Substrate Induced Thermal Decomposition of Perfluoro-Pentacene Thin Films on the Coinage Metals

The thermal and chemical stability of perfluoropentacene (PFP) thin films grown by organic molecular beam deposition onto the (111)-oriented surfaces of the coinage metals copper, silver, and gold have been studied by means of temperature dependent X-ray photoelectron spectroscopy (XPS) and Near-Edge X-ray absorption fine structure spectroscopy (NEXAFS). Under vacuum conditions, PFP multilayers are completely desorbed at 425 K while molecules in contact with the Au(111) surface remain intact up to 500 K. By contrast, PFP that is in contact with Cu(111) is distinctly distorted and becomes partially defluorinated already upon thermal desorption of multilayers. A pronounced defluorination of PFP also takes place on Ag(111) at temperatures around 440 K, while further heating causes a complete cracking and defluorination. Additional measurements carried out on a regularly stepped silver surface demonstrate that steps are active sites that promote defluorination already at lower temperatures. van der Waals corrected density-functional (DFT-D) calculations show that PFP, though being weakly adsorbed on all three metal surfaces, exhibits a reduced energy barrier for defluorination, in particular on copper and silver, thus reflecting their catalytic activity. The calculations reveal further that defluorinated molecules are covalently bound to the substrate, leading to a notable bending of the molecular backbone. The present study highlights the importance of also considering chemical reactions when theoretically analyzing molecule/metal interactions and indicates that fluorinated aromatic molecules, though offering interesting electronic properties, actually exhibit a limited stability in contact with some electrode surfaces like silver due to catalytic effects.