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 (sigma-orientation) or along the surface (lambda-orientation). Initially, the films exhibit predominantly sigma-orientated islands. Increasing the film thickness, elongated, fibrous lambda-orientated islands nucleate on top of the sigma-oriented islands and dominate the growth front due to their fast 3D growth mode. Upon annealing at 150 degrees C, the fibers disappear. We argue that these observations can be understood as a competition between surface energy (favoring the (sigma-orientation) and growth kinetics (favoring the lambda-orientation). (c) 2005 Elsevier B.V. All rights reserved.