Yb 3+ -Mn 2+ dimer based upconversion nanocrystals (UCNCs) have great prospects in biological and photonic applications. In this work, the broadband upconversion luminescence (UCL) of Yb 3+ -Mn 2+ dimer in perovskite nanocrystals is systematically engineered under multi-wavelength excitations. Under the excitation of 980, 940 and 915 nm lasers, KCdF 3 :Yb 3+ /Mn 2+ UCNCs generate yellow broadband UCL from Yb 3+ -Mn 2+ dimer at room temperature and achieve maximum intensity as Mn 2+ concentration reaches 7.5 mol%. By introducing Er 3+ ions into the KCdF 3 host matrix, the UCL color can be further tuned to pure red with high spectral purity, and the red-to-green (R/G) UC intensity ratio is promoted dramatically. The bidirectional energy transfer (ET) processes between Yb 3+ -Mn 2+ dimer, Mn 2+ and Er 3+ ions facilitate color modulation, supported by power dependence and lifetime measurements. Especially, the ET efficiency is evaluated under the multi-wavelength excitations, confirming the high efficiency of ET processes from Yb 3+ -Mn 2+ dimer to Er 3+ ions. Moreover, the obtained KCdF 3 :Yb 3+ /Mn 2+ /Er 3+ UCNCs present a maximum magnetization intensity of 2.01 emu/g when Mn 2+ concentration reaches 98.5 mol%. Both the tunable UCL and excellent paramagnetism properties indicate that these KCdF 3 :Yb 3+ /Mn 2+ /Er 3+ UCNCs have the capability of bifunctional biomarkers of nanomaterials and can be a candidate for bio-related applications.
Due to the lack of intermediate levels and the large energy mismatch with Yb3+, the achievement of efficient upconversion (UC) of Eu3+ is a great challenge. Herein, intense UC emission of Eu3+ is demonstrated via controllable and efficient unidirectional energy transfer from Yb3+–Mn2+ dimers in perovskite fluoride KCdF3:Yb3+,Mn2+,Eu3+ synthesized by a facile solvothermal method. The UC emission color of KCdF3:Yb3+,Mn2+,Eu3+ can be well tuned from yellow to orange and ultimately turned into Eu3+ pure red emission by controlling the Eu3+ concentration. Moreover, a significantly prolonged Eu3+ UC emission lifetime is observed in this system, and it is ascribed to the genetic effect of the sustained resonance energy transfer from Yb3+–Mn2+ dimers to Eu3+. The pump power and temperature dependent UC behaviors further suggest that the phosphor exhibits excellent chromaticity stability. Owing to the long-lived and chromaticity stable UC luminescence, the as-prepared KCdF3:Yb3+,Mn2+,Eu3+ phosphor may be a highly suitable candidate for lifetime-encoded information encryption, anti-counterfeiting, etc.
In contrast to simply static fluorescence, the introduction of time-resolved visualized fluorescence in materials can significantly improve their anti-counterfeiting performance; however, this remains challenging with photon upconversion (UC) luminescent materials. Herein, we demonstrate a color-tunable UC luminescent perovskite fluoride KCdF3:Yb3+,Mn2+,Ln3+ (Ln3+ = Er3+, Ho3+, Tm3+) with both long (∼20–40 ms) and short (∼0.4–2 ms) emission lifetimes. Under steady 980 nm laser excitation, color-tunable steady UC fluorescence from yellow to red (or green) and yellow to blue (including UC white emission) could be easily achieved by tuning the doping concentration of Mn2+ and Ln3+ in the system; whereas, an unalterable yellow UC afterglow was observed by the naked eye when the excitation source was removed. Moreover, the upconversion nanoparticles (UCNPs) exhibited excellent pump power- and temperature-dependent luminescence stability. Static and dynamic photoluminescence analysis provided an understanding of the unique UC luminescence in this system, and a bidirectional energy transfer UC mechanism between Mn2+ (or Yb3+–Mn2+ dimer) and Ln3+ is presented. The multiple anti-counterfeiting prototype using the unique UC luminescent materials demonstrated a high throughput authentication rate without the need for complex, time-gated decoding instrumentation, suggesting their high potential for anti-counterfeiting application, and also providing new insights for the design of photon UC luminescence materials.
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