Rubrene Microcrystals: A Route to Investigate Surface Morphology and Bulk Anisotropies of Organic Semiconductors

Rubrene single crystals were grown by deposition near thermal equilibrium conditions which yields distinct microcrystals with sizes up to 100 μm. Such crystallites are rather spherical and exhibit various low index surfaces which enable detailed surface studies as well as directional-dependent bulk measurements. The analysis of the crystal habit reveals characteristic geometries for the confining surfaces which offer an unambiguous identification of their orientation. Atomic force microscopy (AFM) measurements demonstrate that the surfaces of the crystallites are ultraflat with a very low density of defects which is accompanied by a pronounced stability against oxidation at ambient conditions as inferred from laser desorption ionization time-of-flight mass spectroscopy (LDI-TOF MS). To exemplify the ability of such crystals to investigate bulk anisotropies, the refractive index tensor of rubrene was precisely determined at a wavelength of λ = 589 nm by employing the Becke line method.