Highly Efficient La₂O₃:Yb³⁺,Tm³⁺ Single-Band NIR-to-NIR Upconverting Microcrystals for Anti-Counterfeiting Applications

Efficient single-band NIR-to-NIR upconversion (UC) emission is strongly desired for many applications such as fluorescent markers, plastic recycling, and biological imaging. Herein, we report highly efficient single-band NIR-to-NIR UC emission in La2O3:Yb3+,Tm3+ (LYT) microcrystals. Under 980 nm laser excitation, LYT exhibits a NIR UC emission at ∼795 nm (Tm3+: 3H4 → 3H6) and blue UC emission at ∼476 nm; the NIR UC emission is dominant, with the intensity ratio of the NIR to blue INIR/Iblue > 100. Remarkably, a high absolute UC quantum yield (UCQY) of 3.4% is obtained for the single-band NIR UC emission of LYT at a relatively low excitation power density of 7.6 W/cm2. This value is much higher than the reported values of a single-band NIR UC for rare-earth-based UC materials in literature, such as the well-known benchmark UC materials of β-NaYF4:Yb3+,Er3+ (∼0.9%, with a excitation power density of 9 W/cm2) and Gd2O2S:Yb3+,Er3+ (∼1.9%, with a excitation power density of 20 W/cm2). The high absolute UCQY of single-band NIR UC emission combined with their facile preparation hints at their potential application in anti-counterfeiting, verified by the proof-of-concept demonstration of fluorescent labeling of a transparent IMT pattern.