[en] A new design of NaYF₄: Eu³⁺ microwires (MWs) with a hexagonal structure were synthesized via a facile hydrothermal method assisted with the sodium citrate, and their microstructure and phase structure have been analyzed. The photoluminescence (PL) mechanism of β-NaYF₄: Eu³⁺ MWs was systematically investigated under 532 nm laser excitation through adjusting MW number and excitation environment including the excited position, laser power, and temperature. The strongest emission peak originated from the ⁵D₀→⁷F₂ hypersensitive transition in this system. Furthermore, owing to the special temperature-dependent optical properties of β-NaYF₄: Eu³⁺ MWs, time-resolved PL spectroscopy with high sensitivity is in situ real-time monitored at 1273 K to reveal the underlying mechanism of stark splitting. Therefore, our study should serve as a promising application in optical thermometry.

[en] The upconversion efficiency could be affected by Li${}^{+}$ ions with changing the local crystal field of nanoparticles. Therefore, a series of NaGdF${}_4$: Yb${}^{3+}$/Tm${}^{3+}$ micro-particles with different Li${}^{+}$ doping concentrations were synthesized and investigated, respectively. The structure and morphology of the synthesized materials were tested by X-ray diffraction and scanning electron microscope. The photoluminescence properties were studied with different concentrations Li${}^{+}$ ions doping under the irradiation of 980 nm laser. The emission peaks of Tm${}^{3+}$ ions are 449, 477, 645, 723, 800 nm which are attributed to high excited states to ground state ${}^3$H${}_6$ transition through a multiple steps resonance energy transfer from excited Yb${}^{3+}$. The enormous increase in upconversion emission in upconversion micro-particles (UCMPs) by introducing Li${}^{+}$ ions was observed by the naked eye. The strongest photoluminescence intensity is the sample with 2.5% doped Li${}^{+}$ ions. Additionally, the practicality of the resultant UCMPs for photoluminescence visible imaging was systematically investigated. These results suggest that the Li${}^{+}$ doped NaGdF${}_4$: Yb${}^{3+}$/Tm${}^{3+}$ is very promising as a screen printing material for anti-counterfeiting recognition.