Branched Coordination Multilayers on Gold

Abstract

A C₃-symmetric tridentate hexahydroxamate ligand molecule was specially synthesized and used for coordination self-assembly of branched multilayers on Au surfaces precoated with a self-assembled monolayer (SAM) of ligand anchors. Layer-by-layer (LbL) growth of multilayers via metal−organic coordination using Zr⁴⁺ ions proceeds with high regularity, adding one molecular layer in each step, as shown by ellipsometry, wettability, UV−vis spectroscopy, and atomic force microscopy (AFM). The branched multilayer films display improved stiffness, as well as a unique defect self-repair capability, attributed to cross-linking in the layers and lateral expansion over defects during multilayer growth. Transmetalation, i.e., exposure of Zr⁴⁺-based assemblies to Hf⁴⁺ ions, was used to evaluate the cross-linking. Conductive atomic force microscopy (AFM) was used to probe the electrical properties of the multilayers, revealing excellent dielectric behavior. The special properties of the branched layers were emphasized by comparison with analogous multilayers prepared similarly using linear (tetrahydroxamate) ligand molecules. The process of defect annihilation by bridging over defective areas, attributed to lateral expansion via the excess bishydroxamate groups, was demonstrated by introduction of artificial defects in the anchor monolayer, followed by assembly of two layers of either the linear or the branched molecule. Analysis of selective binding of Au nanoparticles (NPs) to unblocked defects emphasized the superior repair mechanism in the branched layers with respect to the linear ones.