Structural chemistry of self-assembled monolayers of octadecylphosphoric acid on tantalum oxide surfaces

Octadecylphosphoric acid ester is shown to self-assemble on amorphous/nanoc rystalline tantalum oxide (Ta2O5) layers deposited by physical vapor deposi tion onto glass substrates. Three complementary surface-analytical techniqu es (angle-dependent X-ray photoelectron spectroscopy, time-of-flight second ary ion mass spectrometry, and atomic force microscopy in lateral force mod e), showed that a 2.2 nm thick, "tails-up"-oriented adlayer is formed, whic h displays local near-hexagonal order, strong P-O-Ta bonding, and the prese nce of(-P-O-)(2)Ta species. A model for the binding and the structural orga nization of the octadecyl phosphate molecules on the tantalum oxide surface is proposed involving direct coordination of the terminal phosphate headgr oup to Ta(V) cations forming a strong complexation bond, two types of bondi ng of the octadecyl phosphate with both monodentate and bidentate phosphate -Ta(V) coordinative interactions, and, locally, the formation of a coincide nce lattice of approximately hexagonal structure defined by both the locati on of Ta(V) cation sites and an intermolecular spacing between the octadecy l phosphate ligands of approximately 0.5 nm. This is very similar to the se lf-assembled monolayer structure of long-chain alkanethiols on gold. The us e of phosphoric acid ester derivatives is believed to have potential for de signing specific interface architectures in sensor technology, in surface m odification of oxide-passivated metallic biomaterials, and in composite met al (oxide)-polymer interfaces.