Enhancement of near‐infrared up‐conversion and blue down‐conversion luminescence in LuNbO₄:Yb³⁺,Tm³⁺ with Ga³⁺ and Ta⁵⁺ substitutions

Abstract

Under a 980-nm excitation, the up-conversion (UC) spectra of LuNbO 4 :Yb 3+ ,Tm 3+ powders exhibited predominantly near-infrared bands (~805 nm) of Tm 3+ through an energy transfer process from Yb 3+ to Tm 3+ . Regarding the down-conversion (DC) luminescence of the powders, the photoluminescence excitation spectra consisted of a broad charge transfer band (270 nm) due to [NbO 4 ] 3− and sharp band (360 nm) of Tm 3+ , while the corresponding emission spectra exhibited a blue emission at 458 nm. Upon substitution of Ga 3+ and Ta 5+ for Lu 3+ and Nb 5+ , respectively, both UC and DC luminescence properties were significantly enhanced. For the Ga 3+ substitution, the increased emission intensity could be explained by the crystal field asymmetry surrounding the Tm 3+ ions induced by the large difference in ionic radius between Ga 3+ and Lu 3+ . For the Ta 5+ substitution, we believe that an M′-LuTaO 4 substructure was formed in the host, which led to the formation of a TaO 6 octahedral coordination instead of a NbO 4 tetrahedral coordination. Consequently, the crystal symmetry of the local structure was modified, and thus the UC and DC luminescence properties were enhanced. The dual-mode (UC and DC) luminescence demonstrates that LuNbO 4 :Yb 3+ ,Tm 3+ has a great potential in the fields of temperature sensing probes, anti-counterfeiting, and bioapplications.