Aggregation mechanism in fullerene thin films on several substrates

Thin films of functional organic compound, fullerene (C60) are fabricated on the (001) surfaces of alkali halides (NaCl, KCl and KBr) and mica by an organic molecular beam deposition (OMBU) technique. Pure powder samples are sublimated from the precisely temperature-controlled Knudsen cell (K-cell) at a pressure of 10(-7) Torr and deposited onto the substrates kept in the range of 25-350 degrees C for 1-10 min. They tended to grow epitaxially. Dependence of crystal sizes and distances between adjacent fine islands on the temperatures of K-cell and substrates is evaluated by transmission electron microscope (TEM) and atomic force microscope (AFM). With increasing K-cell temperature and decreasing substrate temperature, size and distance tend to decrease. The relationship between these rates and the substrate temperature represents the activation energies for crystal growth of 6.2-11 kJ/mol and surface diffusion of 3.7-11 kJ/mol. Particularly in the case of C60 films formed on the rough surface of alkali halides, although molecules tended to start the nucleation at the edge of steps, they formed the lines of nuclei arranging along to the edge on the wide terrace. The numerical evaluation of the distances between the neighboring nuclei represented the diffusion length of C60 molecules along the one-dimensional step edge and along the normal direction to the step edge. (C) 1998 Elsevier Science S.A. All rights reserved.