Yb 3+ and Ho 3+ co-doped AB(MoO 4 ) 2 (A = Na, Li, K; B = Y or Bi) single crystals were successfully grown by the Czochralski method, and the effects of Li + and K + doping on the structural and optical properties of NaY(MoO 4 ) 2 (NYM) crystals were systematically investigated. The crystals confirmed with zircon-type tetragonal structure (space group 4). First-principle calculations revealed the band structure features of NYM crystals and elucidated the differences in bandgap and oxygen defect formation compared with NaBi(MoO 4 ) 2 (NBM). Judd-Ofelt calculations were conducted and the spectral transition properties of Ho 3+ were revealed. Photoluminescence studies demonstrated that the crystals exhibited high-purity red down-conversion emission under 450 nm excitation and typical two-photon up-conversion green emission under 980 nm excitation. Li + /K + doping effectively suppressed the green emission and enhanced the red component, with a more pronounced effect for K + . Temperature-dependent spectra (300-540 K) indicated good repeatability and thermal stability, with the K + -doped crystal retaining 55% of its room-temperature emission intensity at 540 K. The activation energies derived from temperature-dependent spectra suggest that Li + and K + incorporation significantly improved thermal stability. Furthermore, optical thermometry analysis revealed that Li + and K + doping significantly enhanced the optical sensing sensitivity. Fluorescence decay curves suggested that Li + and K + doping increased lattice disorder, suppressed Ho 3+ -Ho 3+ cross-relaxation, and consequently modified the red-to-green emission ratio. These results indicate that Yb,Ho:NYM crystals and their Li + /K + -doped counterparts exhibit outstanding performance in optical anti-counterfeiting, near-infrared solid-state lasers and optical thermometry applications.
The pure and intense orange upconversion luminescence of Eu3+ was efficiently achieved in NaYF4 and NaLuF4 nanocrystals through the upconversion sensitization of doped Yb3+ and Mn2+ ions dimer for the first time. A shell-coating strategy was performed to further enhance the emission intensity. In this work, a novel upconversion mechanism for energy transfer from the Yb3+–Mn2+ dimer to the Eu3+ ion was proposed by analyzing the upconversion luminescence spectra, lifetimes and pumping photons measurements of the obtained nanocrystals in detail. The applications of the nanocrystals for CT imaging and ratiometric pH sensing in physiological pH range were performed. This study also provides a new upconversion spectral region at around 592 nm for applications in multicolor imaging, multiplexed encoding and detection.
In this work, a label-free fluorimetric method for sensitive and selective detection of Cr(III) was developed by utilizing NaY/GdF 4 :Yb,Er upconversion nanoparticles (UCNPs) and citrate-modified silver nanoparticles (Cit-AgNPs) mixed system. The photophysical and morphological properties of the prepared UCNPs were characterized by various spectroscopic and microscopic techniques. The fluorescence of amino-modified UCNPs can be quenched due to the Cr(III) induced aggregation of Cit-AgNPs by means of inner filter effect. The observed quenching in the fluorescence intensity of the UCNPs is found to be dependent on the concentration of Cr(III). The fluorescence decreases linearly within the investigated concentration of 0.5–40 μM Cr(III) with a limit of detection 34 nM. The practical applicability of the present sensing method is ascertained by detecting Cr(III) in spiked tap water and instant tea samples.
The majority of the fabrication procedures of lanthanide-doped materials involve thermal treatment that often results in crystallite regrowth, stabilizing the specific crystal structure and resulting in luminescence enhancement. The efficiency and intensity of up-conversion luminescence are closely related to the structure and synthesis process of the materials. Herein, well-crystallized and pure tetragonal NaY(MoO4)2 microcrystals with a uniform octahedral shape have been successfully synthesized via an environmentally friendly hydrothermal method, followed by annealing treatment. The phases, structures, morphologies, and compositions of the synthesized products annealed at 500–1000 °C remain unchanged, indicating high thermal stability. Furthermore, the NaY(MoO4)2:Yb3+/Er3+ microcrystals exhibit strong green emission when irradiated using infrared (980 nm) or ultraviolet (378 nm) wavelengths. Upon 980 nm excitation, up to 37-fold luminescence enhancement is achieved when the samples are annealed at about 700 °C. Interestingly, the high colour purity of the strong green emission is not only independent of the dopant concentration and heat treatment temperature, but it is also independent of the excitation conditions, including power and wavelength, and this makes it particularly suitable as a green safety signal light and luminescent security ink in paintings. As-prepared safety inks with NaY(MoO4)2:Yb3+/Er3+ microcrystals were used for visual fingerprint recognition printed on A4 paper with three-level fingerprint security features, significantly increasing the difficulty of illegal imitation and enhancing the levels of anti-counterfeiting.
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