The discovery of negative differential conductance (NDC) in a single molecule and mechanism controlling this phenomenon are important for molecular electronics. We investigated the electronic properties of a typical radical molecule 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrrolin-1-yloxy (CTPO) on an Au(111) surface using low-temperature scanning tunneling microscopy (STM) and inelastic electron tunneling spectroscopy. Large NDC was observed in single CTPO molecules at the boundary of the crystal monolayer. The origin of observed NDC is revealed as the inelastic electron–phonon scattering during tunneling, and the strong spatial variation of the NDC over the single molecule illustrates the nature of the localized radical group. In addition, the NDC can be transformed into a positive differential conductance peak by tuning coupling strengths between different tunneling channels. An empirical multi-channel model has been developed to describe the competition between the valley-shaped NDC and peak-shaped positive differential conductance. The unique electronic property and giant conductance change observed in this radical molecule is valuable for designing novel molecular devices in the future.
This article is Open Access
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