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

Octadecylphosphoric acid ester is shown to self-assemble on amorphous/nanocrystalline tantalum oxide (Ta₂O₅) layers deposited by physical vapor deposition onto glass substrates. Three complementary surface-analytical techniques (angle-dependent X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry, and atomic force microscopy in lateral force mode), showed that a 2.2 nm thick, `tails-up'-oriented adlayer is formed, which displays local near-hexagonal order, strong P-O-Ta bonding, and the presence of (-P-O-)₂Ta species. A model for the binding and the structural organization of the octadecyl phosphate molecules on the tantalum oxide surface is proposed involving direct coordination of the terminal phosphate headgroup to Ta(V) cations forming a strong complexation bond, two types of bonding of the octadecyl phosphate with both monodentate and bidentate phosphate-Ta(V) coordinative interactions, and, locally, the formation of a coincidence lattice of approximately hexagonal structure defined by both the location of Ta(V) cation sites and an intermolecular spacing between the octadecyl phosphate ligands of approximately 0.5 nm. This is very similar to the self-assembled monolayer structure of long-chain alkanethiols on gold. The use of phosphoric acid ester derivatives is believed to have potential for designing specific interface architectures in sensor technology, in surface modification of oxide-passivated metallic biomaterials, and in composite metal (oxide)-polymer interfaces.