Regulation of color tunability of Dy³⁺/Eu³⁺-based double molybdate phosphors with high thermal stability

Abstract

In this paper, a series of NaLu(MoO₄)₂:Dy³⁺ green phosphors and NaLu(MoO₄)₂:Dy³⁺,Eu³⁺ multi-color tunable phosphors were prepared by an EDTA–2Na assisted hydrothermal method. The emission of Dy³⁺ ions at two characteristic peaks at 469 nm and 573 nm was observed for the NaLu(MoO₄)₂:xDy³⁺ phosphor excited at 355 nm. Through calculation and data fitting, the concentration quenching of Dy³⁺ ions in the NaLu(MoO₄)₂ lattice is dominant due to the dipole–dipole interaction. The variable temperature emission spectra show that the luminescence intensity of the NaLu(MoO₄)₂:0.01Dy³⁺ phosphor at 423 K decreases to 59% of that at 298 K, while the NaLu(MoO₄)₂:0.01Dy³⁺,0.07Eu³⁺ phosphor at 423 K remains at 97% compared to that at room temperature, indicating that the thermal stability of the phosphor is greatly improved by the introduction of Eu³⁺ ions. In addition, by adjusting the ratio of Dy³⁺ to Eu³⁺ ions in the host, the luminescence color gradually changes from green to yellow and finally to orange-red, providing insights into color tuning in the visible range toward various applications. In summary, the Dy³⁺ and Eu³⁺ co-doped NaLu(MoO₄)₂ phosphor is a phosphor with good optical properties and excellent thermal stability, which can achieve color tunability and has potential application prospects in the fields of color display and lighting.

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1. Introduction

Rare earth luminescent materials have great application value in optoelectronic devices, white light diodes (W-LED), solar cells, optical temperature sensors, biomedicine and other fields due to their advantages of high conversion efficiency, narrowband emission, long lifetime, and good optical stability. They therefore have sparked intensive interest in the development of luminescent materials with excellent luminous properties and stable physical and chemical properties in recent years.^1–4 It is well known that the luminescent properties of rare earth phosphors are closely related to the crystal field environment in the main crystal lattice, which makes it extremely important to explore novel matrixes with stable structures, excellent properties and effective excitation by doping ions. With excellent luminescence performance and stable physical and chemical properties, molybdate has a wide range of application prospects in photocatalysts, phosphors, lasers, etc., attracting great interest and wide attention from researchers.^5–7 Dy³⁺ ions usually have two distinct characteristic emission peaks in the blue (470–500 nm) and yellow (560–600 nm) regions, which correspond to the ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 transitions, respectively.^8,9 Monika et al. reported that strong emission was observed for the Dy³⁺ doped Sr₂CeO₄ nanophosphor, and the emission intensity of the phosphor was enhanced by co-doping Li⁺ ions.¹⁰ Unfortunately, since Dy³⁺ ions lack the characteristic emission in the red region in addition to the blue and yellow components, the color rendering index (CRI) for the Dy³⁺-based phosphors is very low, far from that demanded for practical luminescent materials.^11,12 To address this problem, another active center with red luminescence is often introduced into the Dy³⁺-based luminescent materials to compensate for the red composition. In this regard, the Eu³⁺ ion is an ideal option because of its orange and red emission originating from ⁵D₀ → ⁷F_J (J = 1, 2, 3, 4) transitions.¹³ Many successes have been achieved by adopting this strategy recently. Liu et al., for instance, synthesized a series of Dy³⁺/Eu³⁺ co-doped KLa(MoO₄)₂ phosphors by a hydrothermal method¹⁴ and they exhibited high multi-color tunable ability from the yellow to red region, crossing the white light area, indicating that this sort of phosphor has broad application prospects in white diodes, full-color displays, photoelectric devices and other fields. Devakumar et al. prepared a series of color-tunable Dy³⁺/Eu³⁺ co-doped CsGd(MoO₄)₂ (CGM) phosphors via a high temperature solid phase reaction.¹⁵ The combination of the comprehensive analysis of the crystal structure, morphology, vibration mode, luminescence characteristics and energy transfer mechanism manifests the fact that the emission color of the phosphor can be easily tuned by altering the content ratio of Dy³⁺ to Eu³⁺. Zhang et al. prepared the Gd-MOF:Eu³⁺,Dy³⁺ phosphors and investigated the temperature sensing properties in the temperature range of 213–453 K. The luminescence color of this phosphor varied from yellow to red as the temperature increased.¹⁶ Wan et al. prepared Dy³⁺/Eu³⁺ co-doped Sr₂Ga₂GeO₇ (SGGO) phosphors with different concentrations of Eu³⁺ by a high temperature solid phase method.¹⁷ Under 344 nm excitation, the emission intensity of Dy³⁺ was attenuated and the emission lifetime was shortened. In contrast, the lifetime of Eu³⁺ was increased. Such a confirmation of energy transfer from Dy³⁺ to Eu³⁺ in the Dy³⁺/Eu³⁺:SGGO phosphors endows them with huge potential application in plant growth as tunable orange radiation resources. Inspired by these studies, a series of Dy³⁺ single-doped and Dy³⁺/Eu³⁺ co-doped NaLu(MoO₄)₂ (NLMO) phosphors were prepared by an EDTA–2Na-assisted hydrothermal method in this paper. The phase purity, morphology, luminescence and thermal stability of the as-synthesized phosphors were studied in detail. The color purity and correlated color temperature (CCT) of the phosphor were calculated and plotted in the Commission Internationale de L’Eclairage (CIE) coordinate diagram, indicating good color characteristics in this studied system. The energy transfer processes between Dy³⁺ and Eu³⁺ ion luminescence centers were explored, too. All the results demonstrated that high efficiency multi-color tunable emission, starting from green to orange and then to red could be easily realized via varying the concentration of Eu³⁺ in the NLMO:Dy³⁺/Eu³⁺ phosphors, indicating their prominent potential applications in the fields of color display and lighting.

2. Experiment

2.1. Synthesis

A series of NLMO:xDy³⁺ (x = 0, 0.01, 0.03, 0.06, 0.09, 0.15, and 0.20) and NLMO:0.1Dy³⁺,yEu³⁺ (y = 0.005, 0.01, 0.07, 0.13, and 0.19) phosphors were prepared by an EDTA–2Na assisted hydrothermal method. Na₂MoO₄·2H₂O with A.R. grade and Lu(NO₃)₃·6H₂O, Dy(NO₃)₃·6H₂O, as well as Eu(NO₃)₃·6H₂O with the purity of 99.99% were employed as raw materials. C₁₀H₁₄N₂O₈Na₂·2H₂O(A.R.) was adopted as an active agent to better control the morphology of the sample and improve the luminescence performance. For a typical preparation process, a stoichiometric amount of Na₂MoO₄·2H₂O (0.006 mol) was dissolved in 10 ml of deionized water and stirred for 20–30 minutes to form solution A. An overall amount of 0.003 mol of Ln(NO₃)₃·6H₂O (Ln = Lu, Dy, and Eu) was weighed and dissolved in 10 ml of deionized water while stirring for 15–30 minutes to form solution B. Subsequently, 0.003 mol EDTA–2Na was dissolved in 10 ml of deionized water to form a solution at lower temperature (60–80 °C) for about 60 minutes to ensure the homogeneity of the solution. Then, solution C was added to solution B, resulting in the immediate appearance of white precipitates. The mixed solution was stirred for about 20 minutes. Sequentially, the precipitate was poured to solution A to form a suspension and stirred for another 20 minutes. Next, the mixture was transferred to a Teflon-lined autoclave with a capacity of 50 mL and the temperature was kept at 180 °C for 12 hours. After naturally cooling to room temperature, the product was centrifuged and washed with deionized water several times. Finally, the white sediment was dried at 80 °C for 5 hours and then calcined at 800 °C for 2 hours in a muffle furnace.

2.2. Measurements and characterization

A high-resolution X-ray powder diffractometer (Bruker D8 Advance) with monochromatic Cu Kα radiation (λ = 0.15406 nm) was used for the PXRD measurement with the 2θ over the range of 15–65° with the step of 0.01°. The morphology of the samples was characterized using an FEI Quanta 400 thermal field emission scanning electron microscope (SEM). The photoluminescence (PL) and photoluminescence excitation (PLE) spectra were analyzed using a Hitachi 2500 fluorescence spectrometer equipped with a 150 W xenon lamp as a light source. The temperature-dependent emission spectrum (T = 298–473 K) and decay lifetimes of the samples were measured with an Edinburgh FLS980 spectrometer under an alternating light source of 150 W xenon and pulsed xenon lamps.

3. Results and discussion

3.1. Crystal structure, phase and morphology analysis

Fig. 1(a) and (b) show the XRD patterns for the NLMO:xDy³⁺ (x = 0, 0.01, 0.03, 0.06, 0.09, 0.15, and 0.20) and NLMO:0.01Dy³⁺,yEu³⁺ (y = 0.005, 0.01, 0.07, and 0.13, 0.19) phosphors, respectively. All the patterns are consistent with that for the standard (PDF#23-1192), with no existence of heterophase, demonstrating not only the effectivity of the preparation method adopted in this study but also the high structural tolerance of the host matrix toward rare-earth ion substitution. According to the theory of defect chemistry, the position occupied by the doping elements in the host lattice can be estimated by the percentage difference (D_r) of the ionic radii, which can be expressed as follows:¹⁸where R_m and R_d are the radii of the host ion and doping ion, respectively, CN is the coordination number which is 8 for the concerned site according to the structure information of the NLMO matrix. Generally, the probability of the doping ion to replace the host ion is inversely proportional to the value of D_r, and therefore, a smaller value of D_r means an easier occurrence of replacement. Based on formula (1), the D_r values for the case of Dy³⁺ replacing Lu³⁺ and Na⁺ are 10.3% and 12.7%, respectively. Therefore, it is rational to assume that the Dy³⁺ ions will substitute Lu³⁺ other than Na⁺ while maintaining the host structure even with a relatively high doping concentration. It is notable that the introduction of Dy³⁺ has resulted in a certain degree of unit cell expansion and reduction of the distance between two randomly adjacent Miller faces of the main lattice without damaging the structure. As the concentration of Dy³⁺ increases, the XRD diffraction peaks shift to the lower angle side compared with the standard pattern, which is illustrated in the inset along with Fig. 1(a). The shift of the diffraction peaks for the Dy³⁺/Eu³⁺ co-doped samples were larger compared to those for the Dy³⁺ single doped ones (Fig. 1(b)). This is mainly ascribed to the larger radius of Eu³⁺ ions (1.07 Å) than that of Dy³⁺ ions (1.03 Å) and both of which are larger than that of Lu³⁺ ions (0.97 Å). When Dy³⁺ or Eu³⁺ ions enter the host lattice, their larger radius will inevitably cause the expansion of the unit cell. This is well in line with the Bragg diffraction equation known as 2d sin θ = nλ. The expansion leads to the increase of distance between two adjacent Miller faces, and in the meantime, the diffraction peaks would shift to the lower angle side. Such good agreement between experimental results and theoretical expectation further confirms easy-doping of Dy³⁺ and Eu³⁺ ions in the NLMO lattice,¹⁹ providing an opportunity of regulating the luminescence color of the phosphors in a vast range and easy way.

Fig. 1 PXRD patterns for (a) NLMO:xDy³⁺ (x = 0, 0.01, 0.03, 0.06, 0.09, 0.15, and 0.20) and (b) NLMO:0.01Dy³⁺,yEu³⁺ (y = 0.005, 0.01, 0.07, 0.13, and 0.19) phosphors.

Fig. 2(a) and (b) show the SEM images of NLMO:0.01Dy³⁺ phosphors at different magnification rates. It can be observed from the images that the particles show good morphology with regular pie-like shape and good uniformity. A large proportion of the particles have very similar diameters which are estimated to be 3–4 μm, further indicating the effectiveness of the synthesis method and the specific parameters adopted for sample preparation. For phosphors that are focused on their optical performances, the morphology is one of the important factors. Therefore, the good morphology of the studied sample boosts its attractive optical properties.

Fig. 2 SEM images of NLMO:0.01Dy³⁺ phosphors with (a) low-magnification and (b) high-magnification, respectively.

3.2. Photoluminescence properties

Fig. 3(a) shows the emission spectra of NLMO:xDy³⁺ (x = 0, 0.01, 0.03, 0.06, 0.09, 0.15, 0.20) phosphors over the range of 445–630 nm excited at 390 nm. Due to the ⁴F_9/2 → ⁶H_13/2 transition of Dy³⁺, the spectra exhibit a strong yellow emission around 575 nm. Other two additional weak emission bands located at 490 nm and 611 nm which correspond to ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_11/2 are observed.²⁰ The blue emission belongs to the magnetic dipole transition which is insensitive to the local environment, while the yellow emission is a super-sensitive electric dipole transition, whose intensity is significantly affected by the crystal field around Dy³⁺ ions.²¹ This characteristic implies that it is highly possible to achieve color-tunable light in this monotype phosphor by varying the concentration of the dopant. In general, the yellow emission predominates when Dy³⁺ occupies at a non-inverted center. In contrast, the blue emission will be stronger when Dy³⁺ is located at a highly symmetric position. Hence, the strong yellow emission at 573 nm for the NLMO:xDy³⁺ series indicates that Dy³⁺ ions occupy a low symmetric site in the NLMO main lattice with no inversion center. In addition, with the increase of Dy³⁺ concentration, the yellow emission intensity gradually increases and it reaches the summit at x = 0.01. Then it began to significantly decline due to the concentration quenching effect with the further increase of Dy³⁺ concentration. The cross relaxation (CR) mechanism originating from the resonance energy transfer (ET) between adjacent Dy³⁺ ions is likely responsible for this effect.²² The PLE spectra of NLMO:0.01Dy³⁺ phosphors monitored at 573 nm are shown in Fig. 3(b). The PLE spectrum consists of a weak wide O²⁻ → Mo⁶⁺ charge transfer (CT) band in the region of 200–350 nm and other 6 sharp peaks located at 351, 366, 390, 426, 453, and 472 nm corresponding to ⁶H_15/2 → ⁶P_3/2, ⁶H_15/2 → ⁶P_7/2, ⁶H_15/2 → ⁶P_5/2, ⁶H_15/2 → ⁴I_13/2, ⁶H_15/2 → ⁴G_11/2, ⁶H_15/2 → ⁴H_11/2 and ⁶H_15/2 → ⁴F_9/2 transitions, respectively.²³ There is a weak excitation peak at about 328 nm ascribed to the ⁶H_15/2 → ⁴G_9/2 transition combined with the CT band. Obviously, the excitation intensities at 351 nm and 390 nm are stronger than those of the other peaks, indicating that the samples are the most suitable to be excited at these wavelengths. Considering that light at 390 nm is more commercially available than the light at 351 nm, we used 390 nm to excite a single-doped sample.

Fig. 3 (a) PL spectra of NLMO:xDy³⁺ (x = 0, 0.01, 0.03, 0.06, 0.09, 0.15, and 0.20) phosphors and (b) PLE spectrum of the NLMO:0.01Dy³⁺ phosphor.

In order to better reveal the concentration quenching mechanism of the NLMO:Dy³⁺ phosphor, the calculation of critical distance (R_c) which is given by the following formula is a common but helpful method:^24,25

where V is the volume of a single cell, Z represents the number of replaceable cations in the host matrix in a unit cell, and x_c is the critical concentration of the activator. For the NLMO matrix, V = 299.4 ų, x_c = 0.01, and Z = 2. As such, the value of R_c is 19 Å, which is much larger than 5 Å, demonstrating that the concentration quench effect is derived from the multipolar interaction. Furthermore, the exact type of interaction can be determined by using Dexter's theory which is expressed as follows:^26,27where I is the emission intensity, x is the doping concentration, A is a constant, and Q may be 6, 8, and 10 corresponding to dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. Based on the emission spectra, log(I/x) as a function of log(x) is shown in Fig. 4. After linear fitting, the slope is −1.77 and consequently the calculated Q value is 5.6 which is very close to 6, demonstrating that the Dy³⁺ ion concentration quenching in the NLMO matrix is mainly caused by dipole–dipole interaction.

Fig. 4 Dependency of log(I/x) on log(x).

Since the optical properties of NLMO:0.01Dy³⁺ are not ideal for the color tunable applications, Eu³⁺ is usually co-incorporated with Dy³⁺ to improve the optical performance. In this regard, a series of NLMO:0.01Dy³⁺,yEu³⁺ (y = 0.005, 0.01, 0.07, 0.13, 0.19) phosphors were prepared by the same method mentioned above. Fig. 5 shows the difference between PLE spectra of NLMO:0.01Dy³⁺,yEu³⁺ phosphors monitored at 573 nm and 614 nm, respectively. As depicted in Fig. 5(a), the shapes and locations of the excited bands are similar to those appearing in the PLE spectra of Dy³⁺-single doped NLMO phosphors. However, the intensities corresponding to the excitation peaks gradually decrease as the concentration of Eu³⁺ rises. This implies that there should exist energy transfer from Dy³⁺ to Eu³⁺ activator. Interestingly, the CT band over 240–340 nm also deteriorates after Eu³⁺ was introduced. This may be because of the increase of Mo–O bond distance caused by unit cell expansion, which makes the charge transition between O²⁻ and Mo⁶⁺ more difficult than the case without Eu³⁺ doped. In contrast, the PLE spectra monitored at 614 nm, as shown in Fig. 5(b), have strong and broad absorption bands, which is attributed to the overlap between Mo⁶⁺ → O²⁻ and Eu³⁺ → O²⁻ charge transfer transitions.²⁸ Particularly, as the former transition was gradually weakened, the latter transition should dominantly contribute to the band. What's more, because Eu³⁺ has high probability of red light emission at about 614 nm, the absorbed energy can be quickly released in the form of red light, leading to the significant boost of absorption ability of this phosphor within the 240–340 nm range. Other several narrow peaks, centered at 361 (⁷F₀ → ⁵D₄), 375 (⁷F₀ → ⁵G₂), 380 (⁷F₀ → ⁵G₃), 393 (⁷F₀ → ⁵L₆), 415 (⁷F₀ → ⁵D₃) and 464 nm (⁷F₀ → ⁵D₂) were observed. These peaks belong to the characteristic f–f transition of Eu³⁺ ions. In addition, relative weak peaks at 351, 366, 426, and 453 nm, corresponding to ⁶H_15/2 → ⁶P_7/2, ⁶H_15/2 → ⁶P_5/2, ⁶H_15/2 → ⁴G_11/2, and ⁶H_15/2 → ⁴H_11/2 transitions of Dy³⁺ are still present in the NLMO:xDy³⁺ phosphors.²³ It is notable that the intensity at 393 nm is comparable to that at 355 nm, implying that the NLMO:0.01Dy³⁺,yEu³⁺ phosphors are appropriate to be excited at both wavelengths. What's more, the excitation intensities present a monotonically increasing trend with the ascent of Eu³⁺ concentration, but the increasing rate becomes significantly slow when the Eu³⁺ concentration exceeds 7 at%, suggesting that the appropriate concentration of Eu³⁺ might be at this doping level.

Fig. 5 PLE spectra of NLMO:0.01Dy³⁺,yEu³⁺ (y = 0.005, 0.01, 0.07, 0.13, and 0.19) phosphors monitored at 573 and 614 nm, respectively.

Fig. 6(a) shows the PL spectra of NLMO:0.01Dy³⁺,yEu³⁺ (y = 0.005, 0.01, 0.07, 0.13, and 0.19) phosphors under the excitation at 355 nm. When compared to the Dy³⁺-single doped NLMO samples, the PL spectra of NLMO:0.01Dy³⁺,yEu³⁺ maintain distinct emission bands at 486 nm and 573 nm, which correspond to the ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 transitions of Dy³⁺. However, their intensities are weakened dramatically. Conversely, the emission intensity at 614 nm experiences a notable enhancement due to the combination of ⁵D₀ → ⁷F₂ transition of Eu³⁺ and ⁴F_9/2 → ⁶H_11/2 transition of Dy³⁺. Additionally, two weaker emission peaks at 655 nm and 703 nm are also observed, which can be ascribed to the ⁵D₀ → ⁷F₃ and ⁵D₀ → ⁷F₄ transitions of Eu³⁺, respectively. In addition, as the concentration of Eu³⁺ increases, both the emissions at 486 nm and 573 nm monotonously decrease while the emission at 614 nm first increases and then decreases with the highest intensity at y = 0.07, as illustrated in Fig. 6(b). Excessive concentration of Eu³⁺ won’t continuously boost and will even reduce the red light emission. This is likely due to the shortened distance between two adjacent Eu³⁺ ions when a larger amount of Eu³⁺ ions is incorporated, consequently enhancing the nonradiative relaxation process. Such considerable variations in PL spectral emission intensities of Dy³⁺ and Eu³⁺ in the NLMO:0.01Dy³⁺,yEu³⁺ (y = 0.005, 0.01, 0.07, 0.13, and 0.19) phosphors indicate that energy transfer and multi-color tunable emission among blue, yellow and red emissions can be realized by appropriately adjusting the ratio of Dy³⁺ and Eu³⁺ ion concentration.²⁹

Fig. 6 (a) PL spectra of NLMO:0.01Dy³⁺,yEu³⁺ (y = 0.005, 0.01, 0.07, 0.13, and 0.19) phosphors; (b) line chart of emission intensity changes at 573 nm and 614 nm as the function of Eu³⁺ concentration.

To uncover the energy transfer process between Dy³⁺ ions and Eu³⁺ ions, an in-depth investigation was conducted into the emission lifetimes of NLMO:0.01Dy³⁺,yEu³⁺ (y = 0, 0.005, 0.01, 0.07, 0.13, 0.19) phosphors. Fig. 7 shows the attenuation curves excited at 355 nm and monitored at 573 nm. For fitting calculations, a double exponential formula was utilized, as detailed below:³⁰

where I_t is the luminescence intensity varying with time, and A₁ and A₂ are constants. Thus, the average life can be calculated by the following formula:^31,32Then, the lifetimes of the NLMO:0.01Dy³⁺yEu³⁺ (y = 0, 0.005, 0.01, 0.07, 0.13, and 0.19) phosphors are 94, 79.4, 61.9, 39.7, 10.53 and 9.12 μs, respectively. Obviously, the lifetime of Dy³⁺ decreases in turn with the increase of Eu³⁺ ion concentration, which resulted in a shortened distance between Dy³⁺ and Eu³⁺ ions, which facilitates the Dy³⁺ → Eu³⁺ energy transfer and subsequently reduces the emission lifetime of Dy³⁺ ions. Furthermore, the efficiency of energy transfer η_τ also rises in line with the increase of Eu³⁺ concentration, which can be estimated by employing the following formula:³³where τ and τ₀ are the emission lifetimes of Dy³⁺ ions when Eu³⁺ is co-doped or not, respectively. Through this equation, the energy transfer efficiencies of NLMO:0.01Dy³⁺ and NLMO:0.01Dy³⁺,yEu³⁺ phosphors with y varying from 0.005 to 0.19 are 15.64%, 25.5%, 57.79%, 88.82% and 90.32%, respectively.

Fig. 7 Decay curves for the NLMO:0.01Dy³⁺,yEu³⁺ (y = 0.005, 0.01, 0.07, 0.13, and 0.19) phosphors.

With the combination of the PL and PLE spectra, as well as the related lifetimes, the mechanism of the energy transfer process in the NLMO:0.01Dy³⁺,yEu³⁺ phosphors was unveiled and is shown in Fig. 8. When excited at 355 nm, electrons of Dy³⁺ at ground state transit to the ⁶P_1/2 state. Successively, a proportion of the excited electrons relaxes nonradiatively to the lower ⁴F_9/2 state and then transits back to ⁶H_J states where J = 15/2, 13/2, and 11/2, accompanied by the light emission at 489, 573, and 678 nm, respectively. Meanwhile, electrons of Eu³⁺ are excited to the ⁵D₄ state and then relax to the ⁵D₀ and ⁵D₁ states, leading to the observation of the characteristic emission of the Eu³⁺ ions corresponding to the ⁵D₁ → ⁷F₁, ⁵D₀ → ⁷F₀, ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions. Furthermore, with the help of energy transition from ⁴F_9/2 state of Dy³⁺ to ⁵D₀ and ⁵D₁ states of Eu³⁺, the number of electrons at these two states sharply increases, and as a consequence, the emission of the Eu³⁺ is amplified, especially for the emission at 614 nm.

Fig. 8 Schematic diagram of electron transition and energy transfer in the Dy³⁺/Eu³⁺ co-doped NLMO phosphors.

3.3. Thermal stability

The application of phosphors in W-LED lighting hinges significantly on their thermal stability. The thermal stability of NLMO:0.01Dy³⁺ and NLMO:0.01Dy³⁺,0.07Eu³⁺ phosphors within the temperature range of 298–473 K has been studied. The fluorescence spectra of these two samples, excited at a wavelength of 355 nm, were recorded at various temperatures and are presented in Fig. 9(a) and (b) respectively. As the temperature increased, the luminescence intensity at 573 nm for NLMO:0.01Dy³⁺ and at 614 nm for NLMO:0.01Dy³⁺,0.07Eu³⁺ phosphors decreased. Specifically, Fig. 9(a) reveals that the NLMO:0.01Dy³⁺ phosphor retained 59% of its initial luminous intensity at 298 K when heated to 423 K. In contrast, the NLMO:0.01Dy³⁺,0.07Eu³⁺ phosphor maintained an impressive 97% of its room temperature luminous intensity at 423 K. Such a remarkable improvement in thermal stability is mainly due to the enhanced structural stability of the matrix caused by the larger radius of Eu³⁺ in comparison to that of Lu³⁺, and successively, the rigidity of the Lu(Dy/Eu)O₈ polyhedron in the lattice is improved. As we know, higher rigidity generally signifies lower phonon energy and thereby the phonon-assisted nonradiative transition process will be weakened. As a result, the thermal stability is improved. Another factor, cross relaxation is also believed to have influence on the thermal stability issue. As the unit cell expands due to the entrance of Eu³⁺ in the lattice, the occurrence of the cross relaxation process seems to be much more difficult and consequently improves the thermal stability. In addition, the increased bonding energy is believed to be an important reason for thermal stability development, too. The bonding energy for the NLMO:0.01Dy³⁺ and NLMO:0.01Dy³⁺,0.07Eu³⁺ phosphors can be calculated by the following formula:³⁴where I₀ is the PL intensity at 298 K, I_T is the PL intensity at different temperatures, A is the constant, K_B is the Boltzmann constant, T is the temperature, and ΔE is the activation energy. Thus, as shown in Fig. 9(c) and (d), the ln(I₀/I_T − 1) and 1/K_BT for the NLMO:0.01Dy³⁺ and NLMO:0.01Dy³⁺,0.07Eu³⁺ phosphors were linearly fitted, and the slopes for them are −0.31 and −0.37, respectively. This indicates that the substitution of Lu³⁺ by Eu³⁺ promotes the bonding energy from 0.31 eV in NLMO:0.01Dy³⁺ to 0.37 eV in NLMO:0.01Dy³⁺,0.07Eu³⁺ phosphors, and consequently, results in the improvement of the thermal stability of the phosphor.³⁵ Under the synergistic effect of the factors discussed above, the incorporation of Eu³⁺ ions has markedly enhanced the thermal stability of the phosphor, implying its promising potential for use in white light illumination and color display applications.

Fig. 9 Temperature-dependent fluorescence spectra for (a) NLMO:0.01Dy³⁺ and (b) NLMO:0.01Dy³⁺,0.07Eu³⁺ phosphors within the temperature range of 298–473 K. The relationship between ln(I₀/I_T − 1) and 1/K_BT for (c) NLMO:0.01Dy³⁺ and (d) NLMO:0.01Dy³⁺,0.07Eu³⁺.

3.4. CIE associated color temperature (CCT)

Based on the PL spectra of NLMO:0.01Dy³⁺,yEu³⁺ (y = 0, 0.005, 0.01, 0.07, 0.13, 0.19) phosphors, the CIE chromaticity coordinates for them were calculated and are illustrated in Fig. 10(a), followed by the calculation of color purity, which is also of importance to assess the applicability of phosphors. With the CIF values, the color purity can be obtained by adopting the following formula:³⁶where (x, y) are the CIE coordinates of the phosphor, (x_i, y_i) are the CIE coordinates of white light (0.3101, 0.3162) standardized by the National Television Standards Committee (NTSC), and (x_d, y_d) are the coordinates under main wavelength monitoring. Furthermore, the CCT of the phosphor can be calculated as follows:³⁷

CCT = −449n³ + 3535n² − 6823n + 5520 (10)

Fig. 10 (a) CIE diagram of NLMO:0.01Dy³⁺yEu³⁺ (y = 0, 0.005, 0.01, 0.07, 0.13, and 0.19) phosphors and (b) naked-eye observation of luminescence performance of the NLMO:0.01Dy³⁺,yEu³⁺ (y = 0, 0.01, and 0.07) phosphors under the excitation of commercially available violet flashlight.

Via the formulae presented above, values of the CIE, color purity and CCT were achieved and are summarized in Table 1. With the increase of Eu³⁺ concentration, the luminous color of the NLMO:0.01Dy³⁺,yEu³⁺ series changes from green to yellow and finally to orange-red. For the CIE parameters, the x value accelerates, but the y value declines. The CCT takes the same trend as the Eu³⁺ concentration increases. However, the color purity features differently. It first increases and then decreases, peaking at y = 0.07 with the altitude value of 65%. All the above experimental and computed results hint that multi-color tunable emission with wide application prospects in the field of color display and lighting is easily realized by adjusting the ratio of Dy³⁺ and Eu³⁺ concentration.

Table 1 CIE, CCT and color purity for the NLMO:0.01Dy³⁺,yEu³⁺ phosphors

Method	CIE	CCT (K)	Color purity (%)
NLMO:0.01Dy^3+	(0.394, 0.453)	4120	31
NLMO:0.01Dy^3+,0.005Eu^3+	(0.429, 0.427)	3316	44
NLMO:0.01Dy^3+,0.01Eu^3+	(0.435, 0.413)	3109	44
NLMO:0.01Dy^3+,0.07Eu^3+	(0.545, 0.367)	1640	65
NLMO:0.01Dy^3+,0.13Eu^3+	(0.497, 0.352)	1779	52
NLMO:0.01Dy^3+,0.19Eu^3+	(0.495, 0.349)	1773	51

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Fig. 10(b) shows the naked eye observation images of NLMO:0.01Dy³⁺,yEu³⁺ (y = 0, 0.01, and 0.07) phosphors under commercially available violet flashlight, respectively. As one can see from the pictures, the phosphor with single doped Dy³⁺ emits dazzling green light and, when a small amount of Eu³⁺ ions (1 at%) was introduced, the color of the phosphor turns to light orange. In particular, when Eu³⁺ ions reach 7 at%, the color of the phosphor changes to red completely. This further demonstrates the great potential of this phosphor in W-LED and color display applications.

4. Conclusions

In summary, a series of Dy³⁺ single-doped and Dy³⁺,Eu³⁺ co-doped NLMO phosphors were prepared by an EDTA–2Na assisted hydrothermal method. The emission of Dy³⁺ ions at two characteristic peaks of 469 nm and 573 nm was observed in the NLMO:xDy³⁺ phosphors excited at 355 nm. Through calculation and data fitting, dipole–dipole interaction takes the main role in the concentration quenching mechanism of Dy³⁺. In addition, the emission color of Dy³⁺ and Eu³⁺ co-doped NLMO phosphors can be adjusted from yellow to green and finally to orange-red by adjusting the ratio of Dy³⁺ to Eu³⁺ ion concentrations. As the concentration of Eu³⁺ increases, ranging from 1 at% to 19 at%, the process of energy transformation from Dy³⁺ to Eu³⁺ significantly enhances with corresponding energy transfer efficiencies of 15.64%, 25.5%, 57.79%, 88.82%, and 90.32%, respectively. The appropriate concentrations of Dy³⁺ and Eu³⁺ were determined to be 1 at% and 7 at%, respectively. The lifetimes of the NLMO:0.01Dy³⁺,yEu³⁺ (y = 0, 0.005, 0.01, 0.07, 0.13, and 0.19) phosphors gradually decreased with the increase of Eu³⁺ concentration. Luminous thermal stability of the studied phosphors was investigated with the NLMO:0.01Dy³⁺ and NLMO:0.01Dy³⁺,0.07Eu³⁺ phosphors as examples. The luminescence intensity at 423 K for the former remained at 59% compared to that at room temperature while the value reached as high as 97% due to the cooperation effect of Eu³⁺, indicating the crucial role of Eu³⁺ in the preservation of luminescence intensity at a higher temperature. The CCT values of the NLMO:0.01Dy³⁺,yEu³⁺ (y = 0, 0.005, 0.01, 0.07, 0.13, and 0.19) phosphors gradually decrease from 4120 K when y = 0 to 1649 K as y = 0.07. In summary, the NLMO:0.01Dy³⁺,yEu³⁺ (y = 0, 0.005, 0.01, 0.07, 0.13, and 0.19) phosphors possess excellent multi-color tunable ability with high performance in the aspect of thermal stability, enabling them to have much potential applications in color displays and various other fields.

Data availability

The data that support the findings of this study are available upon reasonable request.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (12264004), the Natural Science Foundation of Jiangxi Province (20202ACBL214020 and 20224BAB204029), the Science and Technology Research Project of Jiangxi Provincial Education Department (GJJ211412), Scientific Research Based Project of Gannan Normal University (2020KY11) and the US National Science Foundation (DMR-1904701).

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Footnote

† These authors contributed equally to this work.

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