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Infrared Absorption Enhancement of C60 by Overlaying Thin Silver Island Films

Volume 58, Number 3 (March 2004) Page 299-303

Wadayama, T.; Aoshima, K.; Kawano, S.; Hatta, A.


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 Tlu (4) and Tlu (3) modes of C60 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) Ag (2) mode via electron transfer from the silver to adsorbed C60, and the latter is assigned to the T1u (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.