Thickness-Dependent Adsorption of Melamine on Cu/Au(111) Films

Supported metal films represent a model binary metallic system wherein the surface properties can be finely tuned with the composition/thickness of the film. Thus, it has raised increasing research interests from various aspects. In this work, the adsorption and assembly behavior of melamine on the Cu films grown on the (22 × √3) reconstructed Au(111) substrate were investigated as a function of Cu thickness. The atomically resolved scanning tunneling microscopy (STM) images in combination with density functional theory (DFT) calculations reveal that the ultrathin submonolayer copper film forms a pseudomorphic (1 × 1) lattice by agglomerating at the subsurface, while the true Cu adlayers start to appear on the three-layer thick films and hold the bulk Cu(111) lattice. Correspondingly, the adsorption and assembly of the melamine molecules significantly differ from those on pure metals, experiencing a gradual transformation from physisorption to chemisorption assemblies with the Cu film thickening. These findings shed light on the self-assembly behavior of organics on binary metallic surfaces. In addition, the evaporated metal films also register a diversified substrate for tuning the properties of adsorbed molecular films.