Re-condensation and decomposition of Tris(8-hydroxyquinoline)-aluminum in a vapor transport ampoule

By choosing various crystalline forms of tris(8-hydroxyquinoline)-aluminum (Alq ₃) source powders and using different transport gases under various inside-pressures p _in, two constitutional supersaturation (CSS) zones were observed in a vapor transport ampoule with a source temperature of 300 °C and a growth period of 5.0 h. From UV-visible spectra, the material in the lower CSS zone (with sidewall temperature T _side≅ 130-270 °C) was the re-condensed Alq ₃, while that in the upper zone (T _side around room temperature) was the decomposed 8-hydroxyquinoline (8Hq). With increasing p _in from 1.3×10 ² to 9.3×10 ⁴ Pa, the decomposition of Alq ₃ into 8Hq was enhanced, and the deposition of 8Hq CSS zone became apparent with the formation of several 8Hq needle crystals. In addition, a dependence of the crystal structure formed in the Alq ₃ CSS zone on the initial phase of the source powder was observed and discussed.