A Raman spectroscopic investigation of 1-alkanethiol self-assembled monolayers at Ag surfaces

摘要

Initial Raman spectra of 1-alkanethiol self-assembled monolayers (SAMs) at mechanically polished (MP) polycrystalline Ag surfaces could not be accurately interpreted for alkanethiol conformational order, because the spectra indicated these monolayers were contaminated. From XPS and Raman spectra of the unmodified MP Ag surfaces, the primary contaminants were identified as graphitic carbon and alkyl hydrocarbons. As determined by XPS, mechanical polishing procedures were adopted which reduced the quantity of these contaminants; nonetheless, significant contaminant band intensity continued to be observed in Raman spectra of these alkanethiol SAMs. The contaminant was more accurately identified as a polyaromatic hydrocarbon (PAH) and its molecular identity was suggested as phenanthrene (phen). Attempts at its removal from the unmodified MP Ag surface by solvent dissolution and electrochemical reductive desorption were unsuccessful. However, this contaminant study was significant, because it initiated efforts to further reduce the quantity of this contaminant at the MP Ag surface. XPS and Raman spectra of chemically polished (CP) polycrystalline (poly) Ag surfaces indicated them to contain substantially less carbon contamination than the MP Ag surfaces. Thus, Raman spectra of short-chain alkanethiol SAMs at CP Ag (poly) and (chemically polished) Ag (111) were interpreted for alkanethiol conformational order. To better understand the signal intensities from CP Ag (poly), Ag (111), and MP Ag, surface enhancement factors (SEFs) at these surface types were quantified and compared to those measured for electrochemically-roughened (ORC) Ag, coldly-deposited Ag (Cold Ag), "thick" room temperature (RT)-deposited Ag, and MP Au. These SEFs were determined by reference of monolayer signal intensities at these surfaces to that from the unenhancing MP Pt surface (i.e., SEF of 1). Relative surface Raman sensitivities were assessed with use of these SEFs, and limits of detection (LODs) were calculated from the normalized S/N values in these surface spectra. Since the single spectrograph and Triplemate were both available for surface and normal Raman spectral acquisition, their relative performances were characterized to determine the advantages/disadvantages of each spectrograph. Specifically, the S/N and S/B values in spectra acquired on both spectrographs were quantified and compared.