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Shape Evolution of Faceted Silicon Nanocrystals upon Thermal Annealing in an Oxide Matrix Zhenyu Yang, Alexander R. Dobbie, and Jonathan G. C. Veinot Department of Chemistry, University of Alberta, 11227 Saskatchewan Drive, Edmonton, Alberta, T6G 2G2, Canada ABSTRACT It is well established that controlled high-temperature annealing of hydrogen silsesquioxane leads to the formation of small spherical silicon nanocrystals (~3 nm). The present study outlines an investigation into the influence of annealing time and temperature. After prolonged annealing, crystal surfaces thermodynamically self-optimize to form a variety of faceted structures (e.g., cubic, truncated trigonal and hexagonal structures). INTRODUCTION The synthesis of silicon nanoparticles/crystals has been a very active research area over the past 15-20 years in part because these materials are not accessible through the application of standard methods used to prepare traditional nanomaterials. In addition, their biocompatibility (compared to Cd-based quantum dots) make them particularly appealing. In this context, their unique properties make them suitable for nanoelectronic devices, in-vivo imaging, and other light-emitting applications [1-4]. Considerable effort has been aimed at controlling particle size and shape. By controlling the surface energy and electron transfer ability of quantum dots, their chemical and physical properties can be directly controlled [5, 6]. Morphological control has been widely studied and colloidal synthetic strategies are well-developed for metal and metal oxide nanocrystals, as well as II-VI and III-V quantum dots. Surfactants, temperature, and concentration allow the preferential growth of crystal faces by altering their relative thermodynamic stability. However, reports applying similar approaches to shape controlled synthesis of silicon nanomaterials are rare and even nonexistent because the strong directional bonding in Si precludes standard colloidal synthesis. The vast majority of the silicon nanoparticles synthesized by the decomposition of silane or other reduction-functionalization strategies are spherical or pseudospherical [7-10]. Synthesis of tetrahedral and cube shaped silicon nanostructures has been achieved using solution-based and nonthermal plasma methods, respectively [11-13]. However, the sizes of these nanostructures are relatively large and these procedures suffered from challenges such as flammable precursors and complicated infrastructure. Furthermore, our experimental understanding of the parameters that govern nanoparticle shape remains limited. Therefore, a new straightforward approach for making small Si-NCs of tailored shapes is appealing. The Veinot research group has established a facile solid-state method that affords welldefined Si-NCs from hydrogen silsesquioxane (HSQ) as a precursor (Scheme 1) [14]. High temperature processing in a slightly reducing atmosphere causes HSQ to disproportionate and provides Si-NCs embedded in an SiO2-like matrix. The size and crystallinity of these NCs may be