Investigation of solid surfaces modified by Langmuir-Blodgett monolayers using sum-frequency vibrational spectroscopy and X-ray photoelectron spectroscopy

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Abstract

Langmuir-Blodgett (LB) monomolecular layers of alkylhydroxamic acids and alkylphosphonic acids on copper and iron substrates have been studied by X-ray photoelectron spectroscopy (XPS) and sum-frequency vibrational spectroscopy. According to the XPS results, the structures of the hydroxamic acid and phosphonic acid Langmuir-Blodgett films are very similar: the thickness of the layer of the hydrocarbon tails is typically 1.9-2.1 nm, while the layer of headgroups is about 0.3-0.35 nm thick. The tilt angle of the carbon chains is estimated to be 20-30° with respect to the sample surface normal, and the molecules are connected to the substrate via their headgroups. Analysis of the P 2p and N Is lines indicates the presence of deprotonated headgroups. The substrate Cu 2p line includes a component which can be assigned to Cu 2+ ions in a thin Cu(OH)2 layer. The deposition of LB layers led to significant decrease of the hydroxide-related signal, which indicates that binding of the headgroups to the surface is accompanied by the elimination of water molecules. The sum-frequency spectra also clearly indicate that well-ordered monolayers can be formed by the Langmuir-Blodgett technique. Since the nonresonant background from the metal substrates renders the analysis of the spectra more difficult, model system samples on glass were prepared. It was found that the alkyl chains of the adsorbed acids predominantly adopt the all-trans conformation and form an ordered structure. Upper limits for the mean tilt angle of the terminal methyl groups are ∼10-20°.

Original languageEnglish
Pages (from-to)8701-8714
Number of pages14
JournalJournal of Physical Chemistry B
Volume110
Issue number17
DOIs
Publication statusPublished - May 4 2006

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ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Surfaces, Coatings and Films
  • Materials Chemistry

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