Low temperature internal friction of thin fullerene films

The lattice vibrations of crystalline solids are generally described by traveling elastic waves. However, the vibrations of fullerene solids are expected to be quite different from typical crystalline solids because of the large molecular mass. In fact, based on measurements of thermal conductivity and specific heat, it appears that the vibrations of compacted fullerene solids are best described as localized. Only below a few Kelvin has evidence for elastic waves been found in these solids where they exist along with the localized tunneling states that are characteristic of amorphous solids. In order to verify the existence of these tunneling defects, the low temperature internal friction of thin fullerene films deposited on a silicon substrate has been measured. Fullerene films were prepared under a variety of conditions with substrate temperatures ranging from 300 K to 500 K. Film grain sizes were characterized using atomic force microscopy, as well as, scanning tunneling microscopy. Grain sizes were found to range between 100 nm and 400 nm. We find that while localized tunneling defects appear to be present in these films, they do not appear to be intrinsic to the fullerene solid. Instead, the tunneling states may be the result of residual disorder that is present in the grain boundaries of the solids.