Enhancing upconversion of Nd3+ through Yb3+-mediated energy cycling towards temperature sensing

Inhibiting concentration quenching in Yb3+-Tm3+ upconversion nanoparticles by suppressing back energy transfer

Lanthanide-doped upconversion nanoparticles are promising for applications ranging from biosensing, bioimaging to solid-state lasing. However, their brightness remains limited by the concentration quenching effect of lanthanide activator ions, which greatly restricts their utility. Here, we develop a heterogeneous core-shell-shell nanostructure based on hexagonal NaYF4, in which Tm3+ activator and Yb3+ sensitizer are separated into the core and inner shell, while the outmost shell is used to suppress surface quenching effects. We show that this design can alleviate the activator concentration quenching effect, resulting in optimal Tm3+ concentration increasing from 1% to 8% at sub-100 W/cm2 irradiance, compared with the canonical core-only NaYF4:Yb3+/Tm3+. Moreover, under high excitation irradiance (20 MW/cm2), the optimal Tm3+ concentration could be further increased to 50%. Mechanistic investigations reveal that the spatial separation of sensitizer and activator effectively suppresses the back energy transfer from Tm3+ to Yb3+, driving the increase of optimal activator concentration. These findings enhance our understanding of lanthanide concentration quenching effect, unleashing opportunities for developing bright upconverting materials.

The first example of polymeric lanthanide tetrakis-trifluoroacetates in chemical solution deposition of up-converting NaGdF4:Yb,Er,Nd thin films

A series of lanthanide tetrakis-trifluoroacetates {(detaH2)2[La2(tfa)8]2(CH3CN)5(H2O)2}n and (detaH2)n[Ln(tfa)4]2n (Ln = Pr-Eu), mixed-ligand complexes [La(tfa)3(CH3CN)(H2O)]n, [Gd(tfa)3(deta)2](iPrOH) and [Yb(tfa)2(deta)2](tfa), as well as detaH2(tfa)2 were isolated and characterized. All lanthanide tetrakis-trifluoroacetates contain two types of 1D anionic chains [Ln(tfa)4]nn- and cavities occupied by detaH22+ cations and solvating H2O and CH3CN molecules. The thermal behavior of (detaH2)n[Ln(tfa)4]2n in air is investigated by TGA, in situ VT-PXRD and total X-ray scattering with PDF analysis, and the formation of metal fluorides occurs upon heating to 300 °C. The application of solution with lanthanide trifluoroacetates and diethylenetriamine (deta) as precursors for chemical deposition of β-NaGdF4:Er,Yb,Nd thin films is reported. The deposited β-NaGdF4:Er,Yb,Nd thin film demonstrates up-conversion luminescence under 980 and 808 nm laser excitation.

Toward Ultra-High Sensitivity Optical Thermometers and Bright Yellow LEDs Based on Phonon-Assisted Energy Transfer in Rare Earth-Doped La2ZnTiO6 Double Perovskite

Double perovskites, a class of ceramic oxides with unique crystal structures and diverse physical properties, show promise for various technological applications including solar cells, photodetectors, and light-emitting diodes (LEDs). Despite limited research on rare earth-doped double perovskites, leveraging their ultrahigh luminous efficiency to achieve bright yellow LED emission and addressing energy transfer challenges between Yb3+ and Nd3+ ions in double perovskite La2ZnTiO6 with moderate phonon energy are explored in this work. Through phonon-assisted energy transfer, an ultrasensitive optical thermometer covering a wide temperature range is developed by utilizing the different temperature responses of Er3+ emission in the visible light region and Nd3+ emission in the near-infrared region based on the luminescence intensity ratio (LIR). All the results demonstrate that the rare earth (Yb-Er, Yb-Nd, and Yb-Nd-Er)-doped La2ZnTiO6 phosphors can be effectively utilized for ultrabright LED illumination and ultrahigh sensitivity self-calibrated temperature sensing. This research underscores the significance of phonon-assisted energy transfer in improving material properties and provides valuable insights for the advancement of multifunctional materials.

Selectively Manipulating Interactions between Lanthanide Sublattices in Nanostructure toward Orthogonal Upconversion

Smart control of ionic interactions is a key factor to manipulate the luminescence dynamics of lanthanides and tune their emission colors. However, it remains challenging to gain a deep insight into the physics involving the interactions between heavily doped lanthanide ions and in particular between the lanthanide sublattices for luminescent materials. Here we report a conceptual model to selectively manipulate the spatial interactions between erbium and ytterbium sublattices by designing a multilayer core-shell nanostructure. The interfacial cross-relaxation is found to be a leading process to quench the green emission of Er³⁺, and red-to-green color-switchable upconversion is realized by fine manipulation of the interfacial energy transfer on the nanoscale. Moreover, the temporal control of up-transition dynamics can also lead to an observation of green emission due to its fast rise time. Our results demonstrate a new strategy to achieve orthogonal upconversion, showing great promise in frontier photonic applications.

Multi-wavelength excitable mid-infrared luminescence and energy transfer in core-shell nanoparticles for nanophotonics

2 μm mid-infrared (MIR) light sources have shown great potential for broad applications in molecular spectroscopy, eye-safe lasers, biomedical systems and so on. However, previous research studies were mainly focused on conventional materials such as glasses, glass-ceramics and crystals, limiting the luminescence intensity and miniaturization of photonic devices. Here we report a new strategy to realize the multiple excitation wavelength responsive MIR emission in a single nanoparticle by employing an erbium sublattice as the sensitizing host. Intense 2 μm emission of Ho³⁺ from its ⁵I₇ → ⁵I₈ optical transition was observed under 808, 980 and 1530 nm excitations. The possible energy transfer mechanism between Er³⁺ and Ho³⁺ ions was discussed. We also designed a core-shell-shell nanostructure by inserting an NaYF₄:Yb interlayer to maximize the absorption of 980 nm photons and enhance the 2 μm emission. The MIR luminescence under 808 nm excitation can be further improved by introducing Nd³⁺ into the outermost shell and attaching indocyanine green dyes. These results present an efficient way for the development of MIR luminescent nanomaterials with great potential in the fields of MIR gain devices, nanosized MIR light sources, and nanophotonics.

Photomodulated cryogenic temperature sensing through a photochromic reaction in Na0.5Bi2.5Ta2O9: Er/Yb multicolour upconversion

Optical temperature sensing of the non-thermally coupled energy levels (N-TCLs) based on fluorescence intensity ratio (FIR) technologies has excellent temperature sensitivity and signal recognition properties. In this study, a novel strategy is established to enhance the low-temperature sensing properties by controlling photochromic reaction process in Na_0.5Bi_2.5Ta₂O₉: Er/Yb samples. The maximum relative sensitivity reaches up to 5.99% K^-1 at cryogenic temperature of 153 K. After irradiation with commercial laser of 405 nm for 30 s, the relative sensitivity is increased to 6.81% K^-1. The improvement is verified to originate from the coupling of optical thermometric and photochromic behaviour at the elevated temperatures. The strategy may open up a new avenue to improve the thermometric sensitivity in photo-stimuli response photochromic materials.

Near-Infrared-to-Near-Infrared Optical Thermometer BaY2O4: Yb3+/Nd3+ Assembled with Photothermal Conversion Performance

Nowadays, the construction of photothermal therapy (PTT) agents integrated with real-time thermometry for cancer treatment in deep tissues has become a research hotspot. Herein, an excellent photothermal conversion material, BaY₂O₄: Yb³⁺/Nd³⁺, assembled with real-time optical thermometry is developed successfully. Ultrasensitive temperature sensing is implemented through the fluorescence intensity ratio of thermally coupled Nd³⁺: ⁴F_j (j = 7/2, 5/2, and 3/2) with a maximal absolute and relative sensitivity of 68.88 and 3.29% K^-1, respectively, which surpass the overwhelming majority of the same type of thermometers. Especially, a thermally enhanced Nd³⁺ luminescence with a factor of 180 is detected with irradiation at 980 nm, resulting from the improvement in phonon-assisted energy transfer efficiency. Meanwhile, the photothermal conversion performance of the sample is excellent enough to destroy the pathological tissues, of which the temperature can be raised to 319.3 K after 180 s of near-infrared (NIR) irradiation with an invariable power density of 13.74 mW/mm². Besides, the NIR emission of Nd³⁺ can reach a depth of 7 mm in the biological tissues, as determined by an ex vivo experiment. All the results show the potential application of BaY₂O₄: Yb³⁺/Nd³⁺ as a deep-tissue PTT agent simultaneously equipped with photothermal conversion and temperature sensing function.

Ultrasensitive Thermochromic Upconversion in Core-Shell-Shell Nanoparticles for Nanothermometry and Anticounterfeiting

Upconversion nanoparticle based ratiometric nanothermometry has shown many advantages including high relative sensitivity, fast temperature response, and high spatial resolution. However, most of the existing designs are on the basis of thermally coupled upconversion emissions, and it remains a challenge to improve the thermo-sensitivity. Here, we report a new nanoplatform of NaYF₄:Yb/Er/Ce@NaYF₄@NaYF₄:Yb/Tm core-shell-shell nanostructure to improve the thermal sensitivity through the nonthermally coupled upconversion emissions. With the increase of temperature, the green upconversion of Er³⁺ shows a decline while the blue upconversion of Tm³⁺ exhibits a rapid increase, leading to a huge contrast in both intensity ratio and emission colors. The maximum relative sensitivity can reach up to 9.86% K^-1 at 303 K. It is further found that introducing Ce³⁺ is able to improve the sensitivity and expand the thermochromic green-to-blue gamut greatly. These results show great potential in ultrasensitive lanthanide-based nanothermometry and anticounterfeiting.