Portland State University. Department of Chemistry
Term of Graduation
Date of Publication
Doctor of Philosophy (Ph.D.) in Chemistry
Nanostructured materials -- Optical properties, Nanosilicon -- Synthesis, Photoluminescence
1 online resource (viii, 96 pages)
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 annealing synthesis method, with the aim of achieving higher crystallinity Si NPs with similar or better size polydispersity. Initially, a [HSiO1.5]n polymer was reduced by Mg powder to produce highly crystalline, diamond lattice Si0 with nano-sized crystalline domains that either fused into larger structures or maintained original particle morphology depending on processing conditions. Attempting to control the processing conditions to leverage the latter result, nanoscale SiO2 template particles of known size and morphology were next metallothermically reduced to try to produce replica Si NPs of equal size and shape. Although some conditions were modestly successful in terms of producing crystalline Si NPs with morphology retention of the template, most synthetic trials only produced fused micron-sized structures of crystalline Si. It is possible that with better spatial control over local heats of reaction, that individual SiO2 nanoparticles can be reduced to individual Si NPs without the fusion of neighboring domains.
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Barnes, James Donald, "Investigations into Size and Surface Control of Silicon Nanocrystals for Improved Optical Properties" (2021). Dissertations and Theses. Paper 5710.