Alkaline zirconium fluorides (A x Zr y F x+y , A = Li, Na, and K), featuring unique crystallographic structures, have recently emerged as a class of attractive hosts for fabricating lanthanide (Ln 3+ )-doped upconversion nanocrystals (UCNCs) that exhibited distinct morphology, upconversion luminescence (UCL) performance, and physicochemical property. In this paper, for the first time the controlled preparation of Yb 3+ /Er 3+ -doped UCNCs is reported based on the trigonal Cs 2 ZrF 6 host, leading to tunable morphology and size of the resulting UCNCs by varying the reaction temperature and time. By further incorporating Na + ions into the Cs 2 ZrF 6 crystal lattice, sub-10 nm Yb 3+ /Er 3+ /Na + tridoped UCNCs with highly improved crystallinity and thus greatly enhanced UCL intensity are obtained. Moreover, these resulting UCNCs display abnormal thermal enhancement of UCL over a temperature range from 333 to 493 K, enabling the fabrication of supersensitive luminescent nanothermometers for temperature sensing. Based on the luminescence intensity ratio of two nonthermally coupled levels (i.e., 4 F 9/2 and 2 H 11/2 ) of Er 3+ , the as-prepared Cs 2 ZrF 6 :Yb/Er/Na UCNCs exhibit an extremely large absolute sensitivity of 177.3% K −1 and a considerably high relative sensitivity of 1.52% K −1 at 333 K. These results unambiguously demonstrate that Cs 2 ZrF 6 is a suitable host material for preparing small-sized Ln 3+ -doped UCNCs as nanothermometer for high-performance ratiometric temperature sensing.
Min Liu,Yuanqi Qian,Huhui Fu,Yaning Zhang,Jingbing Lu,Qi Xiong,Dapeng Jiang,Huamin Kou,Anhua Wu,Liangbi Su
Lanthanide-doped ultrasmall upconversion nanocrystals exhibit unique optical properties that differ significantly from bulk materials, making them highly promising for diverse applications. However, their widespread applications have long been impeded by extremely weak photoluminescence due to severe surface quenching effect. Herein, we present a Li+-doping-induced phase engineering strategy to achieve significantly enhanced upconversion luminescence in ultrasmall Cs2ZrF6:Yb/Er nanocrystals (NCs). While initial Li+ doping improves crystallinity, yielding up to a 63-fold emission increase, further doping triggers a phase transition to form heterophase nanocrystals consisting of trigonal Cs2ZrF6 and tetragonal LiYbF4. The synergistic effect of improved crystallinity and reduced local symmetry around lanthanide ions results in a 302-fold boost in emission intensity, even as the particle size decreases to ~6.1 nm. Moreover, these ultrasmall nanocrystals display anomalous anti-thermal quenching behavior, with luminescence intensity increasing as temperature rises from 303 to 483 K, driven by defect-mediated energy repopulation. This work not only offers a robust approach for fabricating ultrasmall, high-brightness NCs but also establishes phase engineering as a pivotal mechanism for modulating local crystal field, paving the way for high-performance, thermally stable ratiometric nanothermometry.
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