Chemical interaction and molecular growth of a highly dipolar merocyanine molecule on metal surfaces: A photoelectron spectroscopy study

The growth and ordering of molecules on surfaces is an intriguing research topic as insights gained here can be of significant relevance for organic electronic devices. While often simple, rigid molecules are employed as model systems, we show results for a highly dipolar merocyanine which is studied on top of Au(100), Ag(100) and Cu(100) metal single crystals. Film thicknesses ranging from sub-monolayer to multilayer regimes are analyzed using UV (UPS) and X-ray photoelectron spectroscopy (XPS). For the monolayer regime, there is strong indication of face-on orientation, with both of the molecules’ sulfur atoms bonding to the metal surfaces. Here, on Ag and Au(100) the sulfur atoms lose some or all of their intrinsic charges due to a charge transfer with the substrate, while on Cu(100) a strong metal-sulfur bond forms. The interaction between the substrate and the molecules can also be seen in the intensity and width of the highest occupied molecular orbital features in UPS. Upon multilayer deposition, a gradual lowering in ionization energy is observed, likely due to the formation of antiparallel dimers followed by an increased charge carrier delocalization due to the formation of an extended molecular aggregate for thicker layers. Interestingly, on Cu(100) the aggregated phase is already observed for much lower deposition, showing the importance of substrate-molecule interaction on the subsequent film growth. Therefore, this study offers a detailed understanding of the interface formation and electronic structure evolution for merocyanine films on different metal surfaces.