Single-molecule fluorescence tracking has been used to examine diffusion of small molecules in sol-gel films in order to identify spatial heterogeneity in the structure and molecular diffusivities for different regions of the film. Fluorescence intensity profiles from single molecules are fit to a two-dimensional Gaussian function to determine their x,y positions with subpixel resolution. Scatter plots and histograms of molecular step sizes indicate that the trajectories conform to the predictions of a two-dimensional random walk. The mean-square step size is shown to be an unbiased estimate of the variance of the step-size probability distribution and a valid statistic for determining the diffusion coefficient from a molecular trajectory. The diffusion coefficients measured for different molecules are subjected to an F test, which showed that the sol-gel film exhibits spatial variation in the diffusion coefficient on a micrometer-length scale. The spatial variation in diffusivities is a measure of structural heterogeneity of these films.

Quantitative deposition of dye molecules onto a substrate has been achieved at very low surface concentrations, in the range of 5 x 10(-8) - 1 x 10(-6) monolayer, using the technique of controlled substrate withdrawal from solution. These small surface populations were determined with high (>96%) efficiency by single-molecule counting using an epi-illumination, fluorescence microscope with charge-coupled device detector. The fluorescence imaging resolution (3sigma) is 0.78 microm; over a uniform excitation area of 67 x 67 microm2, a large number (>7,500) of spatially resolved channels are available for counting individual molecules. At low coverages, the number density of fluorescence spots on the surface agrees with the expected surface concentration of molecules, based on the concentration of dye in solution and the solution film thickness predicted from theory. When the surface density of molecules is high enough that fluorescence spot overlap is likely to occur within the optical resolution of the instrument, the observed fewer number of spots can be corrected for overlap through a site occupation model based on Poisson statistics.

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Nanoscale metal oxide particles have been synthesized by using a novel method which combines laser vaporization of metal targets with controlled condensation in a diffusion cloud chamber. The following oxides have been synthesized: ZnO, SiO2, Fe2O3, Bi2O3, PdO, NiO, AgO, TeO, Sb2O3, TiO2, ZrO2, A12O3, CuO, In203, SnO2, V2O5 and MgO. With this method, the size of the particles can be conveniently controlled by careful control of the degree of supersaturation which is accomplished by adjusting the temperature gradient, total pressure, and partial pressure of the metal vapor generated by laser vaporization in a diffusion cloud chamber. The microscale structures of the SiO2 and A1203 particles exhibit interesting web-like matrices with a significant volume of vacancies. These materials may have special applications in catalysis and as reinforcing agents for liquid polymers.

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