Formation and structure of self-assembled monolayers of alkanethiolates on palladium

J. Christopher Love, Daniel B. Wolfe, Richard Haasch, Michael L. Chabinyc, Kateri E. Paul, George M. Whitesides, Ralph G. Nuzzo

Research output: Contribution to journalArticlepeer-review

Abstract

The adsorption of n-alkanethiols onto polycrystalline thin films of palladium containing a strong (111) texture produces well-organized, self-assembled monolayers. The organization of the alkane chains in the monolayer and the nature of the bonding between the palladium and the thiol were studied by contact angle measurements, optical ellipsometry, reflection absorption infrared spectroscopy (RAIRS), and X-ray photoelectron spectroscopy (XPS). The XPS data reveals that a compound palladium-sulfide interphase is present at the surface of the palladium film. The RAIR spectra, ellipsometry data, and wetting properties show that the palladium-sulfide phase is terminated with an organized, methyl-terminated monolayer of alkanethiolates. The local molecular environment of the alkane chains transitions from a conformationally disordered, liquidlike state to a mostly all-trans, crystalline-like structure with increasing chain length (n = 8-26). The intensities and dichroism of the methylene and methyl stretching modes support a model for the average orientation of an ensemble of all-trans-conformer chains with a tilt angle of ∼14-18° with respect to the surface normal and a twist angle of the CCC plane relative to the tilt plane of ∼45°. The SAMs are stable in air, although the sulfur present at the surface oxidizes in air over a period of 2-5 days at room temperature. The differences in chain organization between SAMs formed by microcontact printing and by solution deposition are also examined by RAIRS and XPS.

Original languageEnglish (US)
Pages (from-to)2597-2609
Number of pages13
JournalJournal of the American Chemical Society
Volume125
Issue number9
DOIs
StatePublished - Mar 5 2003

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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