Atomic force microscope study of photo-polymerized and photo-dimerized epitaxial C-60 films

We have used an atomic force microscope to analyze the surface of C-60 thin films, which have been grown epitaxially on a mica surface and illuminated with a 514.5 nm laser light at different temperatures. Two regimes of light-induced polymerization are observed. If the illumination temperature T-p is lower than 320 K, the illuminated surface shows polymer chains typically six molecules long with an intermolecular distance of 9.2 +/- 0.1 Angstrom. Because of the strain associated with the shorter inter-C-60 distance in the polymer compared to the bulk, the surface is buckled with a period of similar to 100 Angstrom and amplitude of similar to 5 Angstrom resulting in a characteristic herringbone three-dimensional structure. For films, illuminated above 350 K, the surface shows primarily:dimers and trimers with an unusually short inter-C-60 distance of 8.8+/-0.1 Angstrom, this being the shortest interbuckyball distance reported so far. We also find large changes in the adhesive forces on the polymerized films compared to unpolymerized ones, suggesting that the polymer is substantially more hydrophyllic than normal C-60. (C) 1997 American Institute of Physics.