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.     

Design,simulation, elaboration and luminescence of Tb3+-doped Ba0.84Gd0.16F2.16 fluoroaluminosilicate scintillating glass ceramics

Extensive researches on scintillators have been executed to satisfy the excellent radiation detection materials in broad applications. However, practical application of conventional scintillators is limited due to the limitations of high cost, time-consuming fabrication process and insufficient radioluminescence. Herein, high density precursor glass doped with Tb³⁺ was designed to absorb X-ray efficiently and produce green emission. Molecular dynamics simulation was used to simulate the phase separation process in melting process. Then, Tb³⁺-doped Ba_0.84Gd_0.16F_2.16 glass ceramics (GCs) with excellent structural and optical properties were elaborated by melt quenching technic and further heat treating. Their structural properties, photoluminescence (PL) and X-ray excited luminescence (XEL) were explored detailedly. The internal quantum efficiency of PL is 64 % in GCs. The XEL intensity is 192 % of that of Bi₄Ge₃O₁₂ (BGO) commercial scintillator. Our results suggest that Ba_0.84Gd_0.16F_2.16:Tb³⁺ GCs might have potential application in X-ray detection.     

Enhanced up-conversion luminescence of Tb3+-Yb3+ doped glass ceramics by doping Li+

Tb³⁺-Yb³⁺ co-doped transparent glass ceramics (GCs) containing Y₂Ti₂O₇ crystal phases were synthesized by the melt crystallization. The light transmittance of GCs in the visible region reached 78%, and the average grain size was 278 nm under the optimal heat treatment conditions (720 °C/2 h). The GCs exhibited greater up-conversion luminescence intensity than precursor glass, and the reason for this result was explained in accordance with the Judd-Ofelt theory. Moreover, the introduction of Li⁺ did not change the crystalline phase of GCs. The emission intensity of the green light of the 8% Li ⁺ doped GCs was significantly enhanced by nearly 4.48 times under 980 nm excitation. The XRD refinement results suggest that the enhanced luminescence intensity is correlated with the change of the Y₂Ti₂O₇ crystal lattice caused by Li⁺ doping. The relevant luminescence mechanism was elucidated. The results suggest that Li⁺ doped transparent GCs open novel avenues for green UC applications.     

Upconverting luminescence based dual-modal temperature sensing for Yb3+/Er3+/Tm3+: YF3 nanocrystals embedded glass ceramic

Yb³⁺/Er³⁺/Tm³⁺ doped transparent glass ceramic containing orthorhombic YF₃ nanoparticles was successfully synthesized by a melt-quenching method. After glass crystallization, tremendously enhanced (about 5000 times) upconversion luminescence, obvious Start-splitting of emission bands as well as long upconversion lifetimes of Er³⁺/Tm³⁺ confirmed the incorporation of lanthanide activators into precipitated YF₃ crystalline environment with low phonon energy. Furthermore, temperature-dependent upconversion luminescence behaviors of glass ceramic were systematically investigated to explore its possible application as optical thermometric medium. Impressively, both fluorescence intensity ratio of Er³⁺: ²H_11/2 → ⁴I_15/2 transition to Er³⁺: ⁴S_3/2 → ⁴I_15/2 one and fluorescence intensity ratio of Tm³⁺: ³F_2,3 → ³H₆ transition to the combined Tm³⁺: ¹G₄ → ³F₄/Er³⁺: ⁴F_9/2 → ⁴I_15/2 ones were demonstrated to be applicable as temperature probes, enabling dual-modal temperature sensing. Finally, the thermal effect induced by the irradiation of 980 nm laser was found to be negligible in the glass ceramic sample, being beneficial to gain intense and precise probing signal and detect temperature accurately.     

Thermal enhancement of up-conversion luminescence in Lu2W2.5Mo0.5O12: Er3+, Yb3+ phosphors

Lu₂W_2.5Mo_0.5O₁₂: Er³⁺/Yb³⁺ phosphors were synthesized through high temperature solid state method. Under 980 nm laser excitation, the Lu₂W_2.5Mo_0.5O₁₂: Er³⁺/Yb³⁺ compounds show thermal enhancement of up-conversion luminescence (UCL), which is attributed to the lattice contraction and distortion from negative thermal expansion (NTE) of Lu₂W_2.5Mo_0.5O₁₂ host enhancing the energy transfer of Yb³⁺ to Er³⁺, eliminating the energy transfer of Er³⁺ to Er³⁺ through Er³⁺ single-doped Lu₂W_2.5Mo_0.5O₁₂ phosphors without thermal enhancement of UCL. The green luminescence intensities at 693 K of the Lu_1.98-xW_2.5Mo_0.5O₁₂: 0.02Er³⁺, xYb³⁺ (x = 0.2, 0.3, 0.4) samples are 4.6, 4.3 and 7.0 times as that of 302 K, respectively. And through fluorescence intensity ratio (FIR) technique, the corresponding maximum absolute sensitivities are 0.00741, 0.00744 and 0.00723, respectively. The green monochromaticity of UCL spectra in Er³⁺/Yb³⁺ co-doped samples increase with the increasing of temperature, and the possible UCL mechanism with temperature was discussed. The results indicate that the Lu₂W_2.5Mo_0.5O₁₂: Er³⁺/Yb³⁺ phosphors can be applied at a high temperature as optical thermometer with a good green monochromaticity.     

Improved photoluminescence and multi-mode optical thermometry of Er3+/Yb3+ co-doped (Ba,Sr)3Lu4O9 phosphors

Contactless optical thermometers have attracted extensive attentions for applications in scientific research and technological fields due to their apparent advantages. Herein, a novel sequence of Ba_3-xSr_xLu₄O₉ (B_3-xS_xLO):Er³⁺/Yb³⁺ phosphors were successfully prepared to investigate the temperature sensing property. By establishing energy transfer from Yb³⁺ to Er³⁺ and regulating the local lattice environment, up-conversion luminescence of Er³⁺ is dramatically improved when excited by 980 nm laser. This can effectively promote signal-noise ratio and reduce the errors in temperature detection. Furthermore, a multi-mode optical thermometry, which includes the fluorescence intensity ratio (FIR) from two thermally coupled levels of ²H_11/2/⁴S_3/2, FIR based on non-thermally coupled system of ²H_11/2/⁴F_9/2 and fluorescence lifetime of ⁴S_3/2 state of Er³⁺, was explored systematically. The fabricated samples exhibit the superior temperature measurement performances containing wide temperature-sensing range, superior signal discriminability, high sensitivity and favorable repeatability, indicative of the enormous utilization prospects of B_3-xS_xLO:Er³⁺/Yb³⁺ for thermometry.     

Broad-scope dual-mode modulation thermometry based on up-conversion phosphor GaNbO4: Yb3+/Er3+

Fluorescence temperature measurement technology has set off another upsurge in non-contact temperature measurement, but still suffers from the large error for single-mode thermometry. Herein, in a broad temperature range of 93–633 K, a dual-mode modulation thermometry based on up-conversion phosphor of GaNbO₄:Yb³⁺/Er³⁺ is realized with the maximum relative sensitivity (S_r) of 11.7% K⁻¹ (93 K) and 13.1% K⁻¹ (123 K), respectively. GaNbO₄:Yb³⁺/Er³⁺ phosphors were synthesized by high temperature solid-state method. The structure, surface morphology and the optical properties were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence (PL). The fluorescence intensity ratio (FIR) readout method based on Er³⁺ thermal-coupled energy level (TCL) and non-thermal-coupled energy level (NTCL) was used to achieve the dual-mode temperature measurement with high temperature resolution and good repeatability in GaNbO₄:5 mol% Yb³⁺ and 5 mol% Er³⁺ phosphors. All the results show that GaNbO₄:Yb³⁺/Er³⁺ phosphors have great application potential in high sensitivity broadband thermometry.     

Up-conversion luminescence and optical thermometry properties of transparent glass ceramics containing CaF2:Yb3+/Er3+ nanocrystals

A series of Yb³⁺/Er³⁺ codoped transparent oxyfluoride glass ceramics with various amounts of Yb³⁺ have been successfully fabricated and characterized. Under 980 nm laser prompting, the samples produce intense red, green and blue up-conversion emissions, and the emission intensities increase with Yb³⁺ concentration and heat treatment temperature. Before losing good transparency in the visible region, optimum emission intensities are obtained for the sample with 25 mol% of Yb³⁺ at a heat treatment temperature of 680 °C. A possible up-conversion mechanism is proposed from the dependence of emission intensities on pumping power. The fluorescence intensity ratio between the two thermally coupled levels ²H_11/2 versus ⁴S_3/2 was measured with the laser output power of 57 mW to avoid the possible laser induced heating effect. The fluorescence intensity ratio values in the temperature range from 295 K to 723 K can be well fitted with the equation: A exp (−∆E/k_BT), where A = 6.79 and ∆E=876 cm⁻¹. The relative temperature sensitivity at 300 K was evaluated to be 1.4% K⁻¹. All the results suggest that the Yb³⁺/Er³⁺ codoped CaF₂ glass ceramics is an efficient up-conversion material with potential in optical fiber temperature sensing.     

Gd-based oxyfluoride glass ceramics: Phase transformation,optical spectroscopy and upconverting temperature sensing

A series of precursor glasses with compositions of SiO₂-Al₂O₃-AlF₃-Na₂O- NaF-Gd₂O₃/GdF₃-YbF₃-ErF₃ were prepared and their crystallization behaviors were investigated. For the samples with high F content, meta-stable hexagonal GdF₃ nanocrystals were preferentially precipitated from glass matrix and decreasing F/O ratio induced phase transformation to cubic NaGdF₄ and finally to hexagonal NaGdF₄. Benefited from its multiple active sites, significant enhanced upconversion luminescence was achieved for Yb/Er co-doped glass ceramic containing hexagonal NaGdF₄ nanocrystals. Importantly, significant temperature-sensitive upconversion fluorescence intensity ratio between Er³⁺: ²H_11/2 → ⁴I_15/2 transition (520 nm) and ⁴S_3/2 → ⁴I_15/2 one (540 nm) was detected owing to the competitive radiation transitions from these two thermally coupled emitting-states. Furthermore, linear temperature-dependent fluorescence intensity ratio between Er³⁺: ⁴F_9/2 → ⁴I_15/2 transition (650 nm) to ⁴S_3/2 → ⁴I_15/2 one (540 nm) was achieved, showing the advantages of high sensitivity, superior signal discriminability as well as excellent thermal stability for temperature determination.     

Simultaneous high transmittance and large tunability of up-conversion photoluminescence in Er3+ doped KNN-based ceramics

0.98(0.94K_0.51Na_0.5NbO₃-0.06SrZrO₃)-0.02Li_0.5La_0.5TiO₃+x mol Er³⁺ ceramics were prepared using a conventional solid-state reaction method. The transmittance of the sample x = 0.5% sintered at 1210 °C reaches 56% in at the wave length of 780 nm and 73% at the wave length of 2000 nm (the thickness of the sample is 0.3 mm). Under 980 nm excitation, two typical emission bands are obtained, which are the green emission band at 510 nm–580 nm and the red emission band at 645–695 nm. Under the irradiation of visible light, the up-conversion luminescence intensity of the sample is significantly reduced, showing a luminescence quenching characteristic. The PL quenching degree (ΔR) of up-conversion emission is up to 78.2% for the sample x = 0.5% sintered at 1210 °C. Besides, a behavior of photosensitive resistance is achieved at room temperature and 350 °C, which suggests that this system has a great application prospect in optical information storage.     

NaSr2Nb5O15:Yb3+, Ho3+, Tm3+ transparent glass ceramics: Up-conversion optical thermometry and energy storage property

Rare earth tri-doped precursor glasses (PGs) were prepared by traditional high-temperature melting method, and NaSr₂Nb₅O₁₅ transparent glass–ceramic (GC) was obtained by subsequent heat treatment. Results exhibit that the up-conversion emission intensity of GC is greatly enhanced compared to PG. Benefiting from the multiple emission bands from Ho³⁺ and Tm³⁺ and their different temperature dependence, multi-ratio optical temperature measurement is realized. The ultimate relative sensitivity (S_r-max) can reach 2.00% K⁻¹ between 298 and 598 K. It provides a possibility for self-reference temperature measurement. Furthermore, under the actual charging and discharge conditions, the GC heated at 750°C has great energy density (W_d = 1.15 J/cm³@600 kV/cm) and high-power density (P_d = 290.4 MW/cm³@600 kV/cm) with ultrafast discharge time (<15.8 ns). The previous results indicate that the obtained GC with good multifunctional properties is expected to be applied in the field of photoelectric conversion.     

Ionic liquid-assisted two-phase synthesis of Lu7O6F9:Yb3+, Er3+ phosphors and their morphological control,color-tunable up-conversion luminescence and temperature sensing behavior

In this work, Lu₇O₆F₉ microcrystals with various novel morphologies, including hand broom-like nanorods, nanoparticles, hexagons, spindle-like nanoparticle aggregates, hexagonal prisms and microrods, were prepared via ionic liquid-assisted two-phase method and following calcination approach. Ionic liquid was used as F^? resource, morphology controller and two-phase solvent. The effect of preparation condition on the phases and morphologies of the precursors as well as the calcined products was studied in detail. The crystallographic structure of Lu₇O₆F₉ was also confirmed by the down-conversion (DC) spectra of Lu₇O₆F₉: Eu³⁺ phosphor with Eu³⁺ ion as the structure probe. Besides, different concentration of Yb³⁺ ions were introduced to the host to obtain Lu₇O₆F₉: Yb³⁺, Er³⁺ phosphors, in case of subsequent investigation on the up-conversion (UC) luminescence properties, UC mechanism and followed temperature sensing behavior. Color-tunable UC emissions were realized and the mechanism was discussed. Furthermore, the optical temperature sensing behavior of orthorhombic Vernier lutecium oxyfluoride was investigated for the first time. The influence of Yb³⁺ content on the sensing sensitivity was also elaborated. These results imply that the as-prepared Lu₇O₆F₉: Yb³⁺, Er³⁺ phosphors could be considered as candidates in color-tunable displaying and optical thermometers.     

Enhanced up-conversion luminescence of Er:LaOF oxyfluoride borosilicate glass ceramics

Er³⁺-doped transparent oxyfluoride borosilicate glass ceramics containing LaOF nanocrystals have been obtained by the high temperature melt-quenching and subsequent heat treatment method. The formation of LaOF nanocrystals in the glass matrix was confirmed by XRD and TEM results. In comparison with the precursor glass, Er³⁺-doped transparent oxyfluoride glass ceramics containing LaOF nanocrystals exhibited efficient up-conversion luminescence. Especially, the green emission intensity was greatly enhanced about nearly 200 times and its up-conversion mechanism can be ascribed to a two-photon absorption process.     

Temperature sensing properties of self-crystalized Ba2LaF7: Tb3+ glass ceramics

Fluorescence intensity ratio (FIR) techniques for temperature sensing based on the thermally-coupled energy levels (TCELs) of two excited states of rare earth ions are widely investigated. However, their performance in lower temperature detection are poor because of thermal decoupling between two emitting levels with relatively large energy gap. On the other hand, most of luminescent thermometer materials so far reported are in powder form, which suffer from severe light scattering and high hygroscopicity. Fortunately, transparent glass ceramics offer an alternative to improve optical property as well as stability of luminescent materials. Hence, herein self-crystallized 20% Tb³⁺ doped transparent Ba₂LaF₇ glass ceramics were synthesized by traditional high-temperature melting method to examine its temperature sensing ability by employing the two low-lying states ⁷F₅ and ⁷F₆ of Tb³⁺, which are thermally coupled even at lower temperature. Under the resonance excitation of ⁷F₅ → ⁵D₄ transition at 543 nm, the emission intensity of ⁵D₄ → ⁷F₆ enhances with the temperature rising from 300 K to 630 K. The maximum relative sensitivity reaches 2.88% K^?1 at 300 K, which is better than the previous results reported. Moreover, the repeatability of the integrated intensity of ⁵D₄ emission of Tb³⁺ under eight consecutive heating-cooling cycles indicates that the sample has a good reliability and reusability. All results suggest that the 20% Tb³⁺ doped transparent Ba₂LaF₇ glass ceramics are one of the excellent candidate materials for optical thermometers.     

Effects of Er3+ concentration on down-/up-conversion luminescence and temperature sensing properties in NaGdTiO4: Er3+/Yb3+ phosphors

Usually, Er³⁺ doping concentration effect on the temperature sensing properties of Er³⁺ containing materials is ignored. In this work, we demonstrated the influence of Er³⁺ concentration and excitation path on the spectral and temperature sensing properties in Er³⁺, Yb³⁺ co-doped NaGdTiO₄ system. The NaGdTiO₄: Er³⁺/Yb³⁺ phosphors were prepared by a high temperature solid state reaction method. Different spectral patterns for down- and up-conversion processes were observed and ascribed to the different excitation and population routes. The concentration quenching behaviors for down- and up-conversion processes were explained via cross relaxations between Er³⁺ ions. Most importantly, the Er³⁺ concentration dependent optical temperature sensing performance was observed and experimentally explained as a fact that the optical transition rate of Er³⁺ in different samples was changed with various Er³⁺ doping concentration.     

Color-tunable up-conversion emission in Y2O3:Yb3+, Er3+ nanoparticles prepared by polymer complex solution method

Abstract

Powders of Y₂O₃ co-doped with Yb³⁺ and Er³⁺ composed of well-crystallized nanoparticles (30 to 50 nm in diameter) with no adsorbed ligand species on their surface are prepared by polymer complex solution method. These powders exhibit up-conversion emission upon 978-nm excitation with a color that can be tuned from green to red by changing the Yb³⁺/Er³⁺ concentration ratio. The mechanism underlying up-conversion color changes is presented along with material structural and optical properties.

PACS

42.70.-a, 78.55.Hx, 78.60.-b     

Optical properties of Er,Yb co-doped YAG transparent ceramics

The transparent polycrystalline erbium and ytterbium co-doped yttrium aluminum garnet (Er,Yb:YAG) ceramics with various Yb contents from 5% to 25% were prepared by the solid-state reaction and the vacuum-sintering technique. The in-line transmittances of the mirror-polished ceramics exceed 80% from the visible band to the infrared band. The samples are very compact with few pores. The average grain size of the Er,Yb:YAG ceramic is about 15 μm. The upconversion luminescence spectra, infrared luminescence spectra and luminescence decay curves of the ceramics were observed and discussed. For 1%Er doped YAG ceramic, the best ion ratio of Yb³⁺ and Er³⁺ is around 15:1.     

Up-conversion luminescence and temperature-sensing properties of Li+, K+ doped Na2Zn3Si2O8: Er3+ phosphors

In this work we detail the preparation of new luminescent Li⁺ and K⁺ doped Na₂Zn₃Si₂O₈: Er³⁺ up-conversion phosphors using the high-temperature solid-phase method. We investigate the phosphors phase structure, elemental distribution, up-conversion luminescence characteristics and temperature sensing properties. Our fabricated samples were found to be homogeneous and when excited using 980 nm light, they emitted wavelengths in the green and red visible wavelength bands, which correspond to two major emission bands of Er³⁺. Doping with Li⁺ and K⁺ increased the luminescence intensity of the Na₂Zn₃Si₂O₈: Er³⁺ phosphor at 661 nm by 36 and 21 times respectively. The highest relative temperature sensitivity (Sa) of the fabricated phosphor reached a value of 19.69% K^?1 and the highest absolute temperature sensitivity (Sr) reached 1.20% K^?1. These values are superior to other materials which utilize up-conversion by Er³⁺ ions as a tool for temperature sensing. We anticipate that these new phosphors will find significant application as components in optical temperature measurement systems.     

Effect of glass-ceramics network intermediate Al2O3 content on up-conversion luminescence in Er3+/Yb3+ co-doped NaYF4 oxy-fluoride glass-ceramics

Effect of alumina as a glass network intermediate on the up-conversion luminescence (UCL) in NaYF₄:Er³⁺/Yb³⁺ co-doped oxy-fluoride glass-ceramics (GCs) was investigated. Combinations of smaller NaYF₄ nanocrystals (10 and 13 nm) and lower Al₂O₃ contents (5% and 10%), as well as larger NaYF₄ nanocrystals (26 and 40 nm) and higher lower Al₂O₃ contents (15% and 20%) were prepared after heat-treatment, respectively. The glass network of intermediate partial replacement of SiO₂ with Al₂O₃ was investigated, and the consequence on the response to the up-conversion of the lanthanide ions was also studied. The UCL properties of Er³⁺ ions were changed in accordance with the addition of Al₂O₃, the red UCL intensity decreased with an increased Al₂O₃ concentration, while the green emission intensity showed opposite tendency. Our results showed that adding Al₂O₃ to 20 mol% is an effective strategy to simultaneous control of the magnitude and luminescence properties of lanthanide ion doped GCs.     

Phase evolution and photoluminescence in Er3+ doped glass ceramics containing lutetium oxyfluoride nanocrystals

Crystalline phase evolution through merely adjusting composition was achieved in silicate glass ceramics containing Lu_nO_n-1F_n+2 (n = 5–10) nanocrystals. Orthorhombic or cubic phase nanocrystals were precipitated in the aluminosilicate glass matrix after thermal treatment together with varying the Na₂O/NaF ratio. Oxyfluoride nanocrystals with quasi-spherical shape show homogenous and dense distribution in glass matrix by transmission electron microscopy measurement. Intense upconversion and mid-infrared emissions were realized in these glass ceramics compared to the precursor glass, and the emission spectral shapes, relative emission intensity and fluorescence decay curves of Er³⁺ in cubic LuOF embedded samples exhibit remarkable differences due to the crystal phase dependent effect in glass ceramics. These results indicate that the crystallization and luminescence properties of oxyfluoride glass ceramics could be modified through the alteration of glass composition, which could be used for the development of novel glass ceramics and design of luminescent properties.