Optical properties and molecular orientation in organic thin films

Abstract

The optical properties and the molecular orientation in thin films of3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and -dimethyl-3,4,9,10-perylenetetracarboxylic diimide (DiMePTCDI) were studied by means ofvariable angle spectroscopic ellipsometry (VASE), atomic force microscopy (AFM), nearedge x-ray absorption fine structure (NEXAFS) spectroscopy, and infrared (IR) and Ramanspectroscopy. VASE reveals that both kinds of film exhibit a strong optical anisotropy. ForPTCDA, the optical constants are found to have much higher values in the substrate planethan perpendicular to it. While the anisotropy measured in the substrate plane onpassivated GaAs(100) is very small for PTCDA a giant anisotropy is observed forDiMePTCDI. This difference in the optical properties is attributed to the differentorientation of molecules in the thin organic films. While the PTCDA molecules lie flaton the substrate with their molecular plane parallel to the substrate surface,the DiMePTCDI molecules are tilted with respect to the substrate surface andare predominantly oriented with their long axis parallel to the [011] directionof the substrate as confirmed by VASE, NEXAFS, and Raman and IR results.