Determination of the C70/C60 ratio in fullerene thin film as a function of the sublimation distance and the substrate temperature using scanning tunnelling microscopy

Thin fullerene films on Au(111) substrates have been prepared by evaporating a mixture of C60 and C70 powders. The variation of the C70/C60 ratio as a function of the sublimation distance and the substrate temperature has been studied by imaging the surface with molecular resolution using scanning tunnelling microscopy (STM). This procedure allows the C60 and C70 molecules on the surface to be counted and a local value of the C70/C60 ratio to be determined. Spectroscopy in UV and visible ranges (UV-VIS) of fullerene solutions in n-hexane obtained by dissolving the fullerene films confirms the STM results. Both methods show an increase of the amount of C70 in the films with increasing substrate temperature and a decrease of the C70/C60 ratio with increasing sublimation distance.