Simultaneous SERS and transport current-voltage I-V measurements in molecular junctions based on C60 have been reported recently.1The vibrational frequencies measured there were found to be slightly shifted by an applied bias. This effect was attributed to a partial reduction of the C60 molecule. The fact that a complete redox process could not be achieved in ref.1 raises the question 2,3 whether the experimental platform utilized in ref. 1 represents the most favorable setup to achieve an efficient bias-driven reduction. The answer presented here is that (i) no matter how high the voltage is, reduction cannot exceed 50% in an experimental setup where the molecule is symmetrically coupled to electrodes, (ii) reduction can be almost complete in cases of highly asymmetric molecule-electrode couplings, but in this case (iii) biases corresponding to current plateaus are required, which common junctions cannot withstand. I next show that, much more than in two-terminal setups, the molecular charge, orbital energies, and vibrational properties can be efficiently controlled in cases of (redox) molecules embedded in STM junctions in electrochemical (EC) environment. The electrochemical setup turns out to provide a unique chance to control the molecular properties. The key role in the unprecedented efficiency of this control is played by the electrolyte gating, which enables a continuous change in the molecular charge by up to an entire electron (complete redox process). To exemplify, I consider EC-STM experimental transport data for the redox unit (viologen), whose essential constituent is 4,4’-bipyridine (44BPY), a molecule of interest for nanotransport.4 Theoretical results revealing changes in vibrational frequencies much larger than in ref. 1 are presented below. 1. Y. Li et al, PNAS 2014, 111, 1282. 2. I. Bâldea, PCCP 2014, 16, 25942. 3. I. Bâldea, PCCP 2015, 17, 15756. 4. I. Bâldea, Nanoscale 2013, 5, 9222.

Unprecedented efficiency to control orbital energies and vibrational properties of single molecules embedded in electrochemical STM junctions

251st National Meeting (2016)

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