On the chloride activation in SERS and single molecule SERS

Abstract

In ‘common (canonical)’ multi-adsorbate, multi-particle surface enhanced Raman scattering (SERS) the vibrational band intensities of adsorbates at roughened electrodes and at colloidal particles are considerably enhanced by one to three orders of magnitude by the presence of chloride or bromide in the electrolyte. We give an extensive review of the literature and present own experiments on the action of chloride on the Raman intensities of pyridine on a copper single crystal and on polycrystalline copper. Only in the latter case the ‘chloride activation’ is observed. In common SERS the creation of active sites by ‘chloride activation’ can be explained well by the formation of stable surface complexes of atomic scale roughness, chloride and the molecular adsorbate, and an electronic charge transfer resonant Raman scattering. So far, also in single molecule SERS (SM-SERS) the ‘chloride activation’ was always essential. The surface complex model can be easily applied for isolated colloidal particles, but a problem is the small number of active sites under the condition of SM-SERS. SM-SERS is also observed with aggregates of particles induced by chloride, but it is not clear whether this is the only role of chloride in this case. After a literature review we propose hypothetical explanations and models.