Mechanical properties of C58 materials and their dependence on thermal treatment

C58 fullerene cages made by electron-impact induced fragmentation of C60 fullerenes have been assembled into several micron thick solid films by low energy cluster beam deposition onto inert substrates held at room temperature under ultrahigh vacuum. The resulting as-prepared material, RT-C58, behaves as an amorphous wide-band semiconductor. Nanoindentation was used to measure its mechanical properties revealing that RT-C58 has a higher elastic modulus E and hardness H than the reference carbon allotropes solid C60 and Highly Ordered Pyrolytic Graphite (HOPG): E(RT-C58) = 14 GPa and H(RT-C58) = 1.2 GPa. This effect can be explained by the unique intrinsic “functionalization” of C58 cages: they comprise reactive surface sites constituted by annelated pentagon rings which give rise to covalently stabilized oligomers, –C58–C58–C58, under our deposition conditions. Annealing, thick RT-C58 films up to 1100 K in ultrahigh vacuum results in HT-C58, a new material with considerably modified electronic and vibrational properties compared to the as-prepared RT-C58 film. The associated molecular transformations, including also partial cage–cage coalescence reactions, raise the overall mechanical hardness of the material: H(HT-C58) = 3.9 GPa.