Importance of Plasmonic Scattering for an Optimal Enhancement of Vibrational Absorption in SEIRA with Linear Metallic Antennas

Abstract

Surface-enhanced infrared absorption (SEIRA) and surface-enhanced Raman scattering (SERS) represent very effective techniques to detect molecular vibrational fingerprints. These techniques can be improved thanks to the use of plasmonic antennas that produce strong resonant near-fields in their vicinity, enhancing the signal of vibrational samples. Here we study the role of plasmonic absorption and scattering of the hosting antennas in the resulting SEIRA signal. Using numerical simulations of the antenna−sample infrared response, we show that the optimal SEIRA signal measured in transmittance (as extinction) is achieved when the spectral maxima of absorption and scattering of the antennas are of similar magnitude. Paradoxically, when the optimal condition for SEIRA is fulfilled, the decomposition of the signal into the contribution from scattering and from absorption show that the vibrational fingerprint is exclusively a result of the scattering, with no contribution from absorption. Using a simple analytical model for the description of the fundamental resonance of linear nanoantennas made of a Drude-type metal, we provide guidelines for controlling the plasmonic light scattering and light absorption properties, thus showing how the optimal condition for SEIRA can be achieved in practical situations. ■ INTRODUCTIONInfrared (IR) vibrational spectroscopy is a powerful tool for the identification of the chemical composition and the molecular geometry via the vibrational fingerprints. However, one big problem hampers the successful application of IR spectroscopy, especially in the case where small amounts of molecules are present. This problem consists in the extremely small IR absorption cross section of molecules which is much smaller than the squared wavelength of the probing IR radiation at the vibrational frequency of the molecule (see Supporting Information). This problem can be overcome by placing the molecule into strongly enhanced electromagnetic near-fields, for example, such of a resonant plasmonic antenna. In this article we will demonstrate that the resonant plasmonic scattering of these hosting antennas is especially relevant for the measurement of enhanced IR vibrational absorption signals.The first findings on enhanced signals of molecules on metal nanoparticles, ca. 50 years ago, were named "anomalous transmission".1 This kind of surface-enhanced IR absorption (SEIRA) of adsorbates on metal−nanoparticle aggregates can reach 3 orders of magnitude and can be modeled by effective media theories (EMT) if the particles are small enough. Usually, the metal nanoparticles in such SEIRA studies have diameters of only a few nanometers, and therefore their light scattering (proportional to the fourth power of the ratio of the particle diameter to the wavelength) almost vanishes in the IR. On the other hand, the optical absorption of these structures even in the IR is strong because the mutual interaction of the nanoparticles broadens and red-shifts their resonances.3 The observed vibrational line-sh...

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“…A further interesting finding of Neuman et al [27] is that near-field scattering leads to having a considerable part of electric field polarized perpendicular to the long axis of the antennas and parallel to the substrate surface in the vicinity of the antennas. The authors show this on the example of an analyte material with cubic crystal symmetry.…”

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