Modification of BaTiO₃ thin films: Adjustment of the effective surface work function

Sputter-deposited BaTiO₃ thin films have been modified with an alkylphosphonic acid and a partially-fluorinated alkylphosphonic acid in order to model the surface composition of similarly modified BaTiO₃ nanoparticles. We present here the surface characterization of these modified films by a combination of X-ray photoelectron spectroscopy (XPS) and UV-photoelectron spectroscopy (UPS). BaTiO₃ layers of average thicknesses ca. 2 nm were prepared by radio frequency (rf) magnetron sputter deposition on Ag films, to avoid charging effects during XPS/UPS characterization. Octadecylphosphonic acid (ODPA) and 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl phosphonic acid (perfluorohexyloctyl phosphonic acid, FHOPA), molecules with quite different molecular dipole moments, were chemisorbed from solution to the BaTiO₃ surface. Polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS) of the modified BaTiO₃ films indicated bidentate bonding of the alkylphosphonic acid to the oxide. Modification of the BaTiO₃ surface with the partially-fluorinated alkylphosphonic acid (versus the normal alkylphosphonic acid) significantly changes the BaTiO₃ interface dipole as revealed by UPS/XPS measurements, which, in turn, changes the frontier orbital offsets between the oxide and the organic modifier.