Tunable concentration-dependent upconversion and downconversion luminescence in NaYF₄: Yb³⁺, Er³⁺@ NaYF₄: Yb³⁺, Nd³⁺...

Lanthanide-doped luminescent nanocrystals display both upconversion luminescence (UCL) and downconversion luminescence (DCL) properties, which offer potential applications in the second near-infrared window (NIR-II) images and biology sensors. Both UCL and DCL are sensitive to concentrations of activators. However, few works reveal the mechanism of concentration-dependent UCL and DCL. Herein, we synthesize core-shell upconversion nanocrystals (UCNCs) NaYF₄: Yb³⁺(20%), Er³⁺ (2%)@NaYF₄: Yb³⁺ (x%), Nd³⁺ (y%) with varying concentration of Nd and Yb ions. The UCL and DCL spectra are recorded under excitation of 980 nm and 808 nm lasers. The results indicate that the luminescence of core-shell UCNCs is influenced by the non-radiative rate between activators (Yb³⁺ and Nd³⁺) and the back energy transfer rate from Er³⁺ ions to activators. UCL tends to be obtained at a relatively low concentration of Yb³⁺ and Nd³⁺ ions (about 5%), whereas NIR emission tends to be obtained at a relatively high concentration of Yb³⁺ and Nd³⁺ ions (not higher than 20%). Dual-mode anti-counterfeiting imaging is successfully fabricated using core-shell UCNCs, which can be detected and distinguished by visible and infrared detectors. The visible versus infrared brightness of dual-mode anti-counterfeiting imaging can be tuned by varying the concentration of activators (Yb³⁺, Nd³⁺). Our work demonstrates concentration-dependent UCL and DCL in core-shell UCNCs, which provides reference to obtain NIR emission in the NIR-II region and adds encrypted dimensions for anti-counterfeiting patterns in the field of file encryption.