Initial growth of 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) on Au(1 1 1): A scanning tunneling microscopy study

The structure and morphology of 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) films deposited on Au(1 1 1) surface in ultrahigh vacuum is studied via scanning tunneling microscopy (STM). In the first monolayer, PTCDA molecules form well-ordered domains with two distinct structural phases. The first phase is characterized by a `herring-bone' arrangement of PTCDA molecules similar to that of a bulk PTCDA molecular crystal. The second phase, which had not been reported so far, is characterized by a molecular arrangement in a square pattern that does not have a known bulk analog. This second phase is less dense than the first. Only the `herring-bone' phase is consistently observed in the second and subsequent layers. The crystallographic orientation of the PTCDA layers, with respect to the Au substrate as well as with respect to each other, is directly determined from the STM images. At coverages above 2 monolayers, the formation of PTCDA islands with good molecular order is observed. Transient effects related to STM-induced dynamic changes in the PTCDA molecular layer are also discussed.