Effect of Thionation on Molecular Interaction with a Metal Bilayer: Diketopyrrolopyrroles on Tl/Si(111)

Adsorption of molecules on metal surfaces or metal atomic layers is influenced by the spatial modulation of atomic structure and of related electronic states. At the same time, the adsorbed molecules affect the surface electronic structure. The strength of this interplay can be modified by functionalization of molecular units, with impact on possible applications of such systems. Here, we study the effect of thionation of diketopyrrolopyrrole (DPP)─substitution of oxygen atoms of the DPP core by sulfur atoms─on interaction with the thallium (abbreviated as Tl) bilayer on the Si(111) substrate. The Tl bilayer forms a moiré structure with spatially varying electronic properties. We use the scanning tunneling microscopy (STM) to investigate adsorbed molecular superstructures and the density functional theory (DFT) calculations to analyze the electronic structure and molecular interactions. STM and DFT reveal selective interaction with specific parts of the surface structure. The results indicate the crucial role of the thionation in the formation of bonding interaction of the DPP molecules with the metal bilayer.