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 struc ture and morphology of thin (<70 ML) crystalline films of 3,4,9,10-per ylenetetracarboxylic dianhydride (PTCDA) on Au(111) surfaces as a func tion of film thickness, substrate temperature, and growth rate. Althou gh the first two PTCDA monolayers grow in a layer-by-layer fashion, th e 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 thick ness, but retain a well-defined film thickness. Furthermore, we find t hat these films have a lattice strain which is independent of film thi ckness. In the EQ regime, the film acquires a three-dimensional morpho logy for thicknesses >2 ML, and the lattice strain decreases rapidly w ith 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 wett ing, and that strain and kinetics play an important role in determinin g molecular organic film characteristics.