The optical properties and the molecular orientation in thin films of 3,4,9,10perylenetetracarboxylic dianhydride (PTCDA) and N, N-dimethyl-3,4,9,10perylenetetracarboxylic diimide (DiMePTCDI) were studied by means of variable angle spectroscopic ellipsometry (VASE), atomic force microscopy (AFM), near edge x-ray absorption fine structure (NEXAFS) spectroscopy, and infrared (IR) and Raman spectroscopy. VASE reveals that both kinds of film exhibit a strong optical anisotropy. For PTCDA, the optical constants are found to have much higher values in the substrate plane than perpendicular to it. While the anisotropy measured in the substrate plane on passivated GaAs(100) is very small for PTCDA a giant anisotropy is observed for DiMePTCDI. This difference in the optical properties is attributed to the different orientation of molecules in the thin organic films. While the PTCDA molecules lie flat on the substrate with their molecular plane parallel to the substrate surface, the DiMePTCDI molecules are tilted with respect to the substrate surface and are predominantly oriented with their long axis parallel to the [011] direction of the substrate as confirmed by VASE, NEXAFS, and Raman and IR results.

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Growth of ordered organic films of 3, 4, 9, 10-perylene-tetracarboxylic-dianhydride (PTCDA), on inorganic substrates of GaAs (001) is investigated by means of low-energy electron diffraction, scanning tunnelling microscopy and atomic force microscopy. The passivation of the sample can be achieved by exposing the substrate to sulphur (the SnS 2 compound) or wet-chemical etching using an S-containing etchant (S 2 Cl 2 ). The sulphur-treated surfaces are less reactive and are suitable as substrates for the growth of epitaxial films. In this work we have compared the growth of organic molecules on substrates prepared by two different methods. The samples prepared by molecular beam epitaxy (MBE) present smoother surfaces compared with chemically treated samples. Therefore, an improvement of the PTCDA molecular order is observed for the MBE samples, which exhibit the formation of crystals.