Mirror Domains and Lateral Electric Fields in Para -Sexiphenyl Thin Films on SiO2: A Low-Energy Electron Microscopy Study

We investigate the structure of para-sexiphenyl (p-6P) on SiO2 as a prototype organic semiconductor (OSC)/dielectric interface. Low-energy electron microscopy (LEEM) reveals crystalline 2D structures of mono- and bilayer p-6P on natively oxidized silicon substrates, with bilayer islands predominantly consisting of mirror domains with counter-directional molecular tilt. In microspot low-energy electron diffraction (μLEED) we observe an unexpected deflection of the electron beam, the magnitude of which increases with sample temperature. Maps of the local work function indicate an adsorption-induced transfer of electrons from the silicon substrate through the native oxide into the p-6P molecules that also increases with temperature. These results suggest that lateral electric fields exist inside the molecular films, arising from substrate-to-molecule charge transfer and the formation of adsorption-induced molecular dipoles. Our work provides a comprehensive structure analysis of p-6P thin films on SiO2 and highlights that lateral electric polarizations can exist in OSC films on dielectrics even in the absence of permanent molecular dipole moments, which potentially affect the charge transport properties of these films.