Island size evolution and molecular diffusion during growth of organic thin films followed by time-resolved specular and off-specular scattering

We report on a combined off-specular and specular x-ray scattering growth study of ultrathin films of the prototypical organic semiconductor diindenoperylene (DIP, C32H16). We investigate the evolution of the in-plane correlation length and the growth kinetics of the films including their dependence on the substrate temperature and the growth rate. We observe a temperature-dependent collective rearrangement of DIP molecules from a transient surface induced to the thin-film phase, which can be rationalized by incorporating a thickness-dependent out-of-plane lattice parameter. We further observe that the nucleation behavior of DIP changes from the first to the second monolayer, which we relate to a difference in the diffusion length of the molecules.