Structure and lattice parameters of thin C-60 films

The structure and lattice parameters of C-60 fullerite films evaporated in vacuum on the (100) cleavage plane of NaCl at the substrate temperature 290 -400 K are investigated in the temperature range 300-5 K by electron-optica l methods. Fullerite films have an fcc lattice at room temperature. The fil m structure changes with the temperature of condensation from epitaxial wit h the (111) orientation to a disordered and highly nanodisperse structure w ith a grain size of 4-5 nm. The crystallographic conditions of conjugation of the (100) surface of NaCl and epitaxial C-60 fullerite films are determi ned, and the four-position type of their structure is established. The fcc- sc transition temperature and the observed jump in the lattice parameter ar e close to the corresponding characteristics of bulk fullerite. The tempera ture dependence of the lattice parameter in the temperature range 100-260 K is used to determine the average linear thermal expansion coefficient alph a of the films. An increase in alpha for small thicknesses is a size effect associated with a considerable influence of the surface. A mechanism of fo rmation of the structure of condensed C-60 fullerite films is proposed on t he basis of the obtained results. (C) 1999 American Institute of Physics. [ S1063-777X(99)01103-2].