Crystalline films of conjugated organic semiconductors offer attractive potential for optoelectronic and electronic applications on flexible substrates. For device applications, the orientation of the molecules with respect to the substrate is crucial. In organic light-emitting diodes and solar cells, a lying orientation between the electrodes is desired whereas organic thin film transistors require in certain configurations that the molecules stand up between source and drain.

Using the small rod-like oligophenylene molecule parasexiphenyl (6P) as an example, it will be demonstrated how the substrate conditions help to tune the molecular orientation [1]. On clean mica(001), the self-organization of crystallites into one-dimensional chains is observed on a wetting layer in which the 6P molecules lie almost flat on the surface [2]. Also the crystallites are composed of lying molecules. On an ion bombarded, amorphous mica surface, the formation of terraced mounds composed of nearly upright standing molecules is observed by atomic-force microscopy(AFM). Quantitative analysis of the mound morphology together with transition state theory calculations reveals the existence of molecule bending during step edge crossing and level dependent Ehrlich Schwoebel barriers [3]. For the same system, the size of the critical nucleus has been determined from island size and capture zone distributions [4] and the subtle interplay of intra- and interlayer diffusion - resulting in elongated hexagonal second-layer islands - has been explored.

Finally, Low Energy Electron Microscopy (LEEM), micro Low Energy Electron Diffraction (µLEED), and AFM have been employed to study the initial growth of 6P on graphene which offers the potential to be used as a transparent flexible electrode. Due to the structural similarity between substrate and 6P one can expect  that the molecules lie flat on the substrate. On Ir(111) supported graphene at 240 K, indeed layer-by-layer growth of lying molecules is observed as it is desired for OLEDs [5]. After formation of a low density layer, the full first monolayer already shows a bulk like structure. Up to at least four monolayers grow in this fashion. The nucleation of the 6P islands occurs at wrinkles in the metal supported graphene layer. Larger islands composed of flat-lying molecules detach from the original nucleation sites and move rapidly as entities across wrinkle free substrate areas [6]. In addition, µLEED reveals the surface unit cells in the different growth stages and at various substrate temperatures. At temperatures above room temperature, again crystalline needles appear on a molecular wetting layer [7] which is in agreement with AFM observations for 6P grown on exfoliated graphene transferred to silicon oxide [8].

This work has been supported by FWF project S9707-N20 and STW and FOM project 04PR2318. Contributions by G. Hlawacek, M. Kratzer, S. Klima, S. Lorbek, Q. Shen, P. Puschnig, D. Nabok, G. Biddau, C. Draxl (Leoben), P. Frank, T. Potocar, A. Winkler (Graz),  F. S. Khokhar, R. van Gastel, B. Poelsema (Enschede), and B. Vasić, R. Gajić (Belgrade) are acknowledged.

[1]G. Hlawacek, C. Teichert, J. Phys.: Condens. Matter 25 (2013) 143202.

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

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

[4] T. Potocar, S. Lorbek, D. Nabok, Q. Shen, L. Tumbek, G. Hlawacek, P. Puschnig, C. Ambrosch-Draxl, C. Teichert, A. Winkler, Phys. Rev. B 83 (2011) 075423.

[5] G. Hlawacek, F. S. Khokhar, R. van Gastel, B. Poelsema, C. Teichert, Nano Lett. 11 (2011) 333.

[6] G. Hlawacek, F. S. Khokar, R. van Gastel, C. Teichert, B. Poelsema, IBM J. Res. Devel. 55 (2011) 15:1.

[7] F. S. Khokhar, G. Hlawacek, R. van Gastel, H.J.W. Zandvliet, C. Teichert, B. Poelsema, Surf. Sci. 606 (2012) 47.

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