Growth and characterization of tris(8-hydroxyquinoline)-aluminum molecular films

Abstract

Various tris(8-hydroxyquinoline)-aluminum (Alq3) molecular solid films were grown on top of indium-tin-oxide (ITO) glass substrates using physical vapor deposition. The effect of changing the growth conditions on the properties of the films was studied. From scanning electron microscopy, an Alq3 planar layer over an ITO-substrate was observed at the initial period, and an Alq3 tubular structure (which becomes dominant at substrate temperature T sub ≧ 90 °C) was found to nucleate from this layer. From X-ray diffraction, the Alq3 planar layer possesses an amorphous character while the Alq3 tubular layer has a triclinic α-phase structure. Based on an Arrhenius plot of the growth rate versus 1/ T sub, the growth behaviors in various T sub-regions were discussed to be dominated by adhesion (for T sub < 90 °C), steric effect (90 °C < T sub < 150 °C), and re-evaporation ( T sub > 150 °C). Then, from optical transmission and photoluminescence spectra performed on the high crystalline Alq3 films, two signals associated with the optical-bandgap E g absorption and the gap-state absorption were determined and discussed in terms of the optical properties of the constituent Alq3 molecules. Finally, from a fit of E g( T) by an effective electron–phonon interaction model, the physical significance of these fitting parameters for the Alq3 molecular solid was investigated.