Mutual energy transfer luminescent properties in novel CsGd(MoO4)2:Yb3+,Er3+/Ho3+ phosphors for solid-state lighting and solar cells

Intensity enhancement of photoluminescence in Tb3+ /Eu3+ co-doped Ca14 Zn6 Al10 O35 phosphor for WLEDs

This article focuses on the effect of monovalent cation doping on the optical properties of rare earth (RE = Eu³⁺ , Tb³⁺ ) co-doped Ca₁₄ Zn₆ Al₁₀ O₃₅ which has been synthesized by a low temperature combustion method. Crystalline phase of the Ca₁₄ Zn₆ Al₁₀ O₃₅ phosphor was examined and confirmed by X-ray diffraction measurement. Under near-ultraviolet light excitation Eu³⁺ -doped Ca₁₄ Zn₆ Al₁₀ O₃₅ phosphor exhibit characterization of Eu³⁺ emission bands that are located at a maximum wavelength (λ_max ) of approximately 470 nm and other peaks centred at 593 nm and 615 nm, respectively. With Tb³⁺ -doped Ca₁₄ Zn₆ Al₁₀ O₃₅ phosphor showing a green emission band centred at 544 nm under near-ultraviolet range. Furthermore, we studied the energy transfer process in Eu³⁺ /Tb³⁺ pair and enhancement in photoluminescence (PL) intensity with doping different charge compensation. Here we obtained the optimum PL emission intensity of the phosphor in broad and intense visible spectral range which may be significant for the fabrication of white light emitting diodes (WLEDs).

Optical Properties Investigation of Upconverting K2Gd(PO4)(WO4):20%Yb3+,Tm3+ Phosphors

Nowadays, scientists are interested in inorganic luminescence materials that can be excited with UV or NIR radiation and emit in the visible range. Such inorganic materials can be successfully used as luminescent or anti-counterfeiting pigments. In this work, we report the synthesis and optical properties investigation of solely Tm³⁺ doped and Yb³⁺/Tm³⁺ co-doped K₂Gd(PO₄)(WO₄) phosphors. The single-phase samples were prepared using a solid-state reaction method. The Tm³⁺ concentration was changed from 0.5% to 5%. Downshifting and upconversion emission studies were performed under 360 nm and 980 nm excitation, respectively. Yb³⁺ ions were used as sensitizers in the K₂Gd(PO₄)(WO₄) phosphors to transfer the captured energy to Tm³⁺ ions. It turned out that under UV excitation, phosphors emitted in the blue spectral area regardless of the presence or absence of Yb³⁺. However, a very strong deep-red (~800 nm) emission was observed when Yb³⁺ and Tm³⁺-containing samples were excited with a 980 nm wavelength laser. It is interesting that the highest upconversion emission in the UV/Visible range was achieved for 20% Yb³⁺, 0.5% Tm³⁺ doped sample, whereas the sample co-doped with 20% Yb³⁺, 2% Tm³⁺ showed the most intensive UC emission band in the NIR range. The materials were characterized using powder X-ray diffraction and scanning electron microscopy. Optical properties were studied using steady-state and kinetic downshifting and upconversion photoluminescence spectroscopy.

Up-Converting K2Gd(PO4)(WO4):20%Yb3+,Ho3+ Phosphors for Temperature Sensing

Inorganic luminescent materials that can be excited with NIR radiation and emit in the visible spectrum have recently gained much scientific interest. Such materials can be utilized as anti-counterfeiting pigments, luminescent thermometers, bio-imaging agents, etc. In this work, we report the synthesis and optical properties of K2Gd(PO4)(WO4):Ho3+ and K2Gd(PO4)(WO4):20%Yb3+,Ho3+ powders. The single-phase samples were prepared by the solid-state reaction method, and the Ho3+ concentration was changed from 0.5% to 10% with respect to Gd3+. It is interesting to note that under 450 nm excitation, no concentration quenching was observed in K2Gd(PO4)(WO4):Ho3+ (at least up to 10% Ho3+) samples. However, adding 20% Yb3+ has caused a gradual decrease in Ho3+ emission intensity with an increase in its concentration. It turned out that this phenomenon is caused by the increasing probability of Ho3+ → Yb3+ energy transfer when Ho3+ content increases. K2Gd(PO4)(WO4):20%Yb3+,0.5%Ho3+ sample showed exceptionally high up-conversion (UC) emission stability in the 77-500 K range. The UC emission intensity reached a maximum at ca. 350 K, and the intensity at 500 K was around four times stronger than the intensity at 77 K. Moreover, the red/green emission ratio gradually increased with increasing temperature, which could be used for temperature sensing purposes.

Blue LED-pumped intense short-wave infrared luminescence based on Cr3+-Yb3+-co-doped phosphors

The growing demand for spectroscopy applications in the areas of agriculture, retail and healthcare has led to extensive research on infrared light sources. The ability of phosphors to absorb blue light from commercial LED and convert the excitation energy into long-wavelength infrared luminescence is crucial for the design of cost-effective and high-performance phosphor-converted infrared LEDs. However, the lack of ideal blue-pumped short-wave infrared (SWIR) phosphors with an emission peak longer than 900 nm greatly limits the development of SWIR LEDs using light converter technology. Here we have developed a series of SWIR-emitting materials with high luminescence efficiency and excellent thermal stability by co-doping Cr³⁺-Yb³⁺ ion pairs into Lu_0.2Sc_0.8BO₃ host materials. Benefitting from strong light absorption of Cr³⁺ in the blue waveband and very efficient Cr³⁺→Yb³⁺ energy transfer, the as-synthesized Lu_0.2Sc_0.8BO₃:Cr³⁺,Yb³⁺ phosphor emits intense SWIR light in the 900-1200 nm from Yb³⁺ under excitation with blue light at ~460 nm. The optimized phosphor presents an internal quantum yield of 73.6% and the SWIR luminescence intensity at 100 °C can still keep 88.4% of the starting value at 25 °C. SWIR LED prototype device based on Lu_0.2Sc_0.8BO₃:Cr³⁺,Yb³⁺ phosphor exhibits exceptional luminescence performance, delivering SWIR radiant power of 18.4 mW with 9.3% of blue-to-SWIR power conversion efficiency and 5.0% of electricity-to-SWIR light energy conversion efficiency at 120 mA driving current. Moreover, under the illumination of high-power SWIR LED, covert information identification and night vision lighting have been realized, demonstrating a very bright prospect for practical applications.

Eu(iii), Tb(iii) activated/co-activated K2NaAlF6 host array: simultaneous approach for improving photovoltaic efficiency and tricolour emission

Rare earth-activated fluoride phosphors have generated attention in recent years in the field of solid-state lighting and solar cell efficiency enhancement.

Structural and Spectroscopic Effects of Li+ Substitution for Na+ in LixNa1-xCaGd0.5Ho0.05Yb0.45(MoO4)3 Scheelite-Type Upconversion Phosphors

A set of new triple molybdates, Li_xNa_1-xCaGd_0.5(MoO₄)₃:Ho³⁺_0.05/Yb³⁺_0.45, was successfully manufactured by the microwave-accompanied sol–gel-based process (MAS). Yellow molybdate phosphors Li_xNa_1-xCaGd_0.5(MoO₄)₃:Ho³⁺_0.05/Yb³⁺_0.45 with variation of the Li_xNa_1-x (x = 0, 0.05, 0.1, 0.2, 0.3) ratio under constant doping amounts of Ho³⁺ = 0.05 and Yb³⁺ = 0.45 were obtained, and the effect of Li⁺ on their spectroscopic features was investigated. The crystal structures of Li_xNa_1-xCaGd_0.5(MoO₄)₃:Ho³⁺_0.05/Yb³⁺_0.45 (x = 0, 0.05, 0.1, 0.2, 0.3) at room temperature were determined in space group I4₁/a by Rietveld analysis. Pure NaCaGd_0.5Ho_0.05Yb_0.45(MoO₄)₃ has a scheelite-type structure with cell parameters a = 5.2077 (2) and c = 11.3657 (5) Å, V = 308.24 (3) ų, Z = 4. In Li-doped samples, big cation sites are occupied by a mixture of (Li,Na,Gd,Ho,Yb) ions, and this provides a linear cell volume decrease with increasing Li doping level. The evaluated upconversion (UC) behavior and Raman spectroscopic results of the phosphors are discussed in detail. Under excitation at 980 nm, the phosphors provide yellow color emission based on the ⁵S₂/⁵F₄ → ⁵I₈ green emission and the ⁵F₅ → ⁵I₈ red emission. The incorporated Li⁺ ions gave rise to local symmetry distortion (LSD) around the cations in the substituted crystalline structure by the Ho³⁺ and Yb³⁺ ions, and they further affected the UC transition probabilities in triple molybdates Li_xNa_1-xCaGd_0.5(MoO₄)₃:Ho³⁺_0.05/Yb³⁺_0.45. The complex UC intensity dependence on the Li content is explained by the specificity of unit cell distortion in a disordered large ion system within the scheelite crystal structure. The Raman spectra of Li_xNa_1-xCaGd_0.5(MoO₄)₃ doped with Ho³⁺ and Yb³⁺ ions were totally superimposed with the luminescence signal of Ho³⁺ ions in the range of Mo–O stretching vibrations, and increasing the Li⁺ content resulted in a change in the Ho³⁺ multiplet intensity. The individual chromaticity points (ICP) for the LiNaCaGd(MoO₄)₃:Ho³⁺,Yb³⁺ phosphors correspond to the equal-energy point in the standard CIE (Commission Internationale de L’Eclairage) coordinates.

Enrichment of Crystal Field Modification via Incorporation of Alkali K+ Ions in YVO4:Ho3+/Yb3+ Nanophosphor and Its Hybrid with Superparamagnetic Iron Oxide Nanoparticles for Optical, Advanced Anticounterfeiting, Uranyl Detection, and Hyperthermia Applications

In this work, we report a polyol route for easy synthesis of upconversion (UC) phosphor nanoparticles, YVO₄:Ho³⁺-Yb³⁺-K⁺, which enables large-scale production and enhancement of luminescence. Upon 980 nm laser excitation, the UC emission spectrum shows a sharp bright peak at ∼650 nm of Ho³⁺ ion; and the luminescence intensity increases twofold upon K⁺ codoping. Upon 300 nm excitation, the downconversion emission spectrum shows a broad peak in the 400-500 nm range (related to the charge transfer band of V-O) along with Ho³⁺ peaks. In addition, the polyethylene glycol-coated UC nanoparticles are highly water-dispersible and their hybrid with Fe₃O₄ nanoparticles shows magnetic-luminescence properties. A hyperthermia temperature is achieved from this hybrid. Both UC and hybrid nanoparticles show interesting security ink properties upon excitation by a 980 nm laser. The particles are invisible in normal light but visible upon 980 nm excitation and are useful in display devices, advanced anticounterfeiting purposes, and therapy of cancer via hyperthermia and bioimaging (since it shows red emission at ∼650 nm). Using UC nanoparticles, detection of uranyl down to 20 ppm has been achieved.

Brilliant Nonlinear Optical Response of Ho3+ and Yb3+ Activated YVO4 Nanophosphor and Its Conjugation with Fe3O4 for Smart Anticounterfeit and Hyperthermia Applications

YVO4:Ho3+/Yb3+ nanophosphors prepared by an effective polyol-mediated route show dual-mode behavior in photoluminescence. Upon 980 nm excitation, the upconversion red emission spectrum exhibits a bright red peak at ∼650 nm, characteristic of the electronic transition of the Ho3+ ion via involvement of two-photon absorption, which has been confirmed by the power-dependent luminescence study. Moreover, at 300 nm excitation, downconversion emission peaks are observed at 550, 650, and ∼755 nm. The nonradiative resonant energy transfer occurs from the V-O charge transfer band to Ho3+ ions, resulting in an improved emission of Ho3+ ions. Moreover, polyethylene glycol-coated nanoparticles make it suitable for water dispersibility; and these particles are conjugated with Fe3O4 nanoparticles to form magnetic-luminescent hybrid nanoparticles. Highly water-dispersible magnetic-luminescent hybrid material attained the hyperthermia temperature (∼42 °C) under an applied AC magnetic field. The specific absorption rate value is found to be high (138 W/g), which is more than that of pure superparamagnetic Fe3O4 nanoparticles. At 300 nm excitation, the high quantum yield value of ∼27% is obtained from YVO4:Ho3+/Yb3+, which suggests that it is a good phosphor material. By employing the neutron activation analysis technique, it is shown that nanophosphor particles can absorb Au3+ up to the ppm level. Interestingly, such nanophosphor also shows the potentiality for anticounterfeiting applications.

Enhancing the static green up-conversion luminescence of NaY(MoO4)2:Yb/Er microcrystals via an annealing strategy for anti-counterfeiting applications

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.

A flexible and portable all-fiber temperature sensor based on the upconversion luminescence of octahedral NaBi(WO4)2:Er3+/Yb3+ phosphors

Octahedral NaBi(WO4)2:Er3+/Yb3+ phosphors were synthesized by a hydrothermal method. Intense green upconversion emissions from both 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions of Er3+ ions were observed and the appropriate energy gap between them is very suitable for temperature sensing requirement based on the fluorescence intensity ratio (FIR) technique. A flexible and portable all-fiber temperature sensing device was established and used to assess the temperature sensing characteristics of NaBi(WO4)2:Er3+Yb3+ phosphors. A maximum absolute sensitivity of 0.014 K-1 is achieved at 423 K and the temperature absolute error is -0.5 K to +0.6 K. The stepwise heating and cooling processes confirm the good stability and recyclability of the all-fiber temperature sensor, which lays the foundation for actual temperature measurement. Based on the high flexibility and accuracy of the temperature sensor, the monitoring of body temperature was realized in real time and continuously, which may provide new development prospects for effective health tracking and improvement of medical care quality.

Insight into the mechanism of intense NIR-to-red upconversion luminescence in Er3+ doped and Er3+–Yb3+ co-doped SrF2 nanoparticles

SrF₂:Yb³⁺,Er³⁺ NPs were synthesized by the hydrothermal method and their luminescence mechanism was discussed in detail, which provided a theoretical basis for further understanding the properties of the materials.

The thermo-optic relevance of Ho3+ in fluoride microcrystals embedded in electrospun fibers

Na(Y1-x-y Ho x Yb y )F4/PAN (NYF-HY/PAN) composite fibers were synthesized using an electrospinning method, and the sub-micron crystals embedded in the fibers had complete hexagonal crystal structures. Under 977 nm laser excitation, strong green and red up-conversion (UC) emission that originated from flexible fibers were due to the radiative transitions (5F4, 5S2) → 5I8 and 5F5 → 5I8 of Ho3+, respectively. The effective green fluorescence emission (539 and 548 nm) can be applied to micro-domain non-contact temperature measurements, realizing rapid and dynamic temperature acquisition in a complex environment without destroying the temperature field. In the temperature range of 313-393 K, the absolute and relative sensitivity of the fibers are 0.00373 K-1 and 0.723% K-1, respectively, which indicates that the NYF-HY/PAN composite fibers have good thermal sensitivity. Composite fibers in which crystallites are embedded have superior properties, with great stability, high sensitivity, and excellent flexibility, providing a reliable reference for developing temperature-sensing materials for the biomedical field.

LuPO4:Yb phosphor with concerted UV and IR thermoluminescent emissions by quantum cutting at high temperatures

Thermoluminescence of LuPO4:0.1%Yb3+ sintered ceramics was investigated and simultaneous infrared 2F5/2 → 2F7/2 and UV-blue (YbCT3+)* → O2- charge transfer emissions of the Yb3+ impurity were observed around 150 °C (423 K) for the first time. Both photons were generated by one excited Yb3+*. LuPO4:Yb3+ was thus proved to be the first system showing the quantum cutting effect in thermoluminescence. Low concentration of the dopant was proved crucial to observe an intense CT emission at so high temperatures. These data revise deeply those reported previously on the thermal quenching of Yb3+ charge transfer luminescence in orthophosphates. In was formerly claimed that CT luminescence of Yb3+ in LuPO4 and similar hosts is quenched below 300 K. Similarly, the thermoluminescent emission of LuPO4:Yb3+ above room temperature was previously reported to appear only in the IR part of the spectrum around 980 nm. Our results fundamentally change this picture and prove that CT luminescence of Yb3+ in orthophosphates appears to be significant even above 150 °C (423 K). We demonstrate the great significance of the activator concentration in its CT luminescence thermal quenching. The Yb3+ impurity ion was found to act both as an electron trap and as a recombination center. Our data open the possibility to generate intense CT luminescence of Yb3+ in orthophosphates at room temperature and above which may make such phosphors rational for applications previously considered unattainable for them.