Self-Assembly of PTCDA Ultrathin Films on Graphene: Structural Phase Transition and Charge Transfer Saturation

The self-assembly of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) on graphene with coverage in the range of 0.3-3 monolayers (MLs) is characterized by a DFT-based ab initio calculation method. For alpha modification mode, with a critical thickness of 1 ML, the growth of PTCDA on graphene follows the Stranski-Krastanov (SK) mode. For beta modification mode, the PTCDA can form two complete MLs on a graphene substrate. From the thermodynamic viewpoint, a modification mode is more stable than beta modification mode. At 1 ML, the PTCDA follows a planar configuration on graphene, which is also almost unperturbed by typical defects in the graphene sheet. The PTCDA adlayer remains its planar and continuous herringbone structure on graphene with three typical defects. For alpha modification mode with 2 or 3 ML coverage, the molecular planes incline to the substrate plane with angles around 90 degrees and 13 degrees, respectively, which indicates that a bulk-like phase appears. This also enhances the lateral intrinsic charge transport characteristics. For a modification mode, the total amount of charge transfer between PTCDA and graphene per 5 root 3 x 5 super cell at 2 MLs saturates with 0.42e, which is 0.1 and 0.06e larger than those of 1 and 3 MLs, respectively.