We describe the synthesis and characterization of alkyl-capped nanocrystalline Si (R ˆ-n-Si) prepared by the reaction of SiCl 4 with Mg 2 Si in ethylene glycol dimethyl ether (glyme) and surface-terminated with various alkyl groups, R ˆ-n-Si (R ) methyl, ethyl, n-butyl, and n-octyl). This reaction produces crystalline nanoparticles with surfaces that can be chemically modified. The resultant crystalline nanoparticles can be suspended in organic solvents or isolated as a powder. The nanoclusters were characterized by transmission electron microscopy (TEM), high-resolution TEM, selected area electron diffraction (SAED), and Fourier transform infrared (FTIR) spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, UV-vis absorption, and photoluminescence spectroscopy. The average cluster size depends on the reflux time of Mg 2 -Si with SiCl 4 , which provided nanoclusters with an average size of 2-5 nm. HRTEM confirms the presence of crystalline nanoclusters, and SAED is consistent with diamond-structured silicon. FTIR spectra are consistent with alkyl surface termination and show very little or no evidence for oxygen on the surface of the nanoclusters, depending on the surface alkyl group. The alkyl termination can be removed by reaction in air at 450 °C, and a Si-O stretch is observed in the FTIR spectra. EPR spectroscopy is consistent with crystalline Si nanoclusters and shows no signal at 4 K. The optical absorption spectra show an absorption edge between 260 and 240 nm, depending on the surface alkyl group, while a strong UV-blue photoluminescence between 315 and 520 nm is observed.
A solution synthesis of crystalline Ge nanoparticles (nc-Ge ) is reported. The metathesis reaction of NaGe with excess GeCl 4 in glyme solvents produces nc-Ge. Metathesis reactions between KGe and excess GeCl 4 or GeCl 2 :(dioxane) in glyme and Mg 2 Ge and excess GeCl 4 in diglyme and triglyme were also investigated. The surface of these nanoparticles is terminated with alkyl groups by reaction with alkyl Li and Grignard reagents. The alkyl-terminated crystalline Ge nanoparticles (R ˆ-nc-Ge) were characterized by Fourier transform infrared spectroscopy, high-resolution transmission electron microscopy, powder X-ray diffraction, UV-vis absorption spectroscopy, photoluminescence, and photoluminescence excitation spectroscopy. The optical properties of R ˆ-nc-Ge made by this method agree with predictions from quantum confinement models.
Solution Synthesis of Germanium Nanocrystals Demonstrating Quantum Confinement.-Colloidal solutions of Ge nanocrystals are prepared from Na germanide and GeCl 4 followed by reaction with MeLi. The resulting Me-terminated nanocrystals show an average particle size of 3.5 nm. The size dependence of the intense absorption, the blue shift of the absorption to the UV, and the monotonic shift of the emission wavelength with excitation wavelength all provide strong evidence for quantum confinement in these Ge nanocrystals. -(TAYLOR, B. R.; KAUZLARICH, S. M.; LEE, H. W. H.; DELGADO, G. R.; Chem. Mater. 10 (1998) 1, 22-24; Dep. Chem., Univ. Calif., Davis, CA 95616, USA; EN)
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