CTR analysis of Rubrene single crystal thin films

Rubrene (5,6,11,12-tetraphenylnaphthacene, C₄₂H₂₈) is a red colored polycyclic aromatic hydrocarbon. As an organic semiconductor, the most promising application is in organic light-emitting diodes (OLEDs) and organic field-effect transistors, which are the core elements of flexible displays. Recently, Professor Y. Wakabayashi (Osaka University, Japan) and his colleagues have studied the near surface structure of Rubrene single crystal by crystal truncation rod (CTR) scattering, which gives a modulated profile in the tail of a series of Bragg peaks (0 0 z). The research group employed coherent Bragg rod analysis (COBRA) rather than conventional curve fitting analysis to determine the electron density profile along the depth. The analysis has shown that the molecules at the surface are slightly expanded along the surface normal direction, while the second or deeper molecular layers are not affected by the existence of the surface. Their research can be extended to applications of other similar organic semiconductors. For more information, see the paper, "Sub-Å Resolution Electron Density Analysis of the Surface of Organic Rubrene Crystals", Y. Wakabayashi et al., Phys. Rev. Lett. 104, 066103 (2010). For information on COBRA, see, for example, "Direct determination of epitaxial interface structure in Gd₂O₃ passivation of GaAs", Y. Yacoby et al., Nature Materials 1, 99 (2002).