Upconversion enhancement and temperature sensing studies in Li+ ions incorporated GdPO4:Tm3+/Yb3+ phosphor

The GdPO4:Tm3+/Yb3+ phosphor codoped with various concentrations of Li + ions were synthesized for upconversion emission and optical thermometry studies. Excitation using 980 nm laser diode results in three upconversion (UC) emission bands with centre wavelengths of 478, 648 and 692 nm. These bands are originated from 1G4→3H6, 1G4→3F4, and 3F3→3H6 transitions of the Tm3+ ion, respectively. Li+ ions modified the local crystal symmetry around dopant ions, resulting enhanced UC emissions. The lifetime of the 1G4 level of Tm3+ ion was studied using a 980 nm laser excitation. The temperature sensing performances of GdPO4:Tm3+/Yb3+ and GdPO4:Tm3+/Yb3+/Li+ based on fluorescence intensity ratio (FIR) technique were evaluated in the temperature range 301-713 K under 980 nm excitation. Non-thermally coupled levels 3F3 (692 nm) and 1G4 (478, 648 nm) were utilized for FIR estimation. A maximum absolute sensitivity of 6.28 × 10-3 K-1 at 653 K and 18.71 × 10-3 K-1 at 713 K were observed for Li + undoped and codoped phosphors respectively. The result indicates that codoping of Li + ions improved the UC emission as well as optical thermometry. Moreover, CIE colour coordinates and anti-counterfeiting application were also exhibited.

Nonlinearity of the Upconversion Response of Er3+ in Y2TiO5:Er3+,Yb3+ Ceramics When Varying the Wavelength of Incident NIR Excitation Radiation

The upconversion response of Er3+ sensitized by Yb3+ in various crystalline hosts and illuminated with a laser light at around 980 nm revealed certain spectral shapes that are typical for each of the crystalline matrices containing the dopants. The purpose of this work was to measure the upconversion response of Er3+ as a dopant in Y2TiO5, sensitized by Yb3+, at different concentrations relative to the substituted Y3+ ion, and to reveal the subtleties of the mechanisms of the energy transfers between them and the lattice. Therefore, we synthesized Y2TiO5 ceramic samples doped with different concentrations of Er3+ and Yb3+, below 10% (mol), in order to minimize the distortion of the lattice. The oxide powders, obtained using the sol-gel method, as well as the ceramics were structurally and morphologically characterized using an X-ray diffraction analysis and scanning electron microscopy. When the ceramic samples were irradiated with an NIR laser light, it was found that, at a wavelength variation of only 2 nm of the incident radiation, from 973.5 nm to 975.5 nm, the upconversion spectra differed significantly. This nonlinearity is notable because it is not present in the case of other crystalline host matrices studied by us since the literature lacks information on this subject. We also correlated this effect with the simulated distribution of the average distances between Er3+ and Yb3+ ions in the host matrix.

Luminescent/Temperature-Sensing Properties of Multifunctional Rare-Earth Upconversion Kevlar Nanofiber Composite under 1550 nm

The unique properties of upconversion nanoparticles (UCNPs) are responsible for their diverse applications in photonic materials, medicine, analytics, and energy conversion. In this study, water-soluble rare-earth upconversion nanomaterials emitting green, yellow, and red light under 1550 nm excitation were synthesized. These nanomaterials were then integrated into water-soluble Kevlar nanofibers (KNFs) to fabricate ultra-thin composite films exhibiting favorable mechanical characteristics. The characterization of the products, along with their luminescent, mechanical, and temperature-sensing properties, was examined. The results indicate that the composite material exhibited varying colors based on the doped nanoparticles when subjected to 1550 nm excitation. The composite showed highly sensitive temperature-sensing properties, excellent luminescent characteristics, and superior mechanical strength. This study suggests that KNFs are effective carriers of UCNPs. This study offers a reference for the utilization of rare-earth upconversion in anti-counterfeiting displays, wearable health monitoring, and remote temperature sensing.

A dual-ratiometric optical thermometry based on Sr2LaF7:Er3+ crystal-implanted pliable fibers

Sr2(La1-xErx)F7/polyacrylonitrile composite fibers with special pliability and excellent crystal dispersibility have been fabricated, which provide the smaller size and appropriate temperature sensitivity. Up-conversion emission shows quadratic dependence of the photoluminescence...

Dual-functional ratiometric fluorescent sensor based on mixed-lanthanide metal-organic frameworks for detection of trace water and temperature

The rapid-response ratiometric sensors are promising materials to detect trace water and temperature. However, the accurately visualized water assay in very narrow-range still remains a challenge. Herein, a novel dual-functional...

Bismuth(III)-Doped NaYbF4:Tm3+ Fluorides with Highly Efficient Upconversion Emission under Low Irradiance

Concentration quenching of upconversion (UC) luminescence (UCL) is a common phenomenon in rare-earth-doped materials that seriously restricts the concentration of the activator and sensitizer and withholds their UC emissions and quantum yields. In particular, it remains a tremendous challenge to develop one novel strategy based on the introduction of trivalent bismuth (Bi³⁺) ions to exceed the typical thulium (Tm³⁺) ion concentration and reach high-efficiency UC under low illumination. In this work, the Tm³⁺ accommodation capacity can be increased from 2.0 to 8.0 mol % in NaYbF₄:Tm³⁺ materials with the assistance of Bi³⁺ ions, which maintains strong UC emissions with large absolute UC quantum yields under low illumination. Specifically, the total upconversion quantum yield (UCQY) of the as-obtained Na(Tm_0.08Yb_0.60Bi_0.32)F₄ (8Tm60Yb32Bi) sample can reach as high as 1.45% upon continuous-wave (CW) laser excitation at 40 W cm^-2. Strikingly, the total UCQY still remains at a high level (0.41%) even though the CW power density decreases to 1.5 W cm^-2. Moreover, the intrinsic mechanism of the breakthrough in the threshold of concentration quenching of UCL by Bi³⁺ ions was also fully explored. These advances in enhancing UC emissions and UCQYs under a low pump power density offer exciting opportunities for important photonic applications.

Electro- and photon-induced cooling in BNT-BT-SBET relaxors with in situ optical temperature sensing

A novel lead-free luminescent ferroelectric (FE) ceramic, ${{\rm Bi}_{0.5}}{{\rm Na}_{0.5}}{{\rm TiO}_3} {-} {0.{06\; \rm BaTiO}_3} {-} {0.{055\;\rm Sr}_{0.7}}{{\rm Bi}_{0.18}}{{\rm Er}_{0.02 \,\square\, 0.1}}$Bi_0.5Na_0.5TiO₃-0.06BaTiO₃-0.055Sr_0.7Bi_0.18Er_0.02◻0.1${{\rm TiO}_3}$TiO₃ (BNT-BT-SBET), is developed with an adiabatic temperature change ($\Delta T$ΔT) of 0.7 K under an electric field ($E$E) of 60 kV/cm at room temperature, an anti-Stokes fluorescence cooling, and a maximum optical $T$T sensitivity of ${0.0055}\;{{\rm K}^{ - 1}}$0.0055K^-1 at 522 K. Interestingly, the electrocaloric response reaches a saturation at permittivity shoulder $T$T of 100°C; meanwhile, the maximized emission intensity of $^2{{\rm H}_{11/2}}{ \to ^4}{{\rm I}_{15/2}}$²H_11/2→⁴I_15/2 occurs. $T$T- and $E$E-tunable enhancement of $^2{{\rm H}_{11/2}}{ \to ^4}{{\rm I}_{15/2}}$²H_11/2→⁴I_15/2 emission intensity is due to the population inversion from the $^4{{\rm S}_{3/2}}$⁴S_3/2 to $^2{{\rm H}_{11/2}}$²H_11/2 states caused by an incoherent regime consisting of FE phase and polar nanoregions in a relaxor matrix.

Dual-Mode Upconversion Nanoprobe Enables Broad-Range Thermometry from Cryogenic to Room Temperature

Noncontact optical thermometers based on the luminescence intensity ratio of two thermally coupled energy levels, exhibiting high sensitivity, excellent accuracy, fast response, and low environment dependence, have attracted great interests in scientific research, life activities, and industrial manufacturing processes. However, the use of optical thermometers in extreme atmospheres (below 150 K) is usually limited by the required large temperature activation because of the relatively big energy difference (200 cm^-1 ≤ ΔE ≤ 2000 cm^-1). Here, we propose a strategy to alleviate the ultralow temperature-sensing problem by exploiting and utilizing the near-infrared (NIR) thermally coupled Stark sublevels of Tm³⁺ (³H₄|0 → ³H₆/³H₄|1 → ³H₆, ΔE ≈ 300 cm^-1) that is much sensitive to minimal temperature variation, especially at ultralow temperatures because of the tiny energy difference. The integration of ultralow temperature-sensitive Tm³⁺ ions and room-temperature-sensitive Er³⁺ ions in an ultrasmall α-NaYbF₄:Tm³⁺@CaF₂@NaYF₄:Yb³⁺/Er³⁺@CaF₂ core/multishell nanoparticle (∼15 nm) as a dual-mode upconversion luminescent nanoprobe enables the broad-range temperature detection from 10 to 295 K. This structure induces ∼14 times NIR emission and ∼sixfold green upconversion luminescence output in comparison with the α-NaYbF₄:Tm³⁺ core and α-NaYbF₄:Tm³⁺@CaF₂@NaYF₄:Yb³⁺/Er³⁺ core/shell/shell nanoparticles. The maximum absolute and relative sensitivities of this dual-mode temperature sensor reach 0.67% and 3.06% K^-1, respectively, showing the advantage of the concurrent utilization of the Tm³⁺ NIR 801/820 nm band ratio and the typical Er³⁺ visible 521/538 nm band ratio for a wide-range temperature-sensing purpose. This work provides a promising strategy to develop accurate and effective, contactless broad-range/ultralow temperature sensors.

Fe3+-codoped ultra-small NaGdF4:Nd3+ nanophosphors: enhanced near-infrared luminescence, reduced particle size and bioimaging applications

Small-sized lanthanide-doped nanoparticles (NPs) exhibiting superior near-infrared (NIR) luminescence properties are highly desired for bioimaging applications. Herein, Fe³⁺ ions are codoped in NaGdF₄:Nd³⁺ nanocrystals via a simple coprecipitation method, which can simultaneously reduce the particle size and enhance the downconverting NIR luminescence of the NPs. The NIR luminescence intensity reaches the maximum for the obtained sub-5 nm NPs when the doping concentration of Fe³⁺ is tuned to 20 mol%, which is ∼1.7 times higher than that of the pristine 8.7 nm NPs without Fe³⁺ doping. After being modified with targeting molecules, the ultra-small NaGdF₄:Nd³⁺,Fe³⁺ NPs were successfully applied as luminescent probes for targeted NIR imaging of tumors in biological tissues. Moreover, they also show great potential as a high contrast agent for T2-weighted MRI imaging.

Small-sized lanthanide-doped nanoparticles (NPs) exhibiting superior near-infrared (NIR) luminescence properties are highly desired for bioimaging applications.

Highly sensitive optical thermometers based on unconventional thermometric coupled levels of Tm3+ following a Boltzmann-type distribution in oxyfluoride glass ceramics

Two highly-sensitive innovative coupled levels of ³F_2,3/¹D₂ and ³F_2,3/¹G₄ governed by a Boltzmann-type distribution in Tm³⁺/Yb³⁺ co-doped β-PbF₂ GCs were proposed.