Characterization of facial and meridional Alq₃ thin films fabricated from physical vapor transport at high substrate temperatures

During a thermal annealing process, the isomeric transformation from meridional (mer) into facial (fac) tris(8-hydroxyquinoline)-aluminum (Alq₃) based on the infrared absorption was observed to be nearly complete around 375 °C. Then, in a physical vapor transport process with post-annealed fac-Alq3 as source powder (at source temperature of 375 °C), the effects from various substrate temperatures (T_sub from 285 to 346 °C) on the transformation from fac-Alq₃ into mer-Alq₃ isomers re-condensed on the substrate were studied again, and an isomeric transformation occurs at T_sub = 316 ± 2 °C. For T_sub below 316 ± 2 °C, the re-condensed mer-Alq₃ isomers tend to aggregate to form close-packed micron-rods, which are composed of α-phase crystallites with an average coherent length L ∼ 46 nm. The photoluminescence (PL) spectra for these as-fabricated mer-Alq₃ films show typical green emission. However, for T_sub above 316 ± 2 °C, the adsorbed fac-Alq₃ molecules on the substrate preserve their corresponding isomeric structure from the post-annealed fac-Alq₃ source and self-organize into flexible submicron-fibers. These samples are dominated by δ-phase crystallites with L ∼ 57 nm. The PL spectra for these as-fabricated fac-Alq₃ films do emit blue luminescence. Besides, Raman scattering for these mer-Alq₃ and fac-Alq₃ molecules are also compared.