Plasmonic angstrom-scale gap-dependent enhancement with gold nanosphere metasurfaces.

This work reports experimental and computational results of gap–dependent surface-enhanced Raman Spectroscopy (SERS) for nanogaps that vary between 4 - 30 angstroms of two different analytes, trans-1,2-bis(4-pyridyl)-ethylene (BPE) and benzenethiol (BZT), pushing the boundary of previous SERS gap size limits. Raman spectroscopy is a valuable, highly sensitive detection tool, and SERS can improve the sensitivity even further. The substrates used in this work are plasmonic metasurfaces with a single layer of a hexagonally close packed gold nanospheres with a ligand surface that sets the nanoscale interparticle gap. The plasmonically enhanced Raman signal increased as the gap-size decreased, as expected, confirmed, and predicted by computational electromagnetic models; though, there was enhancement quenching that was measured for gap sizes below 1 nm. Therefore, in addition to using a "local" computational model, a quantum "nonlocal" model was used to account for electron tunneling effects, which can explain the plasmonic enhancement quenching that was observed for the sub 1 nm gap sizes. This work adds to the growing field of angstrom-scale optics, suggesting that in this range, plasmonic models would require a nonlocal/quantum model to accurately predict optical enhancement.