Negative differential resistance in the scanning-tunneling spectroscopy of organic molecules

In the interpretation of scanning-tunneling spectroscopy data on molecular nanostructures the tunneling conductance is often assumed to be proportional to the local density of states of the molecule. This precludes the possibility of observing negative differential resistance (NDR). We report here the observation of NDR in the current-voltage $(I-V)$ characteristics of a self-assembled monolayer of $4-p-\mathrm{terphenylthiol}$ molecules on the Au(111) surface measured using a platinum tip. We argue that the NDR arises from narrow features in the local density of states of the tip apex atom and show that depending on the electrostatic potential profile across the system, NDR could be observed in one or both bias directions.