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Synthesis and Photoluminescence of Ultra-long Aligned Si Nanotubes and SiO_x Nanotubes

Author: ZhouMing
Tutor: XieErQing
School: Lanzhou University
Course: Condensed Matter Physics
Keywords: electrospinning ordered arrangement TUFT process SiO_x nanotubes Si nanotubes luminescence
CLC: TB383.1
Type: PhD thesis
Year: 2010
Downloads: 113
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Abstract


Si nanotubes and SiOx nanotubes are compatible with Si based microelectronic existing widely in information industry and have prospective applications in the optical materials, nanodevice, glass fiber, catalysis and biological field, so they are research hotspot and frontier field nowadays. Meanwhile, the assembly system of nanomaterials also became the research emphasis at present. It emphasized that people designed, assembled and created new system according to their own will, and made this new system have expectable characteristics. Therefore, ultra-long and aligned semiconductor nanofibers and nanotubes have obvious advantages in assembly and application of nanomaterials. The thesis mainly introduced the preparation of ultra-long aligned polymer nanofibers and doped semiconductor nanofibers by electrospinning, and the fabrication of aligned Si and SiOx nanotubes by TUFT process, as well as the investigation of PL mechanism of nanotubes.1. Electrospinning with two parallel electrodes is used for fabricated aligned PVP nanofibers. The orientation degree and the length of aligned nanofibers are enhanced by improving the electrospinning technological process. The highly oriented nanofibers were 200 nm in diameter and their length was up to several millimeter. There are three basic requirements for obtaining highly aligned PVP nanofibers in this process:(i) using the suspended sheet metal as parallel electrode; (ii) choosing a suitable gap width and a suitable needle tip-to-target distance; (iii) the jet emerging from the Talylor’s cone has only one and stabilized in the effect of electric field.2. Using PVP as carrier, undoped and Co-doped TiO2 nanofibers array were fabricated by electrospinning. The nanofibers were 250 nm in diameter. After annealing, the nanofibers decreased and transferred to a succession of nanoparticle, this is relate to the pyrolysis of PVP nanofibers. The doping of Co promoted the TiO2 transformation process from anatase to rutile. CoTiO3 is formed when TiO2 transferred rutile, this result indicates that Co2+didn’t enter into the crystal lattice of rutile. And the further investigation of existing form of Co in anatase of TiO2 is still needed. Meanwhile, the Co-doped TiO2 didn’t show the room-temperature ferromagnetism by VSM, which may be related to the precision of equipment.3. Using the aligned suspended PVP nanofibers array as template, aligned ultra-long silicon oxide (SiOx) nanotubes with very high aspect ratios have been prepared by plasma enhanced chemical vapor deposition process. In fabrication, the U-shape frame not only offered convenience for transferring the aligned nanofibers, and also made the integrity degree of coating layer well. The inner diameter and wall thickness of tubes were controlled respectively by baking the electrospun nanofibers and by coating time without sacrificing the orientation degree and the length of arrays. Considering that the SiOx nanotubes are ultra-long and have a smooth tube wall, more remainder of PVP pyrolysis is left in the tubes channel when the diameter of nanotubes decreased. The micro-PL spectrum of SiOx nanotubes shows a strong blue-green emission with a peak at about 514 nm accompanied by two shoulders around 415 and 624 nm. The blue-green emission is caused by the defects in the nanotubes.4. Using aligned suspended polyvinyl pyrrolidone nanofibers array as template, aligned ultra-long (about 4 mm) silicon nanotubes have been prepared by a hot wire chemical vapor deposition process. The inner diameter and wall thickness of Si tubes are controlled respectively by baking the electrospun nanofibers and by coating time. So the outer diameter is easily controlled without sacrificing the orientation degree and the length of arrays. Furthermore, it is found that the integrity degree of coating layer is different at different zones of reaction chamber, which is caused by the different concentration and motion state of dissociated gas in various zones. The tube wall is composed of nano-particle or nano-pillar. and the inner surface of the wall is smoother than the outer surface of the wall. The PL spectra of the thinner Si nanotubes show three light emission bands in the red, green and blue regions. And the luminescence mechanism is explained according to the quantum confinement-luminescence center process and radiative recombination from the defect centers.

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