From Molecules to Thin Solid Films: Vibrational Characterization of Evaporated Organic Films

The use of surface-enhanced Raman scattering (SERS) and resonance Raman scattering (SERRS) for vibrational characterization of physisorbed molecules at submonolayer coverage is emphasized and illustrated with the example of vanadyl phthalocyanine (VOPc). SERS and SERRS produced by three different surfaces with physisorbed VOPc are compared with the corresponding unenhanced spectra of VOPc aggregates obtained by evaporation. Silver and gold island films, with plasmon absorptions well separated in the visible region, and a SERS-active surface of Ag-coated Sn spheres that presents enhancing properties for most of the visible region of the spectrum were used. Thin solid films of VOPc were gradually grown onto the SERS-active surface of Ag-coated Sn spheres from thicknesses of 1 to 65 nm. The intensity of the SERS signal decreased with increasing film thickness, and the depolarized SERS spectra were replaced by a typical polarized spectra of the thin-film samples deposited onto alkali halide or glass substrate. Solid-state splittings of some vibrational frequencies were confirmed by their absence in the molecular (SERS) spectra.