Effect of Heat-Treatment and Composition on Structure and Luminescence Properties of Spinel-Type Solid Solution Nanocrystals

The compositional dependence of the structure and properties of spinel-type solid solutions, Zn(Al,Ga)₂O₄ was investigated by comparison with samples hydrothermally prepared and those after heat treatment at 1000 °C in air. Nanocrystalline spinel-type solid solutions in the whole composition range in the ZnAl₂O₄–ZnGa₂O₄ system were directly formed from the aqueous precursor solutions of ZnSO₄, Al(NO₃)₃ and Ga(NO₃)₃ under hydrothermal conditions at 180 °C for 5 h in the presence of tetramethylammonium hydroxide. The incorporation of aluminum into the lattice, Zn(Al _x Ga_1–x )₂O₄, resulted in lower crystallinity of the spinel. The relationship between the lattice parameter of as-prepared samples and the Al atomic ratio in the spinel composition was slightly apart from the ideal linear relationship that was obtained in the samples after heat treatment at 1000 °C. The optical band gap of both as-prepared solid solutions and those heat treated linearly increased from 4.1∼4.2 to 5.25 eV by the incorporation of aluminum ion into the lattice, Zn(Al _x Ga_1–x )₂O₄. Two main broad-band emission spectra centered at around 360 and 430 nm in the range of 300∼600 nm were observed in the spinel solid solutions under excitation at 270 nm, thought their broad-band emission spectra and their peak wavelengths subtly changed depending on the composition and heat treatment.