Figure 4.

Optical enhancement of nanoparticle clusters with coordination numbers from 1 to 7. a) Comparison between the enhancement factors obtained for each sample, normalized to the enhancement produced by a single particle excited with a 633 nm laser line. Inset: SERS spectra of benzenethiol on the pentagonal bipyramid (CN 7). b) Near-field intensity (log thermal scale) for low-CN clusters in air at 633 nm along planes passing by the centers of the lower spheres. The maximum intensity relative to the incident intensity |E_ext|² is 14, 10 100, 5400, and 4000 for CN=1–4, respectively. c) Experimental data fitting with the following expression EF=CN + A G, where G represents the number of gaps and A is a constant factor reflecting the effective enhancement of the gap relative to the surface of an isolated sphere.