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.
Highlights
Nanoparticles attached just above a flat metallic surface can trap optical fields in the nanoscale gap
Atomic-scale morphological features on the gold interfaces further localise the plasmonic hotspot resulting in roughly five-fold higher field enhancements[29, 30], but while most molecules in the gap contribute very weakly to the final SERS signal, emission is dominated by the individual molecules closest to the centre of these hotspots due to the highly nonlinear Raman enhancement[31]
At nanometre length-scales the molecular spacers act as molecular tunnelling junctions (MTJs), providing low-energy tunnelling pathways for excited charge carriers travelling between the metal surfaces
Summary
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. The resulting highly localised plasmonic hotspot gives field enhancements >600 allowing for single-molecule sensitivities[1, 4, 5, 7,8,9,10,11,12] Besides their SERS enhancements, plasmonic structures have gained considerable attention for their ability to generate hot charge carriers as a result of plasmonic non-radiative relaxation pathways, which in turn can be used to induce or aid chemical reactions on or near the surface of a plasmonic structure[13,14,15,16,17,18]. We focus on the generation and transport of hot charge carriers and show the crucial role of molecular binding to the plasmonic substrate as well as the effect of the interaction strength between the charge carriers and the molecules
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