Orientational order of ultrathin C60 films on graphite

Low-energy electron diffraction and electron energy-loss spectroscopy have been applied to study C-60 thin films on a graphite substrate. Well-ordered C-60 overlayers with close-packed face-centred cubic (fcc)(lll) lattice structure have been prepared on graphite at 380 K. Cooling of the C-60 multilayers to 100 K resulted in a (2 x 2) superlattice. This structure transformation has been identified as the surface orientational order-disorder phase transition, in analogy to the bulk orientational fee-simple cubic (sc) transition. We found that this transition was either completely absent or severely frustrated in a (sub)monolayer region. By comparison with data available from other substrates, we propose that the strong C-60-graphite interaction, revealed by our annealing measurements, together with the lack of appropriate corrugation on the graphite surface: may be responsible for suppression of the surface orientational ordering of the first monolayer of C-60 on graphite. (C) 1999 Elsevier Science B.V. All rights reserved.