Quantifying Efficiency of Remote Excitation for Surface-Enhanced Raman Spectroscopy in Molecular Junctions

Surface-enhancedRaman spectroscopy (SERS) is enabled by localsurface plasmon resonances (LSPRs) in metallic nanogaps. When SERSis excited by direct illumination of the nanogap, the background heatingof the lattice and electrons can prevent further manipulation of themolecules. To overcome this issue, we report SERS in electromigratedgold molecular junctions excited remotely: surface plasmon polaritons(SPPs) are excited at nearby gratings, propagate to the junction,and couple to the local nanogap plasmon modes. Like direct excitation,remote excitation of the nanogap can generate both SERS emission andan open-circuit photovoltage (OCPV). We compare the SERS intensityand the OCPV in both direct and remote illumination configurations.SERS spectra obtained by remote excitation are much more stable thanthose obtained through direct excitation when the photon count ratesare comparable. By statistical analysis of 33 devices, the couplingefficiency of remote excitation is calculated to be around 10%, consistentwith the simulated energy flow.