Infrared Absorption Enhancement of C₆₀ by Overlaying Thin Silver Island Films

Normal-incidence infrared absorption has been observed for silver overlaid C₆₀ thin films formed on surface-oxidized Si(111) substrates as a function of the silver or C₆₀ film thickness. The absorption spectra exhibit bands at 1429 and 1180 cm^-1 due, respectively, to the infrared active T_1u (4) and T_1u (3) modes of C ₆₀ in multi-layers. Additionally, two bands appear at 1442 and 1370 cm^-1. The former band is caused by activation of the infrared inactive (Raman active) A_g (2) mode via electron transfer from the silver to adsorbed C₆₀, and the latter is assigned to the T _1u (4) mode red-shifted by the charge transfer. These bands are all enhanced in intensity dependent either upon the silver or C₆₀ thickness, i.e., the largest absorption enhancement is obtained for 25-monolayers-thick silver and 12-nm-thick C₆₀. Under atomic force microprobe inspection, the average size and height of the islands in the overlaid 25-monolayers-thick silver change with underlying C₆₀ film thickness. The influence of the C₆₀ film structure upon the silver film and in turn the absorption intensity is strongly suggested.