Authors: Uibel, Rory H.; Harris, Joel M.

Source: Applied Spectroscopy, Volume 58, Issue 8, Pages 220A-244A and 887-1022 (August 2004) , pp. 934-944(11)

Publisher: Society for Applied Spectroscopy

Abstract:

Resolution of the reaction steps and the associated component Raman spectra during the formation or desorption of self-assembled monolayers is challenging because intermediate adsorbate populations are present at low concentrations and their spectral bands overlap. By collecting Raman spectra versus applied potential into a two-dimensional data set, one can utilize multivariate statistical techniques to resolve the component concentration profiles along with their corresponding Raman spectra. In situ surface-enhanced Raman spectroscopy (SERS) spectra were collected during the potential-dependent formation and desorption (-1.50 to -0.70V versus Ag/AgCl) of n-hexanethiolate monolayer at a polycrystalline Ag electrode. Resolution of the pure component spectra from these components was accomplished by using self-modeling curve resolution (SMCR), which does not require a physical model. For monolayer adsorption, the potential-dependent Raman spectra could be described by three significant eigenvectors; the eigenvectors could be rotated into a set of pure component spectra and concentration profiles using a linear least-squares step to find a common plane in the space of the eigenvectors representing the linear combination of the real-component responses. The convex hull surrounding the data in the plane and positive amplitude criteria were utilized to identify the coordinates of the pure component responses. The C-S stretching vibrations of the resolved spectra show that the initial adsorbate is a gauche conformer, which allows the hydrocarbon chain to lie on the metal surface; a second phase arises at higher coverage with trans C-S conformation, where the hydrocarbon chains are oriented off the surface plane, and a final complete monolayer is formed with a well-ordered, all-trans C-S configuration. In contrast, desorption studies showed only two surface phases, the initial well-ordered monolayer and the low-density phase dominated by gauche conformations. The results illustrate the utility of self-modeling curve resolution to unravel interfacial reaction mechanisms and intermediate structures from two-dimensional SERS data, without requiring prior knowledge of a physical model for the process.

Keywords: SELF-ASSEMBLED MONOLAYERS; SURFACE-ENHANCED RAMAN SPECTROSCOPY; SERS; SPECTROELECTROCHEMISTRY; FACTOR ANALYSIS; SELF-MODELING CURVE RESOLUTION; SMCR

Document Type: Research article

DOI: http://dx.doi.org/10.1366/0003702041655395

Affiliations: 1: Department of Chemistry, University of Utah, 315 South 1400 East, Salt Lake City, Utah 84112-0850

Publication date: 2004-08-01

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