Adsorption characteristics of 1,3-propanedithiol on gold: Surface-enhanced Raman scattering and ellipsometry study

Abstract

The adsorption behavior of 1,3-propanedithiol (1,3-PDT) on a colloidal gold surface has been investigated by means of surface-enhanced Raman scattering (SERS). 1,3-PDT chemisorbed dissociatively on gold as a monothiolate. Moreover, we could clearly identify the S-S stretching band that reveals the formation of multilayers of 1,3-PDT on the colloidal gold surface. As for the case of 1,2-ethanedithiol. the intermolecular disulfide bond appeared to form even at a submonolayer coverage limit, implying that the adsorption of alpha,omega-alkanedithiol on gold does not take place homogeneously from the early stages of self-assembly. On the other hand, referring to the ab initio calculated vibrational frequencies of AuSCH2CH2CH2Cl, we could also analyze the concentration-dependent SER spectral features of 1,3-PDT in terms of the conformational isomerism around the two C-S bonds. Upon the increase of the bulk concentration of 1,3-PDT, a transition from a GG to GT conformation appeared to occur on the sol surface near the monolayer coverage limit; herein the first letter designates the conformation with respect to the C-C bond near the gold surface. On the basis of the concentration-dependent SER peak intensities, the 1,3-PDT-derivatized Au particles appeared to consist in nearly contact configurations. A separate ellipsometry measurement performed with vacuum-evaporated gold substrate revealed that at best one and one-half layered film could be assembled on gold in polar and protic solvents while up to three layers were assembled on gold in n-hexane solution of 1,3-PDT.