Aerosol synthesis and characterization of silicon nanocrystals

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Synthesis and processing of optically active silicon nanocrystals are explored from an aerosol science perspective. Spark ablation, laser ablation and thermal evaporation in inert atmospheres are employed alternatively as vapor phase sources of nanocrystals. Nanocrystals generated employing these techniques comprise a highly polydisperse and morphologically diverse aerosol. After collection on a solid substrate, samples of these nanocrystals exhibit wide-band visible photoluminescence. A system for size classification of the initial polydisperse nanocrystal aerosol is demonstrated employing differential mobility analysis. Working at low nanocrystal concentrations (around [...]) size control within 15% to 20% is achieved in the 2 to 10 nm size regime with a radial differential mobility analyzer at the expense, however, of low throughputs which make optical studies challenging. Seeking higher throughputs, the physics of aerosol size classification by this technique is investigated in detail by self-consistent numerical simulations of the particle transport inside the differential mobility analyzer. Our results lead to the identification of critical design characteristics required to maximize the analyzer performance from the viewpoint of semiconductor nanocrystal synthesis. With the guidance of these theoretical predictions, an optimized differential mobility analyzer design is suggested. This instrument has its parameters chosen to perform high resolution, high throughput size classification of nanocrystals in the 0.5 to 10 nm range. Optical characterization studies on polydisperse and size-classified silicon nanocrystal samples are performed. Results suggest that at least two mechanisms for light emission are at work in aerosol synthesized silicon nanocrystals. X-ray photoelectron measurements on size-classified silicon nanocrystals reveal that an oxide layer with thickness in excess of several nanometers forms on the silicon nanocrystals within a few minutes of air exposure. In order to preserve and control the surface chemistry of the nanocrystals, a system for anaerobic transfer of the size- classified silicon nanocrystals is designed and built. The system couples the nanocrystal synthesis experiment with the ultra high vacuum chamber of a surface analysis system via a load lock high vacuum chamber. Optical characterization capabilities are also installed. Preliminary results on nanocrystal synthesis and characterization using this in situ setup are presented and discussed