Thermally activated layer inversion of ultrathin pentacene/para-sexiphenyl organic heterostructures is observed using a combination of reflectance difference spectroscopy and scanning tunneling microscopy. The heterostructures are formed by deposition of sub-monolayer pentacene (PEN) on top of well ordered para-sexiphenyl (p-6P) layers on Cu(110) at 15 K. When the sample temperature is raised, these heterostructures invert, with pentacene molecules diffusing through the para-sexiphenyl buffer layer and getting in direct contact with the substrate. The observed irreversible inversion demonstrates that the p-6P/PEN/Cu(110) is energetically preferred over PEN/p-6P/Cu(110). Furthermore, the onset temperature of the inversion increases with the layer thickness of para-sexiphenyl indicating a corresponding increase of the kinetic barrier for the inversion. Our results demonstrate the strong influence of the configuration of organic heterostructures on their thermal stability, especially for the very thin layers.
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