Plasmonic tunnel junctions for single-molecule redox chemistry

Abstract

Nanoparticles attached just above a flat metallic surface can trap optical fields in the nanoscale gap. This enables local spectroscopy of a few molecules within each coupled plasmonic hotspot, with near thousand-fold enhancement of the incident fields. As a result of non-radiative relaxation pathways, the plasmons in such sub-nanometre cavities generate hot charge carriers, which can catalyse chemical reactions or induce redox processes in molecules located within the plasmonic hotspots. Here, surface-enhanced Raman spectroscopy allows us to track these hot-electron-induced chemical reduction processes in a series of different aromatic molecules. We demonstrate that by increasing the tunnelling barrier height and the dephasing strength, a transition from coherent to hopping electron transport occurs, enabling observation of redox processes in real time at the single-molecule level.We acknowledge financial support from EPSRC grants EP/G060649/1, EP/I012060/1, EP/L027151/1, ERC grant LINASS 320503. F.B. acknowledges support from the Winton Programme for the Physics of Sustainability. S.J.B. thanks the European Commission for a Marie Curie Fellowship (NANOSPHERE, 658360). M.K. thanks the European Commission for a Marie Curie Fellowship (SPARCLEs, 7020005). P.N. acknowledges support from the Harvard University Center for the Environment (HUCE). R.C. acknowledges support from the Dr Manmohan Singh scholarship from St John’s College. C.C. acknowledges support from the UK National Physical Laboratories. R.S. acknowledges computational resources provided by the Center for Computational Innovations (CCI) at Rensselaer Polytechnic Institute