Tuning localized plasmon cavities for optimized surface-enhanced Raman scattering

Abstract

Mesostructured metallic substrates composed of square pyramidal pits are shown to confine localized plasmons. Plasmon frequency tuning is demonstrated using white light reflection spectroscopy with a wide range of structure dimensions from 400 to 3000 nm. Using a simple plasmon cavity model, we demonstrate how the individual pit morphology and not their periodicity controls the resonance frequencies. By measuring the surface-enhanced Raman scattering ͑SERS͒ signals from monolayers of benzenethiol on the same range of mesostructures, we extract a quantitative connection between absorption, field enhancement, and SERS signals. The match between theory and experiment enables effective design of plasmon devices tailored for particular applications, such as optimizing SERS substrates. DOI: 10.1103/PhysRevB.76.035426 PACS number͑s͒: 73.20.Mf, 42.70.Qs, 72.15.Rn, 81.07.Ϫb Raman scattering is a crucial spectroscopic technique for identifying molecules through their vibrational resonances and has increasingly important applications in monitoring low concentrations of impurities or trace biomolecules.1-3 It has also been suggested for direct monitoring of coupling of molecular distortions and electronic transport in molecular electronics. [4][5][6] However, the terribly weak Raman cross section has always made such application problematic. The enormous enhancement in cross section when the molecules are held close to a metal surface with nanoscale roughness 7,8 has driven the hope that surface-enhanced Raman scattering ͑SERS͒ will become a viable and reproducible diagnostic. While improvements have been made in terms of enhancement factor, reproducibility, and in understanding that plasmons underpin such enhancements, 9 it is unclear how to precisely design nanostructures to optimize the Raman signatures. Over the last five years, we have shown that mesostructured metal films comprised of arrays of voids form excellent surfaces for localizing plasmons while retaining strong coupling to external light.10-13 Recently, we showed, using angularly resolved SERS on such void substrates, that incident photons are transducted both into and out of molecules via plasmons, 14 giving hope that reproducible substrates can be designed for specific applications. While most SERS research has focused on using the nanoscale junctions between metallic particles such as colloids 15 or lithographic arrays 16 that have broad plasmon resonances which can only be tuned through control of shape anisotropy or gap dimensions, the voids show strong sharp tunable plasmon resonances. Previous work has shown that optimized samples possess plasmon absorption that lies between the laser wavelength and outscattered Raman emission; 17-19 however, they are unable to make clear the quantitative link between plasmon resonant absorption and SERS emission.In this paper, we present calibrated spectroscopic measurements on systematically engineered plasmonic mesostructured metal surfaces. We show the quantitative connection between the resonance arising from ...

Self-Assembled Plasmonic Pyramids from Anisotropic Nanoparticles for High-Efficient SERS

“…The relative standard deviation of Raman intensities for 1074 cm -1 peak from tip-to-tip was calculated to be 1.6, 2.6 and 3.4% for RD-, OH-and NS-based pyramidal substrates, respectively. These values are much lower than those reported for gold pyramidal arrays (22%) [44], Klarite (10%) [21], silver pyramidal arrays (9%) [43] and gold nanopillar films (7%) [52], confirming the good reproducibility of our self-assembled pyramid film substrates. Fig.…”

“…The unique property of the pyramidal structure is claimed to be responsible for controlling the propagating surface plasmons, which provides a hot spot for remarkably enhanced and uniform SERS intensity [21,22]. As a result, this sparked concerted efforts by a number of researchers to utilize Klarite design as a starting point towards the fabrication of idealized SERS substrate.…”

Capturing Irradiation with Nanoantennae: Plasmon‐Induced Enhancement of Photoelectrolysis

“…The size of the surface cavity was 480 nm with 55 an inner diameter of 600 nm. This is very suitable for enhancing visible light absorption and matching the Au-LSPR, as it has been shown that when the laser wavelength is comparable with the size of the particle under investigation, the strongest SERS signal is observed 85,[87][88][89] as will be demonstrated here.…”

Plasmonic band gap cavities on biharmonic gratings

“…The center of the absorption does not change as a function of the incidence angle, as would be expected from a localized state. 11 Figures 5͑c͒ and 5͑d͒ show the normal incidence wavelength dependent reflectivity. In Fig.…”