Splitting upconversion emission and phonon‐assisted population inversion of Ba2Y(BO3)2Cl:Yb3+, Er3+ phosphor

Upconversion (UC) peak of ⁴S_3/2→⁴I_15/2 transition of Er³⁺ is close to that of ²H_11/2→⁴I_15/2 transition. The UC emission splitting of Er³⁺ caused by coordination fields of host results in that it is difficult to confirm which transitions (⁴S_3/2→⁴I_15/2 or ²H_11/2→⁴I_15/2) are responsible for the splitting UC emission peaks. In this work, the UC luminescence peaks located at 524, 540, 551, 565, 662, 677, and 683 nm were observed in the Ba₂Y(BO₃)₂Cl:Yb³⁺, Er³⁺ phosphor upon the 980 nm excitation. The 524 and 540 nm UC emissions intensity were increased, while the 551 and 565 nm UC emissions intensity were decreased with the temperature increasing from 323 to 573 K, which is attributed to the phonon‐assisted population inversion from the ⁴S_3/2 to ²H_11/2 level. The temperature dependence of UC emission spectra demonstrated that the 524 and 540 nm UC emissions are from ²H_11/2→⁴I_15/2 transition, and 551 and 565 nm UC emissions are from the ⁴S_3/2→⁴I_15/2 transition. Temperature sensing property was characterized by the UC intensity ratio of the ²H_11/2→⁴I_15/2 transition to ⁴S_3/2→⁴I_15/2 transition. The Ba₂Y(BO₃)₂Cl:Yb³⁺,Er³⁺ phosphor has potential application as the non‐contact temperature sensor.     

Efficient near-infrared to visible and ultraviolet upconversion in polycrystalline BiOCl:Er3+/Yb3+ synthesized at low temperature

Er³⁺/Yb³⁺ co-doped BiOCl poly-crystals were synthesized by the conventional solid state method at 500 °C, which exhibited good crystalline and low phonon energy. Under 980 nm excitation, the samples showed intense red upconversion (UC) luminescence (Er³⁺: ⁴F_9/2→⁴I_15/2) as well as other four UC emission bands, including ultraviolet (UV) emission at 380 nm, violet emission at 411 nm, green UC emissions at 525 and 545 nm and near-infrared (NIR) emission between 800 and 850 nm, corresponding to the transitions of ⁴G_11/2, ²H_9/2, ²H_11/2, ⁴S_3/2 and ⁴I_9/2→⁴I_15/2 of Er³⁺, respectively. Interestingly, including the violet and green UC emissions, the red one originated a nearly three-photon process in this system, and a possible UC mechanism was proposed for the enhanced red emission.     

Er3+ cross-section spectra of Er3+/Pr3+:Gd3Ga5O12 single-crystal: Pr3+-codoping effect

Fluorescence and absorption spectra at 530 nm (²H_11/2→⁴I_15/2), 560 nm (⁴S_3/2→⁴I_15/2), 660 nm (⁴F_9/2→⁴I_15/2), 980 nm (⁴I_11/2→⁴I_15/2), 1530 nm (⁴I_13/2→⁴I_15/2), and 2710 nm (⁴I_11/2→⁴I_13/2) of Er³⁺ in Gd₃Ga₅O₁₂ single-crystal codoped with Pr³⁺ have been measured. Judd-Ofelt analysis yields the intensity parameters Ω₂ = (0.68 ± 0.03) × 10⁻²⁰ cm², Ω₄ = (0.60 ± 0.07) × 10⁻²⁰ cm², and Ω₆ = (0.90 ± 0.17) × 10⁻²⁰ cm². A comparison with previously reported values of Er³⁺-only doping case shows that Pr³⁺-codoping causes slight change of both Ω₂ and Ω₄, while onefold increase of Ω₆. From calculated radiative rates and measured fluorescence spectra, Er³⁺ emission cross-section spectra were calibrated at first. Then, the absorption cross-section spectra were calculated using McCumber relation. In parallel, the absorption cross-section spectra were also obtained from the measured absorption spectrum, and compared with those obtained from the McCumber relation. The comparison shows that both methods give consistent result of absorption cross-section spectrum. Further comparison with Er³⁺-only doping case shows that Pr³⁺-codoping causes considerable change of Er³⁺ cross-section value. In spectrally mixing regions of Er³⁺ and Pr³⁺, Pr³⁺ emission affects little the determination of Er³⁺ emission cross-section as Pr³⁺ fluorescence is much weaker than Er³⁺ fluorescence due to low Pr³⁺ concentration.     

Phase evolution,structure, and up-/down-conversion luminescence of Li6CaLa2Nb2O12:Yb3+/RE3+ phosphors (RE = Ho,Er, Tm)

Garnet-type Li₆Ca(La_0.97Yb_0.02RE_0.01)₂Nb₂O₁₂ (RE = Ho, Er, Tm) new phosphors were successfully synthesized via solid reaction at 900°C for 5 hours, whose course of phase evolution, macroscopic/local crystal structure and up-/down-conversion (UC/DC) photoluminescence were clarified. Mechanistic study and materials characterization were attained via XRD, Rietveld refinement, DTA/TG, electron microscopy (FE-SEM/TEM), and Raman/reflectance/fluorescence spectroscopies. The phosphors were shown to exhibit UC luminescence dominated by a ~ 553 nm green band (⁵F₄/⁵S₂ → ⁵I₈ transition) for Ho³⁺, a ~ 568 nm green band (⁴S_3/2 → ⁴I_15/2 transition) for Er³⁺ and a ~ 806 nm near-infrared band (³H₄ → ³H₆ transition) for Tm³⁺ under 978 nm laser excitation, with CIE chromaticity coordinates of around (0.31, 0.68), (0.38, 0.60) and (0.17, 0.24), respectively. Analysis of the pump-power dependence of UC intensity indicated that all the emissions involve a two-photon mechanism except for the ~ 486 nm blue emission of Tm³⁺ (¹G₄ → ³H₆), which requires a three-photon process. The DC luminescence of these phosphors is featured by dominant bands at ~ 553 nm for Ho³⁺ (green, ⁵F₄/⁵S₂ → ⁵I₈ transition), ~568 nm for Er³⁺ (green, ⁴S_3/2 → ⁴I_15/2 transition) and ~ 464 nm for Tm³⁺ (blue, ¹D₂ → ³F₄ transition). The UC and DC properties were also comparatively discussed.     

Fabrication and Upconversion Luminescent Properties of Er3+‐Doped and Er3+/Yb3+ Codoped La2O2CN2 Nanofibers

La₂O₂CN₂:Er³⁺and La₂O₂CN₂:Er³⁺/Yb³⁺ upconversion (UC) luminescence nanofibers were successfully fabricated via cyanamidation of the respective relevant La₂O₃:Er³⁺ and La₂O₃:Er³⁺/Yb³⁺ nanofibers which were obtained by calcining the electrospun composite nanofibers. The morphologies, structures, and properties of the nanofibers are investigated. The mean diameters of La₂O₂CN₂:Er³⁺ and La₂O₂CN₂:Er³⁺/Yb³⁺ nanofibers are 179.46 ± 12.58 nm and 198.85 ± 17.07 nm, respectively. It is found that intense green and weak red emissions around 524, 542, and 658 nm corresponding to the ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2, and ⁴F_9/2→⁴I_l5/2 energy levels transitions of Er³⁺ ions are observed for La₂O₂CN₂:Er³⁺ and La₂O₂CN₂:Er³⁺/Yb³⁺ nanofibers under the excitation of a 980‐nm diode laser. Moreover, the emitting colors of La₂O₂CN₂:Er³⁺ and La₂O₂CN₂:Er³⁺/Yb³⁺ nanofibers are all located in the green region. The upconversion luminescent mechanism and formation mechanism of the nanofibers are also proposed.     

High recoverable energy storage density in nominal (0.67-x)BiFeO3-0.33BaTiO3-xBaBi2Nb2O9 lead-free composite ceramics

A series of novel lead-free energy storage ceramics, (0.67-x)BiFeO₃-0.33BaTiO₃-xBaBi₂Nb₂O₉ (BF-BT-xBBN), were fabricated by traditional solid-state reaction, where bismuth layer-structured BaBiNb₂O₉ was incorporated into perovskite-structured BiFeO₃–BaTiO₃ ceramic as an additive. The addition of BaBi₂Nb₂O₉ increased the relaxor behavior and breakdown strength of BF-BT ceramics due to the formation of polar nanoregionals (PNRs), inducing enhanced energy storage performance. The composite ceramics, with x = 0.08, showed a large recoverable energy density (W_rec) of 3.08 J/cm³ and an outstanding energy storage efficiency (η) of 85.57% under an applied electric field of 230 kV/cm. Moreover, the composite ceramics exhibited excellent thermal stability and high stability toward different frequencies in a temperature range of 20–100 °C and a frequency range of 0.1–1500 Hz. These results demonstrate great potential of novel BF-BT-xBBN composite ceramics for next-generation energy storage applications.     

Color‐Tunable Up‐Conversion Emission and Infrared Photoluminescence and Dielectric Relaxation of Er3+/Yb3+ Co‐Doped Bi2Ti2O7 Pyrochlore Thin Films

The color‐tunable up‐conversion (UC) emission and infrared photoluminescence and dielectric relaxation of Er³⁺/Yb³⁺ co‐doped Bi₂Ti₂O₇ pyrochlore thin films prepared by a chemical solution deposition method have been investigated. The pyrochlore phase structure of Bi₂Ti₂O₇ can be stabilized by Er³⁺/Yb³⁺ co‐doping. Intense color‐tunable UC emission and infrared photoluminescence can be detected on the thin films excited by a 980 nm diode laser. Two UC emission bands centered at 548 and 660 nm in the spectra can be assigned to ²H_11/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ ions, respectively. A Stokes infrared emission centered at 1530 nm is due to ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ ions. The dependence of UC emission intensity on pumping power indicates that the UC emission of the thin films is a two‐photon process. The thin films also exhibit a relatively high dielectric constant and a low dissipation factor as well as a good bias voltage stability. Temperature‐ and frequency‐dependent dielectric relaxation has been confirmed. This study suggests that Er³⁺/Yb³⁺ co‐doped Bi₂Ti₂O₇ thin films can be applied to new multifunctional photoluminescence dielectric thin‐film devices.     

Dual-mode dichromatic SrBi4Ti4O15: Er3+ emitting phosphor for anti-counterfeiting application

Fluorescent materials have been widely used for anti-counterfeiting of important documents and currencies, wherein their anti-counterfeit abilities could be improved through multi-mode excitation. Herein, dual-mode-excited double-colour-emitting Er³⁺doped SrBi₄Ti₄O₁₅ up-conversion (UC) phosphors (SBTO: Er³⁺) were synthesised, and their UC spectra included green (2H11/2/4S3/2 → 4I15/2) and red (4F9/2 → 4I15/2) emissions from Er3+ ions under 980 or 1550 nm excitation. However, the green emission colour of phosphors was independent of dopant concentration under 980 nm laser irradiation; whereas the final emission colour was dominated by red emission and significantly affected by contents of Er3+ under 1550 nm excitation. These observations demonstrated potential application in dual-mode double-colour anti-counterfeiting. The possible UC mechanisms and emission characteristics of the phosphors using different 980 and 1550 nm irradiation source were contrastively investigated, and some fluorescent security patterns were also designed to demonstrate the potential applications in anti-counterfeiting and concealing important information.     

Intense upconversion luminescence and energy‐transfer mechanism of Ho3+/Yb3+ co‐doped SrLu2O4 phosphor

A series of novel SrLu₂O₄: x Ho³⁺, y Yb³⁺ phosphors (x=0.005‐0.05, y=0.1‐0.6) were synthesized by a simple solid‐state reaction method. The phase purity, morphology, and upconversion luminescence were measured by X‐ray diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence (PL) spectroscopy. The doping concentrations and sintering temperature were optimized to be x=0.01, y=0.5 and T=1400°C to obtain the strongest emission intensity. Under 980 nm laser diode excitation, the SrLu₂O₄:Ho³⁺, Yb³⁺ phosphors exhibit intense green upconversion (UC) emission band centered at 541 nm (⁵F₄,⁵S₂→⁵I₈) and weak red emission peaked at 673 nm (⁵F₅→⁵I₈). Under different pump‐power excitation, the UC luminescence can be finely tuned from yellow‐green to green light region to some extent. Based on energy level diagram, the energy‐transfer mechanisms are investigated in detail according to the analysis of pump‐power dependence and luminescence decay curves. The energy‐transfer mechanisms for green and red UC emissions can be determined to be two‐photon absorption processes. Compared with commercial NaYF₄:Er³⁺, Yb³⁺ and common Y₂O₃:Ho³⁺, Yb³⁺ phosphors, the SrLu_1.49Ho_0.01Yb_0.5O₄ sample shows good color monochromaticity and relatively high UC luminescence intensity. The results imply that SrLu₂O₄:Ho³⁺, Yb³⁺ can be a good candidate for green UC material in display fields.     

Deep red upconversion photoluminescence in Er3+-doped Yb3Ga5O12 nanocrystalline garnet

The nanocrystalline single-phase Er³⁺-doped Yb₃Ga₅O₁₂ garnets have been prepared by the sol-gel combustion technique with a crystallite size of ≈30 nm. The presence of Yb³⁺ in garnet hosts allows their efficient excitation at the ≈977 nm wavelength. The Er³⁺ doping of Yb₃Ga₅O₁₂ garnet host results in deep red Er³⁺: ⁴F_9/2 → ⁴I_15/2 upconversion photoluminescence (UCPL) emission. The dominance of the red UCPL emission over the green Er³⁺: ⁴F_7/2/²H_11/2/⁴S_3/2 → ⁴I_15/2 component was investigated using the measurement of the steady-state and time-dependent Er³⁺ and Yb³⁺ emission spectra in combination with the power-dependent UCPL emission intensity. The proposed upconversion mechanism is discussed in terms of the Er³⁺ → Yb³⁺ energy back transfer process as well as Yb³⁺(Er³⁺) → Er³⁺ energy transfer and Er³⁺ ↔ Er³⁺ cross-relaxation processes. The studied Er³⁺-doped Yb₃Ga₅O₁₂ garnet may be utilized as a red upconversion emitting phosphor.     

Design and preparation of white light emission from up-conversion transitions in Er3+/Tm3+/Yb3+ co-doped cubic zirconia single crystals

High-quality cubic YSZ crystals were designed with various contents of Er³⁺, Tm³⁺ and Yb³⁺ to produce white light emission, and grown by the optical floating zone method. The up-conversion luminescence spectra of the crystals under 980 nm laser irradiation show three distinct groups of emission peaks at ∼473 nm (blue) generated by the Tm^{3+ 1}G₄→³H₆ transition, 531 and 547 nm (green) from the Er^{3+ 2}H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions, and 640 and 662 nm (red) from the Er^{3+ 4}F_9/2 → ⁴I_15/2 transition. The optical power curve obtained by plotting the up-conversion luminescence intensity against the laser power shows that the blue emission involves a three-photon process, whilst both the green and red emissions are the results of two-photon processes. Overall white light emission was observed with the crystal prepared with 0.05 mol% Er₂O₃, 0.5 mol% Tm₂O₃ and 2.0 mol% Yb₂O₃, and this crystal is suitable for use in highly efficient white light emission devices.     

Ultrasensitive optical thermometer based on abnormal thermal quenching Stark transitions operating beyond 1500 nm

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⁺: ⁴I_13/2 → ⁴I_15/2 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⁺: ⁴F_9/2 → ⁴I_15/2 transition and 1.5 μm near-infrared (NIR) Er³⁺: ⁴I_13/2 → ⁴I_15/2 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 I_β to I_α and I_γ to I_α. 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.     

Microwave Sol–Gel Synthesis of CaGd2(MoO4)4:Er3+/Yb3+ Phosphors and Their Upconversion Photoluminescence Properties

CaGd₂(MoO₄)₄:Er³⁺/Yb³⁺ phosphors with the doping concentrations of Er³⁺ and Yb³⁺ (x = Er³⁺ + Yb³⁺, Er³⁺ = 0.05, 0.1, 0.2, and Yb³⁺ = 0.2, 0.45) have been successfully synthesized by the microwave sol–gel method, and the crystal structure refinement and upconversion photoluminescence properties have been investigated. The synthesized particles, being formed after heat‐treatment at 900°C for 16 h, showed a well‐crystallized morphology. Under the excitation at 980 nm, CaGd₂(MoO₄)₄:Er³⁺/Yb³⁺ particles exhibited strong 525 and 550‐nm emission bands in the green region and a weak 655‐nm emission band in the red region. The Raman spectrum of undoped CaGd₂(MoO₄)₄ revealed about 15 narrow lines. The strongest band observed at 903 cm^?1 was assigned to the ν₁ symmetric stretching vibration of MoO₄ tetrahedrons. The spectra of the samples doped with Er and Yb obtained under 514.5 nm excitation were dominated by Er³⁺ luminescence preventing the recording Raman spectra of these samples. Concentration quenching of the erbium luminescence at ²H_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2 transitions in the CaGd₂(MoO₄)₄:Er³⁺/Yb³⁺ crystal structure was established to be approximately at the 10 at.% doping level.     

Enhanced upconverted luminescence and the optical thermometry behavior of Er3+-doped BaYbF5 transparent glass ceramics

Transparent SiO₂ - Al₂O₃ - Na₂O - CaO - BaF₂ - YbF₃ glass ceramics (GC) doped with Er³⁺ ions were successfully fabricated by a melt-quenching technique with subsequent heat treatment. The formation of BaYbF₅ nano-crystalline phase was confirmed by X-ray diffraction and transmission electron microscopy. Compared to the precursor glass (PG), the clearer Stark splitting and greatly enhanced up-conversion (UC) emission in GC indicate that Er³⁺ ions mainly enter into BaYbF₅ nanocrystals with low phonon energy after crystallization. The temperature dependent on purple UC emission ratio (which is due to the Er^{3+ 4}G_11/2→⁴I_15/2 and ²H_9/2→⁴I_15/2 transitions) and common green UC emission ratio with low-power excitation in BaYbF₅ GC have been studied respectively. In addition, the UC mechanisms in PG and GC are illustrated and analyzed. The outstanding properties of Er³⁺-doped BaYbF₅ transparent GC may present potential applications in all-solid-state UC lasers and optical fiber temperature sensors.     

Preparation,Structure, and Up‐Conversion Luminescence of Yb3+/Er3+ Codoped SrIn2O4 Phosphors

SrIn₂O₄, which shows lower phonon energy than CaIn₂O₄, is not only a good photocatalyst but also can be an excellent up‐conversion (UC) host to exhibits UC luminescence. In this work, Yb³⁺ and/or Er³⁺ doped SrIn₂O₄ phosphors were synthesized, and their UC luminescence properties were studied and compared with those in the CaIn₂O₄ host. The structure of SrIn₂O₄: 0.01Er³⁺ and SrIn₂O₄: 0.1Yb³⁺/0.01Er³⁺ samples were refined by the Rietveld method and found to that SrIn₂O₄: 0.1Yb³⁺/0.01Er³⁺ showed increasing unit cell parameters and cell volume, indicating In³⁺ sites were substituted successfully by Yb³⁺ and/or Er³⁺ ions. From the UC luminescence spectra and diffuse reflection spectra, Er³⁺‐doped SrIn₂O₄ showed very weak luminescence due to ground state absorption of Er³⁺; Yb³⁺/Er³⁺ codoped SrIn₂O₄ presented strong green (550 nm) and red (663 nm) UC emissions which were assigned to energy transfer from Yb³⁺ transition ²F_7/2→²F_5/2 to the Er³⁺ transition ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2. Comparing with CaIn₂O₄, Yb³⁺/Er³⁺ codoped SrIn₂O₄ showed obvious advantages with higher UC luminescent intensity. The pumping powers study showed that UC emissions in Yb³⁺/Er³⁺ codoped SrIn₂O₄ were attributed to energy transfer of Yb³⁺→Er³⁺ with a two‐photon process. The possible UC luminescent mechanism of Yb³⁺/Er³⁺‐doped SrIn₂O₄ was discussed.     

Sensitization effect of Nd3+ ions on Yb3+/Nd3+ co-doped oxyfluoride glasses and study of their optical,fluorescence, and upconversion abilities for visible laser and NIR amplifier applications

The development of laser technology has created intense demand for optical confinement materials with high performance. Herein the authors have been investigated Yb³⁺-singly doped and Yb³⁺/Nd³⁺-codoped SiO₂-based oxyfluoride glasses in terms of their optical absorption, and their near-infrared (NIR) and up-conversion (UC) emissions including emission decay profiles. Under 808 nm laser diode (LD) excitation, four NIR emission bands were observed i.e., (Nd³⁺: ⁴F_3/2 → ⁴I_9/2, Yb³⁺: ²F_5/2 → ²F_7/2, Nd³⁺: ⁴F_3/2 → ⁴I_11/2, and Nd³⁺: ⁴F_3/2 → ⁴I_13/2) in co-doped glasses. NIR emission cross-sections [(σ_emi) stimulated, (σ^M_emi) from Mc-cumber theory] were calculated for ²F_5/2 → ²F_7/2 (～1030 nm) transition of Yb³⁺ ion. σ_emi was found to be highest (26.27 × 10^?21 cm²) for the Yb³⁺: ²F_5/2 → ²F_7/2 transition in N2 glass. UC emission spectra recorded at 980 nm LD show bands centered at 500, 536, 595 & 610, and 664 nm, attributed to ⁴G_9/2 → ⁴I_9/2, ⁴G_7/2 → ⁴I_9/2& ⁴G_7/2 → ⁴I_11/2, ⁴G_5/2 → ⁴I_9/2, and ⁴G_9/2 → ⁴I_13/2 transitions, respectively. Decay profiles were analyzed for Yb³⁺: ²F_5/2 → ²F_7/2 (～1030 nm) and Nd³⁺: ⁴F_3/2 → ⁴I_11/2 (～1057 nm) transitions at 808 nm LD. Energy transfer (ET) process from Nd³⁺ to Yb³⁺ in present glasses were detailed.     

Bright red upconversion luminescence from Er3+ and Yb3+ co-doped ZnO-TiO2 composite phosphor powder

ZnO-TiO₂ composites co-doped with Er³⁺ and Yb³⁺ ions were successfully synthesized by powder-solution mixing method and their upconversion (UC) luminescence was evaluated. The effect of firing temperature, ZnO/TiO₂ mixing ratio, and dopant concentration ranges on structural and UC luminescence properties was investigated. The crystal structure of the product was studied and calculated in detail by means of X-ray diffraction (XRD). Also, the site preference of Er³⁺ and Yb³⁺ ions in the host material was considered and analyzed based on XRD results and UC luminescence characteristics. Brightest UC luminescence was observed in the ZnO-TiO₂:Er³⁺,Yb³⁺ phosphor fired at 1300 °C in which the system consisted of mixed phases; Zn₂TiO₄, TiO₂, RE₂Ti₂O₇ and RE₂TiO₅ (RE = Er³⁺ and/or Yb³⁺). Under the excitation of a 980 nm laser, the two emission bands were detected in the UC emission spectrum, weak green band centered at 544 and 559 nm, and strong red band centered at 657 and 675 nm wavelengths in accordance with ²H_11/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ ion, respectively. The simple chemical formula equations, for explaining the site preference of Er³⁺ and Yb³⁺ ions in host crystal matrix, were generated by considering the Zn₂TiO₄ crystal structure, its crystal properties, and the effect of Er³⁺ and Yb³⁺ ions to the host crystal matrix. The UC emission intensity of the products was changed by varying ZnO/TiO₂ mixing ratios, and Er³⁺ and Yb³⁺ concentrations. The best suitable condition for emitting the brightest UC emission was 1ZnO:1TiO₂ doped with 3 mol% Er³⁺, 9 mol% Yb³⁺ fired at 1300 °C for 1 h.     

A new molybdate host material: Synthesis,upconversion, temperature quenching and sensing properties

A new upconversion (UC) host material YbMoO₄ with 0–100 mol% Er³⁺ doping was obtained using a facile coprecipitation method. A pure tetragonal phase of YbMoO₄ was synthesized, which was dependent on the pH value of the reaction mixture and the sintering temperature. The existence of pentavalent molybdenum was confirmed in YbMoO₄ by thermal-reduction of hexavalent molybdenum. Under a 976 nm laser diode excitation, both green and red UC emissions were observed from Er³⁺:YbMoO₄, which corresponded to the ²H_11/2/⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 transitions of Er³⁺ with the strongest luminescence appearing at a mole ratio of Er:Yb=1:10. The two-photon absorption UC process was responsible for the green and red emissions. The temperature-dependent green UC emission of Er³⁺:YbMoO₄ was observed, which was rationalized using the thermal quenching model. The fluorescence intensity ratio (FIR) of green UC emissions was studied as a function of temperature and its high thermal sensitivity implied that the Er³⁺:YbMoO₄ material is a promising prototype for applications in optical temperature sensing.     

A Novel Scheme to Obtain Y2O2S:Er3+ Upconversion Luminescent Hollow Nanofibers via Precursor Templating

Y₂O₃:Er³⁺ hollow nanofibers were prepared by calcination of the monoaxial electrospinning‐derived PVP/[Y(NO₃)₃+Er(NO₃)₃] composite nanofibers, and then Y₂O₂S:Er³⁺ hollow nanofibers were synthesized by sulfurization of the as‐obtained Y₂O₃:Er³⁺ hollow nanofibers via a double‐crucible method using sulfur powders as sulfur source. X‐ray diffraction (XRD) analysis shows that the Y₂O₂S:Er³⁺ hollow nanofibers are pure hexagonal phase with the space group of . Scanning electron microscope (SEM) observation indicates that the Y₂O₂S:Er³⁺ hollow nanofibers are obvious hollow‐centered with the outer diameter of 176 ± 25 nm. Upconversion emission spectrum analysis manifests that Y₂O₂S:Er³⁺ hollow nanofibers emit strong green and weak red upconversion emissions centering at 526, 546, and 667 nm, respectively. The green emissions and the red emission are, respectively, originated from ²H_11/2/⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_l5/2 energy levels transitions of the Er³⁺ ions. The emitting colors of Y₂O₂S:Er³⁺ hollow nanofibers are located in the green region in CIE chromaticity coordinates diagram. The formation mechanism of the Y₂O₂S:Er³⁺ hollow nanofibers is also advanced. This preparation technique can be applied to prepare other rare‐earth oxysulfides hollow nanofibers.