The distributions of enhancement factors in close‐packed and nonclose‐packed surface‐enhanced Raman substrates

Abstract

A method employing photochemical hole burning, previously developed to measure the distribution of Raman enhancement factors on a nanostructured substrate for surface‐enhanced Raman scattering, is used to compare the enhancement distributions of benzenethiol adsorbed on substrates optimized for 532 nm laser excitation consisting of close‐packed (CP) or nonclose‐packed (NCP) nanospheres. The ensemble‐averaged Raman enhancement factor was 2.8 times smaller for the NCP substrate. The measured distributions revealed additional information. For instance, 92% of the molecules on the CP substrate and 93.6% of the molecules on the NCP substrate had Raman enhancements below average. The minimum enhancements on both substrates were ~104, but on the NCP substrate the maximum enhancement was 1.2 × 108, whereas on the CP substrate the maximum was 2 × 1010. The Ag‐coated nanospheres form hemisphere‐on‐cylinder mushroom‐like structures on both lattices, but on the NCP lattice, one third of the molecules are on the flat regions between the mushrooms. The flats on the NCP lattice have enhancements of ~104, showing they are part of a resonant plasmonic structure. The highest NCP enhancements of ~108 are tentatively associated with regions at the bases of the mushrooms, whereas the highest CP enhancements of 2 × 1010 are tentatively associated with gaps between nanospheres where 0.0025% of the molecules reside. Copyright © 2011 John Wiley & Sons, Ltd.