Investigations into Size and Surface Control of Silicon Nanocrystals for Improved Optical Properties

Abstract

The discovery of visible photoluminescence (PL) from nanocrystalline porous silicon in 1990 led to extensive research into the mechanisms of the emergent properties, and optimization of these properties, for use in applications. The widespread use of silicon nanoparticles (Si NPs) in commercial applications is currently limited by three main factors: 1) poor radiative recombination efficiency of the interband transition, 2) instability of the interband photoluminescence, and 3) a lack of scalable methods for producing Si NPs that are both highly crystalline and size monodisperse. To address these limitations, this dissertation correlates changes in the photoluminescence properties of hydrogen passivated silicon nanoparticles (H-Si NPs) with changes in the surface structure, as well as develops new synthesis methodology to produce larger, more crystalline Si NPs. Red photoluminescent H-Si NPs were prepared by high temperature reductive annealing of a [HSiO1.5]n polymer derived from HSiCl3, followed by an aqueous HF etching procedure to isolate them in colloidal form. The H-Si NPs were then subjected to different chemical and physical environments and the changes to the photoluminescence spectra were then related to the changes seen in other spectroscopic measurements. First, the stability of the interband transition of H-Si NPs was probed using the free radical (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO) under different lighting conditions and the photoluminescence spectra of these samples were monitored over time. The TEMPO radical was observed to increase the interband emission intensity, but with a large hypochromic shift that is correlated to the significant oxidation of the Si NP surface. We propose that this shift is due to core shrinkage of the Si NP upon oxidation, and not an emergent electronically active defect state from the resultant surface oxidation. Second, a surface treatment for Si NPs was developed to stabilize the interband transition using purposeful oxidation. A survey of chemical environments that have been shown in the literature to promote oxidation of Si was conducted, and based on spectroscopic results, benzoyl peroxide (BPO) was identified as a reagent that can be used to oxidize the surface of the H-Si NPs without causing a significant hypochromic shift in the interband transition. It was observed that the surface reaction in the presence of BPO was accelerated by continuous 365 nm irradiation, resulting in an increase in interband transition intensity with no shift in the emission energy. Finally, the application of metallothermic reduction of silicon oxides was probed as a potential alternative to the high temperature reductive