Crystalline films of conjugated organic semiconductors offer attractive potential for optoelectronic and electronic applications on flexible substrates. Due to the complexity and anisotropy of the molecular building blocks, novel growth mechanisms can occur as is demonstrated for the growth of the rod-like oligophenylene molecule parasexiphenyl (6P) on mica surfaces. On clean mica(001), the self-organization of crystallites into one-dimensional chains is observed on a wetting layer where the 6P molecules lie flat with their long molecular axis parallel to the surface [1].

Here, we demonstrate by atomic force microscopy that on an ion bombarded mica surface, the formation of terraced mounds composed by almost upright standing molecules is observed. In inorganic growth systems such a mound morphology is frequently due to a kinetic effect, the so-called Ehrlich Schwoebel barrier for step-edge crossing [2]. Quantitative analysis of the mound morphology together with transition state theory calculations revealed the existence of molecule bending during step edge crossing and level dependent step edge barriers [3]. A lower barrier due to a larger molecular tilt angle (with respect to the surface normal) in the first layer results in the completion of one monolayer before mound formation starts. This is convincingly demonstrated by transverse shear microscopy measurements.

By temperature and rate dependent growth experiments we also determined the size of the critical nucleus to be significantly larger than one. These findings are again complemented by force-field calculations revealing the size of an energetically stable island of upright standing molecules.

Our analysis shows that procedures developed and verified for inorganic systems [2] can be successfully applied to organic thin film growth. However, we have also demonstrated that the complexity and anisotropy of the molecular building blocks lead to additional effects [3] that are not observed in atomic inorganic growth systems.

[1] C. Teichert G. Hlawacek, A. Andreev, H. Sitter, P. Frank, A. Winkler, N.S. Sariciftci, Appl. Phys. A 82 (2006) 665.

[2] T. Michely and J. Krug; Islands, Mounds and Atoms (Springer, Berlin 2004).

[3] G. Hlawacek, P. Puschnig, P. Frank, A. Winkler, C. Ambrosch-Draxl, C. Teichert, Science 321 (2008) 108.

  This work has been funded by Austrian Science Fund (FWF) within NFN “Organic Thin Films” Projects S9707 + S9714 as well as P19197.