Silicon nanoparticles synthesized in the gas phase are studied. From time-resolved photoluminescence measurements we determine, quantitatively, the size-dependence of the oscillator strength of the nanoparticles. We investigate experimentally the absorption and photoluminescence emission of nanoparticle ensembles with a broad size distribution. Using a model which accounts for size-effects in both oscillator strength and quantum-confinement, we are able to calculate absorption and emission spectra of ensemble samples. From these results we have determined, whether silicon nanoparticles should be regarded as indirect or direct semiconductors. Moreover, we systematically study the influence of the particle size-distribution on the optical spectra.
Thin films of indium tin oxide nanoparticles are studied using charge-coupled device thoermoreflectance. High resolution sub-micron thermal images confirm that percolation in current conduction induces strongly inhomogeneous heat loads on the thin film. We experimentally show that the inhomogeneous current densities induce thousands of “micro-hotspots” that can be 20% hotter than the average Joule heating in the thin film layer and show comparable behavior in a resistor network. In addition to the percolation induced “micro-hotspots,” we report major hotspots, with non-Joule behavior, whose temperature response is greater than I2. We demonstrate that a temperature dependent resistor can account for an effective exponent larger than 2. Finally, it is shown that while ambient molecules modify the thin film conductivity by at least 20%, current conduction and percolation effects remain largely unchanged, but such chemical reactions can be nonetheless detected with thermoreflectance.
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