Title: Important impact of the experimental platform on the efficient control of electronic and vibrational properties of molecular junctions

Authors: Ioan Bâldea

Addresses: Theoretische Chemie, Physikalisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 229, D-69120 Heidelberg, Germany; Institute of Space Sciences, National Institute for Lasers, Plasma, and Radiation Physics, RO 077125, Bucharest-Măgurele, Romania

Abstract: A recent work reported simultaneous surface-enhanced Raman scattering (SERS) and transport current-voltage I-V measurements in electromigrated molecular junctions based on C₆₀. The vibrational frequencies measured there were found to be slightly but significantly shifted by the 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 that experiment raises the question addressed in this work on whether the experimental platform utilised (electromigrated junctions) represents the most favourable setup to achieve a(n almost) complete 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 the (source and drain) 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. We next discuss 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 scanning tunnelling microscope (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, we consider EC-STM experimental transport data for the redox unit (viologen), whose essential constituent is 4,4´-bipyridine (44BPY), a molecule of special interest for the charge transport at the nanoscale.

Keywords: molecular electronics; molecular junctions; electrochemical STM; scanning tunnelling microscopy; EC-STM junctions; surface Raman enhanced spectroscopy; SERS; redox processes; bipyridine; viologen; electronic properties; vibration; molecular charge; orbital energy; electrolyte gating; nanotechnology.

DOI: 10.1504/IJNT.2016.079672

International Journal of Nanotechnology, 2016 Vol.13 No.8/9, pp.685 - 691

Published online: 07 Oct 2016 *

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