Layer-by-layer assemblies consisting of alternating layers of nitrilotris(methylene)triphosphonic acid (NTMP), a polyfunctional corrosion inhibitor, and zirconium(IV) were prepared on alumina. In particular, a nine-layer (NTMP/Zr(IV))4NTMP stack could be constructed at room temperature, which showed a steady increase in film thickness throughout its growth by spectroscopic ellipsometry up to a final thickness of 1.79 ± 0.04 nm. At higher temperature (70 °C), even a two-layer NTMP/Zr(IV) assembly could not be prepared because of etching of the alumina substrate by the heated Zr(IV) solution. XPS characterization of the layer-by-layer assembly showed a saw tooth pattern in the nitrogen, phosphorus, and zirconium signals, where the modest increases and decreases in these signals corresponded to the expected deposition and perhaps removal of NTMP and Zr(IV). Time-of-flight secondary ion mass spectrometry (ToF-SIMS) confirmed the attachment of the NTMP molecule to the surface through PO-, PO2-, PO3-, and CN- signals. Increasing attenuation of the Al signal from the substrate after deposition of each layer was observed by both XPS and ToF-SIMS. Essentially complete etching of the alumina by the heated Zr(IV) solution was confirmed by spectroscopic ellipsometry, XPS, and ToF-SIMS. Atomic force microscopy revealed that all the films were smooth with Rq roughness values less than 0.5 nm.
Diwan, Anubhav, Bhupinder Singh, Christopher J. Hurley, Matthew R. Linford
Surface and Interface Analysis