Usually, the substrate transmittance serves as reference. With IR micro-spectroscopy the far-field extinction spectrum of one particle can be measured, for example in normal transmittance with a transparent substrate. 7 Such big objects not only absorb but also scatter IR light. For the electric field parallel to the layer, near-field enhancement is strongest in narrow gaps between islands and can be estimated from the ratio of particle width to gap width.Ĭonsiderably higher signal enhancement can be achieved with micrometre long plasmonic nanowires, called nanoantennas, under resonance conditions. The plasmonic excitation is localised at the surface region of the metal particles and near-field enhancement benefits from low electronic damping, especially in Cu, Ag and Au in the IR and in the case of larger crystalline grains. Near-field enhancement is an important issue for the coupling. The asymmetry is considered to be the consequence of a Fano-type effect from the dipolar interaction of the vibrational excitation with the very broad plasmonic resonance of the metal particle ensemble. 5 Vibrational lines with the strongest enhancement appear with an asymmetric shape, see Figure 1. not scattering) metal islands and the change in adsorbate line-shape with metal-filling factor. 5, 6 Model calculations based on effective dielectric functions (for example, on the Bruggeman model) are able to reasonably describe the enhanced vibrational spectra for very small (i.e. Maximum plasmonic enhancement is found for metal–island layers near percolation. The strength of the electro-magnetic enhancement strongly depends on the IR excitation spectrum of the metal islands (in fact it is a plasmonic excitation) and, thus, on metal–film morphology. 2, 3, 4 This effect is called surface-enhanced infrared absorption (SEIRA). The enhancement factor can reach the order of 1000 if the molecules are adsorbed onto Au or Ag island films. 1 found that the IR absorption of molecules on metal films can be enhanced even more. It is well known that a good metallic substrate in an IR reflection–absorption spectroscopy (IRRAS, with p-polarised light under grazing incidence) measurement enhances vibrational signals up to two orders of magnitude compared with normal transmittance or reflectance on insulators. This situation can be improved by local field enhancement. In a sensing application in which a lower number of molecules is present, IR spectroscopy is hampered by the rather tiny vibrational absorption cross section which is much smaller than the IR wavelength squared. In such a measurement, typically, the focal spot amounts to 1 cm 2, which means that, for a monolayer about 10 15 molecules contribute to the measured signal. Furthermore, modern spectrometers allow very fast measurements and, for example, reasonable monolayer spectra can be acquired today within a second. High spectral resolution goes along with a dynamic range of at least four orders of magnitude in photometric sensitivity. Infrared (IR) spectroscopy of vibrational excitations is an extremely powerful technique because it combines sensitive chemical analysis with an almost non-destructive probe. Kirchhoff Institute for Physics of the Heidelberg University, Im Neuenheimer Feld 227, 69120 Heidelberg, Germany Introduction
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