A scanning tunneling microscope (STM) is used to study individual Ag doping centers in a monolayer of C60 molecules supported on a thin Al2O3 film grown on the NiAl(110) surface. Vibronic states of the doping centers are observed with differential conductance (dIdV) spectroscopy. The double-barrier nature of the junction results in bipolar transport: same states participate in charge transport at both bias voltage polarities. Identification of the dIdV features corresponding to bipolar conduction enables a new mode of vibrational spectroscopy with STM.

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