Free-standing Silicon Quantum Dot Photoluminescence

Abstract

This thesis focuses on the photoluminescence (PL) of free-standing silicon quan- tum dots (QDs). Large changes in the emission spectrum were found to occur when the QDs were exposed to different environments while undergoing short-wavelength laser irradiation, a phenomenon that was the main focus of this thesis. In particular, the PL can change in intensity by orders of magnitude, either increasing or decreas- ing, depending on the gases or vapors present in the surrounding atmosphere. The presence of chemical species with -OH groups produced especially strong and rapid effects, when in the presence of oxygen. This process was found to feature a rapid reconstruction and oxidation of the Si-QD surface, as measured by Fourier transform infrared spectroscopy (FTIR) and electron spin resonance (ESR). A model for the physical and chemical changes that silicon QDs undergo during these surprisingly strong changes in the PL intensity was developed, factoring in the results from a range of experiments. The effects reported here suggest that silicon QDs could be used to "sense" changes in the surrounding atmosphere. Therefore, a silicon-QD- based fiber optic sensor was demonstrated and its viability for detecting ethanol and water vapors was established. Finally, the outstanding challenges and potential for future research were discussed in light of improving the selectivity and detection limits for sensors based on the luminescence of free-standing silicon QDs.