Direct Measurement of the Adhesion and Friction of Smooth C60 Surfaces

Using a surface forces apparatus, measurements were made of the static adhesion and dynamic friction forces between two smooth solid crystalline surfaces of C 60 deposited as uniform layers on mica substrates. Between molecularly smooth surfaces, the surface energy (adhesion energy per unit area) was unusually low, whereas the static friction force was very high. However, the kinetic friction force between both smooth and rough C 60 surfaces was “normal”, and the same as previously measured between rough surfaces. Some of these unusual properties can be rationalized in terms of the large size and nearly perfect spherical shape of the C 60 molecule. The results and analysis suggest that C 60 and other large spherical molecules with diameters greater than 5−10 Å interact more as macroscopic particles than as conventional molecules.