J. D. Stout

Silica Nanotubes and Nanofiber Arrays

Sample Preparations: Multilayers of PDADMAC (Mw = 200K±350K g/ mol, Aldrich), and PSS (Mw = 70 000 g/mol, Aldrich) were deposited onto silica colloids (Snowtex, nominal diameter 70±100 nm). 3 g of silica colloid, previously dried for 12 h at 400 C, was dispersed in 500 mL of a polymer/ salt solution, comprised of 0.02 M PDADMAC and 0.1 M NaCl in Millipore Q water. This adsorption solution was left standing for 30 min, then centrifuged at 4300 rpm and the supernatant was removed. 500 mL water was added and the solution was sonicated and centrifuged. The supernatant was then removed to rinse the unadsorbed polyelectrolyte from the colloids. A total of three 500 mL washings were performed after the adsorption of each polymer layer. A small amount (~50 mg) of the coated colloid was then removed for characterization and dried at 65 C for 12 h prior to measurement. The remaining colloid was then dispersed in 500 mL of a similar solution of the oppositely charged polymer (0.02 M PSS and 0.1 M NaCl), and the adsorption and washing steps repeated, until two layers of PDADMAC and two layers of PSS had been sequentially deposited. An insoluble PEC for reference was prepared by adding 2 mL of 20 wt.-% PDADMAC aqueous solution slowly to 200 mL of a 0.01 M PSS solution, under vigorous stirring. A precipitate formed immediately as a thick milky suspension in solution. This solution was centrifuged, and the supernatant removed by pipette. To remove uncomplexed polyelectrolyte, the complex was then washed with water, agitated to disperse, and centrifuged again as described for the colloids, for a total of three washings. f-Potential Measurements: 30 mg of each dried colloid sample was suspended in 15 mL of 1 mM NaCl solution. The pH of each solution was found to be in the range of 7.4 to 7.7 for the multilayered samples, and 8.2 for the bare silica colloid. Electrophoretic mobilities were measured on a Microelectrophoresis Apparatus Mk II (Rank Brothers, Bottingham) and converted to f potentials using the Smoluchowski equation. NMR Measurements: C CP MAS NMR of the carbon spectra were recorded on a Chemagnetics CMX-300 spectrometer for the colloid coated with four layers, the precipitated complex, and both bulk polyelectrolytes. A total suppression of sidebands (TOSS) sequence with background suppression, a spinning speed of 3 kHz, and a contact time of 500 ls were used. Singleand double-quantum H MAS NMR spectra were acquired either on a Bruker ASX500 or DRX700 spectrometer equipped with a 2.5 mm fast MAS probe.

Direct synthesis of silicon nanowires, silica nanospheres, and wire-like nanosphere agglomerates

Elevated temperature synthesis has been used to generate virtually defect free SiO2 sheathed crystalline silicon nanowires and silica (SiO2) nanospheres which can be agglomerated to wire-like configurations impregnated with crystalline silicon. The SiO2 passivated (sheathed) crystalline silicon nanowires, generated with a modified approach using a heated Si–SiO2 mix, with their axes parallel to 〈111〉 are found to be virtually defect free. Modifications to the system allow the simultaneous formation of SiO2 nanospheres (d∼10–30 nm) as virtually monodisperse gram quantity powders which form large surface area catalysts for the selective conversion of ethanol to acetaldehyde.

Measurement of low-level gain in a visible chemical laser amplifier

High precision zero power gain measurements are used to demonstrate that the energy transfer sequence SiO*(b 3 ∏) + Na(3 s 2 S) →SiO(X 1 Σ + ) + Na*(4 d 2 D); Na*(4 d 2 D) → Na(3 p 2 P) + hv(569 nm); Na*(4 d 2 D) + hv(569 nm)→Na(3p 2 P) + 2hv(569 nm) represents a viable visible chemical laser amplifier candidate. Dual beam radiometry is used to advantage in a sensitive and stable measurement system, which provides correction for: 1. source intensity fluctuations, 2. variations in source spectral density, 3. variations in detector spectral response, and 4. gain variations in detector and amplification stages. Experiments are performed to determine the amplification of a probe beam at 569 nm by an extended path length (nominal gain length ∼5 cm) reaction energy transfer zone. For these quantitative gain measurements, the stability of the probe signal sets a limit on the sensitivity of the measurement. Considerable effort was expended to insure equivalent path lengths for probe and reference beams to compensate for source output variations so that the reference signal possesses the same temporal behavior as the probe, and the probe and reference optical systems image the same portion of the source on the detector. A gain coefficient, conservatively estimated as 0.8 to 1.5×10 - 3 cm - 1 , was measured. Based on this result, a Rigrod analysis indicates an expected full laser cavity output power between one and ten watts.