SERS substrates fabricated by island lithography: The silver/pyridine system

A range of pseudo-random silver structures, where there is a choice of clustered spheres or pillars or tori, have been fabricated on silicon using the method of island lithography combined with electroless plating. Pyridine has been adsorbed on these structures and the surface-enhanced Raman scattering spectrum (SERS) measured using 633 nm laser radiation. Measurements of SERS spectra as a function of pyridine solution concentration have enabled an adsorption isotherm to be obtained and the standard free energy of adsorption to be determined (24 U mol(-1)), in good agreement with the literature. The substrates are found to give a uniform signal, as sampled over the prepared area, for both saturation coverage (+/-12%) and for a fraction of a monolayer, ca. 0.08, (+/-31%). It is concluded that, in the system studied, the effective area of the SERS "site" must be large compared with the area occupied by the adsorbed pyridine, so that the SERS signal is proportional to the surface coverage, averaged over the adsorbent. The formalism due to Tian and co-workers has been adopted to determine G, the SERS enhancement factor. G is calculated as follows: G = (scattering intensity per adsorbed molecule)/(scattering intensity per solution molecule). G from 1.9 x 10(6) (pillars) to 2.5 x 10(7) (tori) have been estimated, and larger values are expected. These silver substrates, which are robust and reproducible, would seem to be good candidates for various analytical applications.