segments. The growth of twinned zigzag SiC nanoneedles was realized by the vapor-solid (VS) reaction, in which (111) plane stacks in (111) [11(2|-)]twinningmanner, creating (11(1|-)) faceting over the nanoneedle surface. The photoluminescence spectraof SiC nanoneedles show considerable blue-shifts of multi-PL peaks relative to the bulk 3C-SiC. The stacking faults and quasi-periodically twinning superlattice result in the existence of similar to 4H-SiC or 6H-SiC nanolayers along the [111] direction in 3C-SiC nanowires. The thickness of these nanolayers is 1~1.5 nm, a right dimension to cause the quantum confinement effects, which can thus explain the blue-shift phenomena. Based on more HRTEM characterizations, the structure defects in SiC nanowires were further investigated. The results indicated that the defects play important roles in determining the morphologies and structures i.e. stacking faults result in formation of branches or junctions, while twins cause kinks, bamboo or a zigzag appearance. The thickness of periodical twins has a linear dependence on the diameter of the nanowires. It is necessary for the formation of twin in the SiC nanowires from the view point of crystal wulff construction.
Keywords/Search Tags:Silicon carbide nanostructures, Vapor deposition method, Vapor-solid mechanism, Self- assembly, Stacking faults, Twin defects, Photoluminescence
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