Negative Differential Resistance in the Scanning Tunneling Spectroscopy of Organic molecules

TL;DR

This paper reports the observation of negative differential resistance in the I-V characteristics of organic molecules measured by scanning tunneling spectroscopy, challenging the usual assumption that conductance spectra directly reflect the local density of states.

Contribution

It demonstrates NDR in STM measurements of organic monolayers and explains its origin from local density of states features at the tip apex.

Findings

NDR observed in STM I-V measurements of organic molecules

NDR arises from narrow structures in local density of states

Electrostatic potential profile influences NDR occurrence

Abstract

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.