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Abstract
Thin films of pentacene were deposited by vacuum sublimation onto amorphous carbon, glass, silicon and mica substrates, then characterized by X-ray and electron diffraction, transmission electron microscopy (TEM) and atomic-force microscopy (AFM). Sub-monolayer films consist of dendritic islands, which change to more compact fractal shapes upon transforming to a multilayer structure (with decrease in surface area) prior to complete coverage of the substrate. Increased crystallite size was obtained by heating the substrate during deposition and by post-deposition annealing. Irradiation with 200-keV electrons was found to destroy the crystallinity of the films after a dose of 0.6 C/cm2 (or 2 C/cm2 if the specimen is cooled to 90 K during TEM observation). We argue that, despite this moderate radiation sensitivity, TEM under near-optimal conditions can image monolayers of pentacene with sub-nm resolution.
Acknowledgments
We thank Dr Al Meldrum and Dr M. Freeman for their interest in this work and Dr F. Hegmann and Dr R. R. Tykwinski for expert advice on pentacene. The project was supported by the Natural Sciences and Engineering Research Council of Canada.