Fig. 3.

Raman images of adsorbates (immersion oil or 4-mercaptopyridine). We fabricated 100-nm-thick (A) and 5-nm-thick (B) silver films by thermal evaporation of silver. Silver nanoparticle films (C) (called compacted nanoparticle films) were formed by covalent attachment of silver nanoparticles from colloidal suspensions to glass substrates modified by (3-mercaptopropyl)trimethoxysilane (MPS). Nanorod arrays (D) were synthesized by electrodeposition from silver chloride solution into porous alumina templates. Cluster–cluster aggregated films (E) were deposited by inducing precipitation from colloidal suspensions of citrate ion stabilized silver nanoparticles by adding fumaric acid to the suspension on the MPS-modified substrates. (F–H) Mirror reaction films were formed by using the Tollens reaction. In F, the Tollens reaction time was 5 min, whereas in G and H the time was 2 min. (C–G) Surfaces were treated by saturated 4-mercaptopyridine solutions. (A, B, and H) Surfaces were untreated and data were recorded with excitation from the exterior for the reasons explained in the text. All images were taken by scanning confocal Raman microscopy with excitation at 632.8 nm. The scan size is shown on the images. Note that each image is internally normalized to its maximum so that relative intensities between images cannot be compared. In particular, the hot spots on fractal surfaces have orders of magnitude more Raman scattering than counterparts of similar color on nonfractal surfaces.