Upconversion Nanocomposite Materials With Designed Thermal Response for Optoelectronic Devices

Spotlight on Luminescence Thermometry: Basics, Challenges, and Cutting-Edge Applications

Luminescence (nano)thermometry is a remote sensing technique that relies on the temperature dependency of the luminescence features (e.g., bandshape, peak energy or intensity, and excited state lifetimes and risetimes) of a phosphor to measure temperature. This technique provides precise thermal readouts with superior spatial resolution in short acquisition times. Although luminescence thermometry is just starting to become a more mature subject, it exhibits enormous potential in several areas, e.g., optoelectronics, photonics, micro- and nanofluidics, and nanomedicine. This work reviews the latest trends in the field, including the establishment of a comprehensive theoretical background and standardized practices. The reliability, repeatability, and reproducibility of the technique are also discussed, along with the use of multiparametric analysis and artificial-intelligence algorithms to enhance thermal readouts. In addition, examples are provided to underscore the challenges that luminescence thermometry faces, alongside the need for a continuous search and design of new materials, experimental techniques, and analysis procedures to improve the competitiveness, accessibility, and popularity of the technology.

Tm3+; Yb3+:Zn2TiO4 near infrared to blue upconversion phosphors for anti-counterfeit applications

Tm3+; Yb3+:Zn2TiO4 samples have been synthesized using a solid state reaction route. The phase, lattice parameters, crystallite size has been examined using X-ray Diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). An intense peak of Yb3+ codoped samples is observed near ∼957 nm due to the 2F7/2 → 2F5/2 transition in diffuse reflectance spectra (DRS), which confirms the presence of Yb3+ ion in the prepared compound. The optical band gap of Yb3+ codoped samples has been calculated using Kubelka-Munk function. The Raman spectra corresponds to incorporation of Tm3+/Yb3+ at the octahedral and tetrahedral site of the spinel host. The emission spectra recorded by using 370 nm excitation wavelength shows intense blue colour band corresponding to the 1G4 → 3H6 transition of Tm3+ ion. The upconversion (UC) emission spectra recorded by using 980 nm laser excitation source shows emission bands due to the 1G4 → 3H6, 1G4 → 3F4 and 3H4 → 3H6 transitions of Tm3+ ion in the host matrix lying in the blue, red and NIR regions respectively. There is effective enhancement of about ∼35 times in the blue UC emission intensity with incorporation of Yb3+ at 3% doping concentration in the prepared sample. The anti-counterfeit application of the optimized upconverting phosphor has been successfully demonstrated.

Electrodeposition of Lithium-Based Upconversion Nanoparticle Thin Films for Efficient Perovskite Solar Cells

In this work, high-quality lithium-based, LiYF4=Yb3+,Er3+ upconversion (UC) thin film was electrodeposited on fluorene-doped tin oxide (FTO) glass for solar cell applications. A complete perovskite solar cell (PSC) was fabricated on top of the FTO glass coated with UC thin film and named (UC-PSC device). The fabricated UC-PSC device demonstrated a higher power conversion efficiency (PCE) of 19.1%, an additional photocurrent, and a better fill factor (FF) of 76% in comparison to the pristine PSC device (PCE = ~16.57%; FF = 71%). Furthermore, the photovoltaic performance of the UC-PSC device was then tested under concentrated sunlight with a power conversion efficiency (PCE) of 24% without cooling system complexity. The reported results open the door toward efficient PSCs for renewable and green energy applications.

A high sensitivity dual-mode optical thermometry based on charge compensation in ZnTiO3:M (M = Eu3+, Mn4+) hexagonal prisms

Optical thermometer based on dual-mode fluorescence intensity ratiometric thermometry has been attracted more attention due to its higher sensitivity. In order to obtain optical thermal probe with high sensitivity, ZnTiO₃ hexagonal prisms with hexagonal perovskite structure were fabricated by using self-assembly method, and Al³⁺ ions were introduced into the crystal lattices of ZnTiO₃ doped with Eu³⁺/Mn⁴⁺ to improve the optical properties. The emission intensity assigned to Eu³⁺ was enhanced about twice with the charge compensation of Al³⁺ between Eu³⁺ and Ti³⁺. While the luminescence ratio between the thermal coupled level of Eu³⁺ revealed poor temperature dependence property. The emission assigned to ²E_g→⁴A_2g (713 nm) transition of Mn⁴⁺ revealed an huge thermal quenching. Using the luminescence ratio between ⁵D₀→⁷F₂ (⁵D₀→⁷F₁) transition of Eu³⁺ to ²E_g→⁴A_2g of Mn⁴⁺, the higher relative sensitivity of 2.7 %K^-1was obtained. The charge compensation of Al³⁺ improved the coordination and reduced the relative sensitivity, Sr =1.85 %K^-1. The results suggested the potential application in optical temperature probes for ZnTiO₃: Mn⁴⁺,Eu³⁺ phosphor.

Thermal enhancement of upconversion emission in nanocrystals: a comprehensive summary

Luminescence thermal stability is a major figure of merit of lanthanide-doped nanoparticles playing an essential role in determining their potential applications in advanced optics. Unfortunately, considering the intensification of multiple electron-vibration interactions as temperature increases, luminescence thermal quenching of lanthanide-doped materials is generally considered to be inevitable. Recently, the emergence of thermally enhanced upconversion luminescence in lanthanide-doped nanoparticles seemed to challenge this stereotype, and the research on this topic rapidly aroused wide attention. While considerable efforts have been made to explore the origin of this phenomenon, the key mechanism of luminescence enhancement is still under debate. Here, to sort out the context of this intriguing finding, the reported results on this exciting topic are reviewed, and the corresponding enhancement mechanisms as proposed by different researchers are summarized. Detailed analyses are provided to evaluate the contribution of the most believed "surface-attached moisture desorption" process on the overall luminescence enhancement of lanthanide-doped nanoparticles at elevated temperatures. The impacts of other surface-related processes and shell passivation on the luminescence behaviour of the lanthanide-doped materials are also elaborated. Lack of standardization in the reported data and the absence of important experimental information, which greatly hinders the cross-checking and reanalysis of the results, is emphasized as well. On the foundation of these discussions, it is realized that the thermal-induced luminescence enhancement is a form of recovery process against the strong luminescence quenching in the system, and the enhancement degree is closely associated with the extent of luminescence loss induced by various quenching effects beforehand.

Self-Calibrated Double Luminescent Thermometers Through Upconverting Nanoparticles

Luminescent nanothermometry uses the light emission from nanostructures for temperature measuring. Non-contact temperature readout opens new possibilities of tracking thermal flows at the sub-micrometer spatial scale, that are altering our understanding of heat-transfer phenomena occurring at living cells, micro electromagnetic machines or integrated electronic circuits, bringing also challenges of calibrating the luminescent nanoparticles for covering diverse temperature ranges. In this work, we report self-calibrated double luminescent thermometers, embedding in a poly(methyl methacrylate) film Er3+- and Tm3+-doped upconverting nanoparticles. The Er3+-based primary thermometer uses the ratio between the integrated intensities of the 2H 11 / 2 → 4 I15/2 and 4S 3 / 2 → 4 I15/2 transitions (that follows the Boltzmann equation) to determine the temperature. It is used to calibrate the Tm3+/Er3+ secondary thermometer, which is based on the ratio between the integrated intensities of the 1G 4 → 3 H6 (Tm3+) and the 4S 3 / 2 → 4 I15/2 (Er3+) transitions, displaying a maximum relative sensitivity of 2.96% K-1 and a minimum temperature uncertainty of 0.07 K. As the Tm3+/Er3+ ratio is calibrated trough the primary thermometer it avoids recurrent calibration procedures whenever the system operates in new experimental conditions.