Pinning of size-selected Co clusters on highly ordered pyrolytic graphite

Deposition and implantation of size-selected Co⁺ _50±5 cluster ions on/in highly ordered pyrolytic graphite (HOPG) have been performed. Cobalt clusters were produced by laser ablation using the second harmonic (532 nm) of a Nd:YAG laser. They were deposited/implanted with energies of 250–4850 eV/cluster and examined using scanning tunneling microscopy (STM). For the highest energies the clusters created craters and wells with residual clusters at the bottom of the wells. Decrease of the impact energy led to formation of bumps whichconsist of damaged graphite areas mixed with fragmented cobalt clusters. Further decrease of the impact energy to 250–450 eV/cluster probably corresponds to the so-called pinning regime, when the impacting cluster creates defects in the surface layer and becomes bound to them. The transition from implantation to pinning with a decrease of impact energy was confirmed by etching experiments showing the depth of the damage introduced by the cluster collisions with HOPG.