For several years we have been trying to combine the concepts of molecular electronic devices and strongly localised (“plasmonic”) fields in optical nanocavities so it is possible to watch what happens to molecules under bias. 

Dr Giuliana Di Martino, who has recently joined the Department of Materials Science, devised a new method to investigate in-operando the materials used in active molecular devices, providing insight into their operation. Compared to standard characterization techniques (such as Scanning Tunnelling Microscopes), her direct contacting of nanostructures is performed in ambient conditions and without feedback currents passing through the molecule for tip positioning, and much closer to realistic device configurations.

Dr Di Martino, working together with the NanoPhotonics Centre in Cambridge, the University of Warwick and UCL and King’s College London, now directly shows that electronic transport measurements miss out a key phenomenon, which is that molecules can twist under bias, changing also their optical signatures as well as their conductance. These nanodevices act as micro-electro-mechanical switches (MEMS) but scaling their active volumes million-fold down to single molecules.

The results, reported in the journal Nature Communications, emphasises that theories of molecular electronics treating leads and molecules as separate objects are incomplete, and inclusion of local potentials, charges, and interfaces is needed even for simple junctions. In the tunnelling regime, the capacitive energy saved by dropping potential across the molecular rings yields elastic potential energy to twist the molecular torsion spring. 

“Our versatile contacting technique can be applied to a wide variety of other nanostructures such as plasmonic dimers or metasurfaces” said Dr Giuliana Di Martino. “Our findings have critical implications in the design of novel devices based on quantum interference that rely on molecular conjugation, since the molecule-lead interactions can disrupt conjugation and modify device functionality.”

The research is funded as part of a UK Engineering and Physical Sciences Research Council (EPSRC), the Winton Programme for the Physics of Sustainability and the Royal Academy of Engineering.

Figure caption: Raman spectroscopy highlights the twist of molecules under bias.