Crystalline Growth of Rubrene Film Enhanced by Vertical Ordering in Cadmium Arachidate Multilayer Substrate

The growth of highly crystalline rubrene thin films for organic field effect transistor (OFET) application remains a challenge. Here, we report on the vapor-deposited growth of rubrene films on the substrates made of cadmium arachidate (CdA) multilayers deposited onto SiO₂/Si­(100) via the Langmuir–Blodgett technique. The CdA films, containing 2<i>n</i>+1 layers, with integer <i>n</i> ranging from 0 to 4, are surface-terminated identically by the methyl group but exhibit the thickness-dependent morphology. The morphology and structure of both CdA and rubrene films are characterized by X-ray reflectivity (XRR), X-ray diffraction (XRD), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and atomic force microscopy (AFM). Crystalline rubrene films, evidenced by XRD and marked by platelet features in AFM images, become observable when grown onto the CdA layer thicker than 5L. XRD data show that vertical ordering, that is, ordering along surface normal, of CdA multilayer substrates exerts a strong influence in promoting the crystalline growth of rubrene films. This promoted growth is not due to the surface energy of CdA layer but derived from the additional interaction localized between rubrene and CdA island sidewall and presumably strengthened by a close dimensional match between the <i>a</i>-axis of rubrene lattice and the layer spacing of CdA multilayer. The best OFET mobility is recorded for 9L CdA substrate and reaches 6.7 × 10^–2 cm² V^–1 s^–1, presumably limited by the roughness of the interface between CdA and rubrene films