Temperature Stability of Three Commensurate Surface Structures of Coronene Adsorbed on Au(111) from Heptanoic Acid in the 0 to 60°C Range

For the first time, accurate quantitative data on the temperature evolution of a surface monolayer formed at the solution solid interface are reported. In addition, a detailed analysis is provided of the structures of three different monolayers formed when coronene in heptanoic acid is in contact with Au(111). All three monolayer structures are well-defined epitaxial structures that are extremely stable for temperature variations between 0 and 60 degrees C. At high concentrations, a dense hexagonal structure with molecular separation of 1.19 +/- 0.04 nm is observed. At reduced concentration, the most often observed structure is an open hexagonal epitaxial structure with one molecule per unit cell and a molecular separation of 1.45 +/- 0.04 nm. This structure is stabilized by solvent molecule adsorption. If the dense phase is exposed to pure solvent, or occasionally with low concentration direct adsorption, then a different hexagonal phase is formed with three molecules per unit cell but exactly the same density (lattice length of 2.46 +/- 0.04 nm). Under some conditions, all three phases can be simultaneously present. It is notable that even when the least stable triangular phase is present on a large fraction of the surface, the low-density hexagonal phase is often observed decorating the reconstruction lines. The energy difference between the two low density phases is due to surface solvent and coronene-adsorbed solvent interactions as the coronene gold interactions in the two phases are essentially the same. The barrier to thermal conversion between the two low density phases must be several kT or greater than 2 kcal/mol.