Study on the monolayer arrangement and optical properties of single-dispersed metallic particles on solid surfaces

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1.Infrared absorption spectral measurements have been performed at normal incidence of radiation on octadecanethiol (ODT) deposited from ethanol-water mixed solutions onto colloidal silver particles (ca. 30 nm in diameter) on Si(111) substrates. The spectra taken as a function of silver mass thickness and ODT concentration. indicate that the absorption bands of ODT are enhanced in intensity in comparison to the case in which no silver particles are present. It is also found that regardless of ODT concentration each absorption band reaches a maximum in intensity at a particular mass thickness of the particles, suggesting an intimate relationship between the absorption enhancement and the aggregation of the particles. This indeed was supported by FE-SEM morphological inspection of the particles.2.Aggregation of silver colloidal particles in the hydrosol through chemisorption of ODT molecules has been investigated on the basis of UV-visible transmission and infrared attenuated total refle … More ction (ATR) experiments. As-prepared silver sol showed a single absorption peak centered at 395 nm due to plasma resonance of the particles. Upon addition of ODT-dissolved ethanol to the sol, the plasma resonance peak showed a redshift accompanying a broadening, an indication of the aggregation of the particles in the solution by ODT adsorption. By use of infrared ATR spectroscopy, it was found that the particles in the solution aggregate on the base plane of a germanium ATR prism upon addition of ODT-dissolved ethanol to the solution. At moderate ODT concentrations, intense aggregation via ODT adsorption takes place on the prism surface, thus making it possible to observe ODT absorption bands.3.Raman scattering from the adsorbed species on the colloidal particles after dropping on Si substrates has been taken using a He-Ne laser (632.8 nm) in order to more clarify the influence of ionic species produced during preparation of the colloidal silver sol upon aggregation of the particles after dropping the particles on the silicon substrate. A Raman band due to nitrate ion was observed. This band almost disappeared by immersing the deposited silicon substrate into pure water far 1 hour. However ; the nitrate band was observed more strongly after immersion of the substrate into the water for 5 hours. This increase of Raman scattering intensity can be explained by assuming that the environmental nitrate ions were diluted or adsorbed nitrate ions were desorbed more or less by the water and thereby aggregation of the silver colloid particles proceeded, leading to an increase of the so-called SERS active sites. Actually, no Raman bands due to ODT could be observed when the deposited silicon substrate was subsequently immersed into ODT-dissolved ethanol This maybe explained by assuming that the SERS active sites were occupied by nitrate ions. Less