Synthesis and optical properties of mullite nanowires

Single-crystalline mullite (3Al2O3·2SiO2) nanowires have been synthesized on silicon substrates by forcing aluminum and chromium chloride powders to react under an ammonia gas flow. The diameter and length of the nanowires have uniform diameters of <100 nm and several micrometers, respectively. High-resolution transmission electron microscope and selected-area electron diffraction analyses indicated that the nanowires were almost structural defect free and had a single-crystalline phase with a 〈0001〉 growth direction. The photoluminescence spectra showed that the mullite nanowires reached an emission peak at the center wavelength of 442 nm originating from the Al-O bonds in preference to the Al-O and Si-O bonds in the mullite bonding structure. Comparison of the photoluminescence between as-synthesized, oxygen-annealed, and plasma-etched nanowires indicates that the nanowires have few defects (i.e., oxygen vacancies).