Raman spectroscopy of self-assembled mono- and multilayer films of alkanethiolate on gold

The recently developed Raman imaging microscope was used to investigate mono- and multilayer films of alkanethiol derivatives, containing simple chromophores, deposited on gold. The spectroscopic intensity of the bands associated with the chromophore did not increase linearly with number of layers, whereas those measured using Fourier transform infrared spectroscopy did. It seems probable that some enhancement mechanism, possibly surface enhanced Raman spectroscopy, is taking place. In an attempt to further elucidate the enhancement mechanism for our samples, a number of films were made in which a single chromophore layer was moved systematically away from the gold surface (or was separated from silver islands using spacer layers), from approximately 4 to 100 Å. These are compared with results obtained for multilayer structures containing chromophores in each layer. The Raman microscope was used in its imaging mode to study the uniformity of multilayers and to study films formed on metal-electrode arrangements on silicon.