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We discuss results on tunneling in barriers consisting of both pure fullerene films and layered composites of fullerenes and dielectric materials. This work focuses on C 60 films, which ranged from 50 to 600 Å in thickness and were layered with both Al 2 O 3 and Ge films 10 to 40 Å in thickness. These studies reveal that for the deposition conditions used here, incomplete C 60 coverage occurred for film thicknesses less than ϳ400 Å. For composites of C 60 with Al 2 O 3 or Ge, we observed isolated clusters of C 60 molecules and Coulomb blockade behavior consistent with the size scale of the clusters. Interesting dynamical effects were also observed in conductance characteristics that were both dramatic and in some cases entirely reproducible.

We discuss the tunnel barrier properties of oxidized ultrasmall Bi droplets on and sandwiched within Al2O3. Scanning tunneling microscopy (STM) and standard sandwich junction studies of these systems have revealed a consistent evolution of tunnel barrier parameters for the composite Bi2O3/Al2O3 barrier. We observe a systematic decrease in barrier height with increasing barrier width similar to the results for progressively oxidized pure Al2O3 and other refractory oxides. Our results suggest that STM, under appropriate conditions, can be employed to evaluate and view the time evolution of the local characteristics of selected tunnel-barrier systems.