K₃LaTe₂O₉:Er: a novel green up-conversion luminescence material

Abstract

A novel green up-conversion luminescence material, K₃LaTe₂O₉:Er, was synthesised via a solid-state reaction method. K₃LaTe₂O₉:Er phosphors were characterised by X-ray diffraction, reflectance spectroscopy, Raman spectroscopy, photoluminescence spectroscopy, up-conversion spectroscopy and temperature sensing performance analysis. The diffraction pattern of the hexagonal K₃LaTe₂O₉:0.02Er microcrystals was indexed with Miller indices and the lattice constants were a = b = 0.60636 ± 0.00018 nm, and c = 1.49543 ± 0.00037 nm. The photoluminescence under 380 nm excitation and the up-conversion luminescence under 980 and 1550 nm pumping were investigated. The influence of Er³⁺ ion concentration and excitation power on the luminescence properties of K₃LaTe₂O₉:Er was also discussed. K₃LaTe₂O₉:Er phosphor presented green down-shifting emission and up-conversion luminescence under 380, and 980 nm excitation and yellow–green up-conversion luminescence under 1550 nm pumping, respectively, and the red emission component was enhanced with the increment in excitation wavelength. The quenching concentration of Er³⁺ ions in K₃LaTe₂O₉:Er was much higher than that in normal phosphors. This result can be attributed to the suppression of energy migration because the shortest (0.606 nm) and average distance (0.9720 nm) between Er³⁺ ions were significantly large in K₃LaTe₂O₉. Therefore, the electric quadrupole–quadrupole interactions between Er³⁺ ions are the dominant energy transfer process in down-shift emission, and the UCL mechanism can be regarded as the excited state absorption in K₃LaTe₂O₉:Er. Furthermore, the doping concentration of Er³⁺ ions influenced the temperature sensitivity of K₃LaTe₂O₉:Er.

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1. Introduction

Rare-earth (RE)-doped luminescence materials play an important role in illumination, display, temperature sensor, security and biomedicine.^1–3 Selecting appropriate materials with excellent luminescence performance based on practical applications is important. In general, the luminescence properties of phosphors are regulated through the selection of matrix materials. Matrix materials can bond doped RE ions, and the symmetry and strength of the crystal field exert significant effects on the optical properties of RE ions. Moreover, the practical applications of matrix materials are largely dependent on their physicochemical properties.

Tellurates with rich chemical structures and unique physical properties have attracted considerable attention. RE-doped tellurite glass with relatively low phonon energy, high reflection index, and excellent thermal stability has become a popular research topic.^4–7 However, tellurate phosphors have seldom been reported. For instance, Sobczyk et al.⁸ studied the optical properties of Y₂Te₄O₁₁:Sm³⁺. Zhang et al.⁹ investigated the structure and luminescence properties of Li₃Y₃Te₂O₁₂:Eu³⁺.

K₃LaTe₂O₉ is a new quaternary tellurite material with medium phonon energy.¹⁰ In the present work, the diffraction pattern of K₃LaTe₂O₉ microcrystals was indexed with Miller indices. The down-shifting luminescence (DSL, λ_em = 380 nm) and up-conversion luminescence (UCL, λ_em = 980 and 1550 nm) of K₃LaTe₂O₉:Er were reported for the first time. In addition, the thermal quenching and temperature sensor of K₃LaTe₂O₉:Er were investigated.

2. Experimental

2.1 Sample preparation

K₃LaTe₂O₉:Er phosphors were synthesized via the solid-state reaction method. The starting raw materials La₂O₃ (99.99%), Er₂O₃ (99.99%), K₂CO₃ (A. R.), and TeO₂ (A. R.) were weighed according to stoichiometric ratio. Secondly, the above-mentioned materials were thoroughly mixed and ground using an agate ball mill for 20 min. Finally, all samples were sintered at 650 °C for 5 h. K₃LaTe₂O₉:xEr samples with different Er³⁺ concentrations were then obtained (x = 0, 2, 8, 14, 20, 26, 32, 38, 44 mol%).

2.2 Characterisation technique

X-ray powder diffraction was performed at 40 kV and 40 mA from 10° to 70° using a SHIMADZU-6000 X-ray generator with Cu Kα (λ = 0.154184 nm) radiation. The DSL and UCL spectra (400 nm to 900 nm) were recorded on a Hitachi F-4600 fluorescence spectrophotometer equipped with a power-tunable 980 and 1550 nm fibre laser diode (LD). The highest available power for the LD was approximately 800 and 700 mW, respectively. The beam of the LD was focused by the convex lens before measurement. The temperature dependence of the DSL spectra was tested by a self-assembly temperature control system with a XMT-4000 programmable temperature controller. The reflection spectra of the samples were obtained with a UV-3600 SHIMADZU UV-Vis-NIR spectrophotometer. The maximum phonon energy of the K₃LaTe₂O₉ host lattice was obtained by a micro-Raman spectroscope (Jobin Yvon HR800, excited by 633 nm He–Ne laser with a laser spot size of 1 μm², in line mapping mode). The scanning electron microscope (SEM) and Energy Dispersive Spectrometer (EDS) of the sample were obtained by JEOL-6360LV field emission gun scanning electron microscopy. The X-ray photoelectron spectroscopy (XPS) spectrum of the sample was obtained by ESCALAB250 surface analysis system.

3. Results and discussion

Fig. 1a shows the XRD pattern of the K₃LaTe₂O₉:0.02Er sample. The diffraction peaks of the present sample are similar to those in ref. 10, indicating that the hexagonal K₃LaTe₂O₉:0.02Er was synthesised. The micrograph and EDS of the K₃LaTe₂O₉ crystal shown in Fig. S1.† The concentration of element in the K₃LaTe₂O crystal shown in Table S1.† The molar ratio of K, La, Te, and O element is 3.0 : 1 : 2.1 : 7.7 (Table S1†). The diffraction peaks in the XPS pattern are assigned to K⁺, O²⁻, Te⁶⁺, and La³⁺, respectively (Fig. S2†). The relative concentration values calculated by elemental sensitivity factor method of K, O, Te, and La atoms are 61.88, 18.47, 12.31, and 7.34%, respectively, which are close to the theoretical calculated values in the K₃LaTe₂O₉. However, the standard PDF data of K₃LaTe₂O₉:Er are not available, and ref. 10 did not provide the index data of K₃LaTe₂O₉. The lattice parameters of K₃LaTe₂O₉:0.02Er were calculated by the least square method in accordance with the data in Table S2,† and the crystal lattice parameters of the K₃LaTe₂O₉:0.02Er sample were obtained: a = b = 0.60636 ± 0.00018 nm, c = 1.49543 ± 0.00037 nm, V = 0.47617 nm³. K₃LaTe₂O₉ is a 3D framework structure (Fig. 1b). K atoms are two sites that coordinate with nine and twelve O atoms forming distorted cuboctahedra, respectively. Te atoms are linked to six O atoms to form [TeO₆] octahedral, and two [TeO₆] octahedral are connected to form a face-sharing [Te₂O₉]⁶⁻ anion group. La atoms are surrounded by six O atoms in regular octahedra, and La³⁺ ions have a high-ordered state in the crystal lattice.

Fig. 1 XRD pattern (a) and crystal structure diagram (b) of K₃LaTe₂O₉:0.02Er.

Fig. 2 shows the Raman spectrum of the K₃LaTe₂O₉:0.02Er sample. The peaks situated at 285, 415, 667, 718, and 832 cm⁻¹ corresponding to the K₃LaTe₂O₉ host shown in ref. 10. Singh¹¹ reported that the range of 500–580 cm⁻¹, 620–680 cm⁻¹, and 780–900 cm⁻¹ are the characteristic peaks of Er³⁺ ion. Therefore, several peaks (remarked as *) are related to Er³⁺ ions. Some weak peaks in Raman spectrum are also observed due to defects in the lattice of K₃LaTe₂O₉:Er.¹² Raman peaks shift to the right side, which results from Er³⁺ doping. The maximum phonon energy of the K₃LaTe₂O₉ host lattice is ℏω = 832 cm⁻¹. Thus, K₃LaTe₂O₉ with an appropriate phonon energy can be considered as an alternative for the luminescence host material.

Fig. 2 Raman spectrum of the K₃LaTe₂O₉:0.02Er sample.

Fig. 3 shows the diffuse reflectance spectra of K₃LaTe₂O₉:xEr (x = 2, 26, 44 mol%). As shown in the figure, the absorption peaks become stronger with increasing Er³⁺ ion concentration. The absorption peaks at 380, 409, 486, 525, 550, 653, 798, 973, and 1537 nm can be assigned to transition from an Er³⁺ ground state ⁵I_15/2 to excited states ²K_15/2, ²H_9/2, ⁴F_7/2, ²H_11/2, ⁴S_3/2, ⁴F_9/2, ⁴I_9/2, ⁴I_11/2, and ⁴I_13/2 levels. The ⁴I_13/2 level of Er³⁺ ions has a strong absorption to 980 nm photons. The absorption efficiency of the ⁴I_13/2 level of Er³⁺ ions to 1550 nm photons reached 89% of the 1537 nm. Therefore, Er³⁺ ions can be effectively sensitised by 1550 nm exciting light. The absorption edge of K₃LaTe₂O₉:Er can be obtained in absorption spectra. The reflectance spectra of K₃LaTe₂O₉:Er was converted into the absorption spectra based on the Kubelka–Munk formula¹³ (inset in Fig. 3):

F(R) = (1 − R)²/2R = K/S, (1)

where R is the reflectance, K is the absorption coefficient, and S is the scattering coefficient. The absorption edges of K₃LaTe₂O₉:xEr are 287, 293 and 301 nm, which correspond to the band gap energies of 4.32, 4.23, and 4.12 eV, respectively (x = 2, 26, 44 mol%). The band gap value decreases with increasing Er³⁺ ion concentration. This phenomenon can be attributed to the fact that the ionic radii of Er³⁺ (0.088 nm) are smaller than that of La³⁺ (0.102 nm), thus resulting in the shrinkage of the lattice matrix and the red shift of the absorption bands after doping Er³⁺ ions.

Fig. 3 UV-Vis-NIR reflectance and absorption (inset) spectra of K₃LaTe₂O₉:xEr (x = 2, 26, 44 mol%).

Fig. 4a shows the excitation and DSL spectra of K₃LaTe₂O₉:Er. The excitation spectrum of K₃LaTe₂O₉:Er presents a strong peak at ∼380 nm, which corresponds to the ⁴G_11/2 → ⁴I_15/2 transition by monitoring the 550 nm emission of Er³⁺ ions. K₃LaTe₂O₉:Er under 380 nm excitation (K₃LaTe₂O₉:Er at 380 nm) exhibits pure green emission (I_G, ²H_11/2/⁴S_3/2 → ⁴I_15/2), and the red irradiation (I_R, ⁴F_9/2 → ⁴I_15/2) is very weak with the green and red fluorescence intensity ratio (I_G/I_R) I_G/I_R = 21.3. K₃LaTe₂O₉:Er at 380 nm has a relatively strong peak at ∼410 nm, which is assigned to the ⁴H_9/2 → ⁴I_15/2 transition of Er³⁺ ions. In addition, the emission spectra present a weak band located at 450–470 nm, which corresponds to the ⁴F_3/2/⁴F_5/2 → ⁴I_15/2 transition. The doping concentration of the optimum Er³⁺ ions is 26 mol% (Fig. 4b). The integrated intensity initially increases with increasing doping concentration, reaches its maximum at around 26 mol%, and then decreases. The quenching concentration is much higher than that of conventional materials.¹⁴ The relationship between the DSL intensity and doping concentration of the luminescence centre can be described by the Van Uitert model.¹⁵

I(C) = C/κ(1 + βC^Q/3), (2)

where C is the concentration of RE³⁺ ions, κ and β are the constants and Q is the interaction types between RE³⁺ ions. Q = 3, 6, 8, and 10 is related to exchange interaction, electric dipole–dipole (D–D), electric dipole–quadrupole (D–Q), and electric quadrupole–quadrupole (Q–Q) interactions. Eqn (2) was fitted to the data in Fig. 4b, and the Q value of K₃LaTe₂O₉:Er at 380 nm is 10.2 ± 0.3 ≈ 10. Therefore, the Q–Q interaction between Er³⁺ ions is dominant for quenching ²H_11/2/⁴S_3/2 levels in K₃LaTe₂O₉:Er. However, ET is usually caused by the D–D interaction in most materials.¹⁶

Fig. 4 Excitation and DSL spectra of K₃LaTe₂O₉:Er (a) and dependence of ²H_11/2/⁴S_3/2 → ⁴I_15/2 green DSL integrated intensity on Er³⁺ ion concentration (b).

The shortest distance between La³⁺–La³⁺ ions is 0.606 nm in the K₃LaTe₂O₉ matrix, which is significantly larger than that of Y³⁺–Y³⁺ in Y₂O₃ (0.351 nm). The average distance d̄ between doping Er³⁺ ions can be expressed as¹⁷

where V is the volume of the unit crystal cell, x is the Er³⁺ doping concentration and Z is the number of molecules in the unit crystal cells. When the Er³⁺ ion concentration is 26 mol%, d̄ = 0.9711 nm, which is also significantly larger than that in Gd₂O₂S:10% Er³⁺ (0.753 nm) and β-NaYF₄:25% Er³⁺ (0.663 nm).¹⁷ Therefore, the shortest distance and average distance between Er³⁺ ions are large in the K₃LaTe₂O₉ matrix. The energy migration is affected by matrix.¹⁸ The K₃LaTe₂O₉ host can provide long lattice sites for Er³⁺ ions, which is helpful in obtaining heavy dopes and effectively reduces the harmful ET process. Moreover, it is beneficial in improving the absorption of incident light.

Under 980 nm pumping, K₃LaTe₂O₉:Er presents the green UCL (K₃LaTe₂O₉:Er at 980 nm) (inset of Fig. 5a). The UCL spectra for K₃LaTe₂O₉:xEr at 980 nm are shown in Fig. 5a (x = 2, 8, 14, 20, 26, 32, 44 mol%). The spectra consist of basically two groups in the visual regions at an interval of 500–800 nm: (1) the strongest green emission at ∼525 nm and ∼550 nm can be attributed to the joint contributions of the Er³⁺ ion ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transition, and (2) the second strongest red emission at ∼665 nm can be attributed to the Er³⁺ ion ⁴F_9/2 → ⁴I_15/2 transition. The emission peaks intensity of K₃LaTe₂O₉:Er at 980 nm are successively enhanced with increasing concentration of Er³⁺ ions. The concentration quenching phenomenon is not observed when the concentration of Er³⁺ ion reaches 44 mol%. Compared with the DSL spectra in Fig. 4a, the red emission is improved significantly, and its I_G/I_R ratio is reduced from 21.3 (DSL) to 2.5–4.7 (UCL, 980 nm). The luminescence colour can be tuned by the doping concentration of Er³⁺ ions and the excitation power under 980 nm excitation. As the Er³⁺ ion concentration continues to increase from 2 mol% to 38 mol%, the red light component is enhanced, and its I_G/I_R ratio is reduced from 3.9 to 2.7. By contrast, the green component can be effectively enhanced and the I_G/I_R ratio is increased from 2.5 to 4.7 with increasing excitation power (Fig. 5e). Under high excitation power pumping, more photons can be absorbed by Er³⁺ ions, which results in Er³⁺ ions populating high levels easily.

Fig. 5 UCL spectra of the K₃LaTe₂O₉:xEr sample under 980 nm (a) and 1550 nm (c) pumping, the dependence of the green UCL intensity on Er³⁺ doping concentration under 980 nm (b) and 1550 nm (d) pumping and the dependence of the I_G/I_R value of the K₃LaTe₂O₉:xEr sample on Er³⁺ doping concentration and pumping current under 980 nm (e) and 1550 nm (f) excitation (x = 2, 8, 14, 20, 26, 32, 38, 44 mol%).

In addition, the dependence of luminescence-integrated intensity on the Er³⁺ ion concentration of K₃LaTe₂O₉:Er at 980 nm is nearly linear when the concentration is low (2–26 mol%), which indicates that the UCL mechanism can be regarded as excited state absorption (ESA). This phenomenon is owed to the long distance between the RE ions in the K₃LaTe₂O₉ structure resulting in the suppression of the ET.¹⁹ K₃LaTe₂O₉:Er at 980 nm presents an approximate nonlinear relation when Er³⁺ > 26 mol%, indicating that the ET between Er³⁺ ion pairs also plays an important role in the UC process (Fig. 5b).

K₃LaTe₂O₉:Er exhibits yellow–green emission under 1550 excitation (K₃LaTe₂O₉:Er at 1550 nm) (inset of Fig. 5b). The UCL spectra in the visible region of K₃LaTe₂O₉:Er at 1550 nm have the same peak positions and shape with different relative intensities compared with those at 980 nm excitation. However, the red emission intensity is significantly improved excited by 1550 nm, and its I_G/I_R ratio is reduced to 0.86–1.17. Whilst a relatively strong NIR emission peak at ∼800 nm is observed, the optimum doping concentration of Er³⁺ ions is 32 mol% at 1550 nm pumping (Fig. 5c). When the Er³⁺ ion concentration is increased from 2 mol% to 44 mol%, the I_G/I_R ratio is reduced from 1.5 to 0.86. On the contrary, the green emission can be significantly enhanced, and the I_G/I_R ratio is increased from 0.86 to 1.17 with increasing excitation power (Fig. 5f). The UCL mechanism of K₃LaTe₂O₉:Er at 1550 nm is mainly based on the ESA process when the concentration of Er³⁺ ions is less than 16 mol%. Subsequently, the ET process begins to dominate (Er³⁺ > 16 mol%, Fig. 5d). Compared with DSL, the red component of K₃LaTe₂O₉:Er is increased, particularly under 1550 nm pumping. However, the red UCL mechanism is still controversial.^20,21

The non-radiation multiphonon relaxation rate (ω_P) between the energy levels of RE ions can be expressed using the Miyakawa–Dexter theory:^22,23

where α, and ω₀ are the constants related to the host properties, ΔE is the energy gap (cm⁻¹) and ℏω is the maximum phonon energy of the host lattice.

The energy gaps of ⁴S_3/2–⁴F_9/2 and ⁴I_11/2–⁴I_13/2 are ΔE ≈ 2980 and 3400 cm⁻¹, corresponding to ΔE/ℏω = 3.6 and 4.1. The strong green emission (²H_11/2/⁴S_3/2 → ⁴I_15/2) and almost no red emission (⁴F_9/2 → ⁴I_15/2) of K₃LaTe₂O₉:Er at 380 nm are observed directly excited at 380 nm, indicating that the non-radiative relaxation ⁴S_3/2 → ⁴F_9/2 is weak (Fig. 4a). However, the non-radiative relaxation probability of ⁴I_11/2 → ⁴I_13/2 is smaller than that of the ⁴S_3/2 → ⁴F_9/2, that is, the ⁴S_3/2 → ⁴F_9/2 non-radiative relaxation process is not the main reason for the red UCL in K₃LaTe₂O₉:Er at 980 nm. Therefore, the mechanism of red intensity enhancement can only be the cross relaxation (CR) between Er³⁺ ions. The red emission intensity is very weak even at high Er³⁺ concentrations for DSL because the CR between Er³⁺ ions is restrained in K₃LaTe₂O₉ owing to the joint contributions of the long distance between the RE ions and the lack of intermediate metastable energy level below the ⁴S_3/2 level. However, the long-level lifetime of the intermediate levels is inspired, and the CR processes significantly enhance the red emission in the UCL compared with DSL. The short-level lifetime of the ⁴I_9/2 level and the small energy gap of ⁴I_9/2–⁴I_11/2 (ΔE ≈ 1950 cm⁻¹, ΔE/ℏω = 2.3) are the major factors influencing the red radiation enhancement of K₃LaTe₂O₉:Er at 1550 nm.^24,25 Therefore, the difference in UCL spectra and quenching concentration for DSL and UCL results mainly from the different energy level population of the Er³⁺ ions caused by the different excitation routes.

The different optimum Er³⁺ concentrations of K₃LaTe₂O₉ under various wavelengths pumping is caused by the different excitation paths, level lifetime, and absorption cross section of levels. The transition model of the Er³⁺ ions in the K₃LaTe₂O₉ phosphor excited at 380, 980, and 1550 nm is established to describe the luminescence process in Fig. 6. Red emission (⁴F_9/2 → ⁴I_15/2) of Er³⁺ ions was very weak in the DSL spectrum under 380 nm pumping (Fig. 4a). The green UCL process for K₃LaTe₂O₉:Er at 380 nm is as follows: ⁴I_15/2 + hν_{980 nm} → ⁴G_11/2, ⁴G_11/2 → ⁴H_9/2 + multiphonon relaxation + hν_{410 nm}, ⁴H_9/2 → ⁴F_3/2/⁴F_5/2 + multiphonon relaxation + hν_{450–470 nm}, ⁴F_3/2/⁴F_5/2 → ²H_11/2/⁴S_3/2 + multiphonon relaxation, ²H_11/2/⁴S_3/2 → ⁴I_15/2 + hν_{525 nm}/hν_{550 nm}. The green UCL process for K₃LaTe₂O₉:Er at 980 nm is as follows: ⁴I_15/2 + hν_{980 nm} → ²I_11/2, ⁴I_11/2 + hν_{980 nm} → ⁴F_7/2, ⁴F_7/2 → ²H_11/2/⁴S_3/2 + multiphonon relaxation, and ²H_11/2/⁴S_3/2 → ⁴I_15/2 + hν_{525 nm}/hν_{550 nm}. Its red UCL mechanism is the ⁴F_7/2 + ⁴I_11/2 → ⁴F_9/2 + ⁴F_9/2 CR process. The red and NIR intensity of K₃LaTe₂O₉:Er at 1550 nm is significantly enhanced. The UCL mechanism is as follows: ⁴I_15/2 + hν_{1550 nm} → ²I_13/2, ⁴I_13/2 + hν_{1550 nm} → ⁴I_9/2, ⁴I_9/2 + hν_{1550 nm} → ²H_11/2, ⁴I_9/2 + multiphonon relaxation → ⁴I_11/2, ⁴I_11/2 + hν_{1550 nm} → ⁴F_9/2, and ⁴F_9/2 → ⁴I_15/2 + hν_{660 nm}, ⁴I_9/2 → ⁴I_15/2 + hν_{800 nm}.

Fig. 6 Transition model of Er³⁺ ions in the K₃LaTe₂O₉ phosphor excited at 380, 980, and 1550 nm.

As shown in Fig. 7a, the K₃LaTe₂O₉:0.26Er sample can nearly recover to its original intensity after cooling down to room temperature from 473 K, indicating that K₃LaTe₂O₉:Er phosphors have a high resistance to heating damage by using a 380 nm xenon lamp as the excitation source. Fig. 7b shows the integrated intensities of K₃LaTe₂O₉:xEr phosphors as a function of temperature. The integrated emission intensities are normalized at room temperature for K₃LaTe₂O₉:xEr (x = 20, 26, 32 mol%). With the successive increase in the heating temperature, the green UCL intensity of different concentration samples gradually decreases. At 473 K, the integrated intensity of the K₃LaTe₂O₉:xEr (x = 20, 26, 32 mol%) samples can retain 52%, 56%, and 58% at room temperature, respectively. Therefore, K₃LaTe₂O₉:Er is a good matrix material to achieve a stable DSL. The higher Er³⁺ ion concentration, the more obvious is the thermal quenching phenomenon of the sample. The thermal quenching mechanism of the luminescent material is usually different, containing ET, CR, and non-radiative transition.²⁶ The integrated intensity curves for various content samples have a similar slope. Thus, the ET rates between Er³⁺ ions causing heating temperature are similar. In addition, the probability of ⁴S_3/2 non-radiative relaxation to near-low energy level is small. Therefore, CR is mainly responsible for the luminescent thermal quenching.

Fig. 7 DSL spectra of K₃LaTe₂O₉:0.26Er at different temperatures (a) and integrated emission intensities of K₃LaTe₂O₉:xEr phosphors as a function of temperature (b) (x = 20, 26, 32 mol%), (λ_m = 380 nm).

According to Boltzmann's distribution, the intensity ratio (R_HS) between the thermal equilibrium levels of ²H_11/2 and ⁴S_3/2, (I_H/I_S), can be expressed as

R_HS = I_H/I_S = C exp(−ΔE/κT), (5)

where C is the constant relating to the host materials, ΔE is the energy gap between ²H_11/2 and ⁴S_3/2 levels, κ is the Boltzmann constant and T is the absolute temperature. The DSL spectra are adopted to characterise the temperature sensor properties and to avoid the thermal effect in UCL caused by the LD.²⁷ Eqn (5) was fit for the experimental data in Fig. 8a, and the curves for the K₃LaTe₂O₉:xEr samples from 303 K to 473 K (x = 20, 26, 32 mol%) were drawn. The fitting data show that the Er³⁺ ion concentrations affect the C and ΔE values. The R_HS values of ²H_11/2 and ⁴S_3/2 energy levels continuously enhance with increasing heating temperature, whereas the green DSL intensity decreases with increasing temperature. Sensitivity is an important parameter to evaluate the sensor performance for the temperature sensing. Sensitivity S can be derived according to eqn (6):

S = dR/dT = R(−ΔE/κT²), (6)

Fig. 8 Relationships between ²H_11/2/⁴S_3/2 level green emission intensity radio and the heating temperature (a), and sensitivity curves of the K₃LaTe₂O₉:xEr samples (b) (x = 20, 26, 32 mol%).

In general, the luminescence intensity caused by doping concentration exerts a slight effect on sensitivity. However, in the present study, the temperature sensitivity of the K₃LaTe₂O₉:Er samples decreases with increasing Er³⁺ ion concentration (Fig. 7b). This is due to that the change of the concentration of Er³⁺ ions may cause the change of the crystal field surrounding Er³⁺ ions, and the change of crystal field will cause the change of the optical transition rates of Er³⁺ ions. Therefore, the variation of radiation transition rate of ²H_11/2 and ⁴S_3/2 levels arouse the change of fluorescence intensity ratio FIR in different Er³⁺ ion doped samples and affect the sensitivity values. In addition, the thermal effects of samples is remarkable when concentration of doped Er³⁺ ions is higher.²⁸ The temperature of samples has an effect on crystal field. The sensitivities of 20 and 26 mol% Er³⁺ ions are similar, corresponding to approximately 0.008 and 0.0075 K⁻¹ at 473 K, respectively. When the Er³⁺ ion concentration reaches 32 mol%, the sensitivity decreases rapidly, and its value is nearly 0.0057 K⁻¹ (Fig. 8b). Therefore, the low Er³⁺ content should be carefully chosen to obtain the optimal sensitivity performance. The previously reports on temperature sensitivities data were provided in Table S3.† On the basis of the experimental data in Fig. 8b, the K₃LaTe₂O₉:Er material can be used as the luminescent thermometer for temperature sensing. In addition, the above-mentioned results show that the Er³⁺ ion concentration is important to the thermal quenching and sensitivity of K₃LaTe₂O₉:Er phosphors.

4. Conclusion

A new green up-conversion luminescence material, K₃LaTe₂O₉:Er, was synthesised. The diffraction pattern of the hexagonal K₃LaTe₂O₉ crystal was indexed with Miller indices and the lattice constants a = b = 0.60636 ± 0.00018 nm, c = 1.49543 ± 0.00037 nm, and V = 0.47617 nm³. The DSL and UCL properties of K₃LaTe₂O₉:Er were studied under 380, 980 and 1550 nm excitation. The shortest distance and average distance between Er³⁺ ions in the K₃LaTe₂O₉ matrix are significantly larger than that in normal phosphors. Therefore, energy migration is significantly suppressed, which is helpful in obtaining heavy Er³⁺ doping.

Acknowledgements

The authors thank the National Natural Science Foundation of China (No. 11504039 and 51502031), Liaoning Provincial Natural Science Foundation of China (No. 2015020206), and Research Program of Application Foundation (Main subject) of Ministry of Transport of PR China (No. 2015329225090). Fundamental Research Funds for the Central Universities (Grant Nos. 3132017061, 3132017066, and 3132016349) for their financial support. The authors thank Prof. Chang Liu for the XPS measurements.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra06191a

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