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.     

Phase transitions and upconversion luminescence in oxyfluoride glass ceramics containing Ba4Gd3F17 nanocrystals

Novel transparent Er³⁺ doped oxyfluoride glass-ceramics containing Ba₄Gd₃F₁₇ nanocrystals were prepared by melt quenching followed by heat treatment of as-prepared glasses. The phase composition and microstructure were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Intense upconversion luminescence (UCL) was detected. Longer characteristic decay times and splitting of the luminescence bands compared to the precursor glass indicated the incorporation of erbium ions in the crystalline phase. The spectroscopic properties of glass ceramics were compared with single phase cubic and rhombohedral Ba₄Gd₃F₁₇ ceramics. The unit cell parameters and atomic positions in the rhombohedral phase were calculated using Rietveld refinement. The local environment of Er³⁺ and the phonon energy of both polymorphs were analyzed using luminescence and Raman spectroscopy. In the glass ceramics, a phase transition from distorted metastable fluorite to ordered rhombohedral Ba₄Gd₃F₁₇ was observed and resulted in the enhancement of the efficiency of UCL.     

Competitive nanocrystallization of Na3ScF6 and NaYbF4 in aluminosilicate glass and optical spectroscopy of Ln3+ dopants

The precipitation of monoclinic Na₃ScF₆ nanocrystals from aluminosilicate glass with specially designed compositions of SiO₂-Al₂O₃-Na₂O-NaF-ScF₃-YbF₃ was achieved for the first time. Impressively, competitive nanocrystallization of cubic NaYbF₄ and monoclinic Na₃ScF₆ has been evidenced to be dependent on Na⁺ content and F/Na ratio in glass. Adopting Er³⁺ and Eu³⁺ dopants as structural probes, optical spectroscopic analyses verified that these emissive centers preferred to partition into NaYbF₄ nanocrystals rather than Na₃ScF₆ ones.     

Ordering of fluorite-type phases in erbium-doped oxyfluoride glass ceramics

In this study, novel transparent Er³⁺ doped glass ceramics were prepared from melt-quenched oxyfluoride glasses with general composition of Na₂O-NaF-BaF₂-YbF₃-Al₂O₃-SiO₂. The crystallization of fluorite (BaF₂, BaF₂-YbF₃, NaF-BaF₂-YbF₃ and Na_0.5-xYb_0.5+xF_2+2x) and distorted fluorite (rhombohedral Ba₄Yb₃F₁₇ and tetragonal NaF-BaF₂-YbF₃) phases was analysed in glass ceramics with different BaF₂ and YbF₃ ratio. The phase composition and microstructure were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Intense red upconversion luminescence (UCL) was detected under near-infrared excitation resulting from three photon upconversion followed by cross-relaxation between Er³⁺ and Yb³⁺ ions.The local environment of Er³⁺ ions in fluorite and distorted phases was analysed using site-selective spectroscopy. The Er³⁺ ions were found to act as nucleation centres in the glass ceramics containing BaF₂. The phase transition from metastable fluorite to rhombohedrally and tetragonally distorted fluorite phases was detected using Er³⁺ ions as a probe.     

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.     

Transparent Sr0.84Lu0.16F2.16: Yb3+, Er3+ glass ceramics: Elaboration,structure, up-conversion properties and applications

The development of optical temperature sensors is of fundamental and industrial importance for various applications. Despite the great advance in optical temperature-sensing techniques, challenges remain to search for novel sensing materials with low cost, easy fabrication and high sensitivity. Here, transparent glass ceramics (GC) embedded with cubic Sr_0.84Lu_0.16F_2.16:Yb³⁺/Er³⁺ nano-crystals were prepared via thermal annealing on the parent glass. The optical and structural properties were investigated. The enhanced emission intensity, obvious Stark splitting and prolonged lifetimes of Er³⁺ confirm the enrichment of Er³⁺ ions into formed Sr_0.84Lu_0.16F_2.16 nano-crystals. The temperature sensing performance of Yb³⁺/Er³⁺ ions in Sr_0.84Lu_0.16F_2.16GC were investigated based on up-conversion intensity ratio (FIR) from thermally coupled emitting states of Er³⁺. High energy difference ($\Delta E$?=?839?cm^?1) and high absolute sensitivity (27.4?×?10^?4?K^?1 at 606?K) are obtained. Our results reveal Sr_0.84Lu_0.16F_2.16GC are excellent host for rare earth ions doping and potential candidate for optical thermometry.     

Transparent sol-gel glass ceramics containing β-NaYF4:Yb3+/Er3+ nanocrystals: Structure,upconversion luminescent properties and optical thermometry behavior

Transparent bulk glass ceramics (GCs) containing $\beta ?\mathit{NaY}{\mathrm{F}}_4:{\mathit{Yb}}^{3+}/{\mathit{Er}}^{3+}$ upconversion nanocrystals were successfully prepared via a new sol-gel route for the first time. The structure, composition and morphology of the as-fabricated glass ceramics are characterized by X-ray diffraction ($\mathit{XRD}$), scanning electron microscopy ($\mathit{SEM}$) and transmission electron microscopy ($\mathit{TEM}$), which confirm the segregation of β-NaYF₄ nanocrystals in silica glass matrix with the maintenance of their crystalline phase and microstructure. More significantly, intense upconversion (UC) emissions can be realized for ${\mathit{Yb}}^{3+}/{\mathit{Er}}^{3+}$ co-doped glass ceramics by profiting from low-phonon-energy environment of erbium ions in β-NaYF₄ nanophase. Furthermore, temperature-dependent UC emission performance of the present GC is systematacially investigated to explore their potential application in optical thermometry. Obviously, owing to intense UC emissions of $\beta ?\mathit{NaY}{\mathrm{F}}_4:{\mathit{Yb}}^{3+}/{\mathit{Er}}^{3+}$ nanocrystals and high transparency, superior chemical/mechanical stability of oxide glassy matrix, the as-fabricated GCs exhibit good temperature sensing performance and good thermostability for precise temperature detecting. It is expected that the preliminary research can give a reference for designing new transparent bulk GCs and may exploit a valid method for developing high-performance optical temperature sensors.     

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.     

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.     

Distribution of Nd3+ ions in oxyfluoride glass ceramics

It has been an open question whether Nd³⁺ ions are incorporated into the crystalline phase in oxyfluoride glass ceramics or not. Moreover, relative research has indicated that spectra characters display minor differences between before and after heat treatment in oxyfluoride glass compared to similar Er³⁺-, Yb³⁺-, Tm³⁺-, Eu³⁺-, etc.-doped materials. Here, we have studied the distribution of Nd³⁺ ions in oxyfluoride glass ceramics by X-ray diffraction quantitative analysis and found that almost none of the Nd³⁺ ions can be incorporated into the crystalline phase. In order to confirm the rationality of the process, the conventional mathematical calculation and energy-dispersive spectrometry line scanning are employed, which show good consistency. The distribution of Nd³⁺ ions in oxyfluoride glass ceramics reported here is significant for further optical investigations and applications of rare-earth doped oxyfluoride glass ceramics.     

Germanate-based oxyfluoride transparent glass-ceramic embedded with Tm3+:Ca2YbF7 nanocrystals for high-performance optical thermometer

Germanate-based oxyfluoride transparent glass-ceramic functionalized by Tm³⁺:Ca₂YbF₇ nanocrystals was newly developed. The tremendously enhanced upconversion emission of ³F_2,3 energy levels by in situ crystallization was extremely beneficial for constructing optical thermometry involving the indirect thermally coupled energy levels (¹G₄ and ³F_2,3) of Tm³⁺ ions. Utilizing the fluorescence intensity ratio technique, the thermometry potentials of PG and GC8 were systematically evaluated based on the emissions from ³F_2,3 and ¹G₄ energy levels. The relative and absolute sensitivities, thermometry resolutions, and repeatabilities were superior to many reported materials. This work provides an avenue for precipitating ternary fluoride nanocrystals containing rare earths in germanate-based oxyfluoride glass, and proposes a promising way to achieve high performance optical thermometry based on the emissions from widely spaced energy levels.     

Near-infrared long-lasting phosphorescence in transparent glass ceramics embedding Cr3+-doped LiGa5O8 nanocrystals

Cr³⁺ doped transparent glass ceramics of SiO₂–Ga₂O₃–Li₂O were fabricated by melt-quenching and subsequent crystallization. X-ray diffraction and transmission electron microscopy analyses evidenced that cubic LiGa₅O₈ nanocrystals were homogeneously precipitated among the silicate glass matrix. The incorporation of Cr³⁺ ions into LiGa₅O₈ nanocrystals was evidenced by absorption, emission and time-resolved luminescence spectra. Impressively, the present Cr³⁺ doped glass ceramics were demonstrated to be a new near-infrared (∼720 nm) long-lasting bulk phosphor whose luminescence can last for more than 2 h after stoppage of UV (250–350 nm) irradiation. The occurring of Cr³⁺ long-lasting phosphorescence in the glass ceramics was confirmed to be mainly due to the precipitation of Cr³⁺:LiGa₅O₈ nanocrystals from glass matrix. The filling/releasing of electrons into/from the intrinsic traps of LiGa₅O₈ nanocrystals through the conduction band of host were proposed to be responsible for the realization of the long-lasting phosphorescence of the investigated Cr³⁺ doped glass ceramics.     

Developing "smart windows" for optical storage and temperature sensing based on KNN transparent ceramics

Smart windows have attracted considerable attention due to their wide applications in optical data storage, switchable sunroof and temperature sensing. The development strategy for smart windows is focused on performance design, enhancement and integration. However, developing integrated multi-functional smart windows in a single material remains a challenge. In this work, we have successfully prepared (K_0.5Na_0.5)_0.95Ba_0.04Er_0.01NbO₃ (4Ba-1Er-KNN) transparent ceramics for potential applications of temperature detection and optical information storage in smart windows. With alternating ultraviolet (UV) illumination and 300 °C thermal stimulation, the prepared 4Ba-1Er-KNN ceramics can not only achieve non-destructive luminescence readout, but also exhibits an ultra-high photochromic (PC) contrast with rapid response time of 1 s. Furthermore, based on the up-conversion (UC) photoluminescence (PL) intensity ratio of Er³⁺: ²H_11/2/⁴S_3/2 thermally coupled levels, excellent low-temperature sensing performance with the maximum relative sensitivity of 0.023 K⁻¹ at 213 K is obtained. The integration between UC PL, PC response and temperature sensing performance makes it possible to develop multi-functional smart windows.     

Self-crystallized Ba2LaF7:Nd3+/Eu3+ glass ceramics for optical thermometry

Eu³⁺/Nd³⁺-codoped Ba₂LaF₇ transparent bulk glass ceramics were successfully fabricated by glass self-crystallization. The structure and morphology of the sample were investigated by X-ray diffraction, transmission electron microscopy (TEM), high-resolution TEM, and selected area electron diffraction. The fluorescence intensity ratios of Nd³⁺ emission at 800 nm to the Eu³⁺ emission at 699 nm (⁵D₀ → ⁷F₄) were measured under 578.3 nm laser excitation in a wide temperature range from 290 to 740 K. A relatively good temperature sensing performance was obtained with a maximum relative sensitivity of 1.02% K⁻¹ at 420 K. Both the emission peaks for temperature sensing were located in the optical window of biological tissue, which is favorable for biomedical applications. The results indicate that Ba₂LaF₇:Nd³⁺/Eu³⁺ glass ceramics have a potential application as temperature probes.     

Enhancing upconversion emission and temperature sensing modulation of the La2(MoO4)3: Er3+, Yb3+ phosphor by adding alkali metal ions

Trivalent Er³⁺-doped La₂(MoO₄)₃ upconversion phosphors with intense green emmision were synthesized at 800 °C by the solid-state reaction route, promoting the development of novel optical thermometry. The color emitted from the samples was minorly affected by the excitation power and doping concentration. Yb³⁺ is a better sensitizer for the La₂(MoO₄)₃: Er³⁺ phosphor and it can enhance the emission intensity when a certain amount is co-doping in the system. The up-conversion luminescent mechanism was investigated using the pump power-dependent UC emission spectra. Alkali metal doping increased the up-conversion emission intensities drastically, and Li⁺ ions can enhance the luminous intensity by more than 20 times. The fluorescence intensity ratio of the transition emission ²H_11/2-⁴I_15/2 and ⁴S_3/2-⁴I_15/2 was used to study upconversion optical temperature sensing. The sensitivity changes from doping with diverse alkali metal ions and their effects on the optimal temperature range are discussed in detail. Alkali metal ions doping extended the temperature range, indicating that this phosphor is a potential candidate for temperature-sensing probes.     

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.     

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.     

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.     

Energy transfer,tunable emission and optical thermometry in Tb3+/Eu3+ co-doped transparent NaCaPO4 glass ceramics

Tb³⁺/Eu³⁺ co-doped glass ceramics containing NaCaPO₄ nanocrystals were successfully synthesized via traditional melt-quenching route with further heat-treatment and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and photoluminescence spectroscopy. The energy transfer process of Tb³⁺→Eu³⁺ was confirmed by excitation and emission spectra and luminescence decay curves, and the energy transfer efficiency was also estimated. The results indicated that the efficient emission of Eu³⁺ was sensitized by Tb³⁺ under the excitation of 378 nm, realizing tunable emission in the transparent bulk glass ceramics containing NaCaPO₄ nanocrystals. Furthermore, optical thermometry was achieved by the fluorescence intensity ratio between Tb³⁺:⁵D₄→⁷F₅ (~542 nm) and Eu³⁺:⁵D₀→⁷F₂ (~612 nm). The maximum absolute sensitivity of 4.55% K⁻¹ at 293 K and the maximal relative sensitivity of 0.66% K⁻¹ at T=573 K for Tb³⁺/Eu³⁺ co-doped transparent NaCaPO₄ glass ceramic are obtained. It is expected that the investigated transparent NaCaPO₄ glass ceramics doped with Tb³⁺/Eu³⁺ have prospective applications in display technology and optical thermometry.     

Controllable competitive nanocrystallization of La3+-based fluorides in aluminosilicate glasses and optical spectroscopy

Glass ceramic has been regarded as an alternative to traditional bulk materials such as single crystal and transparent ceramic. The nucleation/growth behavior of glass ceramic via crystallization is an important topic but is seldom studied so far. In the present work, a series of La³⁺-based oxyfluoride aluminosilicate glasses are designed to understand their nanocrystallization processes upon heating. Impressively, controllable LaF₃, α-NaLaF₄ and β-NaLaF₄ phase-competitive crystallization in glasses is achieved and structural/spectroscopic characterizations confirm the key role of Al/Si ratio to determine the release of Na⁺ ions from glass network to participate in crystallization and phase transformation. Furthermore, the developed glass ceramics are evidenced to be ideal hosts for lanthanide dopants (such as Eu³⁺ and Yb³⁺/Er³⁺), which can effectively incorporate into the precipitated fluoride crystal lattices by substituting La³⁺ ions. As a consequence, incoherent LED-excitable upconverting devices are constructed to demonstrate their promising application as emitting media in display.