Crystalline Growth of Rubrene Film Enhanced by Vertical
Ordering in Cadmium Arachidate
Multilayer Substrate
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Abstract
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 SiO<sub>2</sub>/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<sup>–2</sup> cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>, presumably limited by the roughness of the interface
between CdA and rubrene films