UV/blue/green converted efficient red-NIR photoluminescence in Cr incorporated MgAl2O4 nanocrystals: Site selective emission tailored through cation inversion and intrinsic defects

Abstract

The UV/visible activated near-infrared (NIR) phosphors have many applications in solid state lighting, night vision devices and bio-imaging. The early research reported the red-NIR emitting phosphors doped with Cr3+ centers upon visible light excitation. Here, in this work the intense red-NIR emission and color tuning is achieved for broad excitation range (UV/blue/green) through Cr dopant induced defect centers and cation inversion in MgCr Al2O4 ( = 0.5, 1, 3, 5 and 10 mol%) nanocrystals. The Cr3+ doped MgAl2O4 nanocrystals were synthesized by combustion method through stoichiometric substitution of Mg by Cr, while most of the Cr3+ ions occupied the octahedral sites of spinel host with the formation of antisite defects, Cr3+ clusters, magnesium and oxygen vacancies. These defect centers were probed through Rietveld refinement, photoluminescence (PL), x-ray photoelectron and nuclear magnetic resonance spectra analyses. At UV excitation, the intrinsic defects played an interesting role in exhibiting the blue-violet emission attributed to host lattice defects and red-NIR emission attributed to strong/weak ligand field octahedral Cr3+ sites, via charge transfer to Cr3+ ions. The PL spectra evinced the enhanced red-NIR emission intensity upon 266 nm excitation than upon blue and green light excitation. Further, the weak ligand field site emission is found to be dominating with increase in doping concentration. Thus, Cr doped MgAl2O4 nanocrystals showed their potency of exhibiting the intense red-NIR emission and color tuning (from red purple to bluish purple and then to red color) upon UV/blue/green excitation.