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

Abstract During a thermal annealing process, the isomeric transformation from meridional ( mer ) into facial ( fac ) tris(8-hydroxyquinoline)-aluminum (Alq 3 ) 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 -Alq 3 into mer -Alq 3 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 -Alq 3 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 -Alq 3 films show typical green emission. However, for T sub above 316 ± 2 °C, the adsorbed fac -Alq 3 molecules on the substrate preserve their corresponding isomeric structure from the post-annealed fac -Alq 3 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 -Alq 3 films do emit blue luminescence. Besides, Raman scattering for these mer -Alq 3 and fac -Alq 3 molecules are also compared.