Genesis of temperature-driven red-shift of charge transfer band edge for Sm3+-doped vanadate self-activated phosphor

The temperature-driven redshift of the charge transfer band (CTB) exhibits promising potential for optical temperature sensing as well as for the design of anti-thermal quenching phosphors. Therefore, it is essential to investigate the displacement mechanism in detail. In this contribution, we created LiCa2.95MV3O12:5%Sm3+ (M = Mg/Zn) phosphors with considerably red-shifted CTB edges upon temperature stimulation and outstanding anti-thermal quenching behavior. To investigate this unusual behavior, gaussian fitting was performed on the excitation spectra and emission spectra at different temperatures to investigate the redshift mechanism. By averaging the peak energy of the lowest excitation and emission peaks, the zero phonon line (Ezp) indicating the electronic energy level location of the charge transfer state (CTS) shows a downward trend is obtained. As well as the energy reduction of the 1A2(1T1)-1B1(1T2) and 1E(1T1)-1B1(1T2) absorption bands in the [VO4]3- group is observed. Therefore, the drop in the CTS electronic energy level is the dominant factor in the temperature-driven CTB redshift. Based on the redshift phenomenon and anti-thermal quenching phenomenon of CTB, the phosphor exhibited exceptional optical temperature measurement performance in all three thermometry modes of excitation intensity ratio (EIR), International Committee on Illumination (CIE) color coordinates, and fluorescence intensity ratio (FIR), demonstrating its broad application prospects in the field of optical temperature sensing as well as guiding the design of anti-thermal quenching phosphors.

Emission color tuning and dual-mode luminescence thermometry design in Dy3+/Eu3+co-doped SrMoO4phosphors

The challenge of building a highly reliable contactless temperature probe with high sensitivity, good temperature-induced color discriminability, and economical synthesis has prompted the research community to work in the field of rare-earth-based luminescence thermometry. Moreover, the fast-growing market for optoelectronic devices has increased the demand for tunable color-emitting phosphors. In this study, Dy3+/Eu3+co-doped SrMoO4phosphors were developed as tunable color-emitting source and dual-mode luminescence thermometer. A facile and cost-effective auto-combustion method was used to synthesize the phosphors. Our work demonstrates a viable scheme for tailoring the emission of single-phase phosphors by precisely controlling the dopant concentrations and by modulating excitation wavelength. The overall emission is tuned from greenish-yellow to white and greenish-yellow to reddish-orange. A detailed energy transfer process from the host to the Ln3+ions and between the Ln3+ions is discussed. Further, anti-thermal quenching in the emission of Dy3+ion is observed when excited with 297 nm. The dual-mode luminescence thermometry has been studied by analyzing the fluorescence intensity ratio of Dy3+and Eu3+ions upon excitation at 297 nm. The maximum relative sensitivity value for 4% Eu3+co-doped SrMoO4:4%Dy3+phosphor is 1.46% K-1at 300 K. Furthermore, the configurational coordinate diagram is presented to elucidate the nature of temperature-dependent emission. Therefore, our research opens up new avenues for the development of color-tunable luminescent materials for various optoelectronic and temperature-sensing applications.

Near-UV excited multifunctional near-infrared phosphors for anti-counterfeiting and ratiometric thermometer applications

Considerable efforts have been devoted to the development of visible light-emitting phosphors for anti-counterfeiting application, but the design of multifunctional near-infrared materials with suitable luminescent properties was lacking. Based on the crystal field theory, a near ultraviolet excitable and near-infrared emitting Ca₂Sn_2(1-x)Al_2(1-x)O₉:2xCr³⁺,2xSi⁴⁺ phosphor was designed. On the one hand, the Ca₂Sn_2(1-x)Al_2(1-x)O₉:2xCr³⁺,2xSi⁴⁺ phosphor showed great potential for applications including anti-counterfeiting and secret signals under a 405 nm near UV laser. The integrated intensity at 150 °C was 75.9%, implying high thermal stability when irradiated with a high power light source. On the other hand, an unexpected red emission component originating from the Cr³⁺-Cr³⁺ dimer was observed and was confirmed by fast luminescence decay and electron spin resonance signals. The different thermal stabilities of Cr³⁺ near-infrared and Cr³⁺-Cr³⁺ red emissions opened up a new opportunity for luminescence ratiometric thermometers.

Eu3+ Single-Doped Phosphor with Antithermal Quenching Behavior and Multicolor-Tunable Properties for Luminescence Thermometry

To date, non-contact luminescence thermometry methods based on fluorescence intensity ratio (FIR) technology have been studied extensively. However, designing phosphors with high relative sensitivity (S_r) has become a research hotspot. In this work, Eu³⁺ single-doped Ca₂Sb₂O₇:Eu³⁺ phosphors with a high S_r value for dual-emitting-center luminescence thermometry are developed and proposed. The anti-thermal quenching behavior of Eu³⁺ originating from the energy transfer (ET) of host → Eu³⁺ is found and proved in the designed phosphors. Interestingly, adjustable color emission from blue to orange can be achieved. Surprisingly, the degree of the anti-thermal quenching behavior of Eu³⁺ gradually reduces from 240 to 127% as the Eu³⁺ doping content increases from 0.005 to 0.05 mol, attributed to most Eu³⁺ being located in the low symmetrical [Ca1O₈] dodecahedral site. According to the differentiable responses of the host and Eu³⁺ to temperature, the maximal S_r value reaches 3.369% K^-1 (383 K). Moreover, the ambient temperature can be intuitively predicted by observing the emitting color. Owing to the excellent performance in optical thermometry, color-tunable properties, and outstanding acid and alkali resistance for polydimethylsiloxane (PDMS) films, the developed Eu³⁺ single-doped Ca₂Sb₂O₇:Eu³⁺ phosphors are expected to be prospective candidates in luminescence thermometers and LED devices in various conditions.

Fluorescence Lifetime-Based Luminescent Thermometry Material with Lifetime Varying over a Factor of 50

Recently, the growing demand for temperature detection is pushing forward the flourishing development of noncontact optical thermometry. Herein, a new red phosphor Sr₂InTa_1-xO₆:xMn⁴⁺ (SIT:xMn⁴⁺) was first constructed and systematically investigated. Based on the fairly rapid decline of the lifetime from 0.403 to 0.008 ms by increasing the temperature from 25 to 450 K, a noncontact optical thermometer can be made from phosphor SIT:0.003Mn⁴⁺ with S_r = 1.396% K^-1 at 375 K and S_a = 0.0012 K^-1 at 300 K. Because the luminescence is based on the outermost 3d orbits of Mn⁴⁺, the lifetime of SIT:xMn⁴⁺ is quite sensitive to the temperature, leading to a rapid decline of the lifetime with the increase in temperature. Moreover, multiple rounds of variable-temperature studies were performed to demonstrate the stability and reversibility of SIT:0.003Mn⁴⁺. This work suggests that Mn⁴⁺-phosphors are promising candidates for application as optical thermometric material.

Systematic Crystallization of NH4Ln(MoO4)2 as a Family of Layered Compounds (Ln = La-Lu and Y), Derivation of Ln2Mo4O15, Crystal Structure, and Photoluminescence

NH₄Ln(MoO₄)₂ (Ln = La-Lu lanthanide, Y) was crystallized via hydrothermal reaction as a new family of layered materials, from which phase-pure Ln₂Mo₄O₁₅ was successfully derived via subsequent annealing at 700 °C for the series of Ln elements excluding Ce and Lu. Detailed structure analysis revealed that the ionic size of Ln³⁺ decisively determined the crystal structure and Mo/Ln coordination for the two families of compounds. NH₄Ln(MoO₄)₂ was analyzed to be orthorhombic (Pbcn space group, no. 60) and monoclinic (P2/c, no. 13) for the larger and smaller Ln³⁺ of Ln = La-Gd and Ln = Tb-Lu (including Y), respectively, where both the crystal structures have a layered topology featured by the alternative stacking of a [Ln(MoO₄)₂]^- three-tier infinite anionic layer and interlayer NH₄⁺. Four types of crystal structures were found for the Ln₂Mo₄O₁₅ series, which are monoclinic (P2₁/a, no. 14) for Ln = La, triclinic (P1̅, no. 2) for Ln = Pr-Sm, triclinic (P1̅, no. 2) for Ln = Eu and Gd, and monoclinic (P2₁/c, no. 14) for Ln = Tb-Yb (including Y). The photoluminescence of NH₄Ln(MoO₄)₂ (Ln = Eu, Tb) and Ln₂Mo₄O₁₅:Eu³⁺ (Ln = La, Gd, Y) was thoroughly investigated in terms of spectral features, quantum efficiency, fluorescence decay, and CIE chromaticity. The thermal stability of luminescence was also studied for Ln₂Mo₄O₁₅:Eu³⁺, and the observed charge-transfer excitation components were successfully correlated with the features of the Mo-O polyhedron/unit.

Optical thermometry based on europium doped self-activated dual-emitting LiCa3ZnV3O12 phosphor

In this study, rare-earth-doped self-activated LiCa₃ZnV₃O₁₂ (LCZV) vanadate phosphors were preparation by a high-temperature solid-state reaction. Their crystal structure, non-contact temperature sensing, and luminescence properties were studied deeply. Excited by ultraviolet light at 340 nm, the emission of [VO₄]^3- group and the Eu³⁺ ions were monitored. The highest strength emission peaks at 470 nm and 610 nm for [VO₄]^3- and Eu³⁺, respectively, provide favorable signal identification for estimating temperature. Due to thermal quenching behavior and energy transfer, the FIR (Fluorescence Intensity Ratio) from Eu³⁺ to [VO₄]^3- exhibits excellent sensitivity performance at 303 K - 523 K. In the meantime, the maximum absolute and relative sensitivities of the obtained phosphors are 0.0068 K^-1 and 1.18 % K^-1, which are overtopped to those reported previously. Furthermore, for the luminescent color of the CIE diagram with a strong temperature effect, the color coordinate could be verified from (0.2871, 0.3416) to (0.4121, 0.3420), which was matched well with the linear equation. Consequently, the Eu³⁺ doped LCZV phosphor not only can be used for high-temperature environmental safety signals but also is an extraordinary viable material in the field of optical sensing.

(3+1)-Dimensional Modulated Structure and Tb3+-Doped Luminescence for Vanadate Na3La(VO4)2

A modulated structure with symmetrical characteristic higher than three-dimensional is a fascinating crystallographic type, and it is randomly encountered by researchers. Herein, we prepared 0.1 mm level single crystals of Na₃La(VO₄)₂ and determined its structure to be a (3 + 1)-dimensional modulated structure using the X-ray diffraction analysis method for single crystals. Its super space group was determined to be Pca2₁(0β0)000. On the other hand, we introduced Tb³⁺ into the Na₃La(VO₄)₂ host lattice to fabricate phosphors Na₃La_1-x(VO₄)₂:xTb³⁺ and studied their photoluminescence properties. Interestingly, for the strong absorption of the Na₃La(VO₄)₂ host lattice in the range of 200-400 nm, the traditional 330-385 nm light is unable to efficiently excite Tb³⁺ ions in the Na₃La(VO₄)₂ host to generate luminescence of Tb³⁺. Instead, Na₃La_1-x(VO₄)₂:xTb³⁺ is suitable to be excited by 487 nm to generate emitting light at 543, 584, and 622 nm, due to Tb³⁺ characteristic 4f → 4f transitions of ⁵D₄ → ⁷F_J (J = 5, 4, 3). Hence, the Tb³⁺-doped Na₃La(VO₄)₂ phosphors have potential applications for white-light-emitting diodes.

Synthesis and optical spectroscopy of Na3Y(VO4)2:Eu3+ phosphors for thermometry and display applications

A new Na3Y(VO4)2:Eu3+ (NYVO:Eu3+) phosphor was prepared using the sol-gel method. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to evaluate phase purity and particle size, respectively. The optical properties were investigated by UV-visible absorption, PL, and PLE spectroscopies. The absorption measurements show the formation of the vanadate host by the presence of its characteristic band in the visible region related to VO4 3- groups. The experimental results show that the NYVO:Eu3+ phosphors exhibit high-brightness and thermally stable emission. Under near-ultraviolet (UV) excitation, both the broadband emission from VO4 3- groups and the sharp peak emissions from Eu3+ ions are observed. The highest luminescence intensity was achieved for an optimal europium concentration of 15 mol%. The study of the chromaticity parameters of these compounds gives a thermally stable hot emission in the red domain, with a color purity of about 85%, which qualifies the NYVO:Eu3+ compound as a potential phosphor for light-emitting diode (LED) applications. Thermal sensing using NYVO:Eu3+ phosphors are based on monitoring the luminescence intensity ratio between the NYVO host emission and Eu3+ luminescence lines. Notably, the optical thermometry of NYVO:Eu3+ was characterized based on the fluorescence intensity ratio of VO4 3- and Eu3+ emissions in the 298-440 K range, with maximum absolute and relative sensitivities of 3.4% K-1 and 0.0032 K-1 respectively and a temperature uncertainty of 0.01. NYVO:Eu3+ can then be considered as a potential red phosphor for application in ultraviolet-pumped white light-emitting diodes and as a potential optical thermometer. It provides new possibilities for the design of multifunctional materials for red light-emitting diodes and for non-contact thermometry.

High performance FIR thermometry on the basis of the redshift of CTB by dual-wavelength alternative excitation in Eu3+:YVO4

Compared with the forbidden 4f transition of rare earth ions, the strong absorption of the charge transfer band (CTB) enabled fluorescence thermometry to have high luminescence efficiency. Based on the temperature induced redshift of CTB, a high performance fluorescence intensity ratio (FIR) thermometry performed by dual-wavelength alternative excitation was studied. By way of the rising and falling edges of CTB in Eu³⁺ doped YVO₄, monochrome sensitivity as a function of excitation wavelength was studied in the range of 303-783 K. The excitation wavelength with the highest positive monochrome sensitivity was determined, as well as that with the negative one. The optimum FIR temperature sensing strategy is proposed, and the theoretical highest relative sensitivity (S_r) is calculated to be 1.86% K^-1, with the lowest uncertainty (ΔT) of 0.1 K at 783 K.

Photoluminescence and optical temperature measurement of Mn4+/Er3+ co-activated double perovskite phosphor through site-advantageous occupation

Because traditional methods based on thermal coupling energy level temperature measurement have large errors, a new temperature sensing strategy is proposed to attain strong sensitivity and excellent signal resolution ability. The rare-earth and also transition metal ions with poles apart thermal quenching channels are used as dual emission centers to find a suitable host to achieve high-efficiency dual-mode emission. In this work, a string of phosphors with NaLaMgWO₆ (NLMW) as the host, the single-doped and double-doped Mn⁴⁺ and Er³⁺ phosphors were adopted by the traditional high temperature solid-state reaction method. The crystallographic structure of the phosphor was analyzed by X-ray power diffraction and Rietveld refinement methods, and the results showed that a pure double perovskite phosphor with a monoclinic structure was successfully prepared. The photoluminescence excitation and emission spectrum properties, CIE chromaticity coordinates and photoluminescence spectra at different temperatures are detailed studied. Excited by ultraviolet light (300 nm), corresponding to the ⁴A₂→⁴T₁ transition of Mn⁴⁺ and the charge transfer from O^2- to W⁶⁺ of Er³⁺. There is no energy transfer between Mn⁴⁺ and Er³⁺. NLMW:Mn⁴⁺/Er³⁺ phosphors were especially sensitive to temperature changes within the scope of 303 K to 523 K. As the temperature increases, the fluorescence intensity of Mn⁴⁺ is thermally quenched faster than Er³⁺. The luminescent intensity ratio of Er³⁺ thermal coupling level and the FIR of Er³⁺/Mn⁴⁺ are used to study the temperature performance. The results show that the maximum relative sensitivity of FIR up to 1.31% K^-1, which is higher than the maximum temperature sensitivity based on the thermal coupling energy level, and which is beyond most of the non-contact temperature measurement materials previously reported, confirming that NLMW:Mn⁴⁺/Er³⁺ phosphors have application potential in non-contact temperature measurement.

Malate-aided selective crystallization and luminescence comparison of tetragonal and monoclinic LaVO4:Eu nanocrystals

With malate (Mal2-) as a new type of chelate, tetragonal (t-) and monoclinic (m-) structured LaVO4:Eu crystals (∼10-60 nm) were selectively crystallized as nanosquares and nanorods via a hydrothermal reaction at 200 °C for 24 h. The effects of the Mal2-:(La,Eu)3+ molar ratio, solution pH and Eu3+ content on the phase structure and crystal morphology were systematically investigated and elucidated. The competition between OH- and Mal2- toward rare earth ions was discussed to play a critical role in phase selection, and the t-phase can only be fabricated at pH ∼ 6-8 with the assistance of Mal2-. The optimal Eu3+ content for luminescence was determined to be ∼5 at% under the VO43- → Eu3+ energy transfer mechanism. Experimental comparison showed that t-(La0.95Eu0.05)VO4 (λex = 275 nm, λem = 620 nm) emits ∼5.3 times as strong as m-(La0.95Eu0.05)VO4 does (λex = 313 nm, λem = 616 nm), while theoretical analysis revealed that the 5D0 level of Eu3+ has a quantum efficiency of ∼80% for the former and ∼70% for the latter. Besides, the t- and m-(La0.95Eu0.05)VO4 nanocrystal phosphors were analyzed to have fluorescence lifetimes of ∼1.53 ± 0.01 and 2.28 ± 0.01 ms for their 620 and 616 nm red emissions, respectively.

Temperature sensitivity modulation through changing the vanadium concentration in a La2MgTiO6:V5+,Cr3+ double perovskite optical thermometer

To fulfil the requirements of operating at low temperature in a harsh environment, the investigation on optical thermometers plays an increasingly important role. In this work, the influence of vanadium concentration on the capability of temperature readout by La2MgTiO6:V5+,Cr3+ luminescent thermometers was investigated for the first time. The presence of V3+ and V5+ was verified by XPS and absorption measurements. In the emission spectra, a blue-green emission region was assigned to both host and V5+ emission. Moreover, a spin-forbidden emission of Cr3+ ions was also detected. Vanadium ions in the +3 oxidation state do not exhibit luminescence, but play a role as a charge compensator. The highest emission intensity was obtained from the sample doped with 0.1% V. Besides, with increasing vanadium concentration, a redshift in the maximum position of the spectrum was observed corresponding to a movement from the greenish blue to yellowish green region in the CIE1931. It was shown that the relative sensitivity (Sr) and the temperature operating range can be easily modified by changing the concentration of vanadium ions. In particular, the outstanding relative sensitivities of 1.71% K-1 (at 187 K) and 1.96% K-1 (at 165 K) obtained from La2MgTiO6:0.1%V5+,Cr3+ and La2MgTiO6:0.05%V5+,Cr3+ demonstrated the enormous potential of this material for thermal sensing application.

Facile modulation the sensitivity of Eu2+/Eu3+-coactivated Li2CaSiO4 phosphors through adjusting spatial mode and doping concentration

Series of Eu²⁺/Eu³⁺-coactivated Li₂CaSiO₄ phosphors were prepared by solid-state reaction technique. All the samples emitted the unique emissions of Eu²⁺ and Eu³⁺ ions when excited by 395 nm, while the strongest emission intensity was received when x = 0.03. On the basis of theoretical discussion, it is evident that crossover relaxation should be responsible for the thermal quenching mechanism which was further proved by the unchanged lifetime at elevated temperature. Besides, through analyzing the inconsistent responses of the emission intensities of the Eu²⁺ and Eu³⁺ ions to the temperature, the optical thermometric properties of the designed phosphors were studied. By selecting different emissions of Eu³⁺ ions and combining with that of the Eu²⁺ ions, adjustable sensitivities were realized in the resultant phosphors. Furthermore, the sensitivities of the studied compound were also found to be greatly affected by the doping concentration. The maximum absolute and relative sensitivities of the synthesized compounds were 0.0025 K^-1 and 0.289% K^-1, respectively. These achieved results implied that the Eu²⁺/Eu³⁺-coactivated Li₂CaSiO₄ phosphors were promising candidates for optical thermometry. Additionally, this work also provided promising methods to modulate the sensitivities of the luminescent compounds by adjusting spatial mode and doping concentration.

Ultra-high sensitivity of multicolor Sm3+-doped LiSrVO4 phosphors for contactless optical thermometers

Multicolor Sm³⁺-doped LiSrVO₄ phosphors with ultra-high S_a and S_r values of 1.076 K⁻¹ and 6.167% K⁻¹, respectively, are promising candidates for contactless thermometers.