Realizing emission color tuning, ratiometric optical thermometry and temperature-induced redshift investigation in novel Eu³⁺-doped Ba₃La(VO₄)₃ phosphors

Abstract

Rare earth-doped Ba₃La(VO₄)₃ phosphors with tunable emitting colors were firstly explored and their photoluminescence properties were systematically investigated. The experimental results show that the Ba₃La(VO₄)₃ phosphors exhibit high-brightness self-activated emission and enable us to sensitize the luminescence of rare earth activators. Under near-ultraviolet (UV) excitation, both the broadband emission from VO₄³⁻ groups and the sharp peak emissions from Eu³⁺ ions are observed in Ba₃La(VO₄)₃:Eu³⁺ phosphors. Modulation of the emitting color from green to red can be realized by adjusting the Eu³⁺ doping concentration, which is assigned to an efficient energy transfer from VO₄³⁻ to Eu³⁺ ions. Notably, the optical thermometry of Ba₃La(VO₄)₃:Eu³⁺ was characterized based on the fluorescence intensity ratio of VO₄³⁻ and Eu³⁺ emissions in the 298–573 K range, with the maximum absolute and relative sensitivities of 0.0515 K⁻¹ and 1.77% K⁻¹ at 298 K. In addition, similar phenomena were observed in Sm³⁺ and Dy³⁺-doped Ba₃La(VO₄)₃ phosphors. These results verify a feasible strategy for varying the emission color and realizing optical thermometry in the single-component phosphors by adjusting the energy transfer between the host and the activator. It provides new possibilities for the design of multifunctional materials for white light-emitting diodes and non-contact thermometry.

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1. Introduction

In recent years, color tunable phosphors have been widely applied in full-color displays, light-emitting diodes (LEDs), optical display devices, and biological technologies owing to their merits of facile preparation, high-brightness color tunable emission, and excellent thermal stability.^1–4 So far, two main approaches, single and co-doping lanthanide ions in a matrix, are used for the multicolor tunable emission property of phosphors.^5–7 In these materials, the multicolor tunable emission could be realized through many different routes, including varying the wavelength of excitation,⁸ controlling the energy transfer from the host/sensitizer to the activator,^9,10 adjusting the crystal field environment around the dopants,¹¹ and changing the temperature¹² and composition modulation.¹³ Color-tunable phosphors, however, are usually obtained by exploring complicated crystal structures with different dopants or using various excitation wavelengths in the above methods. Hence, we turned our attention to develop appropriate single-doped phosphors with single phase and simple crystal structure under an identical wavelength excitation.

For luminescent materials, vanadate compounds have attracted attention as self-activated phosphors and rare earth doping hosts due to their excellent chemical stability and favorable luminescence properties.^14–16 The photoluminescence (PL) properties of Sm³⁺-doped Sr₃La(VO₄)₃ and Eu³⁺-doped Ca₇(VO₄)₄O phosphors with multicolor emissions were reported by Du et al.^17,18 The crystal structure and luminescence properties of Ca₄La(VO₄)₃O:Eu³⁺ phosphors were also investigated as well.¹⁹ All these phosphors exhibit self-activated efficient luminescence from 400 nm to 750 nm originating from VO₄³⁻ groups, with high quantum yield. Furthermore, the PL results of Ba₃(VO₄)₂:3xSm³⁺ microparticles revealed a bifunctional platform for optical thermometry and safety signs via energy transfer from the VO₄³⁻ group to Sm³⁺ ions.²⁰ However, as far as we know, few studies have been concentrated on Eu³⁺-doped and self-activated Ba₃La(VO₄)₃ phosphors, especially emission color tuning and temperature sensing. Thus, the photoluminescence properties of Eu³⁺-doped Ba₃La(VO₄)₃ phosphors have been further investigated.

Nowadays, the fluorescence intensity ratio (FIR) strategy of optical thermometry has attracted considerable interest for temperature sensors thanks to their satisfactory advantages, such as high sensitivity, noninvasion, and rapid response, etc. There are numerous reports on optical thermometric characterization based on the FIR method with co-doped rare earth ions, such as Eu³⁺/Tb³⁺, Pr³⁺/Tb³⁺ and Eu³⁺/Mn⁴⁺.^21–23 The optical sensing performance using the FIR of VO₄³⁻/Eu³⁺ in LiSrVO₄ phosphors was investigated in detail in our recent work.²⁴ In addition, single-phased LuVO₄:Eu³⁺ phosphors with color tuning emission were reported as well.²⁵ Regrettably, the temperature sensing by manipulating the energy transfer from VO₄³⁻ to Eu³⁺ in LuVO₄:Eu³⁺ phosphors was not investigated yet. As far as we know, the considerable research efforts have been devoted to the vanadate phosphors for lighting and display, but these materials were relatively less studied as ratiometric thermometers.

In this study, the photoluminescence and temperature-dependent luminescence properties of Eu³⁺-doped Ba₃La(VO₄)₃ phosphors were systematically investigated. Notably, the FIR of VO₄³⁻/Eu³⁺ could be served as a highly sensitive temperature sensor in the 298–573 K range with comparable sensitivity performance. In addition, it is found that the excitation spectra of Ba₃La_0.95(VO₄)₃:0.05Eu³⁺ monitoring at 617 nm emission of Eu³⁺ shift to lower energy with the increase of temperature. The possible temperature quenching mechanisms involved in the present system were discussed as well. It is expected that this work may provide a new perspective for designing multifunctional materials.

2. Experimental details

2.1 Sample preparation

All the studied samples of Ba₃La_1−x(VO₄)₃:xEu³⁺ (x = 0–0.05) phosphors were synthesized by a high temperature solid-state reaction method. The stoichiometries of Eu₂O₃, La₂O₃, BaCO₃ and NH₄VO₃ as starting materials were mixed and well ground for about 30 min in an agate mortar. Then the mixtures were transferred to an alumina crucible and calcined at 500 °C for 4 h and subsequently at 1000 °C for 8 h in air. After the samples were cooled down to room temperature, they were ground into a fine powder for consequent measurements. Moreover, the Ba₃La(VO₄)₃:Sm³⁺ and Ba₃La(VO₄)₃:Dy³⁺ phosphors were synthesized in a similar procedure.

2.2 Characterization

The phase of samples was determined using an X-ray diffractometer XD-2 (PERSEE), with Cu–K_α radiation (λ = 1.5406 Å), operating at 36 kV and 20 mA. Photoluminescence spectra at room temperature were obtained using a high-resolution spectrofluorometer FLS 920 (Edinburgh Instruments) equipped with both continuous (450 W) and pulsed xenon lamps. High-temperature PL spectra were recorded using the same spectrofluorometer interfaced with a computer-controlled electrical furnace in the range of 298–573 K.

3. Results and discussion

3.1 Crystal structure analysis

Fig. 1(a) illustrates the XRD patterns of Ba₃La_1−x(VO₄)₃:xEu³⁺ (x = 0, 0.005, 0.01, 0.03, 0.05) phosphors. It can be found that nearly all the diffraction peaks of the studied samples can be indexed to the Ba₃La(VO₄)₃ data (JCPDS no. 40-0111). Besides, there are new minor BaVO₃ (JCPDS no. 26-0205) impurity phases (marked with *) observed in the XRD patterns of the samples, which are weak enough to be neglected in our research. Revealed by the enlarged view of XRD patterns at the most intense diffraction peaks (see Fig. 1(b)), it can be seen that the strongest peak (105) shifts gradually towards a higher angle with increasing Eu³⁺ concentration, which is due to the substitution with a smaller radius of Eu³⁺ (0.947 Å, coordinated number CN = 6) than that of La³⁺ (1.032 Å, CN = 6).²⁶ When Eu³⁺ substituted La³⁺, the interplanar distance d value decreased. Therefore, the diffraction angle θ increased according to Bragg's law. It is verified that Eu³⁺ had been successfully entered into the Ba₃La(VO₄)₃ lattice at the La³⁺ sites.

Fig. 1 (a) XRD patterns of Ba₃La_1−x(VO₄)₃:xEu³⁺ (x = 0–0.05) samples and the standard XRD data of Ba₃La(VO₄)₃ (JCPDS no. 40-0111). (b) Partially enlarged XRD patterns of Ba₃La_1−x(VO₄)₃:xEu³⁺ (x = 0–0.05). (c) Schematic illustration of the Ba₃La(VO₄)₃ unit-cell structure and the coordination environments of Ba/La and V.

The Ba₃La(VO₄)₃ phosphor crystallizes in the monoclinic phase with space group R3̄m, as determined by the XRD measurement. The standard values for the crystal are a = 5.75271 Å, c = 21.04729 Å and V = 603.21 ų. The crystal structure in Fig. 1(c) reveals that there are two different kinds of lattice sites for Ba and La cations, surrounded by six oxygen ions in an octahedron configuration and ten oxygen ions in a polyhedron with 13 faces. Moreover, the V atoms are coordinated by four oxygen atoms in a tetrahedral configuration.²⁷

3.2 Room-temperature luminescence properties of the Ba₃La(VO₄)₃ host

The self-activated host optical properties are fascinating for luminescent materials. The excitation of the photoluminescence (PLE) and PL spectra of Ba₃La(VO₄)₃ phosphors is presented in Fig. 2(a). The PLE spectrum exhibits a broad band, in the range of 250–380 nm, which can be decomposed into two sub-bands peaking at 313 nm (Ex₁: ¹A₁ → ¹T₂) and 340 nm (Ex₂: ¹A₁ → ¹T₁) attributing to the V⁵⁺ → O²⁻ charge transfer (CT) transitions of VO₄³⁻ groups.²⁸ Under 320 nm excitation, the PL spectrum exhibits a broad emission band (400 nm to 650 nm) with the maximum emission peaking at 500 nm. This emission band is ascribed to the CT transitions of the VO₄³⁻ groups.²⁹ Meanwhile, the emission band can be well de-convoluted into two Gaussian peaks, which are attributed to the ³T₂ → ¹A₁ (Em₁ = 488 nm) and ³T₁ → ¹A₁ (Em₂ = 530 nm) transitions of VO₄³⁻ groups as shown in Fig. 2(b) as Em₁ and Em₂, respectively. It can be seen that Ba₃La(VO₄)₃ is an effective self-activated host, which could exhibit intense green light. It is noted that the PL and PLE spectra of Ba₃La(VO₄)₃ are very similar to those of Sr₃La(VO₄)₃, except that the maximum of the excitation and emission bands is shifted to longer wavelengths (320 nm and 500 nm in Ba₃La(VO₄)₃ compared with 350 nm and 520 nm in Sr₃La(VO₄)₃.²⁷

Fig. 2 (a) The excitation and emission spectra of the Ba₃La(VO₄)₃ samples; the green bands named Em₁ and Em₂ are obtained by the Gaussian fitting of the emission spectrum and the green bands named Ex₁ and Ex₂ are obtained by the Gaussian fitting of the excitation spectrum. (b) Schematic illustration of excitation and emission processes for VO₄³⁻.

3.3 Room-temperature luminescence properties of Eu³⁺-doped Ba₃La(VO₄)₃ phosphors

As for the Eu³⁺-doped Ba₃La(VO₄)₃ phosphors, we take Ba₃La_0.99(VO₄)₃:0.01Eu³⁺ as an example. The excitation spectrum monitored at 617 nm corresponding to the ⁵D₀ → ⁷F₂ transition of Eu³⁺ consists of two distinct regions as shown in Fig. 3. In the UV spectral region (250–350 nm), the broad absorption band located at 320 nm is assigned to the V⁵⁺ → O²⁻ CT band as described above. Besides, the relatively weak sharp lines in the range of 370–500 nm are ascribed to the 4f–4f transitions of Eu³⁺ ions. According to the Dieke diagram, these transitions are identified and located at 360, 378, 391, 412, 463 and 531 nm, which are due to the intra-configurational transitions of Eu³⁺:⁷F₀ → ⁵G₄, ⁷F₀ → ⁵L₆, ⁷F₀ → ⁵D₃, ⁷F₀ → ⁵D₂ and ⁷F₀ → ⁵D₁, respectively.³⁰ The observation of the V⁵⁺ → O²⁻ CT band in the excitation spectra of Eu³⁺ implies that energy transfer from VO₄³⁻ groups to Eu³⁺ occurs in Ba₃La_0.99(VO₄)₃:0.01Eu³⁺. In addition, it is worth noting that the host absorption from VO₄³⁻ groups dominates in the excitation spectrum, illustrating that the studied phosphors can be efficiently excited by the near UV chips.

Fig. 3 Excitation spectrum of Ba₃La_0.99(VO₄)₃:0.01Eu³⁺ phosphors monitored at 617 nm and PL emission spectra of Ba₃La_0.99(VO₄)₃:0.01Eu³⁺ phosphors excited at 320 and 391 nm, respectively.

The emission spectra of Ba₃La_0.99(VO₄)₃:0.01Eu³⁺ upon excitation of 320 and 391 nm are also presented in Fig. 3. At room temperature, the sample shows intense green and red luminescence from host and Eu³⁺ (peaking at 592, 619, 655 and 704 nm) by exciting into the CT band, whereas sharp emission red emission from Eu³⁺ can be only observed under 391 nm excitation. It is well known to us that the transition of ⁵D₀ → ⁷F₁ (592 nm) belongs to the magnetic dipole transition of Eu³⁺, which is less influenced by the surrounding environment and obeys the transition selection rule of Δl = 0, Δs = 0. However, the ⁵D₀ → ⁷F₂ (617 nm) transition is a typical electric dipole transition and is sensitive to the site symmetry of Eu³⁺ ions.³¹ The emission intensity ratio of ⁵D₀ → ⁷F₂/⁵D₀ → ⁷F₁ transitions can be applied as a good determination for the site symmetry of the rare-earth ion in the structure.³² The intensity of ⁵D₀ → ⁷F₂ transition is 2.5 times that of ⁵D₀ → ⁷F₁ transition in the case of Ba₃La_0.99(VO₄)₃:0.01Eu³⁺. It implies that Eu³⁺ ions occupy the low-symmetry lattice sites without inversion symmetry. The energy transfer efficiency η_ET from VO₄³⁻ to Eu³⁺ can be evaluated by the following equation:³³

where I₀ and I represent the emission intensities of the Ba₃La(VO₄)₃ host and Ba₃La_0.99(VO₄)₃:0.01Eu³⁺ phosphors, respectively. Accordingly, η_ET can be determined to be about 66.34%.

In order to investigate the influence of the Eu³⁺ concentrations on the PL properties of the Ba₃La_1−x(VO₄)₃:xEu³⁺ (x = 0.005–0.05) phosphors, their PL spectra were recorded as well, as shown in Fig. 4(a). Under 320 nm excitation, the luminescence intensities of Eu³⁺ continuously increase with the increasing Eu³⁺ concentration up to 5 mol%. The values of CIE chromaticity coordinates and images of Ba₃La_1−x(VO₄)₃:xEu³⁺ phosphors under 365 nm UV lamp excitation are presented in Fig. 4(b). Obviously, it was observed that the emission color of the phosphor can be modified by controlling the Eu³⁺ concentration. As shown in the CIE coordinate diagram in Fig. 4(b), the emitting color can be gradually modulated from green to orange and then to yellowish brown and finally red. The images of the sample in Fig. 4(b) also show that the emission color can be tuned from green to red. Correspondingly, the CIE coordinates can be tuned from (0.2442, 0.3946) to (0.6077, 0.3499) with the increase of the Eu³⁺ concentration, which is attributed to the VO₄³⁻ → Eu³⁺ energy transfer.

Fig. 4 (a) PL spectra of the Ba₃La_1−x(VO₄)₃:xEu³⁺(x = 0.005–0.05) samples excited at 320 nm. (b) CIE chromaticity coordinates and images of Ba₃La_1−x(VO₄)₃:xEu³⁺ (x = 0.005–0.05) phosphors.

In order to further understand the energy transfer process, the decay lifetime of (VO₄)³⁻ in the Ba₃La_1−x(VO₄)₃:xEu³⁺ phosphors is shown in Fig. 5. It can be found that the lifetime of the samples changes a little with the various Eu³⁺ concentrations. This phenomenon indicates that the energy transfer process between (VO₄)³⁻ and Eu³⁺ is not due to cross relaxation between ³T₁,³T₂ → ¹A₁ of (VO₄)³⁻ and ⁷F₀ → ⁵D_J(J = 0, 1 or 2) of Eu³⁺, but because of the exchange or super exchange mechanism reported by H. J. Seo et al.^34,35 Similar phenomena have been reported previously for Sr₃La(VO₄)₃:Eu³⁺ phosphors.²⁷

Fig. 5 Decay curves (λ_ex = 320 nm, λ_em = 500 nm) of (VO₄)³⁻ in the Ba₃La_1−x(VO₄)₃:xEu³⁺ phosphors. The inset shows the luminescence lifetime for the phosphors with different Eu³⁺ concentrations.

It is worth mentioning that the Ba₃La(VO₄)₃ host can also sensitize the other rare earths activators, such as Sm³⁺ and Dy³⁺. The PL properties are dependent on the Sm³⁺ or Dy³⁺ concentration and the temperature, as shown in Fig. S1 and S2.† Further work on other RE³⁺-doped Ba₃La(VO₄)₃ phosphors is being carried out in our further study.

The possible mechanisms involved in the energy transfer process and emission color tuning were proposed, as shown in Fig. 6. When Ba₃La_1−x(VO₄)₃:xEu³⁺ is excited by 320 nm into the VO₄³⁻ groups, electrons are first excited from the ground state to the excited state. Then the excited electrons relax to the lower excited level, resulting in radiative green emission and eventually return to the ground state. When the Eu³⁺ ions are doped into it, energy absorbed by VO₄³⁻ groups can be partially transferred to Eu³⁺via the nonradiative resonant process. This process can populate the excited levels of ⁵D₀ through the nonradiative resonant process and subsequently give rise to the radiative transitions from ⁵D₀ to ⁷F_J (J = 0, 1, 2, 3, 4). The energy transfer efficiency can be manipulated by adjusting the doping concentration of Eu³⁺ ions, and thus the red and green emission intensity ratio can be tuned, which enables us to control the emission color from green to red (see Fig. 6).

Fig. 6 Energy transfer scheme for the color tuning Ba₃La_1−x(VO₄)₃:xEu³⁺ phosphors.

3.4 Temperature-sensing performance of Eu³⁺-doped Ba₃La(VO₄)₃ phosphors

Traditional ratiometric luminescent thermometry materials are based on thermally coupled levels (TCLs) of rare earth ions, such as Er³⁺ (²H_11/2/⁴S_3/2), Eu³⁺(⁵D₁/⁵D₀), Nd³⁺ (⁴F_5/2/⁴F_3/2), Tm³⁺ (³F_2,3/³H₄), Dy³⁺ (⁴I_15/2/⁴F_9/2), Ho³⁺ (⁵S₂/⁵F₄), etc.³⁶ In such thermometry materials, the smaller TCL gap is better for absolute sensitivity (S_A) and is detrimental to relative sensitivity (S_R). In addition, a smaller energy gap will result in overlapping of their two emission peaks, which is unfavorable for the detection of the emission peak signal. Another strategy for the luminescent temperature sensing method is utilizing two different rare earth ions as two luminescence centers. However, this method is mostly used for temperatures below 320 K.²² Hence, the FIR method based on single rare earth ion doping is a hopeful approach for optical thermometric characterization in a wide temperature range. In this work, the FIR of the V⁵⁺ → O²⁻ CT band and the f–f transition of Eu³⁺ are dramatically changed with the increase of temperature. The ratiometric temperature sensing was studied based on the temperature dependence of emission spectra, shown in Fig. 7(a). As depicted in Fig. 7(a), the profile of the emission spectrum hardly changes with increasing temperature, whereas the FIR of the VO₄³⁻ group and Eu³⁺ ions decreases gradually owing to the thermal quenching effect by the thermal activation via the crossover point of the excited and ground levels.³⁷ The activation energy ΔE can be obtained according to the Arrhenius equation as described below:³⁸where I₀ and I are the emission intensities at initial temperature and measured temperature T, respectively, A is a constant and K denotes the Boltzmann constant. According to eqn (2), a plot of ln[(I₀/I) − 1] versus 1/KT and a slope of its linear fitting should give the activation energy (ΔE) corresponding to the thermal quenching effect of Eu³⁺, as depicted in Fig. 7(b), which shows the ΔE of 0.257 eV upon 320 nm excitation.

Fig. 7 (a) The temperature-dependent PL spectra of the Ba₃La_0.99 (VO₄)₃:0.01Eu³⁺ phosphor excited at 320 nm. (b) Dependence of Eu³⁺ emission intensities on temperature and their linear fit.

Interestingly, it is observed from the inset of Fig. 8(a) that the integrated emission intensity of the VO₄³⁻ group is dramatically dropped with the increase of temperature, while the Eu³⁺ emission intensity exhibits a relatively weaker decrease in the temperature. Therefore, ratiometric optical thermometry between VO₄³⁻ (400–580 nm) emission and Eu³⁺ (⁷F₀ → ⁵D₂) emission (FIR = I_VO4³⁻/I_Eu³⁺) could be employed as a sensitive thermometric probe. As shown in Fig. 8(a), the FIR value reveals a remarkable decrease as the temperature increases. The FIR changing with various temperatures can be fitted as an exponential curve in the 298–573 K range, which could be described as follows:

Fig. 8 (a) Temperature-dependent intensity ratio (I_VO4³⁻/I_Eu³⁺) and exponential fitting. The inset shows the integrated emission intensity of the Eu³⁺:⁵D₀ → ⁷F₂ and VO₄³⁻ group as a function of temperature. (b) Absolute sensitivity S_A and relatively sensitivity S_R values at different temperatures.

The sensitivity of optical thermometry is the rate of change of FIR in response to the variation of temperature T.³⁶ There are two key parameters, i.e., the relative sensitivity (S_R) and absolute sensitivity (S_A), to quantify the temperature sensing properties of the material for practical applications. S_R and S_A are defined as the following expressions, respectively:^39,40

The temperature dependence of S_A and S_R values was calculated and is displayed in Fig. 8(b). As presented in Fig. 8(b), the maxima of the absolute sensitivity and relative sensitivity are 0.0515 K⁻¹ and 1.77% K⁻¹ at 298 K, both being comparable to those of other reported Eu³⁺-doped optical temperature sensing materials, whose thermometric properties were characterized based on the TCL of Eu³⁺ (⁵D₁ and ⁵D₀).^41–47 Comparison results are summarized in Table 1. Furthermore, the Ba₃La_0.99(VO₄)₃:0.01Eu³⁺ phosphor exhibits a good repeatability after several cycling processes. The above results demonstrate its potential applications in temperature sensing with comparable sensing performance.

Table 1 Optical thermometric properties of Eu³⁺ ion-based doped luminescent materials

Dopant	Host	Temperature range (K)	S R (K^−1) @temperature	Ref.
Eu^3+	Ba3La(VO4)3	298–573	1.77%@323 K	This work
Eu^3+	CaEu2(WO4)4	300–500	1.4%@300 K	41
Eu^3+	BiF3	303–443	0.0034%@443 K	42
Eu^3+	Gd2Ti2O7	303–423	0.95%@423 K	43
Eu^3+	NaEuF4	298–523	0.43%	44
Eu^3+	Ca2.94Eu0.04Sc2SiO12	123–273	1.35%@273 K	45
Eu^3+	EuFDC	12–320	0.33%@300 K	46
Eu^3+	YBO3	333–773	1.8%@333 K	47

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3.5 Red-shift phenomenon in the excitation spectrum

Significantly, a continuous redshift of the charge transfer band in PLE was observed in the 298–573 K range. The maximum of the V⁵⁺ → O²⁻ CT band in Fig. 9(a) band shifts from 320 nm at RT to 336 nm at 573 K, with a redshift of 16 nm and unchanged shape of the PLE spectra. Simultaneously, the decrease of PLE intensity with increasing temperature is in accordance with the observation of PL due to the thermal quenching effect.^48,49 It is worth mentioning that such a temperature-induced red-shift phenomenon of the CT excitation band in Fig. 9(a) is beneficial for UV absorption with longer wavelengths. And this redshift phenomenon can be explained by the alteration in the relative location of the transition from the ground state to the CT excited states,^50,51 as depicted in Fig. 9(b). Under 320 nm excitation, the VO₄³⁻ groups are promoted from their ground state ¹A₁ to the excited states ¹T₁ and ¹T₂, and this process is called the optical absorption transition. At lower temperature, the transition to the excited states ¹T₁ and ¹T₂ starts from the lowest vibronic level ν₁ of the ground state ¹A₁. However, this transition mainly occurs from the higher thermally populated vibronic level ν₂ of the ground state at higher temperature. The thermal population of vibronic levels of the ground state leads to an excitation redshift, i.e. a shift of the excitation peak towards lower energy as the temperature increases.

Fig. 9 (a) The temperature-dependent PLE spectra of the Ba₃La_0.9(VO₄)₃:0.1Eu³⁺ phosphor monitored at 617 nm. (b) Schematic configurational coordinate diagram for Ba₃La(VO₄)₃:Eu³⁺, and an illustration of the excitation, emission, and nonradiative transition processes.

The relationship between the maximum peak position of the V⁵⁺ → O²⁻ CT band and temperature is given in Fig. 10(a), manifesting an almost linear relationship. Further analysis of the data indicates that the maximum peak position (λ) shifting can be well fitted with a linear curve λ = 0.0614T + 301.01, as shown in Fig. 10(a). The slope of the curve represents the peak changes with the temperature, revealing the absolute temperature sensitivity S_A (Δλ/ΔT). The relative sensitivity S_R can be further calculated according to ,⁵² as depicted in Fig. 10(b). Thus, the values of S_A and S_R are determined to be 0.0614 nm K⁻¹ and 0.01923% K⁻¹@298 K, respectively. Compared with previously reported materials based on the spectral shift, the value of S_A (0.0614 nm K⁻¹) is much larger than 0.033 nm K⁻¹ in CaWO₄:Ho³⁺ and 0.007 nm K⁻¹ in LaF₃:Nd³⁺.^53,54 The luminescence results on the Ba₃La_0.99(VO₄)₃:0.01Eu³⁺ phosphor demonstrated a potential temperature sensor based on the fluorescence intensity ratio, as well as the spectral shift, giving a pathway for new luminescence materials designing.

Fig. 10 (a) The relationship between the maximum peak position of the V⁵⁺ → O²⁻ CT band and temperature. (b) Relative sensitivity S_R values at different temperatures.

4. Conclusions

In summary, Eu³⁺-activated Ba₃La(VO₄)₃ phosphors were successfully synthesized by a high temperature solid state reaction method. The PL spectrum consists of a broad emission band from VO₄³⁻ groups and several sharp emission peaks owing to the ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3, 4) transitions of Eu³⁺. By changing the Eu³⁺-doping concentration, it is possible to tune the emission color varying from green (0.2442, 0.3946) to red (0.6077, 0.3499), accordingly. The temperature-dependent luminescence properties of Ba₃La_0.99(VO₄)₃:0.01Eu³⁺ phosphors illustrate that the emission intensity ratio between VO₄³⁻ (400–580 nm) and Eu³⁺ (⁷F₀ → ⁵D₂) can be applied as a thermometric probe with comparable temperature sensing performance. The maximum absolute sensitivity and relative sensitivity are found to be 0.0515 K⁻¹ and 1.77% K⁻¹@298 K, respectively. In addition, the V⁵⁺ → O²⁻ CT band shifts toward the longer wavelength gradually with elevating temperature, showing that the studied material can be explored for temperature sensing based on the spectral shift. These results suggested that the Eu³⁺-activated Ba₃La(VO₄)₃ phosphors may provide promising applications in tunable emission materials and non-contact optical thermometry.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This project was funded by the National Natural Science Foundation of China (11674044, 11604037, and 11704054), the Chongqing Research Program of Basic Research and Frontier Technology (No. CSTC2016JCYJA0113, CSTC2016JCYJA0207, and CSTC2017jcyjAX0046), the Scientific and Technological Research Program of Chongqing Municipal Education Commission (No. KJQN201800619, KJ1600406, and KJZD-K201800602), the Wenfeng High-end Talents Project of Chongqing University of Posts and Telecommunications (W2017-06 and W2016-30) and the Scientific Research Foundation For Doctor of Chongqing University of Posts and Telecommunications (A2008-59).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9dt01917k

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