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Y2O3 codopedwith different Zr4+ (10% mol, 30%mol, and 49%mol) and constant Er3+ (2% mol) ions were successfully prepared via the laser annealing method. According to the XRD data, the lattice parameters decrease with the increase of Zr4+, and the grain size increases with Zr4+. The crystal space group switches from Ia-3 to Fm-3 m at a Zr4+ concentration of 49% mol. As the concentration of doped Zr4+ increases, the upconversion emission of Er3+ decreases. The nonequivalent substitution of Y3+ by Zr4+ creates lattice defects, which affect the fluorescence intensity of Er3+ ions in the material. The temperature sensing behavior of Y88%Zr10%Er2%O1.55 crystals based on the fluorescence intensity ratio between 4S3/2→4I15/2 and 4I9/2→4I15/2 transitions of Er3+ is studied. The results show that after codoping with Zr4+, the 4S3/2→4I15/2 and 4I9/2→4I15/2 transitions satisfy the Boltzmann distribution formula at temperatures above 333K. And the temperature sensing relative sensitivity can reach 2.96% K-1. [ABSTRACT FROM AUTHOR]

A multifunctional double‐perovskite‐type Ca2LnSbO6 (Ln = La, Y, Gd, and Lu) phosphors with Eu3+ and Mn4+ co‐doped were successfully synthesized and designed to simultaneously apply to temperature sensing and plant growth LED. With temperature varying from 303 to 523 K, the integrated intensity of Mn4+ ions (2Eg → 4A2g) markedly decline swifter comparing to Eu3+ ions (5D0 → 7F2) due to the thermal quenching effect. It implies that Mn4+ is susceptive to the temperature in this work. Eu3+ exhibits high heat resistance, which can be selected as a calibration parameter for the fluorescence intensity ratio temperature sensor. The maximum relative sensitivity (Sr) values of Ca2LnSbO6:1%Eu3+, 0.5%Mn4+ (Ln = La, Y, and Gd) phosphor are estimated as 2.19% K−1 at 443 K, 1.68% K−1 at 503 K, and 1.80% K−1 at 483 K, respectively. They are superior to many recent emerging phosphors. Moreover, upon 365 nm excitation, Ca2LnSbO6:1%Eu, 0.5%Mn (Ln = La, Y, Gd, and Lu) phosphors display a wide red emission peaking at 696, 680, 677, and 684 nm, respectively. They come from the overlapped of 2Eg–4A2g of Mn4+ and 5D0–7F4 transition of Eu3+. Their full widths at half maximum are 34, 35, 35, and 29 nm, respectively. They are all perfectly overlapping the absorption curves of chlorophyll a and chlorophyll b, and their fabricated red phosphor‐converted light‐emitting diode device ulteriorly corroborated the promising applications for plant growth. Markedly, the systematic analysis of their crystal chemistry and site preferentially occupancy is first reported and discussed by the bond valence theory. [ABSTRACT FROM AUTHOR]

Here, Bi3+, Er3+ co‐activated gadolinium phosphors with multimode emission properties are prepared, which can emits blue, green, and orange light under the excitation of ultraviolet, 980 and 1550 nm, respectively. Moreover, BaGd2O4:Bi3+, Er3+ can show multicolor luminescence under different excitation conditions, such as pump light source, ambient temperature, working current, and other factors. Based on the dynamic luminescence characteristics, the dynamic anti‐counterfeiting experiments are designed based on the phosphor. At the same time, the material also shows multimode temperature sensing characteristics. Under the excitation of 980 nm laser, three strong up‐conversion signals Er3+ ions are generated at 528 nm (2H11/2), 555 nm (4S3/2), and 668 nm (4F9/2), which have different temperature dependences. Based on the fluorescence intensity ratio between thermal‐coupled energy levels (2H11/2/4S3/2) and nonthermal‐coupled energy levels (2H11/2/4F9/2) of Er3+ ions, respectively, the dual‐mode temperature thermometer was constructed with high‐temperature sensitivity. In addition, the fluorescence lifetime of Bi3+ ions also has a strong temperature dependence, which can be used as another temperature detection signal, greatly improving the stability of thermometers under harsh conditions. Therefore, the material has a bright prospect in the field of anti‐counterfeiting and temperature sensing. [ABSTRACT FROM AUTHOR]

A series of Na0.5Bi2.5Nb2−xTaxO9 (NBN2−xTx):Er3+/Yb3+ upconversion luminescence (UL) phosphors were prepared by a solid‐state sintering method, and the lattice structure of the prepared samples was studied by X‐ray diffraction. Under 980‐nm laser excitation, the UL properties were investigated by a spectrophotometer from room temperature to 723 K, and the optimal concentration of co‐doping Er3+/Yb3+ is 0.14 and 0.26 by replacing Bi3+ sites in NBN2−xTx host, respectively. With the increase of Ta5+ concentration, the color of UL gradually changes from green to yellow corresponding color coordinates from (0.301, 0.682) to (0.441, 0.549) under 980‐nm laser excitation. This color change is attributed to the decrease of lattice unit volume leading to the increase of cross relaxation of Er3+ to Er3+ and energy back transfer from Er3+ to Yb3+. Using the luminescence intensity ratio technique, the maximum absolute sensitivities (SA) are obtained as 0.00674 K−1 at 538 K and 0.00603 K−1 at 573 K, and relative sensitivities (SR) are obtained as 0.01174 K−1 at 303 K and 0.01244 K−1 at 303 K based on 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 energy level transitions of Er3+ in NBN2:0.14Er3+/0.26Yb3+ phosphors under 980‐ and 1550‐nm laser excitations, respectively. The results suggest that the NBN2−xTx:Er3+/Yb3+ phosphors are promising candidates for temperature sensing with different UL colors. [ABSTRACT FROM AUTHOR]

The doping of transition metal ions in the up‐conversion (UC) luminescent material doped with Yb3+/Ln3+ is a facile way to increase their UC luminescence intensities and alter their colors. In this study, La2MgTiO6:Yb3+/Mn4+/Ln3+ (Ln3+ = Er3+, Ho3+, and Tm3+) phosphors showing excellent luminescence properties were prepared by a solid‐state method. The sensitivity of the La2MgTiO6:Yb3+/Ln3+/Mn4+ phosphor was double that without Mn4+, because Mn4+ affects the UC emissions of Ln3+ via energy transfer between these ions. Moreover, Mn4+ also acts as a down‐conversion activator, which can combine with UC ions to achieve multi‐mode luminescence at different wavelengths. Under 980 nm excitation, these samples emit green light (from Er3+ and Ho3+) and blue light (from Tm3+). In contrast, under 365 nm excitation, they emit red light (from Mn4+). Further testing revealed that the La2MgTiO6:Yb3+/Mn4+/Ln3+ phosphors have potential applications in temperature sensing and anti‐counterfeiting. [ABSTRACT FROM AUTHOR]

The doping of transition metal ions in the up‐conversion (UC) luminescent material doped with Yb3+/Ln3+ is a facile way to increase their UC luminescence intensities and alter their colors. In this study, La2MgTiO6:Yb3+/Mn4+/Ln3+ (Ln3+ = Er3+, Ho3+, and Tm3+) phosphors showing excellent luminescence properties were prepared by a solid‐state method. The sensitivity of the La2MgTiO6:Yb3+/Ln3+/Mn4+ phosphor was double that without Mn4+, because Mn4+ affects the UC emissions of Ln3+ via energy transfer between these ions. Moreover, Mn4+ also acts as a down‐conversion activator, which can combine with UC ions to achieve multi‐mode luminescence at different wavelengths. Under 980 nm excitation, these samples emit green light (from Er3+ and Ho3+) and blue light (from Tm3+). In contrast, under 365 nm excitation, they emit red light (from Mn4+). Further testing revealed that the La2MgTiO6:Yb3+/Mn4+/Ln3+ phosphors have potential applications in temperature sensing and anti‐counterfeiting. [ABSTRACT FROM AUTHOR]

The hosts with low maximum phonon energy (MPE) are preferred since the nonradiative consumption of the luminescence centers in them are low. Among the low MPE hosts, the oxide ones are more favored owing to their excellent stability and easy synthesis. In this work, the optical and spectroscopic properties of BaGd2O4:Er3+ phosphor were studied. The MPE of BaGd2O4 host was observed from Eu3+ phonon sideband (PSB) spectrum and Raman spectrum to be 477 cm−1 which does not second to the fluoride hosts. The refractive index, which is indispensable for Judd–Ofelt calculation, was confirmed from the both approaches of the Eu3+‐probe and the band gap energy, and the similar refractive indices were confirmed, therefore the average refractive index 2.01 was used in the Judd–Ofelt calculation. The Judd–Ofelt parameters of Er3+ in BaGd2O4 host was confirmed to be Ω2${\Omega _2}\;$= 7.91 × 10−21 cm2, Ω4${\Omega _4}\;$= 2.36 × 10−21 cm2, and Ω6${\Omega _6}\;$= 9.00 × 10−22 cm2. Furthermore, the internal quantum efficiencies for 4F9/2 and 4IJ (J = 9/2, 11/2, and 13/2) levels were determined. Finally, the optical temperature sensing properties were studied in detail, and the temperature calibration curve was experimentally derived, meanwhile the maximum absolute sensitivity was confirmed to be 0.0028 K−1. [ABSTRACT FROM AUTHOR]

Pyroxene‐type phosphors were widely developed due to the advantages of high chemical stability, luminous efficiency, and low production cost. In this contribution, a series of Eu2+/Tb3+ co‐doped Ca0.75Sr0.2Mg1.05Si2O6 (CSMS) phosphors with pyroxene structure were successfully synthesized by the solid‐state method. Under the 340 nm excitation, the emission peaks of the phosphor show a redshift with the increase of Eu2+ concentration. The emitting color of Eu2+/Tb3+ co‐doped samples shows a redshift attributed to the energy transfer from Eu2+ to Tb3+. Simultaneously, acquired thermometer exposes superbly temperature‐sensitive properties (Sa and Sr having maximum values 4.7% K−1 and 0.6% K−1, respectively) over the cryogenic temperature range (77–280 K). Furthermore, it has good stability and precision at cryogenic temperatures, indicating that CSMS:0.03Eu2+/0.03Tb3+ phosphor is a very promising fluorescent material suitable for cryogenic temperature sensing. [ABSTRACT FROM AUTHOR]

In recent years, rare earth ions doped optical materials have been extensively utilized in anticounterfeiting, temperature measurement, and other fields. However, it is difficult for single‐mode photoluminescence to meet the increasingly complex anticounterfeiting needs in practice. In this article, Yb3+/Er3+ codoped Y2WO6 multifunctional microparticles have been designed and prepared, which can emit multimode luminescence and are used for anticounterfeiting and temperature measurement. Under excitation at 254, 365, and 980 nm, Y2WO6:Yb3+/Er3+ microparticles can emit blue, green, and yellow‐green luminescence, respectively. The multicolor emission is helpful to improve the security of anticounterfeiting in multimode. In addition, the upconversion, downconversion luminescence, and downconversion lifetime attenuation of this material can be used for fast responsive and noncontact temperature measurement. Among the three temperature measurement methods, the material has the highest sensitivity under the downconversion temperature measurement method, which is 1.25 × 10–2 K–1 (at 303 K). The results suggest that the Y2WO6:Yb3+/Er3+ microparticles have excellent applications in the domain of multimode anticounterfeiting and temperature measurement. [ABSTRACT FROM AUTHOR]

Light with wavelength longer than 1500 nm has great potential to afford deep bio‐tissue penetration due to its extremely weak photon scattering and undetectable autofluorescence in vivo. Here, in order to satisfy the requirements for thermometry during the tumor hyperthermia process, an ultrasensitive optical thermometer operating beyond 1500 nm is developed by employing the thermally coupled Stark sublevels of Er3+: 4I13/2 → 4I15/2 transition based on fluorescence intensity ratio (FIR) technology in Yb3+ and Er3+ codoped BaY2O4. Compared with the typical upconversion (UC) material β‐NaYF4: Yb3+/Er3+ and Y2O3: Yb3+/Er3+, BaY2O4: Yb3+/Er3+ shows more intense red Er3+: 4F9/2 → 4I15/2 transition and 1.5 μm near‐infrared (NIR) Er3+: 4I13/2 → 4I15/2 transition induced by its larger phonon energy and higher quenching concentration of Er3+. An equivalent four‐level model is proposed to investigate the temperature characteristics of the NIR emission, from which four Stark transitions are separated from the raw spectra, named α, β, γ, and δ respectively. Then, the NIR thermal sensing performance have been developed by utilizing the FIR of Iβ to Iα and Iγ to Iα. More importantly, an ultra‐high sensitivity for optical thermometry has been obtained through the combination of transition β and γ, especially in the physiological temperature region. Furthermore, the detection depth of NIR light in bio‐tissues is assessed by an ex vivo test, demonstrating that the maximal detection depth of NIR emission can reach to 8 mm without any influence on optical thermometry. These findings indicate that Yb3+ and Er3+ codoped BaY2O4 is a remarkable contender for optical thermometry in deep tissue with ultra‐high sensitivity. [ABSTRACT FROM AUTHOR]

Hexagonal NaYF4: Er3+/Yb3+ (β‐NaYF4) microprisms with uniform particle sizes were synthesized via a microwave‐assisted hydrothermal method (MWHM). Nucleation and crystal growth were significantly surpassed owing to the microwave irradiation. Cubic‐NaYF4: Er3+/Yb3+ (α‐NaYF4) nanoparticles were transformed into β‐NaYF4 microprisms with reaction time in between 10 and 120 minutes at 180°C. The shape of the particles was enhanced via controlling the nucleation process with optimized irradiation duration and heating rates. The size distribution of β‐NaYF4 microprisms was further improved via regulating the concentration of chelating agents and pH values in the precursor solution. β‐NaYF4 microprisms displayed red and green color emissions for 980 nm excitation owing to the 4F9/2 and 2H11/2/4S3/2 → 4I15/2 transitions of Er3+ respectively. The ratio (2H11/2/4S3/2 → 4I15/2) between green fluorescence intensities (FIR) of β‐NaYF4 microprisms was recorded with the variation in temperature from 303 K to 550 K. The recorded FIR values obeyed Boltzmann distribution and the distribution was used to evaluate the sensitivity. The optimum absolute sensitivity was achieved as 0.0044 K−1 at 490 K with promising resolution, reversibility, and stability. Furthermore, the power‐dependent variations in FIR values implied the suitability of β‐NaYF4 microprisms in measuring the laser‐induced heating effects. [ABSTRACT FROM AUTHOR]

The transparent Er3+‐Yb3+‐doped fluoro‐aluminosilicate glass‐ceramic (GC) was prepared by melt‐quenching. The crystal phase, morphology, and up‐conversion (UC) luminescence of as‐produced GC were characterized by X‐ray diffraction, scanning electron microscopy, and fluorescence spectrophotometry, respectively. The results show that BaYF5 nanocrystals were uniformly distributed in the glass matrix of the as‐produced GC. When the as‐produced GC was subjected to heat treatment, the crystallinity was increased, but the crystal identity remains unchanged. Such heat‐treatment doubled the intensity of the UC luminescence, and this enhancement was ascribed to the increased incorporation of both Er3+ and Yb3+ ions into the lower phonon energy environment of BaYF5 nanocrystals. Furthermore, the heat‐treated GC was stable against further crystallization, and consequently its UC luminescence was stable at the application temperature. The heat‐treated GC was found to possess an outstanding temperature‐sensing capability. [ABSTRACT FROM AUTHOR]

The development of noncontact thermal probe based on stable inorganic materials of trivalent lanthanide (Ln3+) doped phosphors with nontoxicity is of vital importance for their promising applications in bio‐medical fields. Here we explore the upconversion luminescence and thermal sensing properties of Er3+, Yb3+ co‐doped oxysulfide in a broad temperature range of 300‐583 K. It was found that constructing an active shell with an optimum concentration of sensitizers is an efficient way to improve both the luminescent intensities and thermal sensitivity. Compared with the core‐only sample, the luminescent intensity of the Y2O2S: Er3+, Yb3+@ Y2O2S: 5%Yb3+ sample is significantly enhanced by 12‐fold at excitation of 980 nm. While further increasing the Yb3+ concentration in the shell activates new quenching pathways of Er3+ → Yb3+ → quencher from the core to the shell. Similar quenching mechanisms are also observed at excitation of 1550 nm. These energy transfer processes and luminescence mechanisms are verified in the fluorescence decay measurements. Furthermore, coating the core sample with an active shell doped by 10% Yb3+ enhances the thermal sensitivity by 30%, holding a high and stable sensitivity more than 50 × 10−4 K−1 in a broad temperature range of 423‐573 K at 980 nm excitation. In addition, at the much safer excitation wavelength of 1550 nm, this sample achieved the maximum sensitivity of 45 × 10−4 K−1 at 503 K. Our work contributes a feasible and versatile way to promote the luminescence and thermal sensing properties of Ln3+‐based materials, combining with the nontoxic oxysulfide host, indicating their potential applications as safe fluorescent and temperature nano‐probes in bio‐field. [ABSTRACT FROM AUTHOR]

Tailoring the local crystal environment around the activators is one of an important way to enhance the upconversion (UC) luminescence intensity. Herein, we substitute the Y3+ lattice with La3+ ion gradually in Y2Ti2O7:Yb3+, Er3+ phosphor and investigate the effect of La3+ concentration on the UC luminescent properties as well as the temperature sensing behaviors. During the phase transformation process from cubic Y2Ti2O7 to monoclinic La2Ti2O7, the La3+ ions also play an important role on the adjustment of size and morphology of particles. Furthermore, the cooperation of La3+ ion is in favor of the crystal growth toward the easy growth directions. The UC luminescence intensities can be enhanced efficiently with the increasing of La3+ concentration. The sensitivity for temperature sensing can be improved by the increasing of La3+ concentration in the Y2Ti2O7 and La2Ti2O7 two‐phase coexistence system and the maximum SA is 45.3 × 10−4 K−1 at 410K when the La3+ concentration is 80%. The maximum SA and corresponding temperature can be adjusted by controlling the La3+ concentration, which means that (Y0.94‐xLax)2Ti2O7: Er3+/Yb3+ phosphors may be applicable to different working environment. [ABSTRACT FROM AUTHOR]

Uniform spindle‐like micro‐rods NaLa(WO4)2:Yb3+,Er3+ phosphors are prepared by the solvothermal method in the text. Controllable morphology of NaLa(WO4)2 crystal can be obtained by adjusting the prepared temperature, PH value, complexing agent content, and solvent ratio. Uniform NaLa(WO4)2:Yb3+,Er3+ micro‐rods of 1.8 μm in length and 0.5 μm in width are synthesized at a low temperature of 120°C. The prepared NaLa(WO4)2:Yb3+,Er3+ phosphors present green upconversion luminescence under 980 nm excitation, luminescence intensity reaches to maximum at the Yb3+ and Er3+ concentration of 6 and 2 mol%. The temperature performance of the NaLa(WO4)2:Yb3+,Er3+ phosphors are evaluated based on thermal coupling technology. Temperature dependence of the two green emissions ratio of Er3+ ion is obtained, and the sensitivity of the sample can be calculated, the maximum sensitivity of NaLa(WO4)2:Yb3+,Er3+ is up to 0.019 K−1 at the sample temperature of 564 K. [ABSTRACT FROM AUTHOR]

We proposed a new strategy for utilizing rare‐earth–free‐activated self‐referencing optical material with dual activators for temperature sensing, which was synthesized by conventional high‐temperature solid‐state method and was scarcely reported. Originating from the different thermal responses of Mn4+ and Bi3+ ions, a Mn4+/Bi3+‐based dual‐emitting fluorescence intensity ratio (FIR) as dual‐modal temperature signal for temperature sensing has been corroborated as a promising temperature sensing method. Due to the outstanding thermal resistance of activator Mn4+ (anti‐Stokes) benefiting from the unique 3dn electronic configurations and strong field strength of host, together with the energy transfer from Bi3+ to Mn4+ ions and excellent thermal quenching of Bi3+, this temperature‐sensitive phosphor displayed both an extensive detection temperature region ranging from 303 to 563 K and excellent absolute and relative sensitivity of 0.0147 K−1 and 1.21% K−1 respectively, both of which are higher than some foregoing reported optical materials. Furthermore, two well‐separated emission peaks at blue and red regions enabled an excellent signal discriminability and accurate temperature detection under the single‐wavelength excitation of 340 nm. In addition, freedom from rare‐earth ions contributed its possibility to be mass‐produced for meeting the needs of economic rationality, nontoxic and convenient synthesis. It is anticipated that this preliminary study would arouse peoples' attention on exploring more novel dual activator‐based optical thermometric materials in absence of rare‐earth ions. [ABSTRACT FROM AUTHOR]

How to improve the sensitivity of the temperature‐sensing luminescent materials is one of the most important objects currently. In this work, to obtain high sensitivity and learn the corresponding mechanism, the rare earth (RE) ions doped Y4.67Si3O13 (YS) phosphors were developed by solid‐state reaction. The phase purity, structure, morphology and luminescence characteristics were evaluated by XRD, TEM, emission spectra, etc. The change of the optical bandgaps between the host and RE‐doped phosphors was found, agreeing with the calculation results based on density‐functional theory. The temperature‐dependence of the upconversion (UC) luminescence revealed that a linear relationship exists between the fluorescence intensity ratio of Ho3+ and temperature. The theoretical resolution was evaluated. High absolute (0.083 K−1) and relative (3.53% K−1 at 293 K) sensitivities have been gained in the YS:1%Ho3+, 10%Yb3+. The effect of the Yb3+ doping concentration and pump power on the sensitivities was discussed. The pump‐power–dependence of the UC luminescence indicated the main mechanism for high sensitivities in the YS:1%Ho3+, 10%Yb3+. Moreover, the decay‐lifetime based temperature sensing was also evaluated. The above results imply that the present phosphors could be promising candidates for temperature sensors, and the proposed strategies are instructive in exploring other new temperature sensing luminescent materials. [ABSTRACT FROM AUTHOR]