Self-organization of phthalocyanines on Al 2 O 3 (1120) in aligned and ordered films
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J.O. Osso´ Max-Planck-Institut fu¨r Metallforschung, 70569 Stuttgart, Germany, and Institut de Cie`ncia de Materials de Barcelona - CSIC, Esfera UAB, 08193 Bellaterra, Spain
F. Schreiber Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford OX13QZ, United Kingdom
M. Garriga and M.I. Alonso Institut de Cie`ncia de Materials de Barcelona - CSIC, Esfera UAB, 08193 Bellaterra, Spain
H. Dosch1 Max-Planck-Institut fu¨r Metallforschung, 70569 Stuttgart, Germany, and Theoretische und Angewandte Physik, Universita¨t Stuttgart, 70550 Stuttgart, Germany (Received 15 December 2003; accepted 8 March 2004)
We studied the self-organization process of F16CuPc films (20–30 ML) on stepped Al2O3 (112¯0) substrates. X-ray diffraction measurements revealed a highly ordered layered structure with the molecules in a nearly upright configuration. The morphology, investigated by atomic force microscopy, consisted of long (several microns) and narrow (20–100 nm) needlelike terraces unidirectionally aligned along one of the main crystallographic directions of the Al2O3 (112¯0) surface. High resolution atomic force microscopy images revealed in-plane molecular order with the molecular stacking direction parallel to the needlelike terraces. Such anisotropic morphology is the result of a self-organization process of F16CuPc in elongated crystallites driven to a preferential orientation by the interaction with the substrate. Spectroscopic ellipsometry showed that these films exhibit anisotropic optical properties correlated with the molecular arrangement.
I. INTRODUCTION
In the last decade it has been shown that organic molecules with extended systems are a versatile and attractive alternative for the fabrication of electronic and optical devices. For the optimization of the electronic and optical properties, the control of growth and structure of the molecular assemblies is required.1–4 One important advance has been the use of organic molecular beam epitaxy (OMBE) as a means to achieve control over the growth of organic films with extremely high chemical purity.1 One of the key factors for the growth of ordered films is the interaction between substrate and molecules. This interaction provides a mechanism to order the first layers in correlation with the surface lattice. Thus, under the appropriate growth conditions, long-range ordered molecular monolayers can be successfully obtained.5–7 a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0258 J. Mater. Res., Vol. 19, No. 7, Jul 2004
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With increasing film thickness, additional difficulties of achieving highly ordered molecular films can arise. Typical problems are the formation of polymorphic structures, roughening and island growth.7–13 For technological applications the formation of thin films with both, smooth surfaces and interfaces as well as high crystallinity is often desirable. Even when a highly ordered structure perpendicular to the surface can be obtained,
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