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.