Comparative X-ray standing wave analysis of metal-phosphonate multilayer films of dodecane and porphyrin molecular square

The nanoscale structures of multilayer metal-phosphonate thin films prepared via a layer-by-layer assembly process using Zr ⁴⁺ and 1,12-dodecanediylbis(phosphonic acid) (DDBPA) or porphyrin square bis(phosphonic acid) (PSBPA) were studied using specular X-ray reflectivity (XRR), X-ray fluorescence, and long-period X-ray standing wave (XSW) analysis. The films were prepared in 1, 2, 3, 4, 6, and 8 layer series on both Si(001) substrates for XRR and on 18.6 nm period Si/Mo layered-synthetic microstructure X-ray mirrors for XSW. After functionalizing the SiO ₂ substrate surfaces with a monolayer film terminated with phosphonate groups, the organic multilayer films were assembled by alternating immersions in (a) aqueous solutions containing Zr ⁴⁺or Hf ⁴⁺ (final metal layer only) cations and then (b) organic solvent solutions of PO ₃-R-PO ₃ ^4-, where R was DDBPA or PSBPA spacer molecule. The Hf ⁴⁺ cation served as the marker for the top surface of the films, whereas the Zf ⁴⁺ cation was present in all other layers. The PSBPA also contained Zn and Re atoms at its midline which served as heavy-atom markers for each layer. The long-period XSW generated by the 0th- (total external reflection) through 4th-order Bragg diffraction conditions made it possible to examine the Fourier transforms of the fluorescent atom distributions over a much larger q _z range in reciprocal space which permitted simultaneous analysis of Hf, Zn/Re, and Zr atomic distributions.