Energy transfer and optical properties of NaLaMgWO₆:Dy³⁺, Tb³⁺ phosphors

Abstract

In this study, we have prepared several NaLaMgWO₆:Dy³⁺, Tb³⁺ phosphors using a high-temperature solid-phase method. X-ray diffraction (XRD) and photoluminescence (PL) were used to analyze the physical phase and luminescent characteristics of these materials. Our results showed that in the NaLaMgWO₆:xDy³⁺ phosphor, an ideal doping concentration of x = 8.0 mol% was discovered, which gave off a noticeably yellowish emission band. In NaLaMgWO₆:yTb³⁺ phosphor, the optimal doping concentration of Tb³⁺ ions were y = 10 mol%. Dy³⁺ and Tb³⁺ singly doped phosphors showed significant yellow and green emission at 574 nm and 547 nm, corresponding to the ⁴F_9/2 → ⁶H_13/2 transitions of Dy³⁺ and the ⁵D₄ → ⁷F₅ transitions of Tb³⁺, respectively. In the NaLaMgWO₆:Dy³⁺, Tb³⁺ phosphor, energy transfer was observed, and the mechanism behind it was thoroughly described. Furthermore, the color of the NaLaMgWO₆ phosphor could be adjusted from yellowish to greenish by varying the Dy³⁺/Tb³⁺ ratio in concentration. These findings demonstrate that co-doped NaLaMgWO₆ phosphors containing Dy³⁺and Tb³⁺ hold great potential for use in gadgets with fluorescent displays.

1 Introduction

White light-emitting diodes (w-LEDs) with phosphor conversion have become a very popular source of solid-state illumination in the past few years. This is due to their remarkable characteristics such as excellent luminance output, a long lifespan, minimal energy use, and environmental sustainability [1]. At present, commercial w-LEDs create light principally by combining blue-emitting InGaN chips with yellow-emitting phosphorescent (YAG: Ce³⁺). However, low color rendering index (CRI) and correlated color temperature (CCT) are caused by the absence of a red component in the visible zone, which makes LED devices less than optimal for display purposes. Therefore, it is crucial and necessary to investigate the regulation of phosphors and phosphors' chromaticity coordinates for different color emission [2,3,4,5].

Phosphors utilize rare earth ions as luminescent centers because of their distinctive electronic structure and rich luminescence hues. Two general pathways for rare earth ion activation of phosphors exist: (1) broadband emission stemming from internal d-f transitions, and (2) the generation of narrowband emission through internal f-f transitions [6,7,8,9,10]. Dy³⁺ ions are often used as activators for doping in phosphors due to their low price, high quantum efficiency, and good thermal stability. Green luminescent materials typically benefit from the activation of Tb³⁺ ions, which is attributed to its ⁵D₄ → ⁷F₅ transition. Generally speaking, the doping of rare earth ions can cause different colors of luminescence, and its CIE color coordinates will also change differently. Therefore, the CIE color coordinates can be used to observe whether multiple rare earth doped luminescent materials can achieve tunable emission [11, 12]. The transition of Dy³⁺ enables it to absorb n-UV light at 364 nm and 386 nm, While in many substrates the excitation spectra of Tb³⁺ and the emission spectrum of Dy³⁺ overlap. Due to this overlap, energy may be transferred from Dy³⁺ to Tb³⁺, creating a flexible green emission strategy [13,14,15,16,17].

Rare earth ions' (Dy³⁺ and Tb³⁺) luminescence properties commonly rely on the host material's crystal structure. To achieve an efficient phosphor, it's crucial to opt for a suitable host material [18, 19]. Lately, study on double perovskite compounds with the general formula AA'BB'O₆ has been extensive. These compounds include various activators and host elements. This is because they possess superb luminescent, chemical, and physical properties. The diversity of crystal structures in AA'BB'O₆ compounds occurs due to variations in A/A' and B/B' ion types [20,21,22,23,24,25,26]. One of the compounds that stands out is NaLaMgWO₆ (NLMWO). This chemical has a monoclinic structure with a double-ordered configuration and a C2/m space group. This arrangement includes alternating layered A-site cation ordering of Na⁺ and La³⁺, and alternating rock-salt B-site cation ordering of Mg²⁺ and W⁶⁺. This arrangement generates a sizable crystal field that enables extensive co-doping of rare earth ions. The doping enables fine-tuning of the luminescent properties to achieve desirable effects. In addition, the ultraviolet region of the NLMWO matrix displays a wide and robust charge transfer band. It effectively transfers its energy to the rare earth activator, resulting in an increase in luminescence efficiency [27]. Consequently, research on rare earth ion-doped NLMWO phosphors has been considerable, especially Dy³⁺ mono-doped NLMWO: Dy³⁺ new yellow-emitting phosphors, as a yellow-emitting material with potential applications. However, no reports exist on the luminescent characteristics and the transmission mechanism for energy between Dy³⁺ and Tb³⁺ in NLMWO: Dy³⁺, Tb³⁺ phosphors. In order to further investigate the luminescence behavior and mutual influence of Dy³⁺ and Tb³⁺ ions in the NLMWO substrate and to achieve energy transfer and tunable emissions, NLMWO: Dy³⁺, Tb³⁺ phosphors were prepared and the experimental results were analyzed in this article.

In this work, a high-temperature solid-state method is used to generate a variety of NLMWO phosphors that are co-doped with Dy³⁺ and Tb³⁺. The produced materials' morphology, luminous characteristics, and energy transfer mechanism were examined. Under the excitation of 389 nm wavelength, it was discovered that the NLMWO: Dy³⁺, Tb³⁺ phosphors may switch between yellow and green emission by altering the ratio of Dy³⁺ and Tb³⁺, while there was energy transfer in the NLMWO system. It has been demonstrated by the findings that the green NLMWO: Dy³⁺, Tb³⁺ phosphor exhibits remarkable photoluminescent characteristics, suggesting it could be a feasible option as a material for w-LEDs.

2 Experiment

An array of NaLa_1−xMgWO₆:xDy³⁺(x = 0.0–12 mol%), NaLa_1-yMgWO₆:yTb³⁺(y = 0.0–12 mol%), NaLa_0.92-yMgWO₆:0.08Dy³⁺, yTb³⁺(y = 0.0–12 mol%) were effectively synthesized by high temperature solid phase technique. The components, Na₂CO₃ (99.8%), La₂O₃ (99.99%), Dy₂O₃ > (99.99%), (MgCO₃)₄·Mg(OH)₂·5H₂O (99.99%), Tb₄O₇ > (99.99%), and WO₃ (99.8%), were precisely ground in an agate mortar after being weighed by stoichiometry. Grinded for half an hour and dried for 30 min in an 80 °C drying oven in an Al₂O₃ crucible. To characterize the sample, first, the muffle furnace was used to heat the dry powder. The powder was heated for 6 h at 900 °C and then further heated for 18 h at 1050 °C. The sample was heated and then cooled in the furnace until it reached room temperature. Next, to carry out the characterization, the burned samples were once more crushed to a powder.

Using an X-ray diffractometer (XRD-6100, Japan), the X-ray diffraction (XRD) patterns of the phosphors were captured using Cu Kα (λ = 0.154056 nm) radiation at 30 kV and 30 mA. The patterns were all gathered in a scanning mode with a step size of 0.01° for the 10–80° 2$\theta$ range. Use a scanning electron microscope (SEM-SU3500), the morphology was examined. An F-7000 fluorescence spectrophotometer was used to evaluate the materials' luminescence spectra at the temperature of the room.

3 Results and discussion

3.1 Crystal structure

NaLa_0.92−yMgWO₆:0.08Dy³⁺, yTb³⁺(y = 0.02, 0.04, 0.06, 0.08, 0.10, 0.12) series phosphors' XRD patterns are shown in Fig. 1. The sample's diffraction peaks match those on the reference card (JCPDF No. 37-0243) for the monoclinic double chalcogenide structure NLMWO, and there are no other impurity phases. It demonstrates that Dy³⁺ and Tb³⁺ were effectively incorporated into the host lattice without changing the structure of the crystals, and all the phosphors have a monoclinic double chalcogenide structure. Figure 1b shows that when rare earth ions are added, the angle of the diffraction peaks increases, according to the Bragg equation ${2}{\text{d}}{\text{sin}}\theta = \text{n} \lambda$, the doping of Dy³⁺ and Tb³⁺ results in a lower lattice parameter and a bigger diffraction angle since the ionic radius of La³⁺ (1.032 Å) is larger than that of Dy³⁺ (0.912 Å) and Tb³⁺ (0.923 Å) [28].

Fig. 1

a XRD patterns and b amplified XRD patterns of NaLa0.92-yMgWO6:0.08Dy³⁺, yTb³⁺ phosphors

Figure 2 shows the Rietveld refined pattern of NaLa_0.84MgWO₆: 0.08Dy³⁺, 0.08Tb³⁺. The starting model of this sample was taken from space group C2/m, and the refinement results obtained a low R convergence factor, where Rwp = 11.34% and Rp = 8.01%. These values are lower than 15%, indicating that the refinement results are reliable and good single-phase NaLa_0.84MgWO₆: 0.08Dy³⁺, 0.08Tb³⁺ phosphors are obtained, and their space group is C2/m.

Fig. 2

Rietveld refinement pattern of NaLa0.84MgWO6: 0.08Dy³⁺, 0.08Tb³⁺ 

Figure 3 shows the Raman spectra of NLMWO and NaLa_0.84MgWO₆: 0.08Dy³⁺, 0.08Tb³⁺. The shifts of the peaks observed in the range of 100 cm⁻¹–250 cm⁻¹ are attributed to the correlation vibrations generated by the A-site cation and the ligand oxygen atoms. The peaks in the ranges of 360–390 cm⁻¹ are attributed to the asymmetric deformation mode. The vibrations at around 800–900 cm⁻¹ correspond to the short-range correlation of the atoms, and the shift of the peaks at that location may correspond to the oxygen octahedral symmetric stretching mode, which has less effect on the radius [29]. The shift of the peaks in Raman spectra also proves that the rare earth ions are successfully doped in.

Fig. 3

Raman spectra of NLMWO and NaLa0.84MgWO6:0.08 Dy³⁺, 0.08Tb³⁺ phosphors in the range of 100–1200 cm⁻¹

3.2 Morphological study

Figure 4 shows the SEM patterns, EDS spectrum and elemental mapping of the samples, which shows that the particle sizes of the samples are between 1 and 2 μm, with irregular shapes, well-defined boundaries, and high crystallinity. The doping of ions has no significant effect on the crystal structure of NLMWO. EDS spectrum shows that Na, La, Mg, W, O, Dy and Tb ions are all present in the sample. The doped Dy and Tb ions completely enter the NaLaMgWO₆ host lattice. The elemental mapping demonstrates unequivocally that the elements are equally distributed throughout the co-doped phosphors for Dy³⁺ and Tb³⁺.

Fig. 4

SEM images, EDS spectrum and elemental mapping of NaLa0.84MgWO6: 0.08Dy³⁺, 0.0.08Tb³⁺ phosphors

3.3 Photoluminescence properties

The excitation spectrum of NaLa_0.92MgWO₆:0.08Dy³⁺, measured at a wavelength of 547 nm, is shown in Fig. 5a. There are two strong peaks and four weak peaks in 350 – 550 nm, and the strong peak at 389 nm corresponds to ⁶H_15/2 → ⁴F_7/2 of Dy³⁺ transition. The spectrum emitted by NaLa_1−xMgWO₆:xDy³⁺ when excited at a wavelength of 389 nm is presented in Fig. 5b. The ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 transitions of Dy³⁺ ions, which are responsible for the weak emission at 482 nm and the strong emission at 574 nm, respectively, are seen in this spectrum. At the same time, the data indicates that as the increases of Dy³⁺ concentration, the emission intensity shows a gradual rise and peaks at x = 0.08. After which, the concentration quenching causes a drop in intensity, therefore, it is inferred that the optimal doping amount of Dy³⁺ is 8% mol. The concentration burst is thought to be triggered by various interactions, including exchange interaction, multipole-multipole interaction, and radiation reabsorption. To identify the type of interaction that leads to concentration burst, one can calculate the critical distance (Rc) between Dy³⁺ ions using the Blasse formula [30]:

$$R_{c} = 2\left[ {\frac{3V}{{4\pi X_{c} Z}}} \right]^{{{\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 3}}\right.\kern-0pt} \!\lower0.7ex\hbox{$3$}}}}$$

(1)

where Z is the total amount of cations in the unit cell, V is the volume of the unit cell, and X_c is the quenching concentration. For NLMWO, Z = 4, X_c = 0.08, V = 482.67 ų. According to the formula, R_c = 7.11 Å, which is larger than 5 Å, demonstrating that the electric multipolar interaction is what causes the concentration quenching [31].

Fig. 5

a Excitation of NLMWO:xDy³⁺ phosphors; b emission spectra of NLMWO: xDy³⁺ phosphors

The NaLa_1−yMgWO₆:yTb³⁺ excitation and emission spectra are shown in Fig. 6a and b, respectively. The excitation spectrum has several weak broadband peaks in the 350–400 nm and strong narrow peaks in the 470–500 nm wavelength ranges. Two faint peaks are seen at 370 and 379 nm, which correspond to the Tb³⁺ ions' transitions from the ⁷F₆ → ⁵L₁₀ and ⁷F₆ → ⁵D₃ energy levels, respectively. Additionally, there is a strong peak observed at 487 nm which corresponds to the transition of the Tb³⁺ ion from the ⁷F₆ → ⁵D₄ energy level. Under excitation at a wavelength of 487 nm, Tb³⁺ ions have three characteristic emission peaks, which are caused by the ⁵D₄ → ⁷F₅ (547 nm), ⁵D₄ → ⁷F₄ (590 nm), ⁵D₄ → ⁷F₃ (623 nm) transitions. The luminous intensity of NaLa_1-yMgWO₆:yTb³⁺ is related to the Tb³⁺ ions doping concentration. As Tb³⁺ ion doping increases, the luminous intensity of phosphors initially rises and reaches its peak at y = 0.10. However, a downward trend is observed thereafter due to concentration quenching.

Fig. 6

a Excitation spectra of NLMWO:yTb³⁺ phosphors; b emission spectra of NLMWO: yTb³⁺ phosphors

The excitation spectra of NLMWO:0.10Tb³⁺ and the emission spectrum of NLMWO:0.08Dy³⁺ are shown in Fig. 7. There exists a slight overlap in spectral range between 450 and 500 nm that is observable. The overlapping part includes the ⁴F_9/2 → ⁶H_15/2 transition of Dy³⁺ at 482 nm and the ⁷F₆ → ⁵D₄ transition of Tb³⁺ at 487 nm. According to Dexter theory, an energy transfer can occur from Dy³⁺ to Tb³⁺, where Dy³⁺ functions as a sensitizer by transferring energy to Tb³⁺. This process enhances the luminescence intensity of Tb³⁺. As a result, Dy³⁺ and Tb³⁺ co-doping was performed in NLMWO, and the energy transfer mechanism of Dy³⁺ and Tb³⁺ in NLMWO was investigated. The Dy³⁺ content was set at 0.08 and the Tb³⁺ doping quantity was varied.

Fig. 7

Excitation of Tb³⁺ and the emission spectra of Dy³⁺

The excitation and emission spectra of NaLa_0.92-yMgWO₆:0.08Dy³⁺, yTb³⁺ are shown in Fig. 8. In Fig. 8a, the excitation spectra observed at the emission wavelength of 547 nm are monitored with both Dy³⁺ excitation peaks and Tb³⁺ ion excitation peaks, and the peak intensity at 389 nm is greater than the peak intensity at 487 nm. Under 389 nm excitation, the emission spectra of NaLa_0.92-yMgWO₆:0.08Dy³⁺, yTb³⁺ sample are illustrated in Fig. 8b. The intensity of the Tb³⁺ characteristic emission peak first rises with the increase in Tb³⁺ concentration, reaches its maximum at y = 0.08, then the intensity gradually decreases. Tb³⁺ concentrations rise, which leads to a steady decline in the intensity of Dy³⁺ emission. This is because as the concentration of Dy³⁺ and Tb³⁺ ions increase, the concentration of activated ions increases, the interaction between Dy³⁺ and Tb³⁺ ions increase, the non-radiative transition increases, Dy³⁺ transfers energy to Tb³⁺ ions, and the luminescence intensity continues to decrease. Tb³⁺ ion concentration It first increases and then decreases. In addition to concentration quenching, the reason for the decrease is that defects caused by the oxygen gap will also capture part of the energy and cause the concentration to decrease [32, 33], none of the energy delivered by Dy³⁺ can make up for these losses. At the same time, yellow-green tunable emission is achieved by changing the ratio of Tb³⁺/Dy³⁺ ion concentration. The following formula may be used to calculate the energy transfer efficiency [34]:

$$\eta_{T} = 1 - \frac{{{\text{I}}_{{\text{s}}} }}{{{\text{I}}_{{{\text{s0}}}} }}$$

(2)

Fig. 8

a Excitation spectra of NLMWO:0.08Dy³⁺; yTb³⁺ phosphors, b emission spectra of NLMWO:0.08Dy³⁺, yTb³⁺phosphors

where I_s and I_s0 denote the intensity of Dy³⁺ in addition to or without Tb³⁺ doping, respectively. The computed value of η_T is shown in Fig. 9. The energy transfer efficiency grows linearly, as can be observed. When the doping amount is 0.12, the conversion capacity is as high as 73%. Its CIE coordinates are shown in the Fig. 10 and Table 1. One can observe that NaLa_0.90MgWO₆:0.08Dy³⁺,0.02Tb³⁺ emits a yellow color. As the concentration of Tb³⁺ rises, the phosphors' emission color shifts from yellow to green. At the same time, it can be seen that its correlated color temperature is low, located near 3000 k, and is suitable for optoelectronic devices and solid-state lighting [35] .

Fig. 9

Energy transfer efficiency between Dy³⁺ and Tb³⁺in NaLa0.92-yDy0.08TbyMgWO6

Fig. 10

CIE chromaticity diagram of NLMWO: 0.08Dy³⁺, yTb³⁺ phosphors

Table 1 CIE coordinates of NLMWO: 0.08Dy³⁺, yTb³⁺ phosphors

4 Conclusion

To sum up, a set of phosphors containing NLMWO: xDy³⁺, yTb³⁺ were produced through solid-phase synthesis at high temperature. XRD results showed that the samples were successfully prepared without impurity phases. SEM and EDS results show that the preparation of the sample is uniform. In the NLMWO: xDy³⁺ phosphors, the best concentration of Dy³⁺ is 0.08 mol, the concentration burst is due to multipolar interactions. Photoluminescence results show that energy is transferred from Dy³⁺ to Tb³⁺, and the conversion efficiency can be as high as 73%. The luminescent color of phosphors under UV stimulation shifts from yellow to green when the Tb³⁺ to Dy³⁺ ratio increases, realizing the yellow-green tunable emission, and the corresponding CIE coordinate changes from (0.3960, 0.4719) to (0.3483, 0.5143), indicating its potential application in w-LEDs.