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 i lluminated with a 514.5 nm laser light at different temperatures. Two regimes of light-induced polymerization are observed. If the illuminat ion temperature T-p is lower than 320 K, the illuminated surface shows polymer chains typically six molecules long with an intermolecular di stance 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, t he surface is buckled with a period of similar to 100 Angstrom and amp litude of similar to 5 Angstrom resulting in a characteristic herringb one three-dimensional structure. For films, illuminated above 350 K, t he surface shows primarily:dimers and trimers with an unusually short inter-C-60 distance of 8.8+/-0.1 Angstrom, this being the shortest int erbuckyball distance reported so far. We also find large changes in th e adhesive forces on the polymerized films compared to unpolymerized o nes, suggesting that the polymer is substantially more hydrophyllic th an normal C-60. (C) 1997 American Institute of Physics.