Simulation and AFM-Measurement Of PTCDA On Ag-supported KBr Films

On a partially KBr-covered Ag(111) substrate thin films of PTCDA molecules exhibit different growth modes, which depend on the thickness of the KBr layer. If no KBr is present, PTCDA form large islands with a herringbone-type substructure; on 1 ML KBr many small, not well aligned PTCDA islands with a large size distribution are formed. On 2ML KBr, again large flat PTCDA clusters are observed, which exhibit a pile-up of PTCDA molecules at the edges of the KBr islands. These growth modes have been analysed with an anisotropic Ising model for a square lattice including next-nearest neighbour interactions. The coupling constants and the adsorbate-substrate interaction strength were derived from electronic-strcuture calculations. The simulations give evidence that large clusters are formed if either the adsorbate-substrate interactions (no KBr) or the lateral inter-molecular interactions (2 ML KBr) dominate. In the intermediate case of 1 ML KBr on Ag(111), both mechanisms compete and a high number of cluster seeds is generated. The presence of the surface steps between regions of different KBr layer thickness enhances this effect.