Structure of 3,4,9,10-perylene-tetracarboxylic-dianhydride grown on reconstructed and unreconstructed Au(100)

The growth of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) has been characterized on Au(100) from the submonolayer range to multilayer films by scanning tunneling microscopy (STM) in ultrahigh vacuum (UHV) and under ambient conditions. Two different structures of PTCDA are found, one of which is the well-known herringbone structure, with the plane of the molecule flat-lying on the substrate surface. This structure is point-on-line coincident with the Au(100) lattice. The second structure which is predominant on this surface consists of closely spaced rods, each rod consisting of closely packed PTCDA molecules. We propose that this structure is associated with the (010) bulk plane of the α-polymorph of PTCDA. The reason for the different growth behavior compared to PTCDA on Au(111) at the very same growth conditions can be found in the nanoscopic structure of the reconstructed Au(100)_hex surface. This reconstruction of the Au(100) surface provides trenches along the Au[011] azimuth which act as a specific adsorption site for PTCDA, leading to very low diffusion rates of this molecule following its deposition. The diffusion rate of first-deposited PTCDA was low enough to enable us to image single molecules at room temperature. These findings, in comparison with results on the reconstructed Au(111) surface, show that not only the substrate material and the symmetry, but also the surface structure on the nanoscopic scale can rule the growth of the organic overlayer.