Charge transfer enhancement in the SERS of a single molecule

摘要

We measured the surface-enhanced Raman scattering (SERS) of individual gold nanoparticle-4-aminobenzenethiol (ABT)-gold film junctions to investigate the charge-transfer (CT) enhancement of the SERS signals. Despite the mild electromagnetic field enhancement (～10~5) and high surface density of the ABT-molecules (～240 molecules/hotspot) at the junctions, we observed the clear spectral and temporal signatures of CT-enhanced single-molecule SERS (SM-SERS). The result reveals that only a small fraction of the molecules at the junction has a significant CT-enhancement of 10~1～10~3, whereas the rest of the molecules are nearly CT-inactive. Furthermore, the result also proves that overall (charge-transfer and electromagnetic) enhancement of 10~6～10~8 is sufficient to observe the SM-SERS of an electronically off-resonant molecule, which disproves the widespread belief that a minimum enhancement of ～10~(14) is required for SM-SERS.