Crystalline Stages Of Rubrene Films Probed By Raman Spectroscopy

Abstract

We report on ex situ Raman characterization of rubrene thin films grown by hotwall epitaxy on cleaved mica substrates. Analysis of the vibrational bands revealed that at earliest growth stages the film is amorphous. In particular, a broad band at 1373 cm⁻¹ proves the amorphous nature of the film. The rubrene molecules in amorphous phase are geometrically distorted, since the appearance of the Raman band at 1606 cm⁻¹ is only infrared active for rubrene molecules with the C_2h symmetry group. Further growth leads to seeding of spherulites in the amorphous matrix and further to their coalescence. Raman bands from isolated spherulites embedded in an amorphous matrix and from coalesced spherulites show polarization dependence (depolarization ratio < 0.6), thus demonstrating their crystalline nature. It is also found that the breathing mode (1003 cm⁻¹) represents the rubrene fingerprint feature independent of layer crystallinity.