  The conductance-voltage spectrum of molecular nanostructures measured by
scanning tunneling spectroscopy (STS) is generally assumed to reflect the local
density of states of the molecule. This excludes the possibility of observing
negative differential resistance (NDR). We report here the observation of NDR
in the scanning tunneling microscope (STM) current-voltage (I-V)
characteristics of self-assembled monolayer (SAM) of 4-p-Terphenylthiol
molecules on gold substrate measured using a platinum probe. We argue that the
NDR arises from narrow structures in the local density of states at 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.
