Impact of Fluorination on Initial Growth and Stability of Pentacene on Cu(111)

Structure, growth, thermal stability, and electronic properties of thin films of the fully fluorinated analogue to the archetypical organic semiconductor pentacene (PEN), perfluoropentacene (PFP), were investigated on Cu(111) at room temperature by scanning tunneling microscopy (STM), low energy electron diffraction (LEED), ultraviolet photoelectron spectroscopy (UPS), and X-ray photoelectron spectroscopy (XPS). In contrast to PEN, where molecules could only be imaged by STM at full monolayer coverage, PFP was seen to stabilize in disordered clusters already in the submonolayer regime. Furthermore, while long-range order was observed for closed PEN molecular monolayers, PFP only formed a disordered first wetting layer. Highly ordered domains were not observed until the formation of the second layer of PFP. In this layer, the molecular planes are inclined to the surface, as supported by additional STM measurements on graphite and theoretical modeling. Careful consideration of the structural details in the transitional growth regime from molecular mono- to multilayers thus emerges as the key ingredient to achieving a deeper understanding of metal/organic interfaces relevant for organic electronic devices.