New strategies for surface-enhanced Raman scattering at transition-metal interfaces: Thickness-dependent characteristics of electrodeposited Pt-groupfilms on gold and carbon

The surface-enhanced Raman scattering (SERS) properties of rhodium films el ectrodeposited onto a SERS-active gold substrate are examined as a function of overlayer thickness, using carbon monoxide and thiocyanate as chemisorb ate probes in aqueous solution. Although the adsorbate Raman signals obtain ed for ultrathin (less than or equal to 10 monolayers, ML) Rh layers emanat e from the Sold template, a significant or even dominant SERS component for thicker films is identified as arising instead from the transition-metal o verlayer itself. The latter contribution was diagnosed and characterized fr om similar experiments undertaken using SERS-inactive gold and carbon subst rates, and from the observation of comparable Raman enhancements obtained w ith green (514.5 nm) as well as red (632.8 nm) excitation. While the surfac e enhancement factors for these layers, of the order of 10(3), are markedly less than those obtained for ultrathin films on SERS-active gold, the sign als even for very weak Raman scatterers (such as adsorbed hydrogen) on pall adium and platinum, as well as rhodium, electrochemical interfaces are dete ctable with a confocal Raman spectrometer. The broad-based potential utilit y of such films for the detailed vibrational characterization of transition -metal ambient interfaces is also pointed out.