Spectroscopic Characterization of Fluorinated Benzylphosphonic Acid Monolayers on AlO_x/Al Surfaces

We recently reported on how the surface energy and work function of AlO_x/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 AlO_x/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 PO stretching bands in the PM-IRRAS spectra indicate that the PO₃ moiety chemisorbs to the AlO_x/Al substrate via a bidentate rather than tridentate or monodentate mode.