Structural Transitions in Organic Adlayers — A Molecular View

Abstract

The structure and dynamics of phase formation of organic monolayers potentiostatically deposited on single-crystal electrode/aqueous electrolyte interfaces has been studied by combining classical electrochemical experiments with in situ Scanning Tunnelling Microscopy (STM). Depending on the potential, uracil, 5-methyluracil, 5,6-dimethyluracil and n, n′-bipyridines form highly ordered physisorbed and/or chemisorbed adlayers on Au(hkl)-electrodes, which were imaged with molecular resolution. The two-dimensional condensed physisorbed films of uracil derivatives display characteristic properties of a hydrogen-bonded network of planar oriented molecules. These pyrimidine bases deprotonate at sufficiently positive potentials and form highly ordered (commensurate, incommensurate) chemisorbed monolayers of perpendicularly oriented surface co-ordination complexes. The positions of the individual molecules and their long-range order are determined by the competing interactions between molecular and substrate co-ordination centres as well as lateral π-stacking. These motifs of molecular phase formation were extended and tested with the chelating bidentate ligand 2, 2′-bipyridine and the non-chelating bridging ligand 4,4′-bipyridine adsorbed on Au(hkl)-electrodes. The dynamics of film formation and dissolution of the various ordered molecular phases and the substrate stability were investigated by in situ STM as a function of electrode potential, temperature and supporting electrolyte. Evidence was found for (hole) nucleation-and growth-controlled (dissolution) formation for uracil and n, n′ - bipyridine adlayers at positive charge densities. Several metastable structures were observed during these transitions. Their appearance and characteristic properties depend strongly on the applied potential perturbation, and are accompanied with major changes of the electrode surface. The chapter concludes with a method to selectively nanostructure an Au(100)-electrode with 2, 2′-bipyridine exploiting the uneven local charge distribution between reconstructed (hex) and unreconstructed (1 × 1) surface patches.