Diffusion-limited island decay of PTCDA on Ag(100): Determination of the intermolecular interaction

The coarsening of two-dimensional islands of the model molecule perylene-3,4,9,10-tetracarboxylic acid dianhydride adsorbed on the Ag(100) surface was investigated by scanning tunneling microscopy. The islands have a quadratic equilibrium shape with rounded corners. The decay curves (island area versus time) can be understood by the classical continuum model according to the Ostwald theory. In the limit of well-separated islands, the asymptotic power law (t^α) exhibits an exponent α=0.67±0.05. This reveals that the coarsening is diffusion limited and that no interfacial barrier exists between island edge sites and terrace sites. From numerical simulations on an ensemble of islands, using the mean-field approach, we determine the average line tension of the island edge at 51±5 meV per molecule. Here from we deduce a next-neighbor intermolecular interaction of 102±10 meV.