Determination of sample morphology of multilayered structures used in surface enhanced Raman scattering experiments

The detailed nature of the morphology of multilayered samples used for surface enhanced Raman scattering measurements is examined by the use of scanning and transmission electron microscopy. The morphology of Raman enhancing 200 Å silver films evaporated at 80 K onto thin films of CaF₂, monolayers of organic molecules on aluminum oxide or polymer films used in the Raman scattering experiments is compared to that of silver island films evaporated at 300 K on SiO substrates. We show that the silver layers deposited directly onto CaF₂ films consist of interconnected polycrystalline island-like structures. These structures are ∼300 A ̊ wide and are located along ridges and valleys of the CaF₂. We argue that this morphology of nearly separated silver particles is better modelled as a collection of random spheriods than as small perturbation roughness on a smooth surface. An estimate of the total pinhole density in the CaF₂ layers used as silver rougheners in some of the samples is obtained by electrical conductivity measurements of AuCaF₂Ag junctions. We conclude from these studies that there is no appreciable molecule-silver contact in samples with configuration molecule-spacer-CaF₂-silver. These samples previously have been shown to give long range ∼30-50 Å falloff of the Raman scattering enhancement as the Raman scattering molecule is spaced away from the silver surface.