Scanning Tunneling Microscopy investigation of multilayer diF-TES-ADT on Au(111)

Organic thin film transistors (OTFT) partially composed of solution processed 2,8-difluoro-5,11-bis(triethylsilylethynyl)-anthradithiophene (diF-TES-ADT) have shown high performance with hole mobilities up to 1 cm$^{2}$/(V s). Pretreatment of the gold electrodes results in growth of large diF-TES-ADT crystals extending well out into the channel of the OTFT. Without electrode pretreatment, the crystal sizes are small and possess a non-preferred molecular orientation. We have chosen to investigate the reasons for the reduced crystal size of these films on untreated gold electrodes by studying a model system generated by vapor deposition of multilayers of diF-TES-ADT on Au(111). The initial wetting layer forms a highly ordered film such that the anthradithiophene backbone is oriented parallel to the substrate and the unit cell is 1.49 nm x 1.25 nm with an included angle of 56.8$^{\circ}$. The second layer is poorly ordered with only weak evidence of crystallinity in small regions. Growth beyond the second layer appears essentially bulk-like and crystalline with domain sizes that are potentially limited by the disordered bilayer growth.