Surface Free Energy and Wettability Determination of Various Fullerene Derivative Films on Amorphous Carbon Wafer

The surface energy of a polished amorphous carbon (a-C) wafer spin-coated with fullerene derivatives is evaluated by the contact angle measurements of test liquids. The contact angles of the epoxy prepolymer droplets are also measured to investigate the effects of fullerene derivatives on the adhesiveness between the epoxy resin and the a-C wafer surface. A marked increase in surface energy is observed when the wafer is spin-coated with the fullerene derivatives having a polar group. The three surface energy components, dispersive, polar, and hydrogen bonding forces, are determined from the contact angles by an expanded Fowkes method. The total surface energy thus obtained varies from 36.2 to 50.4 mN/m, depending on the nature of fullerene derivatives. The major factor, which increases the surface total energy, is found to be the polar component of the fullerene derivatives. Microscopic structures of the thin films of fullerenes on the wafer have no essential effects on the surface energy and the wettability between the a-C wafer and the epoxy resin. In the present study, we demonstrate that the fullerene derivative with a polar group acts as an adhesive in the fixation between the epoxy resin and the a-C wafer: a polar group of the fullerene derivative has large affinity to the epoxy resin, and the fullerene core, to the a-C surface.