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Transparent, flexible, and conductive graphene sheets form an ideal substrate for the fabrication of organic light-emitting diodes. In order to obtain an optimal final device, it is vitally important to understand the underlying nucleation and growth processes. Here, the growth of para-sexiphenyl (6P) thin films on Ir(111)-supported graphene and on Ir(111) has been investigated. Special attention has been given to directed and concerted diffusion processes of 6P molecules on graphene grown on Ir(111). From the movement of large islands, which are formed by flat-lying molecules, across wrinkle-free graphene areas, the activation barrier for the diffusion of 6P molecules along step edges of 6P islands has been estimated to be approximately 0.26 eV. For the case of 6P growth on Ir(111), ramified islands formed by upright-standing molecules are found. Here, heteronucleation, in combination with particularities in the shape of possible smallest stable clusters, is identified as the root cause for the measured critical nucleus size of zero.
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