Size Controlled Synthesis of Silicon Nanocrystals within Inverse Micelles
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Size Controlled Synthesis of Silicon Nanocrystals within Inverse Micelles Keith Linehan and Hugh Doyle Tyndall National Institute, University College Cork, Lee Maltings, Cork, Ireland ABSTRACT Alkyl-terminated Si nanocrystals (NCs) are synthesized at room temperature by hydride reduction of SiCl4 within inverse micelles. Highly monodisperse Si NCs (2 – 6 nm) are produced by variation of the cationic quaternary ammonium salts used to form the inverse micelles. Transmission electron microscopy imaging confirms the NCs are highly crystalline, while FTIR spectra confirm that the NCs are passivated by covalent attachment of alkanes, with minimal surface oxidation. The photoluminescence intensity of the Si NCs exhibits an inverse relationship with the mean NC diameter, with a quantum yield of 12 % recorded for 2 nm NCs. INTRODUCTION Research interest in the preparation of oxide-free Si NCs has been steadily increasing, because the momentum requirements due to the indirect band gap structure of bulk Si are relaxed in the 1−5 nm size range as a result of quantum confinement effects.[1-2] This allows for their excellent photophysical properties, such as size-tunable and narrow luminescence and high quantum yields, to be combined with the richness of silicon surface chemistry. A large variety of both physical and chemical methods have been reported for the preparation of Si NCs, including physical and chemical vapor deposition, thermal annealing, laser induced-heating of gaseous precursors, electrochemical etching of crystalline silicon wafers, thermal decomposition of silane precursors and the reduction of silicon halides by various reducing agents.[3-8] The preparation of Si NCs within inverse micelles was first reported by Wilcoxon et al., which resulted in 2 – 10 nm Si NCs with hydrogenated surfaces.[9] This method was later refined using a Pt-catalyzed hydrosilylation process to chemically passivate the nanocrystal surface and allows addition of various functionalities.[10-12] More recently, alkyl trichlorosilanes have been utilized as both reductant and ligand, resulting in formation of both alkyl- and alkene-functionalized Si NCs.[1314] However, despite the number of reports describing the preparation of high quality Si NCs, no studies have been carried out on the effect of the surfactant in determining the size and shape of the resultant Si NCs. Here we report a simple method for the synthesis of Si NCs within inverse micelles having well defined core diameters ranging from 2 to 6 nm. Through controlling the size of the nanocrystals, their resulting photophysical properties can be tuned. EXPERIMENT The synthesis of the Si NCs was adapted from previously reported methods and carried out in an inert atmosphere glove-box.[10] In a typical preparation, 2.74 mmol of the surfactant was dissolved in 100 mL anhydrous toluene. 0.1 mL (0.87 mmol) SiCl4 was then added to the solution and left to stir for 30 min. Si NCs were formed by the dropwise addition of 6 mL of 1 M lithium aluminum hydride in THF over a period of 2 min. The
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