Confocal Raman Microscopic Study of the SERS‐Effect Spatial Localization Over Nanostructured Self‐Assembled Silver Films

ABSTRACT The localization of the SERS effect from air nitrogen molecules over nanostructured self‐assembled silver films has been experimentally investigated. In our previous experiments on surface‐enhanced Raman scattering from myoglobin on such SERS substrates, the intensity of the scattering spectrum depended on the thickness of the analyte. At very low analyte concentrations, giving an estimated monolayer on the substrate, little or no SERS was observed; at higher concentrations, analyte crystallites as thick as 0.5 to 1 μm gave an intense SERS spectrum. Since the plasmon resonance field of a single nanoparticle is mainly localized near its surface on the scale of a few nanometers, the electromagnetic and chemical mechanisms of SERS are localized at the same distances. We hypothesized that for a rough metal with a developed surface consisting of crystallites of noble metals with subwavelength sizes, the total field of these particles can localize the SERS effect at larger distances from the substrate. While the literature data on this issue for continuous films are contradictory, we present the results of a systematic study of our hypothesis. Experiments with the SERS effect from air nitrogen molecules exclude any volume or chemical effects from the analyte. The measured scale of the long‐range SERS contrasts with the traditional idea of the extremely fast attenuation of SERS at distances of units to tens of nanometers. The observed characteristic space scale of ~140 nm exceeds the characteristic scales of inhomogeneities in the substrate nanostructure. An explanation of this fact should apparently be sought in terms of the mechanism of excitation, scattering, and localization of hybrid plasmon–polariton waves under conditions of a real optical scheme, taking into account the morphology and structure of silver films. The experimental methods (Z‐Scan) used in the work allow measuring the real point distribution function for confocal microscopes.