Narrow Ultraviolet and Broad Tunable Near-Infrared Dual Emissions from Gd3+-Doped Mgal2o4

Abstract

Narrow Ultraviolet (UV) and broad tunable near-infrared (NIR) dual-band emitting single-phase phosphors are highly advantageous due to their multifunctionality. However, their development, especially in single material systems, remains difficult due to the complexity of achieving multiple luminescent properties through single metal ion doping and defect engineering. Engineering dual-modal emissions adds further scientific significance and technological challenge. In this study, we report narrow-band UV-B and broadband tunable NIR emissions from Gd-doped MgAl2O4 phosphors, synthesized via a co-precipitation and molten salt method. Their luminescent properties have been investigated through down-conversion across UV, visible, and NIR regions. Excited at 275 nm, their emission spectra display two distinct bands: a sharp UV-B band at 314 nm and a broad intense NIR emission from 600–850 nm peaking at ~730 nm. The NIR emission can be red-shifted and tunable when excited at longer excitation wavelengths, with peak maxima observed at 770 and 790 nm upon excitation at 312 and 390 nm, respectively. Additionally, visible-to-UV-B and UV-A up-conversion emissions with peaks around 290, 302, 330, and 350 nm have also been observed upon excitations at 390 and 450 nm, attributed to defects such as oxygen vacancies and interstitial oxygen, which create intermediate bands enabling two-photon energy transfer to Gd3+. DFT-based calculations have identified mid-gap states created by defects like VAl3-, VMg2-, VO+, VO2+ and Oi2-, correlating with the observed emissions. These findings provide new insights into the defect-related mechanisms governing both down-conversion and up-conversion luminescence in Gd-doped MgAl2O4 phosphors. This study excites researchers to explore dual emitting single-phase phosphors across UV and NIR ranges.

Keywords: Defect, Narrow-band UVB, Tunable NIR, Up-conversion