Figure 5: Quenching of plasmonic enhancement at subnanometre gaps.

(a) SERS enhancement factors measured for Au dimers shown in Fig. 3c with various gap distances. Largest enhancement is obtained for dimer with gap distance ≈6.7 Å. The horizontal error bars represent the uncertainties in retrieving the gap-width from TEM images. The vertical error bars originate from the uncertainties in the reference Raman measurements. Reproduced from ref. 70. Copyright 2014 Nature Publishing Group. (b) Gap distance dependence of photoluminescence intensity in STM-based measurements (experimental configuration schematically illustrated in inset). (c) Corresponding PL spectra for different gap distances in these STM-based measurements. (b,c) Images are reprinted with permission from ref. 72. Copyright 2014 American Chemical Society. (d) Four-wave mixing photon count rate as a function of gap between two 60-nm-diameter gold nanoparticles. The inset shows a detailed view on a log–log scale. Reprinted with permission from ref. 71. Copyright 2007 American Physical Society. (e) Third harmonic generation as a function of SAM thickness in particle-on-film geometry (inset), comparing the measurements (black line) with the classical (blue line) and QCM (red line) calculations. Insets show near-field distributions simulated using QCM and local Drude models. Reprinted with permission from ref. 73. Copyright 2014 American Chemical Society.