To expand the catalogue of available interactions for the efficient self-assembly of highly-ordered nanoscale structures, we have investigated the formation of high-stability supramolecular networks constructed with anionic carboxylate species and sodium cations. Our experiments demonstrate that these two-component approaches can steer organic molecules towards efficient self-assembly, even with molecules that do not show a strong tendency towards long-range, two-dimensional ordering when deposited alone. Biphenyl-3,3′,5,5′ -tetracarboxylic acid (BTA) on Cu(100) serves as a model system to illustrate this effect. Ionic structures have been resolved with molecular and atomic resolution using scanning tunneling microscopy (STM). Chemical shifts in the Na 1s, C 1s, and O 1s core level binding energies, measured by X-ray photoelectron spectroscopy, confirm the active chemical interactions inferred from the STM results.

 

Ionic self-assembly has been achieved on the Cu(100) surface with terephthalic acid, trimesic acid, as well as BTA. We have shown that the carboxylate and sodium chloride undergo a replacement reaction producing a new salt with long range periodic structure. Chemical shifts in the sodium 1s photoelectron peak have been observed upon addition of the organic species to the surface, confirming a direct interaction. Resulting extended network structures demonstrate very high stability, maintaining their supramolecular structure up to at least 165 °C. The formation of new structures illustrates the interplay between adsorbate-substrate and ionic interactions and opens new possibilities for ionic self-assemblies at surfaces with highly-ordered structure and specific chemical function.