Correlation Between Linear Optical Constants and Raman Enhancement in Phthalocyanine Thin Solid Films with Incorporated Silver Clusters

V. Linss, O. Stenzel, D. R. T. Zahn
Technische Universität Chemnitz, Institut für Physik
D - 09107 Chemnitz
Germany

Surface-enhanced Raman spectroscopic (SERS) experiments on thin films of free base phthalocyanine (H₂Pc) with embedded silver clusters are reported. The thin films were procduced by thermal evaporation of H₂Pc together with different amounts of silver (8, 15 and 26 vol.%). From transmission electron microscopy it turned out that the average silver cluster diameter changes from approximately 4 to 7 nm with increasing silver concentration. The plasmon wavenumber of the silver clusters was obtained from spectrophotometric measurements and was close to 22000 cm^-1 (approximately 450 nm), nearly independent of the silver filling factor. Raman spectra of H₂Pc were recorded at two different exciting laser wavelengths (441,6 and 568,2 nm), being either close to or well seperated from the plasmon resonance. The intensity of the Raman scattering increased with increasing silver filling factor in the films, while the lineshape of the Raman spectra was not seriously influenced by the cluster incorporation. A systematic correlation between the intensity of the plasmon absorption line and the Raman enhancement could be established and is discussed in terms of an electromagnetic enhancment mechanism.