The effect of fluence on the hardening of C60 films irradiated with He and N ions

The hardness (H) and the Young modulus (E) of thin C60 films deposited on Si substrate and ion irradiated are studied via the nanoindentation technique. C60 films 170 nm thick were irradiated with 170 keV N and 30 keV He ions. The hardness and the Young modulus were measured as a function of the fluence phi and of the transferred nuclear (Sn) plus electronic (Se) energy density σt=phi(Sn + Se). The results show an increase of H from 0.33 GPa for the pristine C60 film to ∼ 15 GPa after the irradiations with the highest fluences. Similarly E raises from 23 to 180 GPa after the irradiations. It is observed that the curves of H and E follow the relation between the number of destroyed C60 molecules measured via Raman spectroscopy and the average deposited energy density σt, reaching the highest values when the films are fully amorphized