Nanocrystalline MgCrxAl2-xO4: Facile synthesis and thermal dependency of photoluminescence

Abstract

MgCrxAl2-xO4 nanoparticles were synthesized by facile co-precipitation method. The x=0.02 sample showed intense emission and considered for studying the effect of annealing temperature (500–1200°C) on the structural and optical properties. On annealing, crystallinity improved and the material gradually turned pink at 1200°C. Hexagonal sheet-like nanoparticles (∼52nm) were observed by HRTEM and the lattice fringes suggested their single crystalline nature. Excitation spectra showed two peaks at 546 and 419nm where the latter was asymmetric, attributable to the trigonal distortion. The photoluminescence consisted of intense zero-phonon line and associated multi-phonon side bands, whose intensities increased with annealing. Higher Dq/B (3.5), confirmed strong crystal field for Cr3+, suggesting that the photoluminescence emission could be associated with spin-parity forbidden transitions. The emission lifetimes (λem=688nm) ranged between ∼20 and 24ms. Other oxide nanoparticles suitable for phosphors, biomarkers, etc. can be conveniently synthesized similarly.