Amorphous silicon powder exhibiting microstructural properties such as nanometer-size, large presence of silicon-hydride groups and high surface/volume ratio has been produced in a plasma enhanced chemical vapor deposition (PECVD) reactor using pure silane gas and low frequency square-wave-modulated (SQWM) rf power (13.56 MHz). The possible crystallinity of nanometer-size domains embedded in the amorphous network was studied with Raman spectroscopy and electron diffraction analysis.

Photoluminescence (PL) in the near IR-VIS region was excited by an Ar laser. The PL intensity exhibits very unusual properties: a supralinear dependence on excitation power and an exponential decrease with pressure (below 50 Pa). The analysis of PL dynamics led us to propose a mathematical model of excitation of the PL. This model involves the existence of an intermediate step, with an extremely long lifetime, that controls the PL dynamics.

Structural characterization of phenylacetylene dendrimers (PADs) makes it possible to explore the relationship between molecular architecture and condensed phase organization. The size and geometry of the PAD series is precisely controlled, with phenylacetylene units emanating from a central phenylene in the manner of a tridendron. The branched molecule rapidly increases in size with each synthetic generation. The “shape-persistent” nature of the phenylacetylene molecule makes it ideal for use in the construction of self-assembling supramolecular systems.

Transmission electron microscopy (TEM) has been used to identify the crystal structure of lower generation PADs, and wide-angle X-ray studies confirm the decrease in crystallinity with size. Hot stage optical microscopy studies of thermal transitions reveal melting points for lower generation PADs, and an apparent glass transition for the amorphous higher generations. This type of structural information is essential to the rational design of self-assembling materials.

We have used solution-phase inorganic chemistry to form ultrathin, covalently bonded layers of TiO2 on silicon wafers. Ellipsometry and x-ray photoelectron spectroscopy (XPS) were used to monitor the growth of the films. The ellipsometric thickness of the TiO2 films increased linearly with the number of reaction cycles, the first absorption cycle resulting in about 3–4 Å of growth, and subsequent cycles resulting in about 1 Å of growth. We attribute this limited growth to the limited number of reactive sites on the surface.

We have developed a new poly-foam process for the cost effective preparation of ceramic nanoparticles. The process utilizes the chemistry of polyurethane reactions and is proven to be effective for forming nanometer size ceramic powders of a great variety of single metal oxides and mixed metal oxides. In general, ceramic powders can be prepared by this process having a range of average particle size between 3 to 50 nm, with very narrow particle size distribution. They are free of hard agglomerates, are chemically pure and uniform, and are essentially spherical in shape.

Transport models have predicted that the thermal conductivity of SiGe alloys could be appreciably reduced by incorporating a discrete 40Å particles with the SiGe grains. Such a thermal conductivity reduction would lead to substantial improvements in the figure-of-merit of thermoelectric materials. This paper reports on recent results on adding 40Å particles to SiGe via a spark erosion process. Thermal conductivity reductions consistent with the transport models have been achieved, however, the improvement in figure-of-merit has not been as large as predicted.

Laser ablation has become widely recognized as an effective technique for the preparation of thin solid films. We have employed an excimer laser (KrF, 248 nm) to deposit well dispersed thin films of aluminum phosphate molecular sieves on a titanium nitride substrate. Results for the ablation of AIPO4-5, AIPO4-H3 and AIPO4-H1 molecular sieve targets are presented. The laser power and repetition rate as well as substrate distance and temperature affect the thin film formation. A subsequent hydrothermal post treatment of the ablated films was found to enhance the surface crystallinity. The molecular sieve thin films were characterized by XRD, SEM, XRF, and FT-IR spectroscopy.

Bimetallic catalysts have been studied intensively because of their unique activity and selectivity. Unsupported alloy catalysts, however, are usually of limited value due to their very small surface areas. We have now developed a sonochemical synthesis of bimetallic alloys that provides both high surface areas and high catalytic activity. We have produced Fe-Co alloys by ultrasonic irradiation of mixed solutions of Fe(CO)5 and Co(CO)3(NO) in hydrocarbon solvents. The alloy composition can be controlled simply by changing the ratio of precursor concentrations. After treatment at 673K under H2 flow for 2 hours, we obtain nearly pure alloys. BET results show that the surface areas of these alloys are large (10-30 m2/g). TEM and SEM show that the alloy particles are porous agglomerates of particles with diameters of 10-20 nm. Sonochemically prepared Fe, Co, and Fe-Co powders have very high catalytic activity for dehydrogenation and hydrogenolysis of cyclohexane. Furthermore, sonochemically prepared Fe-Co alloys show high catalytic selectivity for dehydrogenation of cyclohexane to benzene; the 1:1 ratio alloy has much higher selectivity for dehydrogenation over hydrogenolysis than either pure metal.

Resonant optical excitation of fractal clusters generates dipolar eigenmodes which are extremely localized within areas much smaller than the wavelength. The localization occurs due to the fractal morphology and accounts for the very high local fields leading to the huge enhancement of various optical effects. In particular, Rayleigh and Raman light scattering and, especially, nonlinear optical processes, such as 2-photon photoemission and degenerate four-wave mixing, are strongly enhanced in fractal clusters. The spatial distribution of the modes shows the frequency and polarization selectivity.

Conventional Nb-Ti superconductors are optimized by thermal and mechanical treatments which ultimately produce a nanostructure consisting of 1-2nm thick ribbons of α-Ti dispersed in a matrix of β-Nb-Ti. Several groups are now investigating artificial dispersions of second phase in Nb-Ti which may develop stronger flux pinning nanostructures. However, these new methods generally require true strains of 30 or more. We avoid these large strains (and their associated problems) by forming our composites from a mixture ∼ -50¼m size Nb and Nb-Ti powders, thus permitting the desired nanostructure to be produced with strains of order 12. Nb pinning particles were found to develop an irregular shape, tending to nanometer thick ribbons at large strains. The final size nanostructure resembles conventional Nb-Ti, except that the “second” phase is Nb rather than α-Ti. The critical current density (Jc) was found to increase by over two orders of magnitude to peak values of 5490 and 1980A/mm2 at 2 and 5T as the particle thickness was reduced.