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We have developed a versatile, one-step melt synthesis of water-soluble, highly emissive silicon nanoparticles using bi-functional, low-melting solids (such as glutaric acid) as reaction media. Characterization through transmission electron microscopy, selected area electron diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy shows that the one-step melt synthesis produces nanoscale Si cores surrounded by a silicon oxide shell. Analysis of the nanoparticle surface using FT-IR, zeta potential, and gel electrophoresis indicates that the bi-functional ligand used in the one-step synthesis is grafted onto the nanoparticle, which allows for tuning of the particle surface charge, solubility, and functionality. Photoluminescence spectra of the as-prepared glutaric acid-synthesized silicon nanoparticles show an intense blue-green emission with a short (ns) lifetime suitable for biological imaging. These nanoparticles are found to be stable in biological media and have been used to examine cellular uptake and distribution in live N2a cells.

Abstract

Visibly emissive silicon nanoparticles (Si NPs) were obtained via annealing of (HSiO 1.5 ) n polymer, followed by chemical etching. The hydrideterminated Si NPs (H-Si NPs) were surface-functionalized via thermal hydrosilylation with 1-decene and were dispersed in straight chain alcohols varying in carbon chain length (C1−C10). The initial red photoluminescence (PL) (λ max,em ∼580 nm) observed from hexane dispersions of the decane-terminated Si NPs (dec-Si NPs) became weaker upon exposure to alcohols smaller than a minimum chain length, commensurate with appearance of strong, blue PL (λ max,em ∼450 nm). A suite of spectroscopic and microscopic techniques was employed to study and correlate the change in Si NP PL with composition and/or size changes to the Si NPs. The results of these studies support that the conversion from red to blue PL originates from dangling bond defect passivation by small alcohol molecules, resulting in an enhanced blue/red PL ratio. The dangling bond defect (Si • ) is supported by reactivity studies of H-Si NPs with the stable radical (2,2,6,6-tetramethylpiperidin-1-yl)oxy (TEMPO) and with n-alkanes. We discuss these experimental results in light of current hypotheses about the origins of Si NP visible light emission, which we demonstrate is markedly influenced by the Si NP surface chemistry. An energy level diagram is proposed to account for the spectral and dynamic features observed, in which the role of surface states is highlighted.

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