jp6b11089_si_001.pdf (5.82 MB)
Spectroscopic Characterization of Fluorinated Benzylphosphonic Acid Monolayers on AlOx/Al Surfaces
journal contributionposted on 2016-12-12, 00:00 authored by William E. Ford, Ffion Abraham, Frank Scholz, Gabriele Nelles, Graham Sandford, Florian von Wrochem
We recently reported on how the surface energy and work function of AlOx/Al substrates can be tuned by self-assembled monolayers of fluorinated and nonfluorinated benzylphosphonic acid derivatives in view of organic electronic applications. In this contribution, we present a thorough investigation of these monolayers by photoemission (XPS) and infrared (PM-IRRAS) spectroscopies, to provide a quantitative understanding of their structural properties (packing density and orientation) and chemical composition. A detailed analysis of XPS chemical shifts makes an assignment of the carbon species present in the SAMs feasible, from the low-binding-energy aromatic carbon (∼284.5 eV) to the highly electronegative fluorine-substituted carbon (∼287.5 eV), whereby an upper limit for the fraction of nonspecific hydrocarbons (<5% for 10 of 11 SAMs) is assessed. The AlOx/Al substrate provides PM-IRRAS spectra of excellent quality within the window of 3400–1050 cm–1, enabling the identification of a number of bands, primarily those associated with aromatic C–H stretching, the benzyl CH2 group, ring vibrations, and C–F and C–O–C stretching vibrations. In particular, a distinct three-band pattern due to the benzyl CH2 group has been identified, involving a Fermi resonance interaction between the symmetric stretching vibration and the first overtone of the CH2 scissoring vibration. Furthermore, the absence of the CH2 scissoring and PO stretching bands in the PM-IRRAS spectra indicate that the PO3 moiety chemisorbs to the AlOx/Al substrate via a bidentate rather than tridentate or monodentate mode.