Upconversion Luminescence Mechanism Research Articles

UV-induced coumarin-based spiropyran gradient photodimerization and photoisomerization to construct near-infrared responsive gradient hydrogel actuators in one-step

In recent years, the fabrication process of near-infrared (NIR) light-driven gradient hydrogels remains complex. Furthermore, NIR light-driven hydrogels based on the photothermal conversion mechanism have continued to face challenges in achieving spatio-temporal control of the internal structure due to the diffusive nature of heat. Therefore, it is sensible to develop NIR light-driven gradient hydrogels based on the near-infrared upconversion luminescence mechanism through a simple method. In this study, a novel photoresponsive coumarin-based spiropyran monomer (SPCMA) was designed and synthesized, followed by the fabrication of a series of double gradient hydrogels (MHMs) with rapid NIR light-driven capability and reusability through fast in situ photopolymerization of the precursor comprising SPCMA, upconversion nanoparticles (MUCNPs), and hydroxyethyl acrylate (HEA). MHMs exhibit excellent mechanical properties with fracture stress of 4.4 MPa and fracture elongation of 1282 %. Compared with hydrogel using N, N′-Methylenebis(2-propenamide) as a crosslinker, MHMs possess outstanding NIR light-driven capability and reversible cycling characteristics. MHMs rapidly transform from a three-dimensional spiral tubular to a sheet-like morphology within 30 s of NIR light irradiation and achieve a larger actuating angle of 450° within 120 s. Additionally, MHMs maintain similar driveability to the original samples after 10 cycles of NIR light-driven and UV light-recovery processes. More importantly, cargoes with seven times the mass of the "gripper" are firmly grasped and easily lifted by the "gripper" prepared using MHMs within 60 s of NIR light irradiation, demonstrating its powerful gripping ability. This work offers a new strategy for the development of biomimetic smart gradient materials.

A novel single near-infrared luminescence of negative thermal quenching materials for optical temperature measurement

In recent years, upconversion (UC) luminescence has gained attention due to its unique optical properties, and the thermal response spectral characteristics of these materials have been widely utilized in various fields including temperature sensing and biomedical research. However, the photoluminescence (PL) intensity of most rare earth ions doped phosphors decreases with increasing temperature, and the visible fluorescence emission penetration is still limited. In the work, the pure phase of Yb3+ and Er3+ ions co-doped Sc2Mo3O12 was successfully prepared. The Sc2Mo3O12: Yb3+, Er3+ sample showed a single emission of UC near-infrared (NIR) light. The optimum doping concentrations of 0.2 for Yb3+ ions and 0.01 for Er3+ ions were achieved. The possible upconversion luminescence (UCL) mechanism was investigated by analysing the UCL spectra, the down-shift (DS) excitation and emission spectra of Sc2Mo3O12: Yb3+, Er3+ phosphor. The excitation path of NIR emission is mainly through Er3+ from 2H11/2 to 4I9/2 energy level. What's more, the temperature-dependent near-infrared UCL properties of sample is explored in the temperature range from 323 K to 673 K. The enhancement of upconversion luminescence intensity shows a linear relationship with increasing temperature. The study paves a new way for non-contact optical temperature measurement with single near-infrared UCL emission.

Thermal enhanced upconversion luminescent of Sc2W3O12:Yb3+/Er3+ for optical temperature measurement

In recent years, people have higher requirements for temperature measurement with the development of optical temperature measurement. Meanwhile, the emission intensity of most fluorescent materials is quenched by thermal effects as the temperature rises, making it challenging to meet the specifications of fluorescent materials employed in the field of pyrometry. In this work, the pure phase of Yb3+/Er3+ ions co-doped Sc2W3O12 phosphor was successfully prepared. Sc2W3O12: Yb3+, Er3+ samples showed strong green emission and weak red emission. The optimal doping concentrations of Yb3+ and Er3+ ions are 0.3 and 0.02, respectively. The potential upconversion luminescence (UCL) mechanism was investigated by analyzing the UCL spectra, downshift emission spectra and power dependence spectra of Yb3+/Er3+ ions co-doped Sc2W3O12. The upconversion emission excitation path is mainly through Er3+ ions from 4I11/2 absorption photon excitation to 4F7/2 energy level, then the photons of 4F7/2 relaxes to green and red emission energy levels and returns to ground state for emitting green and red light. In addition, according to the UCL green light intensity increasing with temperature, it serves as an optical temperature sensor. The temperature sensitivity of the sample was explored in the temperature range of 298 K–673 K. This study provides new materials and ideas for non-contact temperature sensing.

Bimolybdate single crystals codoping with trivalent ytterbium and erbium ions for high-stable and high-sensitive optical thermometry

Replacing conventional powder polycrystalline materials with crystalline materials is an effective strategy to improve the sensing sensitivity, responsiveness, and anti-interference capabilities of the fluorescence intensity ratio (FIR) based optical temperature sensing techniques. In this work, Yb3+/Er3+ codoped bimolybdate single crystals, NaBi(MoO4)2 (NBM) with different Er3+ ion doping concentrations, were grown by the Czochralski method, in which is expected to be used for optical temperature sensing applications. The structure of the crystals was fully characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, and transmission electron microscopy (TEM). Under 980 nm laser excitation, the crystals demonstrated notable green and slight red upconversion (UC) emissions, and the mechanism of UC luminescence was thoroughly investigated. The luminescence intensity was found to be contingent upon the doping concentration, which gradually decayed with increase of Er3+ ions. The optical temperature sensing performance of Yb3+/Er3+ codoped NBM single crystals was evaluated by the FIR technique based on the thermally coupled level of Er3+ ions (2H11/2/4S3/2). The experimental findings revealed that the 10 at% Yb3+, 10 at% Er3+: NBM crystal exhibited highest sensor sensitivity among the synthesized crystals, with a maximum absolute sensitivity of approximately 1.22 % K−1 at 510 K. Furthermore, the temperature sensing characteristics of the crystals appeared to be dependent on the doping concentration of Er3+ ions. In summary, the Yb3+/Er3+ codoped NBM single crystal showcased excellent sensor sensitivity and held considerable promise as materials for optical temperature sensing applications.

Experimental Optimization, Design Synthesis, and Up-Conversion Luminescence Properties of Y4GeO8: Er3+/Yb3+ Red Phosphors

The Er3+/Yb3+ co-doped Y4GeO8 crystal powders were successfully synthesized us-ing a high-temperature solid-phase method. The crystal structure of the obtained phosphors was confirmed to be pure Y4GeO8 through X-ray diffraction (XRD) analysis. A regression equation correlating Er3+/Yb3+ doping concentrations with luminescent intensity was estab-lished based on the optimized theoretical model derived from experimental design. The op-timal concentrations of Er3+ and Yb3+ under 980 nm laser excitation were determined as 7.41% and 21.34%, respectively, while under 1550 nm laser excitation, the concentrations were 2.66% and 17.42%, respectively. The fluorescence emission spectra of the up-conver-sion samples were measured, revealing intense green and red emissions with peaks at 542, 546, and 654 nm under 980 nm excitation, and peaks at 546, 557, and 663 nm under 1550 nm excitation. These peaks correspond to transitions from 2H11/2 to 4I15/2, 4S3/2 to 4I15/2, and 4F9/2 to 4I15/2 energy levels. The relationship between up-conversion luminescence and laser op-erating current for the optimal samples under 980 nm and 1550 nm was investigated, uncov-ering that up-conversion luminescence occurs through both two-photon and three-photon processes. A detailed analysis and discussion of the up-conversion luminescence mecha-nisms were conducted. Furthermore, the relationship between up-conversion fluorescence and temperature for the optimal samples was studied, revealing excellent temperature-sens-ing characteristics under 980 nm and 1550 nm laser excitations. The calculated illumination region coordinates for the optimal samples under 980 nm and 1550 nm wavelength excitations were (0.5558, 0.4362) and (0.5256, 0.4687), respectively. The research highlights the potential of rare-earth ion-doped up-conversion luminescent materials for diverse anti-coun-terfeiting applications. In particular, the Y4GeO8: Yb3+/Er3+ phosphors, incorporating a dual-excitation mechanism, enhance the security of anticounterfeiting strategies in multifaceted scenarios. The study underscores the promising developments in this field.

The single near-infrared upconversion luminescence of a novel KYb (MoO4)2: Er3+ phosphor for the temperature sensing

The rare earth doping upconversion luminescence (UCL) materials exhibit multimodal emission, which has huge potential applications. However, it is still a difficult problem to obtain a single emission by modulating UCL, especially the modulation of single red light and single near-infrared light. In this work, we report a novel UCL KYb (MoO4)2: Er3+ phosphor, which can achieve a single near-infrared emission (804 nm) under rare earth Er3+ doping. Its pure phase was successfully prepared by high-temperature solid-state method, and its optimal Er3+ doping concentration was found to be 0.1 %. Excited at 980 nm laser, KYb (MoO4)2: Er3+ exhibit a single near-infrared UCL attributed to the 4I9/2 → 4I15/2 transition of Er3+ ions. To further explain its UCL mechanism, the power dependence, ultraviolet-visible-near-infrared absorption spectrum, excitation spectrum and emission spectrum of KYb (MoO4)2: Er3+ was measured. Due to the excellent near-infrared emission, the temperature-dependent UCL properties of the KYb (MoO4)2: Er3+ material is investigated in the temperature range from 298 K to 673 K. The linear relationship between the UCL and the temperature, And the sensitivity of KYb (MoO4)2: Er3+ phosphor reaches the maximum value at 673 K, which is 0.0078 K−1. The single near-infrared luminescence with good penetrability and large sensitivity makes KYb (MoO4)2: Er3+ have a promising application prospect in the fields of non-contact temperature sensing and bioimaging.

Optical thermosensor using Cr3+ doped NaGdF4: Yb, Er microcrystals

ABSTRACT In this work, NaGdF4: 20%Yb, 2%Er microcrystals with varying contents of Cr3+ doping were obtained by conventional hydrothermal process. The microcrystal phase structure, morphology, upconversion luminescence mechanism, as well as temperature sensing behaviour under different treatment methods were researched. Based on the thermally coupled 2H11/2 and 4S3/2 levels of Er3+ and the dependence of fluorescence intensity ratio (FIR) on temperature, the optical temperature sensing behaviour of micrometer crystals were experimentally demonstrated. The NaGdF4: 20%Yb, 2%Er, 7%Cr microcrystals treated with high-temperature annealing show the best temperature sensing behaviour within the temperature span of 298–498 K. NaGdF4: 20%Yb, 2%Er, 7%Cr microcrystals with high temperature annealing presented the feasibility and promising properties for optical temperature sensing. The endeavour of emerging materials could lay the groundwork for future sustainable photonics engineering industrial growth.

Exploring energy mechanisms involved in the luminescence reduction due to Ce3+ doping via core-multishell upconversion nanoparticles highly doped with Ho3+ and Yb3+

Upconversion nanoparticles (UCNPs), particularly those that are highly doped, possess outstanding properties and have demonstrated tremendous application potential. However, novel fine-tuning strategies are urgently required to enhance the upconversion luminescence (UCL) performance of UCNPs. In this study, a series of core–multishell UCNPs highly doped with Ho3+ and Yb3+ ions were synthesized via the oleate route. The X-ray diffraction (XRD), transmission electron microscopy (TEM), and UCL spectra and decay times, and pump power dependency of the synthesized UCNPs were used to examine their morphology, size, and the UCL properties. Ce3+ ions were introduced to enhance Ho3+-based red UCL, and their doping strategy was also optimized based on summarized UCL trends and detailed UCL mechanism analysis. NaHoF4:Gd3+@NaGdF4:Yb3+,Ce3+@NaGdF4:Yb3+ UCNPs that had 10 mol% Ce3+ dopants in their middle shells exhibited better UCL properties than the larger traditional NaGdF4:Yb3+,Ce3+,Ho3+ UCNPs, probably because of the combined influence of the energy interactions between the Ce3+ ions and surface groups, sensitizing energy enhancement by active shells, and the positive cooperation between the Ce3+ ions and highly-doped core–shell structures. In this study, the phenomenon of UCL decrease resulting from Ce3+ ion doping was explored. Using unique core–multishell structures with highly-doped activators and sensitizers placed within and outside the structures, respectively, the UCL decrease due to Ce3+ ion doping was attributed to a competition for photons. Doping the UCNPs with Ce3+ ions near the UCNP surfaces favors Ho3+-based red UCL. Our study findings can deepen the understanding of the Ho3+-based UCL mechanisms and increase the potential uses of NaHoF4 materials in photo-induced therapy.

Upconversion Nanoparticles Based Sensing: From Design to Point-of-Care Testing.

Rare earth-doped upconversion nanoparticles (UCNPs) have achieved a wide range of applications in the sensing field due to their unique anti-Stokes luminescence property, minimized background interference, excellent biocompatibility, and stable physicochemical properties. However, UCNPs-based sensing platforms still face several challenges, including inherent limitations from UCNPs such as low quantum yields and narrow absorption cross-sections, as well as constraints related to energy transfer efficiencies in sensing systems. Therefore, the construction of high-performance UCNPs-based sensing platforms is an important cornerstone for conducting relevant research. This work begins by providing a brief overview of the upconversion luminescence mechanism in UCNPs. Subsequently, it offers a comprehensive summary of the sensors' types, design principles, and optimized design strategies for UCNPs sensing platforms. More cost-effective and promising point-of-care testing applications implemented based on UCNPs sensing systems are also summarized. Finally, this work addresses the future challenges and prospects for UCNPs-based sensing platforms.

Preparation and strong green upconversion luminescence of Ca9Y(VO4)7: Yb3+/Er3+ phosphor

Exploring novel oxysalt compounds remains significant due to their affordability and versatile applications, such as serving as laser crystals, piezoelectric crystals, frequency-doubling crystals, and luminescent materials. Specifically, this study introduces a new near-single green upconversion luminescent Ca9(VO4)7: Yb3+/Er3+ material, synthesized through high-temperature solid-phase methods. When subjected to 980 nm near-infrared laser irradiation, the Ca9(VO4)7: 0.3 Yb3+, 0.02 Er3+ phosphor demonstrates robust green luminescence at 531 nm (2H11/2 → 4I15/2) and 558 nm (4S3/2 → 4I15/2), accompanied by faint red luminescence at 665 nm (4F9/2 → 4I15/2). The optimal doping concentrations for Yb3+ and Er3+ ions, yielding the highest upconversion luminescence intensity, are determined to be 0.3 and 0.02, respectively. The investigation elucidates the upconversion luminescence mechanism through the downward shift spectrum in photoluminescence and the pumping power dependence of upconversion green-red luminescence. The primary excitation pathway identified for the upconversion process in Ca9Y(VO4)7: 0.3 Yb3+, 0.02 Er3+ involves Yb3+ (2F5/2) + Er3+ (4I11/2) → Yb3+ (2F7/2) + Er3+ (4F7/2). This research contributes to the exploration of new materials, injecting vitality into modern industries and production processes.

Improvement of upconversion and temperature sensing properties in Ho3+/Yb3+ co-doped Ca0.5Gd(WO4)2 phosphor via incorporation of Bi3+

A series of Ca0.5Gd(WO4)2: Ho3+/Yb3+/Bi3+ phosphors prepared by solid-state reaction method were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy disperse spectroscopy (EDS), elemental mapping, upconversion (UC) spectroscopy and decay time analysis. Under 980 nm excitation, four up-converted emission bands of Ho3+ were observed in all samples. The concentration quenching effect of the UC emission appears by changing the concentration of Ho3+ in Ca0.5Gd(WO4)2: x mol% Ho3+, 40 mol% Yb3+ and Yb3+ in Ca0.5Gd(WO4)2: 1 mol% Ho3+, y mol% Yb3+, and the optimal concentration of Ho3+ and Yb3+ were determined to be 1 and 40 mol%, respectively. Compared to Bi3+-absent sample, the upconversion luminescence (UCL) intensities of green emission (540 nm) and red emission (644 nm) in Ca0.5Gd(WO4)2: 1 mol% Ho3+, 40 mol% Yb3+ via 15 mol% Bi3+ doping were increased by 2.5 and 3.6 times, respectively. The involved UCL mechanism based on the power dependent UC emission spectra were analyzed in detail. Additionally, the temperature sensing performances based on non-thermally coupled levels (NTCLs) 5F4(5S2) → 5I8, and 5F5 → 5I8 of Ho3+ ions and thermal quenching properties of the samples were also investigated in the range of 298–573 K. The maximum relative sensitivity (SR) of Ca0.5Gd(WO4)2: 1 mol% Ho3+, 40 mol% Yb3+ and Ca0.5Gd(WO4)2: 1 mol% Ho3+, 40 mol% Yb3+, 15mol% Bi3+ reached 0.0094 K-1 and 0.0112 K−1 at 298 K, respectively. The results indicated that doping Bi3+ ions can enhance the UCL intensity and optical temperature sensing of Ca0.5Gd(WO4)2: Ho3+/Yb3+.

Understanding and Effective Tuning of Red‐to‐Green Upconversion Emission in Ho‐Based Halide Double Perovskite Microcrystals

AbstractHo‐based lead‐free double perovskite microcrystals (MCs), Cs2NaHo1‐xYbxCl6, are synthesized and demonstrated to be promising optical temperature sensing materials by understanding the concentration‐dependent upconversion (UC) luminescence mechanisms. By optimizing the dopant concentration, a dominant emission shifting from red to green is successfully achieved in an activator‐rich system. During this process, the red to green emission intensity ratio (R/G) can be changed from 12.5 (pure red light) to 0.14 (pure green light). It is clarified that the cross‐relaxation (CR) process between activator and the energy transfer process between Yb3+ and Ho3+ are the key mechanisms of UC luminescence intensity and R/G ratio variation. Moreover, the intensity of red and green UC emission shows strong dependence on temperature, resulting from the competition between CR and energy transfer in the emission levels. The high‐sensitivity Ho‐based thermometer can be obtained by Cs2NaHo0.99Yb0.01Cl6 MCs with bright red UC luminescence, whose fluorescence intensity ratio thermometer gains maximum relative sensitivities reaching 0.68% K−1 at 400 K. The multicolor UC luminescence quantitative modeling mentioned here is crucial for understanding the concentration‐dependence UC luminescence characteristics and to optimize the design of the Ho‐based thermometer. This study can be an extension of the application range for perovskite UC materials.

Impact of varying Li+ concentration on the upconversion emission and temperature sensing characteristics of YVO4: Tm3+/Yb3+ phosphor

A series of Li+ codoped YVO4: Tm3+/Yb3+ phosphors were prepared by Pechini sol–gel reaction technique. Structural study of the material confirms the crystal structure of the prepared samples and also gives information about the crystallite size and phase purity with the variation of Li+ ion concentrations. Emission peaks around ∼ 476, and ∼ 799 nm corresponds to the 1G4→3H6, and 3H4→3H6 transitions, which could be seen under irradiation of NIR (980 nm) laser light. Li+ (10 mol%) codoped sample shows maximum upconversion. Li+ doping significantly increases the up-conversion emission intensities by more than 5 times for emission peaks around 476 nm and 799 nm.The possible explanation for the increase in UC (Upconversion) emission intensity is attributed to both improved crystallinity and crystal field modification around Tm3+ ions. The variation of UC emission spectra with respect to pump power was observed to investigate the underlying UC luminescent mechanism in YVO4: Tm3+/Yb3+ codoped with Li+ (10 mol%). The multiphoton involvement is observed in the power-dependent study of upconverted blue and NIR emission bands. Widely used fluorescence intensity ratio (FIR) technique was used to evaluate the temperature sensing capability of the optimized Li+ codoped phosphor. The FIR study of Stark components 3F2, and 3H4 levels indicates that this material is an efficient material for temperature sensing. The optimized sample, doped with Li+, exhibits relative sensitivity of 0.78% K−1 at a temperature of 548 K. This material have utility in both light-emitting diodes and as a temperature-sensing probe.

Modulated upconversion luminescence of WO3: Yb3+/Ho3+ through non-equivalence substitution of Bi3+ ions

The upconversion luminescence (UCL) of inorganic materials doped with activators and sensitizers has great application, but its luminescence intensity always impose restrictions on its development. Herein, an obvious enhancement in luminescence intensity of the WO3: Yb3+, Ho3+ phosphor was realized through non-equivalence substitution of Bi3+ ions. The single pure phase of the synthesized WO3: Yb3+, Ho3+, and Bi3+ phosphors has been confirmed by the XRD pattern. The morphology of phosphors and valence state of elements have been performed by SEM and XPS techniques. The sample on excitation with 980 nm performs forceful green emission and weak red emission at 543 nm and 662 nm, separately. The concentrations for doped Yb3+ and Ho3+ to obtain the optimal UCL in WO3 are 8% and 1%, respectively. The optimized doping concentration for Bi3+ ions is 3%. Compared to samples without Bi3+ ions, the Yb3+/Ho3+/Bi3+ doped WO3 obtains 1.59 and 2.33 times increase in emission intensity of green and red, respectively. We also explored the UCL mechanism and enhancement mechanism on the basis of the power dependency, the UCL and the downshift luminescence. These results based on Yb3+/Ho3+ UCL phosphors research is of great significance.

Recent advances in rare earth ion-doped upconversion nanomaterials: From design to their applications in food safety analysis.

The misuse of chemicals in agricultural systems and food production leads to an increase in contaminants in food, which ultimately has adverse effects on human health. This situation has prompted a demand for sophisticated detection technologies with rapid and sensitive features, as concerns over food safety and quality have grown around the globe. The rare earth ion-doped upconversion nanoparticle (UCNP)-based sensor has emerged as an innovative and promising approach for detecting and analyzing food contaminants due to its superior photophysical properties, including low autofluorescence background, deep penetration of light, low toxicity, and minimal photodamage to the biological samples. The aim of this review was to discuss an outline of the applications of UCNPs to detect contaminants in food matrices, with particular attention on the determination of heavy metals, pesticides, pathogenic bacteria, mycotoxins, and antibiotics. The review briefly discusses the mechanism of upconversion (UC) luminescence, the synthesis, modification, functionality of UCNPs, as well as the detection principles for the design of UC biosensors. Furthermore, because current UCNP research on food safety detection is still at an early stage, this review identifies several bottlenecks that must be overcome in UCNPs and discusses the future prospects for its application in the field of food analysis.

Enhanced upconversion luminescence and single-band red emission in β-Ba2ScAlO5: Yb3+/Er3+ phosphor through doping Cu2+

This study reports on a single-band red upconversion emission of Er3+ in a pure-phase β-Ba2ScAlO5: Yb3+/Er3+ fluorescent powder by doping Cu2+. The optimum concentration of Cu2+ ions in β-Ba2ScAlO5: Yb3+/Er3+ powder was 0.20. The red and green emission intensity of β-Ba2ScAlO5: Yb3+/Er3+ sample with optimum concentration of Cu2+ ions increased by 19.1 and 3.6 times, respectively, relative to the β-Ba2ScAlO5:Yb3+/Er3+ material lacking Cu2+ ions. The red-green integral intensity ratio of the β-Ba2ScAlO5:Yb3+/Er3+/Cu2+ phosphor reached 141.9. The potential upconversion luminescence (UCL) mechanism was explored by analyzing the down-shift luminescence (DS) spectra, upconversion luminescence spectra, and upconversion fluorescence lifetime decay curves of Yb3+/Er3+/Cu2+ triple-doped β-Ba2ScAlO5. Enhanced red emission and green upconversion emission are achieved by energy transfer between defect band by the oxygen vacancy and Er3+ energy levels. This study's findings establish the foundation for developing novel phosphors with single UC red light emission, which has potential applications in illumination displays, optical thermometry, and biomedicine.

Infrared to visible up-conversion luminescence of SrF2:Ho particles upon excitation of the 5I7 level of Ho3+ ions

The visible and near-infrared (NIR) up-conversion luminescence (UCL) of a concentration series of fluoride phosphors SrF2:Ho (CHo = 1.3, 3.5, 4.8, 7.4, 8.9, 10.8, 13.0, 13.6 mol.%) upon excitation of the 5I7 level of Ho3+ ions are presented. The dependences of the UCL intensity corresponding to the 5F3→5I8, 5S2(5F4)→5I8, 5F5→5I8, 5I4→5I8, 5F3→5I6, 5I5→5I8, 5F5→5I7, 5I6→5I8 transitions on the concentration of Ho3+ ions were revealed. The UCL energy yield, color coordinates, and correlated color temperature (CCT) values were estimated. The maximum values of UCL energy yields of 0.03% (380–780 nm) and 0.09% (380–1100 nm) were registered for SrF2:Ho (3.5 mol.%) sample. All studied fluoride phosphors are characterized by a red glow of UCL. This glow corresponds to CCT of 6711–7474 K upon incident power density of 28 W/cm2. The UCL mechanisms of SrF2:Ho phosphors were proposed. The SrF2:Ho phosphors can be used as laser beam viewers of the two-micron radiation of thulium and holmium commercial lasers.

Controllable crystal form transformation and temperature sensing properties studies of K3Sc0.5Lu0.5F6: Ho3+, Yb3+ with cryolite structure

AbstractIn this work, a method for controllable regulation of crystal form was proposed for the first time and the effects of different crystal forms on the up‐conversion luminescence properties of Ho3+, Yb3+ co‐doped K3Sc0.5Lu0.5F6 (KSLF) phosphors was explored. KSLF:Ho3+, Yb3+ phosphors were synthesized by the high temperature solid‐state reaction method, and X‐ray diffraction (XRD) results illustrate that the compounds can obtain monoclinic phase and cubic phase at different synthesis temperatures. In addition, according to the power‐dependent up‐conversion luminescence (UCL) intensity, the UCL mechanism and the electronic transition process are discussed. Simultaneously, the temperature sensing behavior of the best samples with different crystal forms at different temperatures was studied by fluorescence intensity ratio technique. All the results show that KSLF:Ho3+, Yb3+ phosphors show great application potential in the field of temperature sensing, and the preparation of KSLF:Ho3+, Yb3+ in different crystal forms can provide a reference value for crystal design and growth.

Distinct Luminescent Thermal Behaviors of Yb3+- and Nd3+-Sensitized Core/Shell Upconversion Nanocrystals

Luminescent materials generally exhibit the thermal quenching due to accelerated nonradiative transitions at elevated temperatures. Recently, the discovery of anomalous thermally induced luminescence enhancement in lanthanide-doped upconversion nanocrystals (UCNCs) has aroused wide attention. The research on this topic not only deepens the understanding on the upconversion luminescence (UCL) mechanism but also promotes frontier applications based on UCNCs. Herein, comparative studies on temperature-dependent UCL properties were performed between Yb3+- and Nd3+-sensitized active-core/active-shell UCNCs. Opposite luminescent thermal behaviors were revealed for these two types of core/shell NCs, and the underlying mechanisms were elucidated with the assistance of temperature-dependent steady/transient spectral analysis under various atmospheres. Yb3+-sensitized active-core/active-shell NCs exhibited an anomalous luminescence enhancement with increasing temperature, which was attributed to the thermally alleviated quenching of surface-adsorbed water molecules. By contrast, Nd3+-sensitized active-core/active-shell NCs showed a normal thermal quenching due to rapidly increased deactivation rates of Nd3+ ions at elevated temperatures, although their UCL intensity was also affected by the quenching effect of surface-adsorbed water molecules. Highly secure anti-counterfeiting patterns were also demonstrated using the nanohybrids of Yb3+- and Nd3+-sensitized core/shell UCNCs as luminescent inks on the basis of their opposite luminescent thermal behaviors. This work provides new insights into luminescent thermal behaviors and energy loss pathways of lanthanide-doped UCNCs.

Nonradiative Energy Transfer in Bi2O3/Tm2O3 Powders under IR Excitation at Liquid Nitrogen Temperature

The presented work shows the study of energy transitions in the NIR and visible regions in the system of Bi2O3 and Tm2O3 powders. Mechanisms of upconversion luminescence and NIR luminescence between two Bi3+ and Tm3+ ions at T = 80 K accompanied with nonradiative energy transfer through the vibrational levels were investigated under IR photoexcitation. The absorption bands of the samples on the reflection spectra were examined in the visible region. The values of the emission cross-section parameters were calculated for the Bi2O3/Tm2O3 complexes.