| Summary: | 博士 === 國立交通大學 === 光電工程系所 === 93 === We successfully fabricate ZnO-based nanowires by vapor transport method. The optical properties of ZnO nanowires are also investigated. Hexagonal ZnO nanowires have been selectively synthesized via vapor-solid process without gold catalysis on a pre-coated ZnO buffer layer. The presence of nanometer-sized pits or hills on the surface of ZnO buffer layer provides nucleation sites to which the zinc vapor is transferred and condensed. Followed by immediate oxidation the ZnO nanowires were grown on the buffer layer. ZnO nanowires can be also synthesized on porous silicon substrates with different porosities via the vapor-liquid-solid method. The texture coefficient analyzed from the XRD spectra indicates that the nanowires are more highly orientated on the appropriate porosity of porous silicon substrate than on the smooth surface of silicon. Vertically well-aligned ZnO nanorods are synthesized without employing any metal catalysts on various substrates including glass, Si(111), and saphire(0001), which were pre-coated with c-oriented ZnO buffer layers, by simple chemical vapor deposition. The epitaxial relationship between ZnO nanowires and various substrates is discussed in detail.
From the temperature dependent photoluminescence spectra, we deduce the activation energies of free and bound excitons. Besides a strong ultra-violet emission at 3.26 eV observed at room temperature, the coupling strength of the radiative transition to LO-phonon polarization field was deduced in use of the Huang-Rhys factor from low temperature photoluminescence spectra to show that single crystalline ZnO nanorods.
The coupling strength of the radiative transition of hexagonal ZnO nanowires to the longitudinal optic (LO) phonon polarization field is deduced from temperature dependent photoluminescence spectra. An excitonic polaron formation is discussed to explain why the interaction of free excitons with LO phonons in ZnO nanowires is much stronger than that of bound excitons with LO phonons. The strong exciton-phonon coupling in ZnO nanowires affects not only the Haung-Ray S factor but also the FXA-1LO phonon energy spacing, which can be explained by the excitonic polaron formation.
We report room-temperature ultraviolet stimulated emission and lasing from optically pumped high-quality ZnO nanowires. Emission due to the exciton-exciton scattering process shows apparent stimulated-emission behavior. Several sharp peaks associated with random laser action are seen under high pumping intensity. The mechanism of laser emission is attributed to coherent multiple scattering among the random-growth oriented nanowires. The characteristic cavity length is determined by the Fourier transform of the lasing spectrum.
Finally, we demonstrate a simple method to achieve the bandgap engineering in core-shell ZnO-MgO nanowires by using Mg diffusion. Furthermore, we report the observation of stimulated emission (SE) from optically pumped ZnMgO nanowires.
|
|---|
