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Structural and optical properties of nanomaterials produced using template technique

Authors Li, Ling
Issue Date 2004
Summary Nanomaterials have attracted extensive attentions because of their significance in both fundamental science and potential applications. There are many conventional methods to produce nanostructures. However, the products fabricated using traditional methods usually have a size distribution and disordering in structures, which might smear the quantum effects in the samples. It is desirable and a challenge to produce well aligned and mono-sized nanostructures. In this thesis, we will report a unique method to produce mono-dispersed nanostructure arrays using nanoporous A1PO4-5 (AFI) single crystals as template. The task of this thesis is to characterize the structure and investigate physical properties of several nano-species formed in the channels of AFI crystals. I) Selenium nanospecies are fabricated in the channels of AFI single crystals. Raman spectra show that four possible types of Se clusters are formed, namely amorphous-like selenium (a-Se), trigonal crystalline-like selenium (t-Se), one dimensional selenium helix (Se chain), and selenium 8-member rings (Se8). The phase stability of these nanostructures is investigated by thermogravimetry, differential scanning calorimetry, and temperature dependent micro-Raman measurements. Phase transition of these confined Se species occurs at temperature range 320 K~350 K. The crystallization temperature for a-Se in the mesochannles and torsion 'melting' point for Se single chains are determined. The energy band gaps of Se chain and rings are blue shifted, which is interpreted as the quantum confinement effect by the AFI matrix. II) The highly-aligned mono-sized ultra small single-walled carbon nanotubes (SWNTs) are formed inside AFI matrix by pyrolysing tripropylamine organic molecules in the channels. Transmission electron microscopy, diffuse x-ray diffraction, and micro-Raman measurements all consistently indicate that the diameter of SWNTs is ~0.4 nm. Strong curvature effects induce mixing of the σ-π unoccupied orbitals, which introduces a variety of interesting material characteristics such as splitting of the G-band, softening of the radial breathing modes. The calculated dielectric functions yield predictions in very good agreement with the experimentally measured resonant Raman spectrum and modulated photo-reflectance spectra. III) Styryl 7 dye molecules loaded AFI single crystals (dye/AFI) are also produced. The intensity of the polarized absorption and polarized photoluminescence (PL) bands of a single dye/AFI crystal is mainly proportional to cos2θ, indicating that the dipole transition moment of the organic dye molecules is highly oriented in the channels. Due to size confinement of the AFI channels, there is a structure deformation in the dye molecules, which leads to a large blue shift in the absorption spectra for the dye/AFI crystal. The hexagonal AFI single crystal serves as a good natural micro-cavity for the lasing action of the adsorbed dye molecules. The micro-cavity mode of the lasing action is demonstrated for the first time on the dye-loaded zeolite crystal prepared by physical diffusion method.
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2004
Language English
Format Thesis
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