Phase stability and transformation of the α to ε-phase of Alq3 phosphor after thermal treatment and their photo-physical properties

In this study, we analyzed changes in the phases of aluminum tris 8-hydroxyquinoline (Alq3) after thermal treatment at different temperatures and their photo-physical properties. We prepared α-Alq3 phosphor via the co-precipitation method. Improvements in the phase purity due to stagnation of the α-phase and the transformation from α to ε-Alq3 were achieved by thermal treatment in an Ar atmosphere. The initial formation, stagnation, improvement in the phase purity, and conversion to ε-Alq3 were confirmed by X-ray diffraction (XRD) analysis. The XRD results were also validated by Fourier-transform infrared spectroscopy and Nuclear magnetic resonance spectroscopy. Ultraviolet-visible (UV-Vis) absorption spectroscopy was conducted in the presence of acidic and basic media at concentrations of 10−6 M to 10−3 M, respectively. The modifications in the UV-Vis absorption spectra indicated changes in the band gap energy (Eg) after thermal treatment. The variations in Eg for α and ε-Alq3 supported the stagnation and transformation of the phase. Photoluminescence (PL) analysis of the as-prepared α-Alq3 determined a λemi maximum at 516 nm. A minor blue shift of Δλ = 2 nm was observed as the PL intensity increased for the annealed α-Alq3. A large blue shift of Δλ = 18 nm as the PL intensity decreased was due to the change in phase from α to ε-Alq3. PL study of the α and ε-Alq3 phases in acidic solvent detected blue shifts, whereas red shifts occurred in the basic solvents due to variations in their dielectric constants. The mechanism related to the solvatochromatic effect on the shifts in PL emission was also determined in this study. Thermogravimetric analysis was employed to determine the thermal stability of the prepared phosphors.