The initial growth of PTCDA on Cu(111) studied by STM

The initial growth of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) was analysed. Ultrathin films with coverages of up to two layers were prepared on a ( 111) orientated copper single crystal by means of vapour deposition in an ultrahigh-vacuum chamber. The films were characterized in situ by scanning tunnelling microscopy (STM). Within the first layer, two different structures were found. Both exhibit a herringbone-like arrangement of the molecules, which is also found in the (102) plane of the alpha and beta bulk phases. The two-dimensional unit cell is given by two molecules which are rotated by about 90 degrees. As an effect of the interaction with the substrate, a voltage-dependent moire pattern was observed for one of these phases. For the second layer, a herringbone phase was found that is denser than the phases of the first layer but less dense than the bulk phases.