Controlled synthesis and characterization of silicon nanocrystals
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In response to the demand for shrinking feature sizes and faster electronics, many resources have been dedicated to the research of nanotechnology. At present, silicon is undoubtedly the building block and key to the microelectronics world. In its bulk form, silicon is an inefficient emitter in the infrared, but as its dimensions shrink to the nanoscale, silicon exhibits unique optical and electrical properties such as size tunable photoluminescence. The more successful methods of the synthesis of silicon nanocrystals include laser ablation and silane pyrolysis; however these methods offer little in the way of particle stabilization which would prevent oxidation and allow for manipulation through dispersion in organic solvents. A novel supercritical fluid synthesis is investigated with respect to various silicon precursors such as diphenylsilane, silicon tetrachloride and trisilane. The electrochemical and luminescent properties of silicon nanocrystals, synthesized via the thermal decomposition of diphenylsilane, were studied. Differential pulse voltammetry of silicon nanocrystals in DMF and acetonitrile exhibit quantized double layer charging as previously reported for Au and CdS nanocrystals. Additionally, electron transfer reactions between positively and negatively charged nanocrystals (or between charged nanocrystals and molecular redoxactive coreactants) occurred that led to electron and hole annihilation, producing visible light. The electrogenerated chemiluminescence spectra exhibited a peak red shifted from the photoluminescence maximum. Single nanocrystal photoluminescence was investigated via Argon laser excitation and confocal microscopy. The single nanocrystals demonstrate stochastic single-step “blinking” behavior and size-dependent PL spectra with line widths approximately only three times greater than those measured for CdSe nanocrystals at room temperature. Investigation of trisilane as a viable silicon precursor in a supercritical fluid synthesis led to the formation of well formed, sub-micron, amorphous silicon colloids in high yield. Manipulation of temperature, pressure and precursor concentration allowed for the synthesis of amorphous silicon particles 60-400 nm in diameter. Polydisperse samples exhibited two dimensional, longviii range orientational order in the absence of translational order which has been compared to the Reverse Brazil Nut Effect. Additionally, metal induced crystallization was observed in amorphous silicon particles annealed in a vacuum evaporator.