Observation of competing modes in the growth of diindenoperylene on SiO2

Diindenoperylene (DIP) thin films deposited on SiO2-surfaces by organic molecular beam deposition in ultra-high vacuum at room temperature have been investigated by atomic-force-microscopy and X-ray scattering as a function of film thickness. We observe the evolution of two distinct molecular orientations; the long axis of the DIP molecule is pointing either along the surface normal (σ-orientation) or along the surface (λ-orientation). Initially, the films exhibit predominantly σ-orientated islands. Increasing the film thickness, elongated, fibrous λ-orientated islands nucleate on top of the σ-oriented islands and dominate the growth front due to their fast 3D growth mode. Upon annealing at 150 °C, the fibers disappear. We argue that these observations can be understood as a competition between surface energy (favoring the σ-orientation) and growth kinetics (favoring the λ-orientation).