Improved luminescence and temperature sensing performance of Ho(3+)-Yb(3+)-Zn(2+):Y2O3 phosphor

Visible to infrared wide wavelength range luminescence of Sr2LaNbO6: Yb3+/Nd3+/Ho3+ phosphor under 808/980 nm excitation and temperature sensing application

Here, the Sr2LaNbO6: Yb3+/Nd3+/Ho3+ phosphors are prepared through solid-state route. The upconversion (UC)/downshifting (DS) luminescence and temperature performances of all samples are investigated in detail. Under 808 nm excitation, the infrared emission band (∼2000 nm) of Ho3+ ions is obtained. Especially, the energy transfer process of Nd3+ → Yb3+ → Ho3+ has increased the emission intensity of ∼2000 nm. Moreover, the infrared emission band presents thermal enhancement. Under 980 nm excitation, the UC emission band 500-960 nm is investigated. The green and red light of Ho3+ ion, near infrared light of Nd3+ ions are achieved. In addition, the possible luminescence mechanism of Sr2LaNbO6: Yb3+/Nd3+/Ho3+ phosphor is discussed. At last, the optical temperature sensing characteristics of the phosphor are studied according to the luminescence intensity ratio (LIR) technology. The maximum relative sensitivity of Sr2LaNbO6: Yb3+/Nd3+/Ho3+ reaches 5.476 % K-1 at 293 K. These results prove that the Sr2LaNbO6: Yb3+/Nd3+/Ho3+ phosphor presents the luminescence in infrared and visible range. The Sr2LaNbO6:Yb3+/Nd3+/Ho3+ phosphor also can be applied to design optical temperature sensor.

Up-Conversion Luminescence and Optical Temperature Sensing Properties of NaLuF4:Yb3+/Ho3+ Micron-Sized Crystals at Low Temperature

Non-contact temperature sensors utilising the fluorescence intensity ratio and the unique up-conversion (UC) luminescence of rare-earth ions have numerous benefits; however, their operational temperature range has remained limited. In this study, NaLuF4:Yb3+/Ho3+ samples were prepared by the hydrothermal method. The samples exhibited exceptional UC luminescence properties at low temperatures. The intensity of the green emission (with peak wavelengths of 540 and 546 nm) gradually decreased with increasing temperature, and the green emissions showed a unique change at low temperatures. In addition, we studied the dependence of the UC luminescence intensity on the excitation power and the variation in the decay lifetime with temperature. The experiments revealed excellent luminous performance and significantly enhanced sensitivity at low temperatures; the maximum absolute sensitivity Sa and relative sensitivity Sr of the 540 and 546 nm thermally coupled energy levels were 1.02% and 0.55% K-1, respectively. The potential temperature sensing properties of Yb3+/Ho3+-co-doped NaLuF4 makes it suitable for temperature sensing applications at temperatures as low as 30 K. This study offers a novel approach for the advancement of temperature sensing technology at low temperatures.

Anti-thermal quenching of luminescence in Y2W3O12:Yb3+/RE3+ (RE = Er/Ho/Tm) and its temperature sensing application

Herein, a series of Y2W3O12:10%Yb3+/x%RE3+ (RE = Er/Ho/Tm) phosphors is prepared via a solid-state reaction. The upconversion and downshift luminescence properties of the phosphors were investigated under an excitation of 980 nm. The bright blue light emission from Tm3+ ion and the green and red light emissions from Ho3+(Er3+) ions were observed. The near-infrared light intensity of NIR-I (Tm3+, ∼850 nm), NIR-II (Er3+: ∼1550 nm; Tm3+: ∼1783 nm) and NIR-III (Ho3+: ∼2050 nm) were analyzed. In particular, the dramatic thermal enhancement phenomenon in visible and NIR regions was exhibited by the Y2W3O12:10%Yb3+/x%RE3+ (RE = Er/Ho/Tm) phosphors. Among them, the green light intensity of Er3+ ions increased 26.77 times, from 303 to 573 K. The NIR-II emission band (∼1783 nm) intensity of Tm3+ ions at 533 K increased 168.7 times compared to that at 313 K. The possible thermal enhancement mechanism is illustrated by the negative thermal expansion (NTE) and Frenkel defect of the Y2W3O12 host. Finally, the optical temperature sensing performances of Y2W3O12:10%Yb3+/x%RE3+ (RE = Er/Ho/Tm) samples are investigated according to the luminescence intensity dependence relationship on temperature. The maximum value of SR reached 4.24% K-1 at 353 K for Y2W3O12:10%Yb3+/0.6%Ho3+ phosphor. The results indicate that the Y2W3O12:10%Yb3+/x%RE3+ (RE = Er/Ho/Tm) phosphors possess anti-thermal quenching properties and are suitable for developing optical temperature sensors.

Optical temperature-sensing properties of trivalent europium with high sensitivities in a wide temperature range

To develop new luminescent materials for optical thermometer, the Eu3+-activated BaY2ZnO5 (BYZ) phosphors were designed. Upon 394 nm excitation, several groups of 5D0-2→7FJ (J = 0-4) transitions are observed, and the dominant emission is located at 625 nm. The temperature-dependent emission spectra reveal that the emission peaks are weakened with different rates, depending on the excited states of Eu3+. The transient decay kinetics, studied at various temperatures, are in agreement with the emission spectral features. The optical temperature-sensing performance is evaluated with two strategies. For the thermally-coupled (TC) levels of Eu3+, the fluorescence intensity ratio (FIR) of the 536 and 593 nm emissions follows the Boltzmann distribution, and the sensor sensitivities rise with increasing temperature. For the non-TC levels of 5D0 and 5D2, the piecewise functions between the FIRs and absolute temperature are utilized, and the highest absolute and relative sensitivities in the BYZ:7%Eu3+ phosphor are obtained to be 0.674 and 2.19% K-1 at 533 K, respectively. Thus, the developed samples can show higher sensitivities at higher temperature.

Development of SrWO4:Ho3+/Yb3+ green phosphor for optical thermometry application

In recent years, the growth of luminescent materials for optical thermometry applications has gained substantial attention due to their non-invasive and contactless nature. This research article presents the synthesis, characterization, and potential application of SrWO4:Ho3+/Yb3+ phosphors as temperature-sensitive materials. The inspection of the structural properties is done by X-ray diffraction and Raman and FTIR spectroscopy. The X-ray diffraction signifies the single tetragonal phase of the materials without any impurity or secondary phase. Nine Raman active modes are detected in the Raman spectra of pure SrWO4 and 10 Raman active bands are present in the doped samples. The morphology and elemental composition are visualised by field emission scanning electron microscopy (FESEM) and energy dispersive X-ray (EDX) spectroscopy. The spectral properties of these phosphors are evaluated by down-conversion and up-conversion photoluminescence, signifying their suitability for solid state lighting and optical thermometry in various temperature regimes. The phosphors can generate green and yellow light under blue and near-infrared radiation, respectively. The sensitivity of 3.19 × 10-3 K-1 ensures the promising prospects of SrWO4:Ho3+/Yb3+ phosphors for precise and reliable temperature sensing in diverse fields.

Upconversion luminescence and optical thermometry behaviors of Yb3+ and Ho3+ co-doped GYTO crystal

Optical thermometry based on the upconversion (UC) luminescence intensity ratio (LIR) has attracted considerable attention because of its feasibility for achievement of accurate non-contact temperature measurement. Compared with traditional UC phosphors, optical thermometry based on UC single crystals can achieve faster response and higher sensitivity due to the stability and high thermal conductivity of the single crystals. In this study, a high-quality 5 at% Yb3+ and 1 at% Ho3+ co-doped Gd0.74Y0.2TaO4 single crystal was grown by the Czochralski (Cz) method, and the structure of the as-grown crystal was characterized. Importantly, the UC luminescent properties and optical thermometry behaviors of this crystal were revealed. Under 980 nm wavelength excitation, green and red UC luminescence lines at 550 and 650 nm and corresponding to the 5F4/5S2 → 5I8 and 5F5 → 5I8 transitions of Ho3+, respectively, were observed. The green and red UC emissions involved a two-photon mechanism, as evidenced by the analysis of power-dependent UC emission spectra. The temperature-dependent UC emission spectra were measured in the temperature range of 330-660 K to assess the optical temperature sensing behavior. At 660 K, the maximum relative sensing sensitivity (Sr) was determined to be 0.0037 K-1. These results highlight the significant potential of Yb,Ho:GYTO single crystal for optical temperature sensors.

Upconversion luminescence properties and optical thermometry based on non-thermal coupling levels of Ho3+, Yb3+, and Tm3+ codoped 12CaO·7Al2O3 single crystals

Compared with the fluorescence intensity ratio (FIR) temperature measurement technology based on the thermal coupling levels (TCLs) of rare earth (RE) ions, non-TCL (NTCL) FIR technology can greatly improve temperature measurement sensitivity because it is not limited by Boltzmann distribution. In this paper, a H o 3+/Y b 3+/T m 3+ co-doped 12C a O⋅7A l 2 O 3 (C12A7) single crystal was grown by the Czochralski method. As the temperature increased from 363 K to 523 K, the upconversion luminescence color of the H o 3+/Y b 3+/T m 3+/C12A7 crystal changed from white to yellow, and exhibited a large temperature dependence under 980 nm excitation. In the temperature range of 363-523 K, the FIR temperature measurement based on different NTCLs exhibited high temperature sensitivity; the maximum absolute sensitivity and relative sensitivity values were 0.0207K -1 and 2.82% K -1, respectively, which are higher than those previously reported based on TCLs of H o 3+ and T m 3+. This provides a strategy to achieve accurate sensitivity of FIR technology. The RE ion doped C12A7 single crystal material has good research and application prospects in the field of temperature sensing and optoelectronics.

Impact of crystal structure on optical properties and temperature sensing behavior of NaYF4:Yb3+/Er3+ nanoparticles

We report a comprehensive study of the structural, morphological, and optical properties, and UC-based ratiometric temperature sensing behavior of (α) cubic and (β) hexagonal phases of NaYF₄:Yb³⁺/Er³⁺ nanoparticles. The α-NaYF₄:Yb³⁺/Er³⁺ and β-NaYF₄:Yb³⁺/Er³⁺ nanoparticles were synthesized using co-precipitation and hydrothermal methods, respectively. Powder X-ray diffraction studies confirmed the phase purity of the samples. The morphological studies show uniform particle sizes of both phases; the average particle size of α-NaYF₄:Yb³⁺/Er³⁺ and β-NaYF₄:Yb³⁺/Er³⁺ was 9.2 nm and 29 nm, respectively. The Raman spectra reveal five sharp peaks at 253 cm^-1, 307 cm^-1, 359 cm^-1, 485 cm^-1, and 628 cm^-1 for β-NaYF₄:Yb³⁺/Er³⁺, whereas α-NaYF₄:Yb³⁺/Er³⁺ shows two broad peaks centred at 272 cm^-1 and 721 cm^-1. The optical property measurements show that α- and β-NaYF₄:Yb³⁺/Er³⁺ phases have distinct upconversion emission and temperature sensing behavior. The upconversion emission measurements show that β-NaYF₄:Yb³⁺/Er³⁺ has higher overall emission intensities and green/red emission intensity ratio. The temperature-dependent upconversion emission measurements show that α-NaYF₄:Yb³⁺/Er³⁺ has higher energy separation between ²H_11/2 and ⁴S_3/2 energy states. The temperature sensing performed utilizing these thermally coupled energy levels shows a maximum sensitivity of 0.0069 K^-1 at 543 K and 0.016 K^-1 at 422 K for β-NaYF₄:Yb³⁺/Er³⁺ and α-NaYF₄:Yb³⁺/Er³⁺, respectively.

Investigation on anomalous thermal enhancement and temperature sensing properties of Zn3Mo2O9:Yb3+/RE3+ (RE = Er/Ho) phosphors

In this work, Yb³⁺/RE³⁺ (RE = Er/Ho) co-doped Zn₃Mo₂O₉ phosphors were synthesized by high-temperature solid-state reactions. Under 980 nm excitation, the upconversion (UC) luminescence thermal enhancement was obtained for Zn₃Mo₂O₉:Yb³⁺/RE³⁺ phosphors. The green emission intensity of the Zn₃Mo₂O₉:Yb³⁺/Er³⁺ sample was increased 5 times from 373 to 573 K. The red emission intensity of the Zn₃Mo₂O₉:Yb³⁺/Ho³⁺ sample was enhanced 7.92 times. The anomalous thermal enhancement of UC emission was induced by the negative thermal expansion (NTE) of the Zn₃Mo₂O₉ host. The energy transfer rate from the sensitizer (Yb³⁺) to the activator (RE³⁺) was enhanced because of the lattice contraction and distortion for NTE materials. Compared with the UC emission of Er³⁺single doped Zn₃Mo₂O₉ sample, the luminescence thermal enhancement was absent, which contributed to proving the physical mechanism. The temperature sensing properties of the Zn₃Mo₂O₉:Yb³⁺/Er³⁺ and Zn₃Mo₂O₉:Yb³⁺/Ho³⁺ samples were also investigated based on the fluorescence intensity ratio (FIR) technology. The absolute sensitivity (S_A) and relative sensitivity (S_R) of Zn₃Mo₂O₉:Yb³⁺/Er³⁺ phosphor reached 0.0060 K^-1 and 0.72% K^-1, which is based on the thermal coupling levels (²H_11/2, ⁴S_3/2) FIR of Er³⁺ ions. In addition, the S_A and S_R of Zn₃Mo₂O₉:Yb³⁺/Ho³⁺ phosphor reached 0.0119 K^-1 and 0.86% K^-1, that is based on the non-thermal coupling levels (⁵S₂/⁵F₄, ⁵F₅) FIR of Ho³⁺ ions. The research results indicate that the Zn₃Mo₂O₉ host shows NET. The Yb³⁺/RE³⁺ co-doped Zn₃Mo₂O₉ phosphors are good materials for highly sensitive optical temperature measurement, which can be used to develop thermally enhanced ratiometric optical thermometers.

Investigation on the upconversion luminescence and ratiometric thermal sensing of SrWO4:Yb3+/RE3+ (RE = Ho/Er) phosphors

SrWO₄ phosphors doped with Ho³⁺(Er³⁺)/Yb³⁺ are successfully prepared by a high temperature solid-state reaction method. The upconversion (UC) luminescence properties of all the samples have been investigated under 980 nm excitation. Strong green emissions are obtained in the SrWO₄:Yb³⁺/Ho³⁺ and SrWO₄:Yb³⁺/Er³⁺ samples with the naked eyes. In a temperature range going from 303 K to 573 K, the UC emission spectra of the phosphors have been measured. Then the temperature sensing properties also have been discussed via fluorescence intensity ratio (FIR) technology. For the SrWO₄:Yb³⁺/Ho³⁺ phosphor, the FIR technologies based on thermal coupling levels (TCLs)(⁵F₄,⁵F₅) and non-thermal coupling levels (non-TCLs)(⁵S₂, ⁵F₄/⁵F₅) are used for investigating the sensitivity. The results show that the maximum absolute sensitivity reaches 0.0158 K⁻¹ with non-TCLs. As for Yb³⁺/Er³⁺ codoped SrWO₄ phosphor, the maximum absolute sensitivity reaches 0.013 K⁻¹ with TCLs (²H_11/2,⁴S_5/2) at a temperature of 513 K. These significant results demonstrate that the SrWO₄:Ho³⁺(Er³⁺)/Yb³⁺ phosphors are robust for optical temperature sensors.

SrWO₄ phosphors doped with Ho³⁺(Er³⁺)/Yb³⁺ are successfully prepared by a high temperature solid-state reaction method.

Achieving high thermal sensitivity from ratiometric CaGdAlO4:Mn4+,Tb3+ thermometers

The pursuit of optical temperature sensing with high thermal sensitivity to discriminate small temperature changes without contact with the subject possesses a crucial technological and scientific significance. Ratiometric temperature detection based on transition metals and lanthanides emerges as a promising strategy to achieve the purpose due to the dopants' distinct thermal quenching rates. In this work, a new CaGdAlO₄:Mn⁴⁺,Tb³⁺ luminescent thermometer was developed. The combination of the highly-thermal-sensitive red emission from Mn⁴⁺ ions with the thermally-robust green emission from Tb³⁺ ions renders the thermometer with a maximum relative thermal sensitivity of 2.3% K^-1 at 398 K. The well-separated red and green channels in digital images enable further evaluation of thermal sensitivity. The estimated thermal sensitivity is 2.23% K^-1 at 398 K from the pixel intensity ratio of red and green channels.

Cryogenic enabled multicolor upconversion luminescence of KLa(MoO4)2:Yb3+/Ho3+ for dual-mode anti-counterfeiting

The rational development of multicolor upconversion (UC) luminescent materials is particularly promising for achieving high-tech anti-counterfeiting and security applications. Here, an Ho³⁺ and Yb³⁺ ion co-doped KLa(MoO₄)₂ material can achieve multicolored UC luminescence by thermally manipulating the electron transition process, which could be developed to execute advanced optical anti-counterfeiting applications. The emission color of this material turns from bright green to deep orange with the temperature controlled from 85 K to 240 K in a cryogenic environment. The maximum absolute sensitivity and relative sensitivity of this temperature-sensing material based on non-thermally coupled levels of Ho³⁺ ions reached 0.049 K^-1 and 4.6% K^-1. And utilizing the thermochromic luminescence properties and high sensitivity for low temperature of the KLa(MoO₄)₂:Yb³⁺/Ho³⁺ UC material, we created KLa(MoO₄)₂:Yb³⁺/Ho³⁺ fluorescent security inks and UC photonic barcodes to realize novel visual reading and digital recognition dual-mode anti-counterfeiting in a secure manner. These results may provide useful enlightenment for the design and modulation of high-sensitivity temperature-sensing materials for high-level anti-counterfeiting applications.

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.

Photoluminescence properties of Er3+ and Eu3+ ions based on oxide host for optical temperature sensing with high sensitivity

Motivated from increasing demands of non-contact optical temperature sensing, the Yb³⁺-Er³⁺ and Eu³⁺ doped NaY₉Si₆O₂₆ (NYS) oxide phosphors were designed by high-temperature solid-state reaction method. The phase purity of the as-prepared samples was checked from XRD patterns. For the upconversion luminescence in NYS:Yb³⁺,Er³⁺, the optimal Er³⁺ doping content was determined to be 1 mol%, and the characteristic emission peaks of Er³⁺ were observed in the region of 500-700 nm. In Eu³⁺ activated NYS phosphors, it has been found that the emissions originating in ⁵D₁ and ⁵D₀ levels demonstrate different change tendencies in intensity with Eu³⁺ doping concentration. By studying the temperature-dependent luminescent spectra, it was indicated that the emissions intensities from different excited states of Er³⁺ change differently with temperature. Two kinds of fluorescence intensity ratio (FIR) strategies were used in NYS:10%Yb³⁺,1%Er³⁺, containing thermally-coupled levels and non-thermally-coupled levels. In the NYS:3%Eu³⁺ phosphor, it was found that the FIR for 577 and 536 nm emissions follows a linear relation with temperature. The high sensitivities in the present phosphors indicate the potential application in optical temperature sensing.

Development of Er3+, Yb3+ Co-Doped Y2O3 NPs According to Yb3+ Concentration by LP-PLA Method: Potential Further Biosensor

As diagnostic biosensors for analyzing fluids from the human body, the development of inorganic NPs is of increasing concern. For one, nanoceramic phosphors have been studied to meet the increasing requirements for biological, imaging, and diagnostic applications. In this study, Y₂O₃ NPs co-doped with trivalent rare earths (erbium and ytterbium) were obtained using a liquid phase–pulsed laser ablation (LP–PLA) method after getting high density Er, Yb:Y₂O₃ ceramic targets by Spark plasma sintering (SPS). Most NPs are under 50 nm in diameter and show high crystallinity of cubic Y₂O₃ structure, containing (222), (440), and (332) planes via HR–TEM. Excitation under a 980 nm laser to a nanoparticle solution showed 525 and 565 nm green, and 660 nm red emissions. The green emission intensity increased and decreased with increasing Yb³⁺ additive concentration, when the red spectrum continuously strengthened. Utilizing this study’s outcome, we suggest developing technology to mark invisible biomolecules dissolved in a solvent using UC luminescence of Er³⁺, Yb³⁺ co-doped Y₂O₃ NPs by LP–PLA. The LP–PLA method has a potential ability for the fabrication of UC NPs for biosensors with uniform size distribution by laser parameters.

NIR light guided enhanced photoluminescence and temperature sensing in Ho3+/Yb3+/Bi3+ co-doped ZnGa2O4 phosphor

The conversion of NIR light into visible light has been studied in Ho3+/Yb3+/Bi3+ co-doped ZnGa2O4 phosphor for the first time. The crystallinity and particles size of the phosphor increase through Bi3+ doping. The absorption characteristics of Ho3+, Yb3+ and Bi3+ ions are identified by the UV-vis-NIR measurements. The Ho3+ doped phosphor produces intense green upconversion (UC) emission under 980 nm excitations. The emission intensity ~ excitation power density plots show contribution of two photons for the UC emissions. The UC intensity of green emission is weak in the Ho3+ doped phosphor, which enhances upto 128 and 228 times through co-doping of Yb3+ and Yb3+/Bi3+ ions, respectively. The relative and absolute temperature sensing sensitivities of Ho3+/Yb3+/5Bi3+ co-doped ZnGa2O4 phosphor are calculated to be 13.6 × 10-4 and 14.3 × 10-4 K-1, respectively. The variation in concentration of Bi3+ ion and power density produces excellent color tunability from green to red via yellow regions. The CCT also varies with concentration of Bi3+ ion and power density from cool to warm light. The color purity of phosphor is achieved to 98.6% through Bi3+ doping. Therefore, the Ho3+/Yb3+/Bi3+:ZnGa2O4 phosphors can be suitable for UC-based color tunable devices, green light emitting diodes and temperature sensing.

Upconversion luminescence and temperature sensing characteristics of Yb3+/Tm3+:KLa(MoO4)2 phosphors

Yb3+/Tm3+ codoped KLa(MoO4)2 phosphors are synthesized by a hydrothermal method. Under 980 nm excitation, the upconversion (UC) emission spectra of the phosphors are observed. The temperature sensing characteristic based on the fluorescence intensity ratio is studied. The maximum sensitivity reaches 2.93% K-1 at 453 K. The sensitivity value of non-thermally coupled levels is higher than that of thermally coupled levels. The results indicate that the KLa(MoO4)2:Yb3+/Tm3+ phosphor could be used in temperature sensors.

Temperature dependence of up-conversion luminescence and sensing properties of LaNbO4: Nd3+/Yb3+/Ho3+ phosphor under 808 nm excitation

LaNbO₄: Nd³⁺/Yb³⁺/Ho³⁺ phosphor was prepared by a conventional high temperature solid-state reaction method. The temperature dependence of up-conversion (UC) luminescence property of LaNbO₄: Nd³⁺/Yb³⁺/Ho³⁺ phosphor under 808 nm excitation and the potential application of exploiting the red-to-green UC emission intensity ratio (I_R/I_G) of Ho³⁺ in temperature sensing were studied. Two-photon processes were confirmed to be responsible for both the green and the red UC emissions at different temperatures by analyzing the excitation power density dependent UC luminescence spectra measured at different temperatures. The energy level diagram was drawn to analyze the UC luminescence mechanism of Ho³⁺. In addition, it was found that the ratio I_R/I_G of Ho³⁺ was independent of the excitation power density of 808 nm laser under the current experimental condition, but it was sensitive to the temperature. And the temperature dependent UC luminescence spectra displayed that the ratio I_R/I_G exhibited a good linear increasing tendency with temperature rising. The obtained temperature sensing sensitivity was 2.04 × 10^-3 K^-1 in the temperature range of 303-693 K. The results suggest that LaNbO₄: Nd³⁺/Yb³⁺/Ho³⁺ phosphor may be a good candidate for application in optical temperature sensors.

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.

Largely enhanced luminescence intensity and improved optical temperature sensing properties in CaWO4-La2(WO4)3: Er3+, Yb3+ via regulating cations composition

High temperature sensing sensitivity and luminescence intensity of phosphors are crucial factors for excellent optical temperature sensing performance. Based on material design, the pure phase and two-phase solid solutions were prepared by regulating the relative content of cations Ca2+ and La3+ in CaWO4-La2(WO4)3, respectively. The up-conversion luminescence (UCL) and optical temperature sensing performance of rare earth ions Er3+/Yb3+ co-doped CaWO4-La2(WO4)3 were studied. As guided by regulating cation composition through partial substituting Ca2+ ions by La3+ ions, the UCL intensity of two-phase solid solutions at 552 nm is much higher than that of pure phase material. The UCL intensity of 0.2La2(WO4)3-0.8CaWO4: 1%Er3+, 5%Yb3+ is as 33.5 times as that of CaWO4: 1%Er3+, 5%Yb3+ material. More importantly, the high temperature sensing sensitivity (0.01026 K-1) is achieved in a wider temperature range 83-683 K in optimal UCL material 0.2La2(WO4)3-0.8CaWO4: 1%Er3+, 5%Yb3+. It is suggested that material design theory can be used as a powerful tool to accelerate discovery of novel optical temperature sensing materials, with implications even for the design of other optoelectronic materials.

Yb/Ho Codoped Layered Perovskite Bismuth Titanate Microcrystals with Upconversion Luminescence: Fabrication, Characterization, and Application in Optical Fiber Ratiometric Thermometry

Optical thermometry has attracted great interest owing to its noncontact and fast responsive properties in practical applications. However, some sensing errors may occur in many optical ratiometric thermometers due to the overlap of emission peaks, suggesting the necessity of developing excellent luminescent materials. Here, we report the fabrication and characterization of Bi₄Ti₃O₁₂:Yb/Ho for ratiometric thermometry. Bismuth titanate was selected as the matrix due to its low phonon energy, high machinability, and satisfactory thermal stability. The temperature sensing was constructed on the intensity ratio of the two upconversion emission bands with wide separation in Bi₄Ti₃O₁₂:Yb/Ho under 980 nm excitation. The wide separation endows the materials with high signal discrimination for temperature detection. The developed materials were characterized in terms of crystal structure, reflectance, and emission spectra for thermometry application. The maximum relative sensitivity was shown to be as high as 2.11% K^-1. More importantly, an optical fiber thermometry was developed based on the fabricated microcrystals, which can find its potential applications in harsh environments.

Attempting to Verify the Existence of ZnY2O4 Using Electron Backscatter Diffraction

Attempts to synthesize ZnY₂O₄ are made via a solid-state reaction in a high-temperature X-ray powder diffraction chamber as well as analyzing Y₂O₃ sinter ceramics pressure infiltrated by ZnO in a scanning electron microscope using energy-dispersive X-ray spectroscopy and electron backscatter diffraction (EBSD). The microstructure of the sinter ceramic is composed of ZnO grains dispersed in an Y₂O₃ matrix. Superimposed EBSD patterns of Y₂O₃ are misindexed as ZnY₂O₄ during the EBSD scan. The literature concerning ZnY₂O₄ is critically discussed.

Photon quantification in Ho3+/Yb3+ co-doped opto-thermal sensitive fluotellurite glass phosphor

Multi-photon-excited thermal-correlated green and red upconversion (UC) emissions have been quantified in Ho³⁺/Yb³⁺ co-doped fluotellurite (BZLFT) glass phosphor under the 978 nm laser excitation. The temperature dependence of the fluorescence intensity ratio (FIR) originated from UC emissions bands centered at 550 nm and 661 nm has been verified in the range of 303-543 K. The net emission photon numbers of ⁵F₄+⁵S₂→⁵I₈ and ⁵F₅→⁵I₈ transition emissions are up to 40.08×10¹² and 68.51×10¹²cps in the 0.4wt.%Ho₂O₃-0.4wt.%Yb₂O₃ co-doped BZLFT case under the 6.95W/mm² laser power density. Furthermore, the quantum yield (QY) and luminous flux are determined to be dependent on pumping power. When the excitation power increases 874 mW, the QY values for 550 nm and 661 nm emissions are as high as 0.94×10^-5 and 1.60×10^-5. In addition, the high photon producing efficiency is conducive to ensuring high feedback to thermosensitive performance. The temperature thermal sensor can be manipulated steadily in medium temperature range, and the relative sensitivity reaches 0.4%K^-1 at 303 K, which is 1 order of magnitude larger than those in several rare-earth-doped materials. Efficient photon conversion ability and high temperature sensitivity indicate that the rare-earth-ion-doped fluotellurite material has a prospective application in the construction of optical temperature sensors based on the FIR technique allowing for self-referenced temperature determination.

Role of chemical pressure on optical and electronic structure of Ho2Ge x Ti2-x O7

Chemical pressure plays a crucial role in determining the electronic properties of the quantum materials. Investigation of electronic structure of Ho₂Ge _x Ti_2-x O₇ (x  =  2, 1.9, 1.75, 1.5 1, 0.5, 0.25, 0.1 and 0) series has been performed. Pyrochlore and Pyrogermanate, Re₂B₂O₇ (Re  =  Ho³⁺, B  =  Ti⁴⁺ and Ge⁴⁺; rare earth titanates and germanates), substituted with increasing amount of Ge⁴⁺ at the Ti⁴⁺ site and vice versa develops structural distortions. Distinct shrinkage effect has been established in the Ho₂Ti₂O₇ matrix upon Ge⁺⁴ substitutions at B site, resulting in the modification of band gap value. Band gap of 5.24 eV drastically drops to 3.92 eV with immediate Ti⁴⁺ substitution in Ho₂Ge₂O₇. Electronic states of Ho³⁺ (4f forbidden transitions) had also been identified. We observe favored sub level transition (Specific Stark component) corresponding to⁵F₅ to ⁵I₈ electronic transition for Ho³⁺ at λ _exc.  =  450 nm. The upper valence band consisted of O 2p state hybridized with Ho 5p and Ti and Ge 4p states and conduction band primarily formed by Ho 5d state hybridized with Ti 3d and Ge 4d states as obtained from density of states (DOS) calculations. Strong hybridization between Ho 5p_1/2 and Ti 3p orbital upon Ti⁴⁺ inclusion in Ho₂Ge₂O₇ has been observed through both theoretical studies using LDA-1/2 and UV-Vis, photoluminescence, ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy. The evolution of total DOS of all studied composition shows that valence band edge is more sensitive than conduction band to composition. These results provide chemical pressure as an excellent tool to tailor the band gap and fine tune the intermediate electronic states in Ho₂Ge _x Ti_2-x O₇.

Temperature-sensing luminescent materials La9.67Si6O26.5:Yb3+–Er3+/Ho3+ based on pump-power-dependent upconversion luminescence

Rare earth ion doped upconversion (UC) luminescent materials could show potential applications in optical temperature sensing.

Upconversion luminescence and favorable temperature sensing performance of eulytite-type Sr3Y(PO4)3:Yb3+/Ln3+ phosphors (Ln=Ho, Er, Tm)

Phase-pure eulytite-type Sr₃Y_0.88(PO₄)₃:0.10Yb³⁺,0.02Ln³⁺ upconversion (UC) phosphors (Ln = Ho, Er, Tm) were synthesized via gel-combustion and subsequent calcination at 1250°C. Their UC luminescence, temperature-dependent fluorescence intensity ratio of thermally and/or non-thermally coupled energy levels, and performance of optical temperature sensing were systematically investigated. The phosphors typically exhibit green, orange-red and blue luminescence under 978 nm NIR laser excitation for Ln = Er, Ho and Tm, respectively, which were discussed from two- and three-photon processes. The 524 nm green (Er³⁺), 657 nm red (Ho³⁺) and 476 nm blue (Tm³⁺) main emissions were analyzed to have average decay times of ~52 ± 2, 260.6 ± 0.7 and 117 ± 1 μs, respectively. It was shown that (1) the Er³⁺ doped phosphor has a better overall performance of temperature sensing with thermally coupled ²H_11/2 and ⁴S_3/2 energy levels, whose maximum absolute (S _A) and relative (S_R ) sensitivities are ~5.07 × 10^-3 K^-1 at 523 K and ~1.16% at 298 K, respectively; (2) the Ho³⁺ doped phosphor shows maximum S _A and S_R values of ~0.019 K^-1 (298-573 K) and 0.42% at 573 K for the non-thermally coupled energy pairs of ⁵F₅/(⁵F₄,⁵S₂) and ⁵I₄/⁵F₅, respectively; (3) the Tm³⁺ doped phosphor has a maximum S _A of ~12.74 × 10^-3 K^-1 at 573 K for the non-thermally coupled ³F_2,3/¹G₄ energy levels and a maximum S_R of ~1.74% K^-1 at 298 K for the thermally coupled ³F_2,3/³H₄ levels. Advantages of the current phosphors in optical temperature sensing were also revealed by comparing with other typical UC phosphors.

Luminescence properties of KBaYSi2O7:Ce/Eu-Tb phosphors for multifunctional applications

In this work, the multifunctional KBaYSi₂O₇:Ce/Eu-Tb (KBYS:Ce/Eu-Tb) phosphors were synthesized by using the conventional high-temperature solid-state reaction, and the luminescence properties were investigated for LED and optical thermometer applications. The single-phase samples were checked by x-ray diffractometer patterns. The energy transfer was found in both Eu²⁺-Tb³⁺ and Ce³⁺-Tb³⁺ codoped samples, which was verified by spectra and decay curves. Correspondingly, the tunable emission was realized by adjusting the dopant concentrations and white light was obtained in the KBYS:4%Eu²⁺,9%Tb³⁺ sample, owing to the energy transfers among Eu²⁺, Tb³⁺, and Eu³⁺. By studying the temperature dependence of the KBYS:2%Ce³⁺,7%Tb³⁺, it has been found that the fluorescence intensity ratio for Tb³⁺ and Ce³⁺ emissions changes with temperature, indicating the potential application in an optical thermometer. The high absolute sensitivity was determined to be 0.012  K^-1. By evaluating the thermal quenching luminescence of the KBYS:4%Eu²⁺,9%Tb³⁺, the thermal quenching temperature T_0.5 was determined to be 555 K, implying high thermally luminescent stability.

Multicolor-tunable up-conversion emissions of Yb3+,Er3+/Ho3+ co-doped Ba3Lu2Zn5O11: crystal structure, luminescence and energy transfer properties

Yb3+-Er3+ and Yb3+-Ho3+ co-doped Ba3Lu2Zn5O11 phosphors were successfully obtained. The structure of the as-synthesized phosphor was determined by Rietveld refinement. The up-conversion (UC) spectra of Ba3Lu2Zn5O11:Yb3+,Er3+ exhibits two prominent emission bands centered at 560 and 663 nm, which originate from the 2H11/2/4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions of Er3+, respectively. Ba3Lu2Zn5O11:Yb3+,Ho3+ revealed a bright green emission at around 553 nm and negligible red emission peaking at 660 nm, which were assigned to the 5F4/5S2 → 5I8 and 5F5 → 5I8 transitions of Ho3+, respectively. The up-conversion luminescence of the samples was studied as a function of the concentration of the dopants. The origin of the UC mechanism is discussed in detail by analyzing energy-level diagrams, the dependence of the UC emission intensity on pump power, and the lifetimes. The results indicate that the Ba3Lu2Zn5O11:Yb3+,Er3+/Ho3+ phosphors are excellent up-conversion emitters and have potential applications in display and illumination technologies.

Highly efficient infrared to visible up-conversion emission tuning from red to white in Eu/Yb co-doped NaYF4 phosphor

Eu/Yb co-doped NaYF₄ phosphors have been synthesized by the combustion method. The Eu doping was fixed and the effect of Yb doping concentration on the structural, morphological and luminescence properties has been investigated. X-ray diffraction analysis revealed that the phosphors consisted of mixed α- and β-phases, but the β-phase was dominant. All elements of the host and dopants, as well as adventitious C, were detected using X-ray photoelectron spectroscopy. The surface morphology showed a microrod-like structure with sharp hexagonal edges. Energy dispersive X-ray spectroscopy spectra proved the formation of the desired materials. The photoluminescence spectra illustrated the optical emission properties of Eu³⁺ in the red region when excited at 394 nm, while, under the same excitation, Yb³⁺ ions gave emission at 980 nm. The up-conversion (UC) emission of Eu/Yb co-doped NaYF₄ produced a white color at the higher concentration of Yb excited by a 980 nm laser, which was made possible by green emission of Er contamination (from Yb source) and blue emission of Eu²⁺ ions. The lifetime of the Eu³⁺ UC luminescence at 615 nm was also affected by the Yb doping concentration. The temperature sensitivity associated with the Er³⁺ peaks at 520 and 542 nm was assessed as a function of temperature and the maximum of 0.0040 K^-1 occurred at 463 K.

Sensitivity improvement induced by thermal response behavior for temperature sensing applications

Overcoming the restriction of the energy gap (700-800 cm-1) in Er3+-doped upconversion (UC) materials to achieve high detection accuracy is crucial for practical temperature detection applications. Herein, we design a feasible route based on the different thermal response behaviors of various hosts to enhance the SA value in a double perovskite NaLaMgWO6:Er,Mo system. The maximum SA value is 222.8 × 10-4@423 K in the NLMW:5%Er3+ host, and this can be promoted to 275.4 × 10-4 K-1@323 K in the NaLaMgWO6:Er,Mo system. The SR values decrease monotonously as the temperature rises, and this is due to the dependency of the SR values on the energy gap. A mechanism that is ascribed to the different thermal response behaviors of the various hosts is proposed, and this mechanism is further proved by investigating the temperature sensing properties of barium gadolinium zincate phosphors that possess the same thermal response behaviors. In addition, this study introduces the idea that a host with a high emission intensity for the 2H11/2 level and a lower emission intensity for the 4S3/2 level is highly suitable for temperature measurements. A thorough investigation of this system offers a strategy to acquire a high SA value and reveals the broad prospects of NaLaMgWO6:Er,Mo in the temperature detection field.

Enhanced optical temperature sensing and upconversion emissions based on the Mn2+ codoped NaGdF4:Yb3+,Ho3+ nanophosphor

In this study, to explore new phosphors for temperature sensing with high detection sensitivity, Yb³⁺/Ho³⁺/Mn²⁺ doped hexagonal NaGdF₄ nanoparticles were designed.

Effects of Tm3+ concentration on upconversion luminescence and temperature-sensing behavior in Tm3+/Yb3+:Y2O3 nanocrystals

The effects of Tm³⁺ concentration on upconversion emission and temperature-sensing behavior of Tm³⁺/Yb³⁺:Y₂O₃ nanocrystals were investigated. Blue and red emissions were observed under 980 nm excitation. Both upconversion emissions and the blue to red intensity ratio were found to decrease with increasing Tm³⁺ concentration. The temperature-sensing performances of the samples were studied, the fluorescence intensity ratio of ¹G_4(a)→³H₆ (477 nm) and ¹G_4(b)→³H₆ (490 nm) transitions from Tm³⁺ ions was chosen as the thermometric index. The results showed that the sensor sensitivity was sensitive to Tm³⁺ ion concentration. The maximum sensitivity of ~32 × 10^-4 K^-1 was obtained for 0.1%Tm³⁺/5%Yb³⁺:Y₂O₃ nanocrystals at 344 K. Moreover, a marked optical induced heating effect was also found in the nanocrystals. The prepared Tm³⁺/Yb³⁺:Y₂O₃ nanocrystals could be used in temperature-sensing probes and in optical heaters.

Effect of doping Ge into Y2O3:Ho,Yb on the green-to-red emission ratio and temperature sensing

The Ge-doped Y₂O₃:Ho,Yb phosphor tunes the G/R ratio, and the G/R ratio has a higher absolute temperature sensitivity.

Inhomogeneous-Broadening-Induced Intense Upconversion Luminescence in Tm3+ and Yb3+ Codoped Lu2O3-ZrO2 Disordered Crystals

Near-infrared (980 nm) to near-infrared (800 nm) and blue (490 nm) upconversion has been studied in 0.2% Tm³⁺ and 10% Yb³⁺ codoped Lu₂O₃-ZrO₂ solid solutions as a function of the ZrO₂ content in the range of 0-50%, prepared by a high-temperature solid-state reaction. The continuous enhancement of upconversion luminescence is observed with increasing ZrO₂ content up to 30%. Analyses of the Yb³⁺ emission intensity and lifetime indicate enlarged absorption of a 980 nm excitation laser and enhanced energy transfer from Yb³⁺ to Tm³⁺ with the addition of ZrO₂. The spectrally inhomogeneous broadening of the dopants in this disordered solid solution is considered to play the main role in the enhancement by providing better matches with the excitation laser line and increasing the spectral overlap for efficient energy transfer from Yb³⁺ to Tm³⁺. In addition, the inhomogeneous broadening is also validated to improve energy migration among Yb³⁺ ions and energy back transfer from Tm³⁺ to Yb³⁺. Hence, it is understandable that a drop in the upconversion luminescence intensity occurs as the concentration of ZrO₂ is further increased from 30% to 50%. This work indicates the possibility of disordered crystals to achieve intense upconversion luminescence for biological and optoelectronic applications.

Upconversion luminescence and temperature-sensing properties of Ho3+/Yb3+-codoped ZnWO4phosphors based on fluorescence intensity ratios

Ho³⁺/Yb³⁺-codoped ZnWO₄phosphors were synthesized using a solid state reaction method. The optical temperature sensing properties of ZnWO₄:0.01Ho³⁺/0.15Yb³⁺phosphors have been discussed by using four-level system and the intensity ratio between the red and green emissions.

Terbium and holmium codoped yttrium phosphate as non-contact optical temperature sensors

A new sensing mechanism is presented and a high relative sensitivity is achieved in our sample YPO₄:3.2%Tb³⁺,0.8%Ho³⁺.