GROWTH AND CHARACTERIZATION OF METAL(II) ALKANEBISPHOSPHONATE MULTILAYER THIN-FILMS ON GOLD SURFACES

Thin films of divalent metal (Zn and Cu) alkanebisphosphonates grow on gold surfaces modified with (4-mercaptobutyl)phosphonic acid by alternate immersion in 5 mM ethanolic solutions of the metal acetate or perchlorate salt and H2O3P(CH2)nPO3H2, n = 8, 10, 12, and 14. Growth of each layer is remarkably fast. Well-ordered multilayers can be deposited with 10-min adsorption steps, and films of 100-layer thickness are easily prepared. Quartz crystal microbalance (QCM) measurements of mass changes are in quantitative agreement with adsorption of individual layers corresponding to bulk stoichiometry (M2[O3P(CH2)nPO3].2H2O) and packing density. Ellipsometric film thicknesses agree closely with measured layer spacings from X-ray diffraction of bulk solids, and alkyl chain tilt angles (31-degrees) for the Zn films are essentially the same as predicted from the crystal structure of Fe(O3PC2H5).H2O. Reflectance infrared spectra show that the alkyl chains are less ordered in the films than in the bulk solids for M2[O3P(CH2)nPO3].2H2O, n = 8, but that they are well-ordered for n = 14. Atomic force microscopy (AFM) images of multilayer films show that they follow the surface topology of the underlying gold and that they can be mechanically removed from the substrate. The depth of etch pits prepared with the AFM tip matches the film thickness measured by ellipsometry.