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Knowledge of percolation in binary composites is critical to the development of new materials with specific electrical and optical properties. This report investigates the detection of percolation in novel two-phase composites consisting of poly(methyl) methacrylate(PMMA) and indium tin oxide (ITO). ITO is a filler of particular interest primarily for possessing optical clarity consistent with PMMA in the visible light range. AC impedance measurements were performed on specimens with varying concentrations of ITO particles to determine the percolation threshold. Percolation was observed when specimens contained 2-3% vol. of nano- sized ITO and 6%-8% vol. of coarse-sized ITO. Thus, the percolation threshold appeared significantly decreased with reduced particle size of the filler. It is speculated that minor agglomerates in the bulk of the specimens may have prevented percolation from occurring at even lower volume fractions of the filler phase.
We report photoluminescence spectra of C60 single crystals grown by vapor phase transport method using either the sealed ampoule technique or the open tube technique. The spectra for both types of samples show similar features, but different line resolution related to the two different
growth techniques. An analysis of temperature and excitation intensity dependencies of the
luminescence spectra is reported. The main structures of the spectra have been interpreted according to a model involving intramolecular polaron-exciton recombinations. In particular, emissions due to
purely electronic transitions of singlet and triplet or the exciton and related vibronic recombinations have been resolved. At low temperature, emission bands due to X-traps have been observed on the high-energy side of the excitonic singlet purely electronic transition.
Visible photoluminescence at 1.62 eV has been observed at room temperature from fluorinated and hydrogenated nanocrystalline silicon (nc-Si:H,F) produced in a typical plasma enhanced chemical vapor deposition system. The use of SiF4-SiH4-H2 mixture, because of the H2 dilution and the presence of SiF4, favours the amorphous - crystalline transition through the etching process of the amorphous phase. The x - ray diffraction measurements give an average grain size of about 100 Å. The presence of these nanocrystals shifts the absorption edge of the films towards higher energy. An energy gap of 2.12 eV is estimated, although the hydrogen content in the material is only 4.5 at. %. The temperature dependence of the photoluminescence behaves similarly to that of porous silicon.
Laser ablation technique has been successfully used for the deposition of CdSe and CdTe/CdSe multilayers on Si(100) and Si(l11) substrates. X-ray analysis showed that CdSe/Si films were highly oriented. Their orientation changed from (100) to (002) by varying the substrate temperature from 473 to 673K. High orientation was also obtained on multilayered polycrystalline structures of CdSe and CdTe on Si(lll). Photoluminescence experiments have also been carried out on the deposited films.
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