NSOM Investigations of the Spectroscopy and Morphology of Self-Assembled Multilayered Thin Films

Near-field scanning optical microscopy (NSOM) and atomic force microscopy (AFM) have been employed to spatially resolve the complex nanoscale morphologies, spectroscopy, and energy-transfer efficiencies of self-assembled multilayered structures composed of alternating layers of α-zirconium phosphate [α-Zr(HPO₄)₂] (ZrP) and dye-labeled poly(allylamine hydrochloride) (dye-PAH) (where dye = Fluorescein (FL), Rhodamine B (RhB), or Texas Red (TR)). Two types of multilayer films have been investigated, namely, glass/anchor/ZrP/dye-PAH and glass/anchor/ZrP/dye-PAH/ZrP/dye-PAH, which were formed by the sequential layer-by-layer adsorption of the charged polyelectrolyte component layers. High- and low-coverage films were investigated. The glass/anchor/ZrP assemblies were shown to consist of a densely packed “tiled” motif of ZrP sheets which lie flat on the surface and cover more than 95% of the area, with average plate sizes of height = 13 (7) Å, width ≈ 150 nm. The dye-labeled polymer layers in glass/anchor/ZrP/dye-PAH and glass/anchor/ZrP/dye-PAH/ZrP/dye-PAH were shown to adhere to the surface of the ZrP sheets and fill in the cracks between the sheets to a lesser extent. The measured heights of these polymer-coated multilayer films are 26(9) and 48(15) Å, respectively. These heights are consistent with theoretical estimates of ideally packed ionic films (28 and 48 Å, respectively). Dual-wavelength fluorescence NSOM imaging at 580 nm and >610 nm and near-field photobleach experiments were used to spatially resolve nanoscopic regions that display energy transfer between the layers.