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Ordered and Robust Ionic Surface Networks from Weakly Interacting Carboxyl Building Blocks

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posted on 14.02.2013, 00:00 authored by Daniel Skomski, Steven L. Tait
Application of self-assembly strategies to technologically relevant materials requires robust interactions between complex organic building blocks at surfaces. However, complex molecules of practical interest are not necessarily in registry with a surface, which may impede self-organization into a desired architecture. In this work, we demonstrate that for an organic system in which neither adsorbate–substrate nor adsorbate–adsorbate interactions are sufficient to yield self-assembly by the organic species alone, a self-organized network possessing high thermal stability is achieved via an ionic bonding protocol. The self-assembled ionic surface network is formed by reaction of tetra-anionic carboxylate species and sodium chloride and achieves a degree of thermal stability not previously demonstrated in self-assembled organic nanostructures at metal surfaces. Thermal stability at 180 °C is demonstrated by molecular-resolution, high-temperature imaging of the supramolecular architectures for several hours. The excellent structural stability is achieved through a cooperative effect of ionic interactions and metal–organic adsorption. Thermodynamically controlled conversion into a single phase, in spite of the nondirectional nature of ionic bonding, is demonstrated here for the first time in ionically bonded networks at the metal surface. The degree of sodium incorporation into the surface networks can be precisely tuned, allowing some structural control in the ionic networks. These results present a strategy for achieving atomically precise supramolecular architectures using molecular carboxylate species that do not show a tendency toward long-range self-organization alone but can be driven toward two-dimensional, highly robust self-assembly by the ionic surface reaction discussed herein.