UV/blue/green converted efficient red-NIR photoluminescence in Cr incorporated MgAl₂O₄nanocrystals: Site selective...

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 inversionWe present the infuence of Cr dopant induced defect centers and cation inversion in Mg_1-xCr_xAl₂O₄(x= 0.5, 1, 3, 5 and 10 mol%) nanocrystals. The Cr³⁺doped MgAl₂O₄nanocrystals 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, 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 MgAl₂O₄nanocrystals 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.