High Potassium Concentrations Nested in Epitaxial Monolayers of a Flexible Lander-Type Molecule on Ag(111)

The structural, electronic, and optical properties of organic adlayers can be tuned to a large extent by incorporating metal atoms. Naturally, the tunability of those properties is limited by the thermodynamic stability of the intercalated phases obtained and the segregation tendency, which often prevents the nesting of high metal atom concentrations in homogeneous epitaxial compound films. Here, we employ scanning tunneling microscopy and low-energy electron diffraction to investigate monolayers of the polycyclic aromatic hydrocarbon tetraphenyldibenzo-periflanthene (DBP, C64H36) epitaxially grown on Ag(111) and intercalated with potassium. This lander-type molecule contains four phenyl substituents that are nearly perpendicular to the aromatic backbone, and its flexibility enables rather complex adlayer structures. We succeeded in preparing highly ordered (mixed) monolayers with up to six potassium atoms per DBP. For increasing K concentrations, we find that DBP changes its shape from a considerably bent geometry (pristine DBP and K2DBP phases) to a molecule with a planar backbone (K6DBP phase), which is known to occur in free DBP molecules. By means of density functional theory (DFT) calculations, it is elucidated that the added K atoms adsorb underneath the molecule and thereby weaken the direct bonding channels between DBP and Ag while adding new bonding channels via the K atoms. This is accompanied by a gradually increasing charge transfer into the lowest unoccupied molecular orbital of DBP. The combination of structural data, results of different spectroscopy methods, and state-of-the-art DFT calculations leads to a comprehensive view on this rather complex host molecule-guest atom-substrate system.