Utilizing Energy Transfer in Mn2+/Ho3+/Yb3+ Tri-doped ZnAl2O4 Nanophosphors for Tunable Luminescence and Highly Sensitive Visual Cryogenic Thermometry

Lanthanide (Ln(3+))-doped upconversion (UC)phosphorsconverting near-infrared (NIR) light to visible light hold very highpromise toward biomedical applications. The scientific findings onluminescent thermometers revealed their superiority for noninvasivethermal sensing. However, only few reports showcase their potentialfor applications in extreme conditions (temperatures below -70 & DEG;C) restricted by low thermal sensitivity. Here, we demonstratethe tailoring of luminescence properties via introducing Ho3+-Mn2+ energy transfer (ET) routes with judiciouscodoping of Mn2+ ions in ZnAl2O4/Ho3+,Yb3+ phosphor. Preferentially, a singular redUC emission is required to improve the bioimaging sensitivity andminimize tissue damage. We could attain UC emission with 94% red componentby a two-photon UC process. Higher temperature annealing brings thecolor coordinates to the green domain, highlighting the potentialfor color-tunable luminescence switch. Moreover, this work investigatesthe thermometric properties of ZnAl2O4/Yb3+, Ho3+ in the range of 80-300 K and influenceof inducing extra ET pathways by Mn2+ codoping. Interestingly,the luminescence intensities for nonthermally coupled (F-5(4),S-5(2)) and the F-5(5) radiative transitions of Ho3+ ions display oppositebehavior at 80 and 300 K, which revealed competition between temperature-sensitivedecay pathways. The codoping of Mn2+ ions is fruitful incausing a fourfold increase of absolute sensitivity. Notably, thecolor tunability from green through yellow to red is helpful in roughtemperature estimation by naked eyes. The maximum relative (S (r)) and absolute sensitivities (S (a)) were estimated to be 1.89% K-1 (140K) and 0.0734 K-1 (300 K), respectively. Even at80 K, a S (a) of 0.00447 K-1 and S (r) of0.6025% K-1 were achievable in our case, which arehigher than most of the other Ln(3+)-based systems. The above-mentionedresults demonstrate the potential of ZnAl2O4/Yb3+,Ho3+ for cryogenic optical thermometryand a strategy to design new Ln(3+)-based UC thermometersby taking advantage of ET routes.

Automated summary

In this study, the luminescence properties of ZnAl2O4/Ho3+, Yb3+ phosphor were tailored by introducing Ho3+-Mn2+ energy transfer routes. The luminescence intensities of Ho3+ ions showed opposite behavior at 80K and 300K, indicating competition between temperature-sensitive decay pathways. The codoping of Mn2+ ions increased the absolute sensitivity by fourfold and allowed for color tunability from green to yellow to red, enabling rough temperature estimation by naked eyes.