Mapping the SERS Efficiency and Hot-Spots Localization on Gold Film over Nanospheres Substrates

Abstract

Noble metal films deposited over two-dimensional arrays of polystyrene nanospheres constitute a confirmed class of efficient and cost-effective substrates for surface enhanced Raman scattering (SERS). In this paper, we perform scanning confocal SERS microscopy to investigate the spatial (lateral) variations of the SERS enhancements on gold films over nanospheres (AuFoN) substrates. By constructing SERS imaging maps with a resolution down to the diffraction limit, the local SERS efficiency is found to vary on two different scales. First, the local SERS efficiency is periodically modulated (intensity ratios of 2−3) by the periodic AuFoN surface topography (as demonstrated by correlation with atomic force microscopy imaging of the same sample area); second, randomly distributed SERS hot-spots are observed, at which the SERS intensity is 1 to 2 orders of magnitude larger than at adjacent regions. Furthermore, these hot-spots exhibit fluctuating behavior, characteristic of single-molecule SERS sensitivity. These results are particularly useful for furthering current understanding of SERS on AuFoN substrates. More generally, the SERS maps provide a direct visual demonstration that in SERS only a fraction of the metallic surface yields the major part of the SERS scattering. The evidence of clear correlations between SERS enhancement and topography can be relevant for the characterization of ordered noble-metal plasmonic structures.

Plasmon-induced charge separation at two-dimensional gold semishell arrays on SiO2@TiO2 colloidal crystals

“…[11][12][13][14][15][16][17][18][19][20][21] Transmission and absorption spectra of the 2D arrays can be easily tailored in the visible range and near infrared range by varying thickness and roughness of the semishells as well as dielectric properties and size of the spheres constituting the colloidal crystal. 12,15,16,20,21 Consequently, the metal semishell arrays have been applied to surface-enhanced Raman scattering (SERS), 11,17,[19][20][21] surface-enhanced fluorescence (SEF), 14,16,20 plasmonic refractometry, 15 and plasmonic enhancement of photocurrents. 18 Nevertheless, to the best of our knowledge, PICS behavior of metal semishell arrays has not yet been reported.…”

“…1. Since strong optical near field is advantageous for PICS, [22][23][24] it is reasonable to expect that semishell arrays, which generate strong near field at junctions between adjacent semishells, 17,19 will exhibit good photoelectrochemical properties. The large metal-semiconductor contact area might also be favorable for PICS.…”

Morphology Effects of Cap-shaped Silver Nanoparticle Films as a SERS Platform

“…MFON has advantages of simplicity in preparation, controllability in the particle size and thickness and is superbly suited for formation of gaps. [18][19][20][21][22]32 Van Duyne et al were some of the earliest workers to demonstrate its utility by showing SERS spectra of BPE and pyridine. 18 Later, they explored the diameter dependence by employing nanospheres with diameters ranging from 390 to 600 nm.…”

“…17 MFON has been demonstrated to be an effective SERS substrate. [18][19][20][21][22] In principle, MFON is prepared with close-packed nanospheres, but metal particles can also be formed atop a monolayer of randomly adsorbed nanospheres. 23 Some of the features associated with random MFON are (1) spheres do not have to be highly monodisperse, (2) it is possible to mix nanospheres with different diameters to modulate extinction spectra, (3) nanosphere adsorption density can be controlled to modulate the interaction among particles, and (4) a random nanosphere monolayer can be formed over surfaces other than a perfectly flat surface, important in industrial applications.…”

Nanodome coins for intracellular surface-enhanced Raman spectroscopy

“…An interesting alternative to the gold nanotriangles are the so-called 'gold-film-over-nanoparticle (AuFON)' substrates [10], where a thicker gold film is directly deposited on the polymer particles without lifting off the polystyrene beads afterwards. These AuFON substrates offer an increased enhancement as compared to the nanotriangles due to a coupled localized surface plasmon resonance in the gap between the polystyrene beads and a more robust surface due to the thicker, uninterrupted gold layer, while still sharing the benefits of a nanosphere-lithography approach with respect to cost and ease of fabrication [11] [12]. However, the presence of the monolayer of polystyrene beads is neither compatible with the further fabrication process, nor with the use of the microcoins for intracellular applications.…”