On the existence of transition-metal fullerides:: Deposition and characterization of TixC60

Titanium fulleride films (TixC60) were prepared by coevaporating Ti and C-60 onto moderately heated substrates (similar to 100 degrees C) in an ultrahigh vacuum system. The TixC60 films were all amorphous according to X-ray diffraction and exhibited a gray, metallic luster different from the brown color of pristine C-60. The compound also appeared to be metallic or semiconducting as no charging was observed in X-ray photoelectron spectroscopy (XPS) in contrast to analyses of pure C-60. XPS of the films showed a maximum titanium content of about 5.3-5.5 at. % (Tisimilar to 3.5C60). For higher titanium contents, carbide formation was observed. The XPS analyses also showed binding energy shifts and line broadening effects consistent with the formation of chemical bonds between Ti and C-60. Raman spectroscopy showed a softening of the A(g)(2) mode upon Ti incorporation (-4 cm(-1)/Ti atom) which is consistent with a partial charge transfer from Ti to C-60. The TixC60 films oxidized immediately upon air exposure. However, XPS and Raman spectroscopy show persistent differences from pristine C-60 which might indicate that another phase, TixOyC60, is formed upon oxidation.