Islandlike film

The optical properties of Ag islandlike films have been studied most extensively since their utilization in surface-enhanced Raman scattering and... 

According to the EMT, the effective optical constants of an islandlike film depend on the packing density of the particles, the optical constants of the surroundings, and the polarizability of the mutually interacting metal islands... 

The studies of optimum conditions for SEIRA measurements [356, 357, 368, 376, 378, 393,410b] showed that the optimum thickness of a metal islandlike film is in the 4-25-mn range, depending on the system and the metal film morphology. For thicker films, the enhancement is reduced and eventually disappears because of absorption from the metal itself. The greatest enhancements were observed in metal nonpercolating deposits with sharply defined islets [357, 388] and for particles with a small cnrvature (e.g., needle-shaped) [357]. Ordered arrays of metal particles prodnce enhancement comparable to that on disordered vapor-deposited island films [350], It is important for analytical purposes that the SEIRA band intensities are finear functions of the film thickness for the first two to three adsorbed monolayers only [357]. 

Besides exploiting the interference effect due to the presence of the thin metal film over- and underlayers, it is possible to obtain an additional enhancement of the band intensities in aU the above-mentioned configurations by using multiple reflections. The maximum value of AR for the multiple-reflection method and the optimum number of reflections depend on the system considered (Section 2.4). However, when a relatively thick (3-20 nm) metal layer participates in the optial scheme, multiple reflections may not improve the quality of the spectrum because of the high absorption by the metal film. The spectrum contrast can be enhanced if the metal layer has islandlike structure, due to an increase in the effective surface and the SEIRA effect (Section 3.9.4). 

Enhancement of absorption bands in the IR spectra of ultrathin films in the presence of discontinnons (islandlike) nnder- and ovemanolayers of Ag and An was discovered by Hartstein et al. [356] in the early 1980s. Although these researchers believed that they observed an increase in the vCH band intensities for p-nitro-benzoic acid (p-NBA), benzoic acid, and 4-pyridine-COOH films, it was recently shown [350] that the spectra reported are in actual fact due to fully saturated hydrocarbons (possibly vacuum pump oil). In any case, this discovery has stimulated various research activities and led to the development of surface-enhanced IR absorption (SEIRA) spectroscopy. To date, the SEIRA phenomenon has been exploited in chemical [357] and biochemical IR sensors (see [357-360] and literature therein), in studying electrode-electrolyte interfaces [171, 361-365], and in LB films and SAMs [364, 366-370]. Other metals that demonstrate this effect are In [371] and Cu, Pd, Sn, and Pt [372-375]. The metal films can be prepared by conventional metal deposition procedures such as condensation of small amounts of metal vapor on the substrate, spin coating of a colloidal solution, electrochemical [388], or reactive deposition [299] (see also Section 4.10.2). 

The main drawback of an OTE is instability and poor conductivity of the islandlike metal film. To circumvent this problem, a Pt grid evaporated on the surface of a ZnSe IRE was used in ATR studies of the electropolymerization of... 

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