Highly ordered thin films of perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA) on Ag(1 0 0)

We report experiments performed on vacuum deposited perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA) films on Ag(1 0 0) by spot profile analysis low energy electron diffraction (SPA-LEED) and scanning tunnelling microscopy (STM). Up to a coverage of 1 ML, PTCDA forms a commensurate c(8 × 8) structure with two molecules per primitive unit cell, which are perpendicularly oriented to each other (four molecules per c(8 × 8) unit cell). The PTCDA islands exhibit an equilibrium shape that corresponds to the symmetry of the Ag(1 0 0) surface. The structural order is stable up to a temperature of ∼670 K, at which a phase transition to a disordered phase occurs. Upon cooling back to room temperature, the order is partly re-obtained, although there is evidence that molecules have partly desorbed and dissociated at the high temperature. At coverages above 1 ML, a second layer orders in a herringbone structure on top of the c(8 × 8)-ordered first layer. The corresponding unit cell is very similar to that of the (1 0 2) plane of PTCDA bulk crystals.