Stable gold clusters around 1 nm in size have been prepared by evaporation of the metal into an organosilicon polymer solution. A controlled particle growth of the entrapped gold particles was achieved by annealing in helium at ∼410 °C. Above 460 °C the growth rate is dramatically increased, concurrent with the onset of pyrolysis of polysilazane to form a porous solid. X-ray diffraction and simulation calculations using Debye functions of model clusters of different sizes and both crystallographic and noncrystallographic symmetry were used to characterize these particles. The exclusive presence of noncrystallographic (decahedral and icosahedral) multiply twinned particles (MTPs) in the as-prepared material is confirmed by EXAFS. The high precision of the XRD experiments and theoretical data fits gives detailed insight into the thermally induced growth process, during which the MTPs progressively disappear and are replaced by singly twinned fcc particles of larger size. This transition occurs in the range ∼2−3 nm. This analysis requires the introduction of a size-dependent contraction for the Au−Au distance in the order of 1−3% compared to bulk gold. This is discussed in the context of theoretical predictions.

ExperimentalRefractive Indices: The ordinary and extraordinary refractive indices of drawn films were measured using an Abbe refractometer. The so-called trirefringence technique [31] was used to determine the three principle refractive indices of the drawn films.UV-vis: Polarized UV-vis measurements were performed on a Perkin Elmer Lambda 9 spectrophotometer in the wavelength range from 400 to 700 nm. Prior to the measurements, the drawn films were coated with a paraffin oil and sandwiched between two glass slides to eliminate the scattering of light at the film surface. The transmittances of the films were measured parallel (T 1 ) and perpendicular (T 2 ) to the drawing direction. The polarizing efficiency (PE) and single-piece transmittance (T sp ) were calculated from these measurements using Equations 1 and 2. PE = (T 2 ± T 1 )/(T 2 + T 1 )(1)Light Scattering in the Forward Direction: A collimated beam of light, originating from a halogen light source, illuminates the sample via an optical fiber. The measurements were performed using a green bandpass filter with a central wavelength of 561 nm and a full width at half maximum of 21 nm. Two linear dichroic polarizers are placed below and above the sample to control the polarization directions of the incident and transmitted light. The light passing through the system is collected by a light collector connected to a photomultiplier tube (PMT), which measures the light intensity in terms of absolute luminance (Cd/m 2 ). The light collector can rotate in a vertical plane over an angle y from 0 to 70. The sample with the polarizers is mounted on a sample holder that can rotate over an angle j from 0 to 360. Consequently, the spatial distribution of the light intensity in the forward direction can be evaluated in spherical coordinates. For a graphical representation of the three-dimensional light intensity distribution, the spherical coordinates y and j were converted into Cartesian coordinates x and y with x = sin y cos j and y = sin y sin j. Relative light intensities were obtained by dividing the absolute intensities by the measured intensity without a sample at x = 0 and y = 0 (i.e., the maximum of the incident light beam).Integral Light Intensity in the Forward and Backward Direction: The ratio of the light scattered in the forward and backward direction was measured with light from a laser (Spectra Physics, 2 mW helium neon laser, wavelength = 540 nm), which is polarized using a dichroic polarizer and subsequently split into two beams by a beam splitter. One beam is directed to the sample and an integrating sphere and the second, reference beam is transmitted directly to a photodiode. The measured intensity in the integrating sphere was divided by the reference intensity to correct for laser intensity fluctuations. The relative intensity of the light in the forward direction is measured by placing the sample in front of the integrating sphere. Subsequently, the same sample is placed at the rear of the integrating sphere and the relative intensity of light sc...

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High surface area, microporous, amorphous silicon imidonitride, characterized by infrared spectroscopy, MAS 29Si NMR and surface area and porosity measurements has been prepared by the treatment of co-oligomers of methylsilazane and dimethyl silazanes with gaseous ammonia at temperatures up to 700°C. The material has a narrow pore-size distribution showing a maximum in the range associated with wide- pore zeolites (ca. 0.72 nm mean). Variation of the organic content of the silazane is a means of controlling the surface area of the resulting solid. The Knoevenagel condensation reaction of benzaldehyde with a series of active methylene compounds has been used to probe the basicity and size-selectivity of these microporous solid base catalysts.

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