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

Abstract

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.

Role of molecular conformations in rubrene polycrystalline films growth from vacuum deposition at various substrate temperatures

“…2(e–h). The morphologies of these highly crystalline rubrene grains growth from CSS region with high enough T sub were similar to those reported results for single crystals growth from hot-wall deposition near thermal equilibrium conditions891011.…”

“…Motivated by a high hole mobility1234 and a long exciton diffusion length56 observed in rubrene (C 42 H 28 ) single crystal, an intense research effort has been devoted to the preparation and characterization of various rubrene molecular solid films suitable for applications in organic electronic and optoelectronic devices78910111213141516. Rubrene molecule consists of a tetracene backbone with four phenyl side rings, and each side ring lies in a plane nearly perpendicular to the tetracene backbone.…”

“…The rubrene molecular solids are formed by weak intermolecular interactions, and hence different growth methods can lead to different polymorphs24. The rubrene crystalline grains growth by vapor condensation exhibit a face-centered orthorhombic structure89111524. The high mobility observed in rubrene single crystal12 is attributed to the slipped-cofacial configuration along the long molecular-axis between two adjacent planar rubrene molecules, and this cofacial π-stack interactions significantly enhances the intermolecular charge transfer integral3.…”

Refractive Index Dispersion and Anisotropic Group Refractive Indices of Rubrene Crystals

“…These were determined at 589 nm [25]. Here, their crystal axis assignments were changed to conform to those in Ref.…”

Vibrational modes and changing molecular conformation of perfluororubrene in thin films and solution

“…23 Different single crystal structures formed by rubrene molecules with a planar tetracene backbone were found, for which an orthorhombic packing turned out to be most efficient in terms of charge carrier mobility. [24][25][26][27] For rubrene thin films, a predominance of the energetically favored twisted conformation has been observed directly after growth on silicon substrates with a native oxide layer (SiO 2 ). 28 For the newly synthesized PF-RUB, however, it is not a priori clear which molecular conformation prevails in thin films or crystals.…”