Optics, SEIRAS

Sampling in surface-enhanced Raman and infrared spectroscopy is intimately linked to the optical enhancement induced by arrays and fractals of hot metal particles, primarily of silver and gold. The key to both techniques is preparation of the metal particles either in a suspension or as architectures on the surface of substrates. We will therefore detail the preparation and self-assembly methods used to obtain films, sols, and arrayed architectures coupled with the methods of adsorbing the species of interest on them to obtain optimal enhancement of the Raman and infrared signatures. Surface-enhanced Raman spectroscopy (SERS) has been more widely used and studied because of the relative ease of the sampling process and the ready availability of lasers in the visible range of the optical spectrum. Surface-enhanced infrared spectroscopy (SEIRA) using attenuated total reflection coupled to Fourier transform infrared spectroscopy, on the other hand, is an attractive alternative to SERS but has yet to be widely applied in analytical chemistry. 

Fig. 5.58. Spectroelectrochemical cells and optical arrangements for SEIRAS in external reflection top) and attenuated total reflection bottom)...

In early in-situ infrared spectroscopy studies of catalyst supported on reflective Au substrates, it was recognized that spectral bands develop distortions due to anomalous optical effects as the catalyst coverage becomes greater than a few monolayers [17, 18, 158]. Different procedures were adopted to ensure that catalyst coverages on Au remained low [15, 17, 18, 157, 158]. Related to these efforts are experiments that examine the electrochemical and optical properties of nanostructured metal electrodes prepared by chemical, vapor, or electrochemical deposition methods (cf. Refs. [79, 137, 141, 170-179] and references therein). Much of the work in this area has targeted SEIRAS measurements. 

The optimum optical conditions (the angle of incidence and polarization of the incident infrared radiation) for ATR-SEIRAS depend on the thickness and morphology of the thin metal film [8-10, 31], Figure 8.5 shows the simulated incident-angle dependence of the band intensity of a model molecule adsorbed on an Ag film for p-polarization, where the metal film was assumed to be a bilayer consisting of a continuous underlayer and a nanoparticle overlayer [11]. 

Two of the many enhanced optical phenomena in surface-enhanced spectroscopy are surface-enhanced Raman scattering (SERS) and surface-enhanced infrared absorption (SEIRA). These two phenomena and now analytical techniques can be described as a new branch of vibrational spectroscopy that deals with the spectra of molecules on specially fabricated nanostructures with the... 

More complicated dependences are observed when two layers are located on the surface of the ATR element. The optical properties of a hemicylin-drical IRE-thin (d < 50 nm) metal hhn-hlm system, called the Kretschmann configuration [84] (Fig. 2.36a), were actively investigated in the seventies and eighties (see, e.g.. Ref. [85]) regarding the possibility of SEW excitation at the metal-outer layer interface. However, even without exploiting this and surface-enhanced infrared absorption (SEIRA) (Section 3.9.4) effects, optical enhancement may be achieved in the ATR spectrum of a layer deposited on metal. Because of this, the Kretschmann configuration has found wide application in the investigation of nanolayers located on the metal surfaces, especially at the metal-solution interface (Section 4.6.3). 

This model explains why SEIRA is observed in both s- and p- polarized IRRAS [384] and ATR [391, 405] spectra and in normal-incidence transmission spectra [377] and why the enhancement is not uniformly spread over each metal island but occurs mainly on the lateral faces of the metal islands [378, 384, 385]. The quasi-static interpretation of the SEIRA also defines the material parameters necessary for excitation and observation of SPR (1) The resonance frequency determined from the general Mie condition must be as low as possible and (2) Ime((Ures) must be as small as possible. The maximum enhancement effect should be observed for the absorption bands near the Mie (resonance) frequency of the particle. As mentioned in Section 3.9.1, the resonance frequencies of metal particles lie in the visual or near-IR range. However, they can be shifted into the mid-IR range by (1) increasing the aspect ratio of the ellipsoids, (2) adding the support to an immersion medium, (3) coating the particles by a dielectric shell [24, 406], or (4) varying the optical properties of the support [24, 349, 350, 384]. As emphasized by Metiu [299], the surface enhancement effect is not restricted to metals but can also be observed for such semiconductors as SiC and InSb. 

To estimate the contribution of SPR in the SEIRA, Osawa and coworkers [382, 384,407] modeled the electric field coupling between the particles within the framework of the EMT. The optical constants of the effective medium, which consists of the Ag oblate ellipsoids with p-nitrobenzoate shells and the... 

Optimum conditions in the case of Ag films on the Si or ZnSe IREs and the ATR geometry were found [403] to be the film thickness of 9 nm and the angle of incidence of 30°-40°. As in ordinary spectra obtained by transparent IRRAS (Section 2.3.1), the j-polarized IRRAS-SEIRA bands are negative, independent of the angle of incidence, while the / -polarized bands are negative at and positive at (pi > cpB, where (Pb is the Brewster angle of the substrate [384]. The maximum band intensity is observed at small and oblique angles of incidence in s- and p-polarization, respectively [384], However, the maximum absorption of 5-polarization is always smaller than that for p-polarization, independent of the metal film thickness and the optical properties of the substrate [359, 360, 384]. The enhancement factor increases as the refractive index of the substrate decreases [350, 384],... 

Figure 7.45. Potential-difference unpolarized ATR-SEIRA spectra of 4-mercaptopyridine (PySH) SAM on 20-nm-thick (80-nm-size particles) Au evaporated electrode in 0.1 M HCIO4. Reference potential was -0.1 V (SCE). Arrows show changes of peaks for positive shift of electrode potential from -0.3 to -1-0.4 V. Spectra were recorded using Bio-Rad FTS 60A/896 FTIR spectrometer equipped with dc-coupled MCTdetector and bandpass optical filter transmitting between 4000 and 1000 cm". Spectrometer was operated in rapid-scanning mode and spectra were collected sequentially during potential sweep at 5 mV s". Sixty-four interferograms were coadded to record each spectrum, which required about 10 s. Reprinted, by permission, from K. Ataka, Y. Hara, and M. Osawa, J. Electroanal. Cham. 473, 34 (1999), p. 37, Fig. 3. Copyright 1999 Elsevier Science S.A.

Interaction of an EM field with a nanostructured metal surface also strongly influences other optical phenomena, namely absorption and luminescence. Surface-enhanced infrared absorption (SEIRA) of monolayers of benzoic acids on thin Ag island films was first observed in 1980s (Hartstein et al. 1980). The EM mechanisms is attributed to a local field enhancement and, correspondingly, to an enhancement of the absorption cross-section. Absorption can be enhanced generally by a factor of 10 -10 (Osawa 2001 Aroca 2006) and SEIRA is used as a complementary technique to SERS in some cases (Aroca 2006). 

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