STM observation of Coulomb staircase behavior through C60 clusters

Two samples of C60 clusters were prepared on highly oriented pyrolytic graphite (HOPG) substrate by thermal evaporation. One is ∼1 nm and the other is ∼15 nm in radius. The current flowing across C60 clusters was measured using scanning tunneling microscope in atmosphere at room temperature. Coulomb staircase behavior was successively observed in the current–voltage (I–V) characteristics. Interestingly, the capacitances between HOPG and C60 clusters, estimated from the I–V characteristics, were almost the same between these two samples. We discussed this result taking into account the relaxation and localization time of electrons injected into C60 clusters.