Structural and optical properties of self-assembled multilayers based on organic zirconium bisphosphonates

Multilayer films of zirconium terphenylbisphosphonic acid (TPBP) and zirconium quarterphenylbisphosphonic acid (QPBP) were prepared on ITO substrates via self-assembly. This deposition technique requires covalent attachment of a phosphonic acid anchoring layer appropriate for the ITO substrate. The functionalized substrate is then alternately exposed to aqueous ZrOCl2 and TPBP/QPBP solutions to deposit multilayer films. Investigation of these films by ellipsometry shows a linear increase of film thickness with the number of deposited layers. The average layer thickness indicates that the molecules of the phosphonic acid are inclined by 45–60° depending on the deposition conditions, this was proven by near-edge X-ray absorption fine structure (NEXAFS) measurements. For optical characterization fluorescence spectra of these films were recorded and analyzed. In order to test the ability of phosphonate films to act as electroluminescent materials, single layer devices have been prepared by using the ITO as anode and evaporating an aluminum cathode on top of the self-assembled multilayer film. Current–voltage and electroluminescence–voltage characteristics were recorded.