Highly ordered thin films of 5,5′′-bis(3′-fluoro-biphenyl-4-yl)-2,2′ : 5′,2′′-terthiophene with two meso-phases

Abstract

The growth process and phase state of 5,5′′-bis(3′-fluoro-biphenyl-4-yl)-2,2′ : 5′,2′′- terthiophene (m-F2BP3T) thin films were investigated by atomic force microscopy (AFM), in-plane and out-of-plane X-ray diffraction (XRD), and selected area electron diffraction (SAED). Two meso-phases (thin film phases) of m-F2BP3T films on SiO₂ surface were obtained in the early stages. The m-F2BP3T films initially exhibited two-dimensional (2D) layers (≤4 ML) followed by three-dimensional (3D) island growth. The film structure evolved two thin film phases in the first four layers and the bulk phase was formed from the fifth layer, which occurred concomitantly with the change of the growth mode. Moreover, the variation of weak epitaxy growth behavior of ZnPc from 2D to 3D growth further reflects that the phase state of the first three layers is different from that of the fourth layer, in spite of ZnPc crystals showing just one orientation corresponding to commensurate epitaxy. The novel phase behavior is closely related to the synergistic effects of the outstanding soft matter properties, limited elasticity of organic molecules, and strain originating from the SiO₂ substrate. This study investigates novel phase behavior in organic thin films and provides significant insight into the mechanism of the phase transition.