Non-thermal plasma synthesis and passivation of luminescent silicon nanocrystals

A novel reactor for the controlled synthesis of small silicon nanocrystals has been developed. A non-thermal plasma is generated in a quartz tube through which a silane containing mixture is flown, resulting in the nucleation and growth of silicon nanoparticles. Given the short residence time in the reactor (<10 ms), very small crystallites are produced, and quantum confinement effects lead to the observation of intense visible photoluminescence when the particles are excited by UV irradiation. The system is capable of producing up to 50 mg/hr of luminescent powder.; The mechanism leading to the formation of small crystallites has been investigated by studying the interaction of the silicon cluster with the surrounding plasma, in particular with argon ions and with atomic hydrogen. Ion and atomic hydrogen densities have been experimentally measured. The particle temperature exceeds the background gas temperature of approximately 100 K, and the instantaneous temperature of very small clusters exceeds the gas temperature of several hundreds of degrees. This behavior likely leads to the formation of high quality crystals.; As-produced silicon nanocrystals have a hydrogen-terminated surface, which is an ideal chemical configuration for grafting alkenes onto the particle surface. Liquid phase treatment of plasma-produced silicon nanocrystals with 1-dodecene leads to the synthesis of a clear and stable colloidal dispersion of silicon particles. Fluorescent quantum yields exceeding 60% have been measured for silicon inks with a peak emission wavelength around 800 run. This is the highest ensemble quantum yield ever reported for the case of silicon.; The disadvantages of the liquid phase passivation scheme, long reaction time and the use of solvents, are overcome by using the in-flight plasma initiated passivation scheme described in this thesis. Various molecules have been successfully reacted with the silicon crystals in the gas-phase, and a silicon ink can be readily obtained without using liquid-phase processing. The process is promising for attaching short molecules to the particle surface, necessary for improving the electrical properties of the quantum dot, and for realizing stable dispersion of silicon particles in water.