Size-dependent Reactivity in the Functionalization of Nanostructured Silicon Surfaces
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Size-dependent Reactivity in the Functionalization of Nanostructured Silicon Surfaces Joel A. Kelly,1 Amber M. Shukaliak,1 Michael D. Fleischauer,2 Jonathan G.C. Veinot1 1
Department of Chemistry, University of Alberta Edmonton Alberta T6G 2G2, Canada 2 NRC- National Institute for Nanotechnology Edmonton Alberta T6G 2M9, Canada ABSTRACT The reactivity of silicon nanocrystals (Si-NCs) in near-UV photochemical hydrosilylation was evaluated as a function of size. Results show that Si-NCs with photoluminescence (PL) in the visible spectral region react faster than Si-NCs with near-IR PL. Fourier-transform infrared (FTIR) spectroscopy suggests this difference in reactivity is due to quantum size effects in the exciton-mediated mechanism proposed for this reaction. We have carried out a detailed comparison of Si-NC reactivity in photochemical and thermal hydrosilylation and determined the conditions under which Si-NCs may be size-selected based on their reactivity. INTRODUCTION Silicon nanocrystals (Si-NCs) have garnered considerable research interest for their sizedependent optical and electronic properties, particularly for in vivo imaging where their biocompatibility is thought to enable their use over potentially toxic II-VI and III-V semiconductor NCs (e.g., CdSe, GaAs) [1-5]. A critical aspect to the utility of these siliconbased materials is tailoring their surface chemistry to protect against unwanted oxidation [6]. This is often achieved through hydrofluoric acid (HF) etching and subsequent hydrosilylation with alkenes or alkynes [7]. Several initiation methods for hydrosilylation have been reported, including catalytic, thermal and photochemical routes. Recently, we investigated the use of near-UV photochemical hydrosilylation for the reaction of Si-NCs exibiting PL in the visible spectral region, with a range of alkenes and alkynes [8]. This choice near-UV irradiation offers several advantages for facile hydrosilylation with broad tolerance for a range of olefins. While in many cases the hydrosilylation of nanostructured Si surfaces has been shown to be analogous to that of molecular silanes and bulk surfaces, its quantum confined electronic properties have lead to the postulation of unique size-dependent reactivity. The reactivity we observed was generally consistent with Stewart and Buriak’s excitonmediated mechanism, originally proposed for visible-light hydrosilylation of luminescent porous Si [9]. Near-UV light corresponds to a direct gap (primarily electronic) transition in Si, rather than the indirect gap (coupled electronic and vibrational) behavior observed at lower energies [10,11]; thus, near-UV light might enable faster, more efficient hydrosilylation through this mechansim. Given their anticipated application as in vivo fluorophores, a natural extension of this study is towards the hydrosilylation of larger Si-NCs with PL in the near-IR “therapeutic window” region where light deeply penetrates tissue [12]. However, here it is shown near-UV hydrosilylation of these “large” Si-NCs occurs at a slower
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