An empirical Compton upscattering model is described that reproduces both the fractional amplitude (rms) versus energy and the soft time lags in the ≈830 Hz quasi-periodic oscillation (QPO) observed in 4U 1608Ϫ52 on 1996 March 3. A combination of two coherent variations in the coronal and soft photon temperatures (with their relative contributions determined by enforcing energy conservation) gives rise to the QPO's energy-dependent characteristics. All input parameters to the model, save a characteristic plasma size and the fraction of Comptonized photons impinging on the soft photon source, are derived from the time-averaged photon energy spectrum of the same observation. Fits to the fractional rms and phase lag data for this kilohertz QPO imply that the spatial extent of the plasma is in the range from ∼4 to 15 km.
Large-area, vertically aligned silicon nanowire ͑SiNW͒ arrays have been successfully synthesized in an aqueous solution containing AgNO 3 and HF on ͑001͒Si substrates. The as-synthesized SiNWs were determined to be perfectly single crystalline with the axis of the wire parallel to the ͓001͔ direction. The typical widths of the SiNWs were in the range of 30-200 nm. The lengths of SiNWs could be tuned from several to tens of micrometers by adjusting the synthesis temperature and time. We measured the formation rate at different reaction temperatures. The activation energy for linear growth of the SiNWs, as obtained from an Arrhenius plot, was found to be about 0.36 eV. In addition, the Si substrates with highly oriented SiNW arrays were found to exhibit significant hydrophobic properties. The water contact angles of the HF-treated SiNW arrays were measured to be about 120-148°, much greater than that with a flat silicon wafer surface ͑ϳ85 to 88°͒.
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