In situ studies of morphology, strain, and growth modes of a molecular organic thin film

We use grazing incidence x-ray scattering to study the molecular structure and morphology of thin (<70 ML) crystalline films of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on Au(111) surfaces as a function of film thickness, substrate temperature, and growth rate. Although the first two PTCDA monolayers grow in a layer-by-layer fashion, the film evolution beyond the second monolayer depends strongly upon the growth conditions resulting in low-temperature [i.e., nonequilibrium (NEQ)] and high-temperature [equilibrium (EQ)] growth regimes. In the NEQ regime, the films roughen monotonically with increasing film thickness, but retain a well-defined film thickness. Furthermore, we find that these films have a lattice strain which is independent of film thickness. In the EQ regime, the film acquires a three-dimensional morphology for thicknesses >2 ML, and the lattice strain decreases rapidly with increasing thickness. We also show that the transition between the NEQ and EQ regimes is sharp and depends upon the balance between the growth rate and substrate temperature. These results suggest that the PTCDA/Au(111) system is thermodynamically described by incomplete wetting, and that strain and kinetics play an important role in determining molecular organic film characteristics.