Yttrium aluminum garnet based novel and advanced phosphor synthesized by combustion route activated by Dy, Eu, and Tb rare earth metals

Abstract

In the present studies, rare earth (Dy, Eu, and Tb) activated garnet-based (Y₃Al₅O₁₂) phosphors were prepared using the combustion method at 550 °C. The formation of the compounds has been checked by powder X-ray diffraction and structural parameters were calculated. The crystallite/particle size has been measured using Scherrer formula as well as by transmission electron microscopy which show that the size of the particles is in the nanorange. In the photoluminescence emission spectra, YAG:Dy_0.05 emits two distinctive colors: blue and yellow, YAG:Eu_0.05 emits red color, whereas YAG:Tb_0.02 emits green phosphor. Thus, the combination of rare earth (Dy_0.05, Eu_0.05, Tb_0.02) with garnet gives BYRG (blue–yellow–red–green) emissions can produce white light. These discussed phosphors exhibit a strong absorption between 340 and 400 nm. The energy transfer mechanism was also discussed. The higher luminescence color purity (95.68%), Color Rendering Index (95), Correlated Color Temperature (5287 K), and Quantum efficiency (93.7%) are calculated, therefore, synthesized Y₃Al₅O₁₂Dy_0.05Eu_0.05Tb_0.02 phosphor material can be used as a WLED phosphor materials in solid-state lighting system.

1 Introduction

There are diverse field of Luminescence, luminescent materials and applications. Modern lighting system totally depends on advanced and novel materials for great efforts for enhancing display quality and visibility [1,2,3]. In the present scenario, the need and requirement of novel phosphor is one of the most important and urgent challenges to synthesize luminescent material for white light emitting diodes (WLED) for solid lighting devices [4,5,6]. Other requirement should be fulfillment by modern technology like that maximum quantum efficiency, high color purity (NTSC), high Color Rendering Index (CRI), cool Correlated Color Temperature (CCT) value, easily availability for WLED application, optimum brightness, Ultra- violet (UV) stability, transparency, and high quenching temperature [7].

A several numbers of Eu³⁺ and Eu²⁺ ion doped phosphors have been synthesized for excellent red and blue luminescence under near-UV excitation respectively, name of few are includes such as Y₃Al₅O₁₂:Eu³⁺ [8], Y₂O₃:Eu³⁺ [9], YVO₄:Eu³⁺ [10], YPO₄:Eu³⁺ [11], Ca₂ZnWO₆:Eu³⁺ [12], LaPO₄:Eu³⁺ [13], GdAlO₃:Eu³⁺ [14], and Ba₂CaZn₂Si₆O₁₇:Eu₃₊ [15] BaMgAl₁₀O₁₇:Eu [16], ZnS:Ag, Al [17], Y₂SiO₅:Ce [18], KMg₄(PO₄)₃:Eu²⁺ [19], RbBaPO₄:Eu²⁺ [20], Ca₃Mg₃(PO₄)₄:Eu²⁺ [21], LiCaPO₄:Eu²⁺ [22], SrZnP₂O₇:Eu²⁺ [23], SrCaP₂O₇:Eu²⁺ [24], NaMgPO₄:Eu²⁺ [25]. Tb³⁺ and Mn²⁺ ions are have found to produce green emission in numerous host compounds [26,27,28,29,30]. Some standard green phosphors have been reported such as Mg₂SiO₄:Tb³⁺ [29], SrAlSi4N7: Pr³⁺ Sm³⁺ Tb³⁺ [27], MgSrAl₁₀O₁₇: Mn²⁺ [28], ZnGa₂O₄:Mn²⁺, Cr³⁺ [29], Li₂Mg_1 − xZrO₄:xTb³⁺ [30], Zn₂SiO₄:Mn [31], Y₂SiO₅:Tb [32], Y₃Al₅O₁₂:Tb [33].

For the developing of best WLED phosphor materials to the single-phase host doped with co-doped rare earth ions and transition metals that operate under ultraviolet or near-ultraviolet excitations numerous efforts have been taken to develop the single-phase host-based phosphor materials for the WELDs [1, 2, 4, 34,35,36]. Without compromising in the stability and color reproducibility, the use of single-phase phosphor compound for fabricating of WELDs and it will be potentially easy for NUV convertible WLEDs [1]. There are several white color emitting phosphors have been reported such as BaZn₂(PO₄):Ce³⁺, Dy³⁺/Tb³⁺ [4], K₂SrP₂O₇:RE³⁺(RE = Sm, Tb, Eu, Dy) [37], Ca₈Gd₂(PO₄)₆O₂:(Ce³⁺/Eu²⁺/Tb³⁺/Dy³⁺/Mn²⁺) [38], LaPO₄:RE³⁺ (RE = Eu, Tb, Dy, Ce) [39], Mg₂₁Ca₄Na₄(PO₄)₁₈:Dy³⁺Tb³⁺Eu³⁺ [40], Zn₂SiO₄:Mn²⁺-Eu³⁺-Dy³⁺ [41], Sr₃Gd(PO₄)₃:Dy³⁺ Eu³⁺ [42], and Sr₂SiO₄ : Dy³⁺ Mn²⁺ [43].

Garnets (YAG) based phosphor materials have some outstanding properties such as good stability, Optimum brightness, high chemical stability, Low energy consumption, long life span, High thermal stability, and Friendly environments [8, 33, 44,45,46,47,48,49,50]. The garnets (YAG) based phosphor are used as hosts with doping rare earth ions for luminescent application [1, 47]. The White-LEDs (W-LEDs) already have been produced by the combination of garnet Y₃Al₅O₁₂:Ce³⁺ phosphor and chips of InGaN [51]. The Y₃Al₅O₁₂ based phosphor materials can behave as photo-luminescence, thermo-luminescence material, electroluminescence, and cathodo-luminescence [44, 52,53,54,55,56]. On the basis of the emission spectra of the rare earth ions, the combination of Dy³⁺, Tb³⁺, and Eu³⁺ in YAG is expected to produce a white light spectrum [50, 53, 57,58,59,60,61]. Therefore, in the present studies, we synthesized Y₃Al₅O₁₂-based phosphors doped with Dy, Eu, and Tb rare earth activators by the combustion method using urea as fuel.

2 Preparation techniques

In the present work, we synthesized YAG:Dy, YAG:Eu, YAG:Tb, YAG:Eu, Dy, and YAG:Eu, Dy, Tb phosphors compounds for WLED applications. The combustion synthesis process is similar as explained in Refs. [59, 60]. Aksh et al. were already used combustion synthesis for synthesizing various aluminates [57,58,59,60,61]. The nitrates (oxidizer) and urea (fuel) ratio were calculated by the techniques designated as previous. In this work, stoichiometric amounts of Y(NO₃)₃·6H₂O, Al(NO₃)₃·9H₂O, Dy(NO₃)₂·6H₂O, Eu(NO₃)₂·6H₂O, Tb(NO₃)₂·6H₂O, and NH₂CONH₂ were taken and carefully mixed. In this present work, we were used 1 h mixing time of the raw material before heating. After regular mixing of raw materials A thick paste is formed. The alumina crucible with thick paste was introduced in a preheated furnace maintained at 550 °C. after 1–2-min exothermic combustion reaction (Eq. 1) crucible was removed outside the furnaces and grind 1 h, the foamy product. Similarly, all other materials were synthesized by combustion route Fig. 1S, synthesized material is listed in Tables 1 and 1S.

Fig. 1

Typical PXRD graph of synthesized garnet samples doped with rare earth metals

Table 1 The various samples synthesized by the urea fuel combustion (UFC) process

$$3\text{Y}{\left({\text{N}\text{O}}_{3}\right)}_{3}\cdot6\left({\text{H}}_{2}\text{O}\right)+5\text{Al} {\left(\text{N}{\text{O}}_{3}\right)}_{3}.9\left({\text{H}}_{2}\text{O}\right) +0.05\text{D}\text{y}{\left({\text{NO}}_{3}\right)}_{2}\cdot6\left({\text{H}}_{2}\text{O}\right) + 0.05\text{E}\text{u}{\left({\text{N}\text{O}}_{3}\right)}_{2}.6\left({\text{H}}_{2}\text{O}\right) + 0.02\text{T}\text{b} {\left({\text{N}\text{O}}_{3}\right)}_{2}\cdot6\left({\text{H}}_{2}\text{O}\right) +{{\text{NH}}_{2}\text{CO}\text{N}\text{H}}_{2}\to {{\text{Y}}_{3}{{\text{Dy}}_{0.05}\text{Eu}}_{0.05}{\text{Tb}}_{0.02}{\text{Al}}_{5}\text{O} }_{12} +\left({\text{H}}_{2}+{\text{N}}_{2}+{\text{N}\text{H}}_{3}+{\text{O}}_{2}+{\text{C}\text{O}}_{2}+{\text{H}}_{2}\text{O}\right)\dots$$

(1)

3 Characterization instruments details

Structural and crystallinity analysis were investigated by as Powder X-ray Diffractometer (PXRD), result of (D8 ADVANCED BRUKER). The surface morphology, EDX (Energy Dispersive X-ray Spectroscopy) mapping, crystallite size, and SAED pattern were observed using field emission scanning electron microscopy (SEM, FEI NOVA NANO SEM™ 450) and transmission electron microscope (TEM, TECNAI G² T30, S-TWIN). Fourier transform infrared spectroscopic (FTIR) studies were recorded by 84,005 (SHIMADZU), with a resolution of 2 cm⁻¹. The range of 500–3800 cm⁻¹. (Sample mixed with KBr in 1:5 ratio) used for identification of homogeneity, purity and for structural properties analysis. Raman Spectra recorded by Raman micro spectrometer RENISHAW in via Raman microscope (He–Ne Laser 633 nm, Ar laser 514 nm). Photo-Luminescence (excitation & emission) spectra were recorded by RF- 5301 PC Shimadzu Spectro-fluorophotometer, hitachi F-4500 florescence spectrometer. De-cay curve recorded by RF-5301 PC Shimadzu Spectro-fluorophotometer. The PL quantum yield (efficiency) of synthesized sample were measured under UV-violet excitation (500 W Xe lamp) using an integrating sphere.

4 Result and discussion

4.1 Structural and crystallinity analysis

All YAG, YAG:Dy, YAG:Eu, YAG:Tb, YAG:Dy Eu, and YAG:Dy, Eu, Tb synthesized samples were measured and analyzed by PXRD. The PXRD pattern of all selected synthesized compounds are revealed in Fig. 1 with comparing the PXRD patterns with standard JCPDS card no. 33-0040 [7, 33, 50, 53]. However, there are some weak diffraction peaks from the impurity phase and effect of doping of Dy, Eu, and Tb. This small amount of Dy, Eu, and Tb does not change the structure of host material (YAG). However, the shift of diffraction angle is detected with addition of Eu³⁺, Dy³⁺, and Tb³⁺ which revealed Fig. 2(a–c). The crystalline size and crystallinity are played important role in the maximum luminescence efficiency and other luminescence properties of synthesized materials [53]. In this case all synthesized phosphors exhibited the formation of sharp diffraction pattern is the consequence of the generation of high improved crystallinity. The structural parameters such as lattice constant (a), inter-planner spacing(d) is calculated by Bragg equation, average crystallite size (D_s) and (D_W−H) is calculated by Debye Sherrer (Eq. 2) and Hall–Williamson method (Eq. 3) respectively. The micro-strain (ε) and dislocation density (δ) is calculated (Eqs. 4 & 5) from the XRD data for the main peak of at [420] (Fig. 2S) [62, 63], all the calculated values are listed in Table 2. Figure 3 shows the typical micro-strain plot of discussed materials. The minimum value of dislocation density (δ), and micro-strain (ε) indicates that observed crystallinity of materials is suitable for luminescence application. The estimated crystallite size of YAG based phosphor is found to increase with Eu, Dy, and Tb doping, larger crystallite size decreases the density of the grain boundaries, which might scatter light generated inside due to enhancement the photo-luminescence properties.

$$D_S=\frac{0.9\lambda}{\beta cos\theta}$$

(2)

Fig. 2

a–c Typical Rietveld refinement curve of YAG & YAG: DyEuTb_0.02, and c cubic crystal structure for YAG:DyEuTb_0.02 sample

Table 2 Typical values of peak angles, FWHM, d-spacing, crystallite size, dislocation density, and macrostrain of YAG:Dy_0.05Eu_0.05Tb_0.02 phosphor

Fig. 3

Typical micro-strain plot of Y₃Al₅O₁₂:Dy, Eu, Tb samples

The peak broadening in of synthesized samples originates not only due to crystallite size but also due to existing strain. Dislocation density (δ) is calculated by Eq. (5).

$$\beta cos\theta = \frac{k\lambda }{{D}_{W-H}} + \epsilon sin\theta$$

(3)

Where, D_W−H = average crystallite size, Β = is the FWHM, ε = micro-strain, θ = angle of diffraction, and λ = wavelength of the X-ray radiation.

$$\epsilon=\frac{\beta cos\theta}4$$

(4)

$$\delta = \frac{1}{{D}_{S}^{2}}$$

(5)

A structural refinement using PXRD data of the synthesized YAG:DyEuTb_0.02 compound by Rietveld method using full prof software was performed to analyze the structural and unit cell parameters of the discussed materials were shown in the Fig. 2(a, b) and refinement parameters were tabulated in Tables 3 and 4. By using the pseudo-voigt function we have refined PXRD data of the YAG and YAG:DyEuTb_0.02 sample by computing the Bragg contribution (χ²) and goodness of fit (GOF) parameters [7, 57]. This GOF is subjected to some radiality factor of R_wp and R_exp, GoF = R_wp/ R_exp. The χ² must be < 5 and GoF must be approach to unity for perfect refinement of PXRD data of samples. In this work, GOF and χ² were calculated and found 1.1860 and 3.7, respectively, for YAG:DyEuTb_0.02 phosphors. The result designates good agreement among the observed and calculated by XRD pattern. Figure 2c shows the crystallographic structure of garnet was described as a 3D structure of polyhedrons, in which tetrahedron (AlO₄) and octahedron (AlO₆) are linked by shard oxygen ions. The Y³⁺ ions are an eight co-ordinated dodecahedral (YO₈) arrangement in the three crystallographic directions. The Dy³⁺, Eu³⁺, and Tb³⁺ ions when incorporated in the lattice goes and occupy the Y³⁺ site. The lattice parameter is calculated from the least square fitting of the XRD peaks, and are found 12.0393Å for doped YAG:DyEuTb_0.02 material and 12.0069Å for undoped YAG materials. The lattice parameters for the doped samples compared to the undoped samples are increased due to different and more ionic radii of rare earth (Dy³⁺ = 0.091 nm, Eu³⁺ = 0.106 nm, and Tb³⁺= 1.18 nm) ions as compared to Y³⁺ (0.1011 nm) ions. Therefore, the Dy³⁺, Eu³⁺, and Tb³⁺ ions are expected to mostly enter into the distorted dodecahedral by exchanging the Y³⁺ (24c) ions [7, 33, 50]. In the garnet structure (complex structure) activators and co-activators are accommodated at different location or position of dodecahedral site, In a local view each Y³⁺ (Dy³⁺, Eu³⁺, and Tb³⁺) ions is surrounded by two nearest AlO₄⁵⁻ (tetrahedral site) and AlO₆⁹⁻(octahedral site).

Table 3 Rietveld refinement crystallographic data of Y₃Al₅O₁₂ and Y₃Al₅O₁₂:Dy_0.05Eu_0.05Tb_0.02.

Table 4 Comparison of refinement crystallographic data for atom parameters of Y₃Al₅O₁₂ and Y₃Al₅O₁₂:Dy_0.05Eu_0.05Tb_0.02.

4.2 Scanning electron microscopy, SEM mapping, and EDX analysis

The FESEM is a significant technique for detecting surface morphology, crystallite size, EXD mapping, and elemental mapping of the synthesized phosphor samples [50, 53, 57]. Figure 4a–c displays FE-SEM micrographs, elemental mapping, and EDX spectra of the prepared YAG:DyEuTb_0.02 phosphor. Figure 4a represent irregular morphology, un-equal sizes, and micro-nano crystallites (agglomerated) of synthesized phosphor sample. By the lineal intercept technique [Heyn’s, technique or Eq. 6] the average particle size [${D}_{SEM}$] is calculated using the FE-SEM image [64].

$${D}_{SEM}=1.56\times \frac{L}{MN}$$

(6)

Where, M, L, and N is the magnification of images, the length of a line drawn on the images, and grain boundaries intercepted by the lines drawn, respectively. The average crystallite size was calculated to be 50–100 nm for the YAG:DyEuTb_0.02 sample. Figure 4b, c and Fig. 3S displays an elemental mapping, individual element mapping, and distribution statistics of an elements of the samples. It represents no other elements observe apart from Y, Al, O, Dy, Eu, and Tb. Figure 4c displays EDX spectra of the YAG:DyEuTb_0.02 sample. it represents perfect elemental composition, and homogeneity of the synthesized sample.

Fig. 4

a–c Typical FESEM surface morphology, energy dispersive X-ray mapping of elements, energy dispersive X-ray (EDX) spectra of YAG:DyEuTb_0.02.

4.3 Transmission electron microscopy (TEM) analysis

TEM is a significant technique for observing information about the crystal structure, particle size, pores or voids, defect, particles distribution, selected area electron diffraction (SAED) analysis, and high resolution (HR) analysis of the synthesized sample. In this study, we were observed and collected TEM images of YAG:DyEuTb_0.02 sample is shown in Fig. 5a–e. Figure 5a & b shows the average crystallite size is found range from 15 to 40 nm, which are like sphere of small nanoparticles, but some agglomerated (100 nm–1 μm) of nanoparticles is observed. The bigger (agglomerated particles) particles having found pores, defect, and dis-similarity due to exothermic reaction of combustion [53, 57,58,59,60,61].

Fig. 5

a, b The photograph of TEM images for the YAG:DyEuTb_0.02, c typical histogram plot indicates distribution of nanoparticles, d Typical SAED image for YAG:DyEuTb_0.02 and e HR image for YAG:DyEuTb_0.02 sample

The ImageJ software is used to estimate the size of particle of TEM image (Fig. 5a & b) in which about 100 particle is considered for plotting the graph is shown in Fig. 5c. The histogram is fitted with the log-normal distribution curve (blue line), the mean particle size of around 15 nm is observed, which matches with the results obtained from the XRD measurement. Figure 5d shows SAED (selected area of electron diffraction) pattern of the sample in which 10 diffraction circular rings are observed at 1st [211], 2nd [220], 3rd [231], 4th [440], 5th [420], 6th [422], 7th [532], 8th [444], 9th [640], and 10th [642] is shown in Fig. 5d. Figure 5e displays a HR-TEM (High Resolution) image of sample in which HR lattice fringes are occupied at different location of image. The interspacing distance between two lattice fringes is measured at different location it varies from 0.504 to 0.824 nm. The measured interspacing distance is perfectly equivalent to the interplanar spacing (d) which is calculated by Bragg equation. Therefore, the SAED pattern and HR image of sample is perfectly verify result of PXRD patterns of YAG:DyEuTb_0.02 samples.

4.4 FTIR analysis

The FTIR spectra displays bands from 550 to 4000 cm⁻¹ for YAG:DyEuTb_0.02 sample is shown in Fig. 6. The FTIR bands located at ~ 786 cm⁻¹ and ~ 687 cm⁻¹ indicates the vibration of Al–O, while the bands at ~ 721 cm⁻¹ and ~ 568 cm⁻¹ is observed due to Y–O groups, the separation and sharpness peaks of Y–O and Al–O indicates perfect crystalline structure YAG:DyEuTb_0.02 [7]. In the spectra, there is does not found any impurity of organic and in-organic elements. Therefore, intensity of emission enhanced by reducing non-radiative relaxations.

Fig. 6

Typical FT-IR spectra of the YAG:DyEuTb_0.02 and inset plot represents zooming area of YAG:DyEuTb_0.02 sample

4.5 Raman analysis

The Raman spectroscopy is an important technique for detecting the frequency of long-wavelength of lattice vibrations of phonons as well as useful for detecting of grain size, disorders of crystals, impurity level, impurity, crystal structure, residual stress, and defect of crystal symmetry [53]. The Raman spectra of YAG:DyEuTb_0.02 sample were observed at room temperature which displayed in Fig. 7. In this spectra, 6 peaks were recorded at ~ 139 cm⁻¹, ~ 867 cm⁻¹, ~ 914 cm⁻¹, ~ 1358 cm⁻¹, ~ 1391 cm⁻¹, and ~ 1579 cm⁻¹. The sharp and highest peak has found at ~ 1391 cm⁻¹, which exhibits maximum crystalline crystal symmetry of synthesized YAG:DyEuTb_0.02 sample. Some other peaks of Raman spectra indicate the cubic spinel anti-symmetry or asymmetric bending vibration of YAG:DyEuTb_0.02 due to the vibrations, oscillation, and scattering phenomenon of multiple optical phonons. Sharpness and intensive bands of Raman spectra evidence of good fluorescence properties of samples [53].

Fig. 7

Typical Raman spectra of the YAG:DyEuTb_0.02 sample

4.6 Photo-luminescence analysis

4.6.1 Photo-luminescence analysis of YAG:Dy

Photoluminescence (PL) is a significant characterization technique for characterize the optoelectronic properties of phosphors and semiconductor materials. Its principle is electrons are excited from the ground state to the conductance state of the material by a Xenon laser source with an energy larger than the bandgap. PL is the important phenomenal for the solid-state lighting application. In this work various phosphor sample were synthesized by combustion synthesis route, synthesized sample are listed in Table 1. Before this work several researchers have been synthesized YAG based phosphor doped with Dy rare earth metals by various method. After intense review of research papers, we were synthesized YAG based phosphor doped with Dy, Eu, and Tb rare earth metals.

Figure 8 displays the PL excitation (λ_em = 482 nm) and PL emission (λ_em = 350 nm) spectra, of Y₃Al₅O₁₂:Dy_0.05 (YAG:Dy) phosphor. In the PL excitation spectra has several excitation peak is originated at around 290 nm (charge transfer), 328 nm (⁶H_15/2 → ⁴M_21/2), 354 nm (⁶H_15/2 → ⁴P_7/2), 369 nm ( ⁶H_15/2 → ⁴P_5/2 ), 390 nm ( ⁶H_15/2 → ⁴I_13/2), 430 nm (⁶H_15/2 → ⁴G_11/2), 455 nm ( ⁶H_15/2 → ⁴I_15/2), and 476 nm (⁶H_15/2 → ⁴F_9/2). The weak band originates due to charge transfer phenomenon and sharp excitation bands originates at lower frequency region due to the f-f transitions of Dy³⁺ions [40, 53, 65, 66].

Fig. 8

Typical PL excitation spectra and emission spectra for YAG:Dy

In the PL emission spectra has two emission peaks is originated at around 482 nm (⁴F_9/2→ ⁶H_15/2) and 577 nm (⁴F_9/2→ ⁶H_13/2) due to magnetic dipole transition and electric dipole transitions of Dy³⁺ ions, respectively. It is noticed that 482 nm (blue emission) peak is more dominant to the 577 nm (yellow emission) peak due high local symmetry site of Dy³⁺ in the crystal structure [66]. It is observed that the values of Y/B ratio for various concentration is 0.8074. Before this work, application of Dy doped various phosphors as a WLEDs application have been already reported by researchers, because they were obtained a ratio of yellow and blue peaks of emission spectra (Y/B) to one or more by appropriate doping of a single Dy³⁺ in many such suggested host compounds and adjusting the ratio of the peaks in the blue and yellow regions [1]. Thus, it is possible to obtain white light in a single Dy³⁺ doped phosphor material by appropriately adjusting the yellow to blue intensity ratio (Y/B). But if the ratio of yellow color and blue color remains one or less then this type of phosphor material is not suitable for making WLEDs. therefore, research is unmoving required to achieve high quality white light with high emission intensity, after the co-doping of Eu and Tb (red and green emitting component) with suitable concentration of Dy³⁺, that high white light can be generated.

The Y/B ratio of Dy³⁺ is related to many factors, co-doping of another sensitizer which due to enhance the intensity of yellow peak, the site symmetry of activators, and electronegativity of the next-neighbor element [66]. In this research work we were used to co-doping of Eu and Tb ions for enhancing the intensity of yellow peak, improve quantum efficiency, increase value of CRI, and enhancing color purity of emitting white color. Therefore, in this work we have taken Eu rare earth elements as a red component and Tb elements as a green components of PL spectra. The decay curve, CIE color co-ordinate, CRI, CCT, and quantum efficiency is calculated of YAG:Dy phosphor sample all values is shown in following Table 1S, Tables 5 and 6. From the analysis of all-result, prepared YAG:Dy sample is not suitable for WLEDs white lighting system.

Table 5 Listed sample code, CIE color coordinate, color purity, CRI, and CCT and critical distance values of phosphors sample

Table 6 Calculated internal and external, and quantum efficiency (QE) of the phosphor materials

4.6.2 Photo-luminescence analysis of YAG:Eu

The PL excitation spectra of YAG:Eu phosphor was observed by monitoring emission wavelength at 594 nm is shown in Fig. 9. The characteristic lines located at 322 nm (⁵L₆ →⁷F₀), 330 nm (⁵L₆ →⁷F₁), 364 nm (⁵L₆ →⁷F₂), 386 nm (⁵L₆ →⁷F₃), 394 nm (⁵L₆ →⁷F₄), 408 nm (⁵D₂ →⁷F₀), 467 nm (⁵D₂ →⁷F₁), and 477 nm (⁵D₂ →⁷F₂) are attributed due to transitions of Eu³⁺ ions [65, 66].

Fig. 9

Typical PL excitation spectra and emission spectra for YAG:Eu

The PL emission spectra YAG:Eu was observed by under the monitoring of excitation wavelength 394 nm, the PL emission spectra exhibits emission bands at 591 nm (⁵D₀→⁷F₁), 594 nm (⁵D₀→⁷F₁), and 599 nm (⁵D₀→⁷F₁) magnetic dipole transition, But the PL emission spectra exhibits emission bands at ~ 612 nm (⁵D₀→⁷F₂), ~ 630 nm (⁵D₀→⁷F₂), ~ 650 nm (⁵D₀→⁷F₃), and ~ 708 nm (⁵D₀→⁷F₄) due to the electric dipole transition, respectively is shown in Fig. 9. In this study, the magnetic dipole transition is stronger than the electric dipole transitions, it indicates that these Eu³⁺ ions occupy strong symmetry sites of crystal [66]. The decay curve, CIE color co-ordinate, CRI, CCT, and quantum efficiency is calculated of Y₃Al₅O₁₂: Eu_0.05 phosphor sample all values is shown in following Table 1S, Tables 5 and 6. It is concluded that the prepared YAG:Eu phosphor material is suitable for red lighting system.

4.6.3 Photo-luminescence analysis of YAG:Tb

Figure 10 displays the PL excitation spectra of YAG:Tb phosphors and it was monitored under the an emission wavelength 547 nm and slit width 1.5 nm. The characteristic lines located at ~ 329 nm (4f₈→4f₇5d₁ transition of Tb³⁺). Meanwhile, the strong excitation band at ~ 307 and 315 nm which was attributed due to the ⁸S_7/2→⁶P_7/2 and ⁸S_7/2→⁶P_3/2, respectively, three more other strong band were found at ~ 353 nm (⁷F₆→⁵L₉), ~ 378 nm (⁷F₆→⁵G₉), and ~ 397 nm (⁷F₆→⁵L₁₀) have been found of transition Tb³⁺ [67]. The PL emission spectra were observed of the Y₃Al₅O₁₂: Tb³⁺ phosphor under the monitoring of an excitation wavelength of 378 nm is shown in Fig. 10. The PL emission spectra of Y₃Al₅O₁₂: Tb³⁺ phosphors characteristic bands located at ~ 485 nm (⁵D₄→⁷F₆), ~ 547 nm (⁵D₄→⁷F₅), ~ 588 nm (⁵D₄→⁷F₄), and ~ 621 nm (⁵D₄→⁷F₃) which due to transitions (electric dipole and magnetic dipole induced) of Tb³⁺ [67]. The YAG:Tb³⁺ phosphor emits green band (~ 547 nm) is stronger than other bands. Additionally, high intensity bands have been observed for Tb³⁺ (< 272 nm) due to Tb-O charge transfer band [67]. The decay curve, CIE color co-ordinate, CRI, CCT, and quantum efficiency is calculated of Y₃Al₅O₁₂:Tb_0.02 phosphor sample all values is shown in following Table 1S, Tables 5 and 6. From the analysis of YAG:Tb phosphor, it can be suitable for green lighting system.

Fig. 10

Typical PL excitation spectra and emission spectra for YAG:Tb compound

4.6.4 Photo-luminescence analysis of YAG:DyEu

The PL excitation spectra of YAG:DyEu phosphor was monitored under an emission wavelength 614 nm is shown in Fig. 11. The characteristic lines located at ~ 346 nm (⁶H_15/2 → ⁴P_7/2), ~ 361 nm (⁶H_15/2 → ⁴P_5/2), ~ 380 nm (⁶H_15/2 → ⁴I_13/2) due to Dy³⁺ and it may some of these characteristic lines located at ~ 361 nm (⁵L₆ →⁷F₂), ~ 380 nm (⁵L₆ →⁷F₃), ~ 395 nm (⁵L₆ →⁷F₄), ~ 464 nm (⁵D₂ →⁷F₁) due to Eu³⁺ ions. Therefore, it is observed that the excitation of Dy³⁺ and excitation spectra of Eu³⁺ is overlapping criterion due to energy gets transfer from higher energy level (Dy³⁺) to lower energy level (Eu³⁺). Therefore, peak intensity of Dy³⁺ is diminished. The PL emission spectra of YAG:DyEu phosphor was monitored at 395 nm is shown in Fig. 11. The PL characteristic bands is located at ~ 482 nm (⁴F_9/2→ ⁶H_15/2), ~ 577 nm (⁴F_9/2→ ⁶H_13/2), ~ 654 nm (⁴F_9/2→ ⁶H_11/2), ~ 591 nm (⁵D₀→⁷F₁), and ~ 612 nm (⁵D₀→⁷F₂) nm due to the transitions of Dy³⁺and Eu³⁺ions [66]. In the PL emission spectra displays two characteristic lines at 482 and 577 nm is observed due to Dy³⁺ transition, and two another characteristic lines at 591 and 614 nm are observed due to Eu³⁺ transition is displayed in Fig. 11.

Fig. 11

Typical PL excitation spectra and emission spectra for YAG:DyEu.

The PL emission spectra of YAG:DyEu phosphor was monitored at 395 nm is shown in Fig. 11. The PL characteristic emission bands are located at ~ 480 nm (⁴F_9/2→ ⁶H_15/2), ~ 575 nm (⁴F_9/2→ ⁶H_13/2), ~ 656 nm (⁴F_9/2→ ⁶H_11/2) due to the transitions of Dy³⁺ and another PL emission bands is observed at ~ 537 nm (⁵D₁→⁷F₁), ~ 595 nm (⁵D₀→⁷F₁), and ~ 612 nm (⁵D₀→⁷F₂) due to transitions of Eu³⁺ions [66]. In the PL emission spectra, it is observed that an overlaps band is found between yellow emission (577 nm) of Dy³⁺ ion and orange red emission (591 nm) of Eu³⁺ ions. Therefore, energy get transfer from Dy³⁺ to Eu³⁺ resultantly, PL emission intensity increases of the Eu³⁺ (⁵D₀→⁷F₂) while the PL emission intensity diminishes of Dy³⁺ (⁴F_9/2→ ⁶H_13/2) [66].

4.6.5 Photo-luminescence analysis of YAG:DyEuTb_1−x

The PL excitation spectra of YAG:DyEuTb_1−x (x = 0.01, 0.02, 0.03, and 0.04) were monitored under an emission wavelength 544 nm is shown in Fig. 12. The characteristic bands located at ~ 277 nm (4f₈→4f₇5d₁) due to Tb³⁺, ~ 350–356 nm [(⁶H_15/2 → ⁴P_7/2) doe to Dy³⁺ ions & (⁵L₆ →⁷F₂) due to Eu³⁺], ~ 382 nm [(⁵D₃→⁷F₆) due to Tb³⁺, ( ⁶H_15/2 → ⁴I_13/2) due to Dy³⁺, and (⁵L₆ →⁷F₃) due to Eu³⁺], 450 nm ( ⁶H_15/2 → ⁴I_15/2) due to Dy³⁺, ~ 464 nm (⁵D₂ →⁷F₁) due to Eu³⁺, ~ 477 nm(⁶H_15/2 → ⁴F_9/2) due to Dy³⁺, and ~ 490 nm (⁵D₄→⁷F₆ ) due to Tb³⁺ ions. The excitation spectra range from 250 to 500 nm several peaks originate by the co-contribution and overlapped transition of Dy ³⁺, Eu³⁺, and Tb³⁺ ions [66, 67]. The PL emission spectra of YAG:DyEuTb_1−x (x = 0.01, 0.02, 0.03, and 0.04) phosphor is observed under the monitoring of an emission wavelength 356 nm is shown in Figs. 4S and 13, & Fig. 5S. The maximum PL intensity is observed for YAG:DyEuTb_0.02, after increases concentration of Tb PL intensity decrease gradually due to concentration quenching. The cause of concentration quenching phenomena is described below (Figs. 17 and 18). The PL emission characteristic bands located at ~ 465 nm (⁵D₂→⁷F₀) due to Eu³⁺, ~ 482 nm (⁴F_9/2→⁶H_15/2) due to Dy³⁺, ~ 544 nm (⁵D₄→⁷F₅) due to Tb³⁺, ~ 577 nm (⁴F_9/2→⁶H_13/2) due to Dy³⁺, ~ 595 nm (⁵D₀→⁷F₁) due to Eu³⁺, ~ 612 nm (⁵D₀→⁷F₂) due to Eu³⁺, ~ 650 nm (⁵D₀→⁷F₃) due to Eu³⁺, and ~ 708 nm (⁵D₀→⁷F₄) due to Eu³⁺ ions. Figures 4S and 13 shows that the PL emission intensity of Dy³⁺, and Tb³⁺ get diminishes (only one peak at ~ 544 nm slightly increases with respect to Tb concentration, after concentration quenching it decreases gradually) and intensity of Eu³⁺ get enhances with respect to increases concentration of Tb³⁺ ions, it is the evidence of the energy getting transfer from Tb³⁺ to Eu³⁺ ions.

Fig. 12

Typical PL excitation spectra for YAG:DyEuTb_(1−x) (x = 0.01–0.04)

Fig. 13

Typical PL emission spectra for YAG:DyEuTb_(1−x)(x = 0.01–0.04)

It is observed that several overlap spectrums are found between blue, yellow, orange, and red PL emission, it indicates the energy transfer takes place from Dy³⁺ to Tb³⁺ and Tb³⁺ to Eu³⁺. The PL emission intensity increases of the Eu³⁺ (⁵D₀→⁷F₂), while the PL emission intensity diminishes of the Dy³⁺(⁴F_9/2→ ⁶H_13/2), and PL emission intensity diminishes of Tb³⁺( ⁵D₄ → ⁷F₅) are shown in Figs. 8, 9 and 10, & 11. These result show that the spectra get clear tuned from blue/yellow to orange/red emission for white light emission.

4.7 PL decay analysis

Figures 14 and 15 displays the decay curves were recorded of various samples such as YAG:Dy, YAG:Eu, YAG:Tb, YAG:Dy, Eu, and YAG:DyEuTb_1−x (x = 0.01, 0.02, 0.03, and 0.04). PL spectra were measured under the monitoring of an excitation and an emission wavelength and those same wavelengths are used for measurement of PL decay. For the determination of fitting parameters double-exponential decay function is used according to Eq. (7) [57, 68]. Where, I(t) is the PL intensity t is the time, A₁ & A₂ is the fitting constants, τ₁ is the short life time decay, τ₂ is the long life time decay. The average lifetime τ_av has been calculated by Eq. (8) and average life time is listed in Table 7 [68].

Fig. 14

Typical photoluminescence decay result for YAG:Dy, YAG:Eu, YAG:Tb, and YAG:Dy,Eu samples

Fig. 15

Typical photoluminescence decay result for YAG:DyEuTb_(1−x) (x = 0.01, 0.02, 0.03, and 0.04 mol)

Table 7 PL Main intensity for one of the peaks, average life time decay, logarithmic values of average life time and energy transfer efficiency of the synthesized phosphor samples

Based on the above Eq. (7), the luminescence lifetimes of samples have determined of all samples, short- and long-life time decay time and fitting constant are listed in Table 1S. YAG based phosphors activated with rare earth metals gives efficient life time decay with respect to concentration of Eu and Tb. The decay curve exhibits are enhanced life time decay. The YAG:DyEuTb_0.02 sample has exhibited more take persistence time with respect to other samples due to after increasing concentration of Tb and PL intensity were decreased as shown in Table 1S.

$$I\left(t\right)= {A}_{1}exp\left(-\frac{t}{{\tau }_{1}}\right)+{A}_{2}exp\left(-\frac{t}{{\tau }_{2}}\right)$$

(7)

$${\tau }_{av}= \frac{{A}_{1}{\tau }_{1 +}^{2}{A}_{2}{\tau }_{2}^{2}}{{A}_{1}{\tau }_{1}+{A}_{2}{\tau }_{2}}$$

(8)

4.8 Critical distance

we have calculated relative distance or critical distance by the Blasse’s Eq. (9) [4, 39, 57] for YAG:DyEuTb_0.02 sample.

$${R}_{0}=2{\left[\frac{3 V}{4\pi {X}_{C}N}\right]}^{1/3}$$

(9)

Where V = 17450.25 volume of unit cell, Xc = 0.12, critical concentration of Eu, Dy, and Tb, N = 8, is the number cation per unit YAG cell. The R_o was Calculated to be about 15.1397 Å. Thus, calculated value of R_o is greater than 5Å. Therefore, it indicates that possibility of energy transfer by exchange interaction mechanism between Dy³⁺, Eu³⁺, and Tb³⁺ ions.

4.9 Energy transfer efficiency

The energy transfer efficiencies are calculated as a function of Eu³⁺/Tb³⁺ sensitizer concentration under 356 excitations. In this work the energy transfer efficiency (η) is calculated using by two methods Eqs. (10) & (11) [4, 69,70,71]. First, by using PL intensity of YAG based activator (Dy), and PL intensity of YAG based activator with sensitizer (Eu and Tb), and second by using average life time decay for YAG based activator and activator with sensitizer. The PL emission intensity of samples and calculated energy transfer efficiency are tabulated in Table 7.

$${\eta }_{ET} = \left[1-\frac{{I}_{Dy}}{{I}_{Dy}+{I}_{Eu}+{I}_{Tb}}\right]$$

(10)

Where, ${{I}_{Dy} = {I}_{S0}, {I}_{S}= I}_{Dy}+{I}_{Eu}+{I}_{Tb}$, and I_S0 is the PL intensity of YAG:Dy_0.05 (absence of a sensitizer), I_s the PL intensity of YAG:DyEuTb_1−x [the presence of a sensitizer (Eu³⁺ and Tb³⁺)], where x is varied from 1 to 4 mol%.

Again, we have calculated the energy transfer efficiency by using average decay time for activator and activator with sensitizer using Eq. (11), where τ_s and τ_s0 are the photo-luminescence average lifetime decay of the activator (Dy³⁺) and activator with sensitizer ions (Dy³⁺/Eu³⁺/Tb³⁺), respectively.

$${\eta }_{ET} = \left[1-\frac{{\tau }_{Dy}}{{\tau }_{Dy}+{\tau }_{Eu}+{\tau }_{Tb}}\right]$$

(11)

Where ${\tau }_{Dy} ={\tau }_{S0,} , {\tau }_{S}= {\tau }_{Dy}+{\tau }_{Eu}+{\tau }_{Tb}$, τ_Dy is the average decay time for YAG:Dy and τ_s is the average time decay for YAG:DyEuTb(1−x). From Fig. 15 and the calculated result is tabulated in Table 7. The energy transfer efficiency for YAG:Dy³⁺-Eu³⁺-Tb³⁺ can reach maximum around ~ 51% and its depends on concentration of sensitizer. This calculated efficiency of energy transfer is estimated by non-radiative transition. We have observed the energy transfer efficiencies by both methods approximately are found same result, that calculated result indicates energy transfer from Dy³⁺ to Tb³⁺ and Tb³⁺ to Eu³⁺.

4.10 Energy transfer mechanism

In this section we are studied Energy transfer mechanism only for YAG:DyEuTb_1−x (x = 0.01, 0.02, 0.03, and 0.04) sample, previously several researchers have been reported for energy transfer mechanism for various rare earth ions and pare of rare earth ions [4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25]. The energy level of Eu³⁺ is a little lower than that of Tb³⁺, therefore, energy transfer through due to nonradiative processes occurs from Tb³⁺ ions to Eu³⁺. On the other hand, the energy level of Dy³⁺ ions higher then Tb³⁺ and Eu³⁺ ions therefore, when the 4f higher energy level of Dy³⁺ is excited then energy transfer to the ⁵D₄ level of Tb³⁺ due to resonance between the two energy levels. The phonon-assisted energy transfer enhances the population of ⁵D₄ levels of Tb³⁺. Thus, the energy transfers from firstly excited ions Dy³⁺ to Tb³⁺ and then Tb³⁺ to Eu³⁺, the schematic energy level diagrams of Dy³⁺, Tb³⁺, and Eu³⁺ ions for obtaining white light emission are shown in Fig. 16.

Fig. 16

Typical Photoluminescence decay result for YAG:Dy, YAG:Eu, YAG:Tb, and YAG:Dy,Eu samples

The PL emission spectra of YAG:DyEuTb_0.02 sample were monitored under the excitation of 356 nm. it is observed that Dy³⁺ emissions partly overlap with the excitation wavelengths of Tb³⁺ and Eu³⁺. After the superpositions of each excitation and emission wavelength of each ions (Dy³⁺, Eu³⁺, and Tb³⁺) reveals that complete PL spectra of the sample. During emission of PL spectra, some transition occurs of the Dy³⁺ ions at ~ 482 nm (⁴F_9/2→ ⁶H_15/2), ~ 577 nm (⁴F_9/2→ ⁶H_13/2), and ~ 612 nm (⁴F_9/2→ ⁶H_11/2). But around 50% energy shift towards the Tb³⁺ and Eu³⁺ ions due to non-radiative process. The Tb³⁺ gives transition due to ⁵D₄→⁷F₅ (~ 547 nm) while, Eu³⁺ gives transition due to ⁵D₀→⁷F₁ (594 nm) and ⁵D₀→⁷F₂ (612 nm). After the combining all weak and strong transition for emission gives one advance complete white spectra of the proposed sample for WLEDs application.

Types of the interaction energy transfer mechanism according to Inohuti-Hirayama (I–H model) and Reisfeld approximation is shown in Eq. (12), [39, 71] where, I_S0 is the PL intensity of YAG:Dy (absence of a sensitizer), I_s the PL intensity of YAG:DyEuTb_{1−x ,} (x = 1–4 mol%), and C is the total concentration of doping elements. The I_so/I_s versus C^n/3 plots are shown in Fig. 17a, b, & c and values are listed in Table 8. The relation is observed when using n = 6, 8, and 10. The highest R² values of the linear fittings occurs when n = 6, corresponding to their best linear fitting.

Fig. 17

Typical Photoluminescence decay result for YAG:DyEuTb_(1−x) (x = 0.01, 0.02, 0.03, and 0.04 mol)

Table 8 PL intensity’s, relative PL intensity’s, PL intensity with respect to concentrations, concentration of activators and sensitizer with activators, logarithmic values concentration of activator and sensitizers of phosphors samples

Again, I–H model and Reisfeld approximation is shown in Eq. (13) [72, 73], where, C is the total concentration of doping elements, τ_so average life time decay for YAG:Dy, and τ_S is the average life time decay of YAG:DyEuTb_1−x. The τ_so/τ_s versus C^n/3 plots are shown in Fig. 17d, e, & f and measured values are listed in Table 8. The highest R² values of the linear fittings occur when n = 6, corresponding to their best linear fitting. Here, both approximations have given same result, therefore, quenching phenomenon and energy transformation process occurs due to dipole-dipole interaction mechanism.

$$\frac{{I}_{S0}}{{I}_{S}}\propto {c}^{n/3}$$

(12)

$$\frac{{\tau }_{S0}}{{\tau }_{S}}\propto {c}^{n/3}$$

(13)

Another theory of energy transfer process of intermolecular exchange (Dexter’s energy-transfer and Reisfeld’s approximation) expressions of multipolar interaction is calculated by Eq. (14) [74]. Where, n = 6 (dipole–dipole interactions), n = 8 (dipole–quadrupole interactions), and n = 10 (quadrupole–quadrupole interactions), C is the doping concentration of the Dy, Eu, and Tb ions. The ln(I/C) versus lnC plots are further illustrated in Fig. 18. where, lnx = lnK–lnβ, k and β are constants, I is the PL intensity of YAG:DyEuTb_1−x (x = 0.01, 0.02, 0.03, and 0.04).

$$I/C = K{\left[{\beta C}^{Q/3}\right]}^{-1}$$

$$\text{ln}\left(\frac{I}{C}\right) = \text{ln}x - \left(Q/3\right)\text{ln}C$$

(14)

Fig. 18

Typical energy transfer mechanism for YAG:DyEuTb_1−x (x = 0.01, 0.02, 0.03, and 0.04 mol) phosphor

Linear equation of one dimension is Y = mx + c, where, M is the slope, by the graph we have observed M = − 1.69248, therefore, after comparison, − Q/3 = − 1.69248. The value of Q = 5.0784 is close to the 6, therefore, in the present case the main cause of concentration quenching is due to dipole- dipole interaction. The energy transfer parameter (intermolecular exchange interaction) is calculated by Eq. (15) [70].

$$Q=\frac{4\pi }{3} {\Gamma }\left[1-\frac{3}{s}\right]{C}_{A}{R}_{0}^{3/s}$$

(15)

Here, Q (Q = 2.02059) depends (energy transfer parameter) on the concentration of acceptors C_A (Eu³⁺ and Tb³⁺) and types of interaction (dominant value of s = 6), R_o is the critical transfer distance between donor acceptors ions, ${\Gamma }\text{x}$ is the gamma function. The energy transfer parameters increase with increasing the concentration of Tb concentrations.

4.11 CIE chromaticity analysis

International Commission on Illumination or CIE color co-ordinate, CCT, and CRI are most useful parameters for lighting application of the phosphors. In this work, YAG based phosphor synthesized with different doping concentration of Dy, Eu, and Tb are summarized in Table 5. The chromaticity coordinate was measured based on the corresponding photoluminescence spectrum of all the samples, and they are also represented in Fig. 19 and one of the sample YAG:DyEuTb_0.02 phosphor is represented in Fig. 6S its indicates pure white region according to NTSC.

Fig. 19

The typical CIE-1931 color chromaticity image for all synthesized phosphors

4.11.1 Color purity analysis

Color purity of the all-synthesized sample is calculated by Eq. (16).

$$Color purity = \frac{\sqrt{{\left(x-{x}_{i}\right)}^{2}+ {\left(y-{y}_{i}\right)}^{2}}}{\sqrt{{\left({x}_{d}-{x}_{i}\right)}^{2}+ {\left({y}_{d}-{y}_{i}\right)}^{2}}}\times 100\%$$

(16)

Where, x and y is the CIE co-ordinate of samples, and x_i = 0.3300 and y_i = 0.3300 co-ordinate of white light. x_d and y_d Is the dominate wavelength co-ordinate samples. Calculated color purity of the samples is listed in Table 5. As we used cooping of Eu and Tb, and enhanced the concentration of Tb ions color co-ordinate are shifted toward center of white region due to enhancing intensity of blue green and red emission peak as shown in Fig. 19. The color purity of phosphor samples (YAG:DyEuTb_1−x) is calculated very less because it emits all the RGB color simultaneously. After subtraction the color purity 95.68% is calculated for YAG:DyEuTb_0.02 sample indicating the pure white color emission.

4.11.2 Correlated color temperature (CCT) analysis

McCamy formula Eq. (17) [75] is used for the calculation of CCT value of all synthesized phosphor.

$$T = {-437n}^{3}+{3601n}^{2}-6861n+5514.31$$

(17)

$$n = \frac{x-0.332}{y-0.1858}$$

Where, x_e = 0.332, y_e = 0.1858 is the epicenter of convergence of the is iso-temperature lines in 1931 chromaticity diagram and x, y is CIE co-ordinate of samples. All the calculated CCT values is listed in Table 5 of the samples. Moreover, when we used cooping of Eu and Tb, and enhanced the concentration of Tb ions then CCT value is decreases with respect to concentration of sensitizer ions. The YAG:DyEuTb_0.02 of phosphor lie above the 5000 K is indicates that sample show cool appearance [58]. Therefore, the triple doped YAG sample is potential candidates (phosphor materials) for cool WLEDS.

4.11.3 Color rendering index (CRI) analysis

The quotatives measurement of the ability of a PL emission spectrum of synthesized phosphor reveals separately colors of different objects faithfully which is called CRI of the synthesized phosphor. It can compare with standard natural light source. If CRI of any light source is found range from 75 to 100 this value indicates an excellent quality of light source [76]. In this observation we were measured CRI from 90 to 95 of the synthesized phosphors. In this work, we have calculated the CRI value of all samples YAG:Dy, YAG:Eu, YAG:Tb, YAG:DyEu, YAG:DyEuTb_0.01, YAG:DyEuTb_0.02, YAG:DyEuTb_0.03, and YAG:DyEuTb_0.04. are listed in Table 5 by Ghosh Eq. (18) [77]. In this research work we have calculated 95.6 CRI value of YAG:DyEuTb_0.02 phosphor sample. This result indicates excellent value of CRI. It may be use in advance solid state lighting system, and displaying purpose.

$$CRI=\frac18\sum_{i=1}^8R_i$$

(18)

4.12 Quantum efficiency (Q.E.) analysis

The PL quantum efficiency (Q.E.) is a significant observation of the phosphor materials and key metric for lighting industries for the resultant of luminescent processes of LEDs, WLEDs, lamps, and displays. Ideally, this value of quantum efficiency should be 1 that indicates reduce losses of energy. The observed Q.E. is tabulated in Table 6, and it can calculate by the Eq. (19). Where, E_sam = PL intensity of excitation spectra of samples, E_{ref =} PL intensity of excitation spectra of reference, I_sam. = PL intensity of emission spectra of the samples, and. The internal (IQE) and an external quantum efficiency (EQE) were calculated by Eqs. (20) & (21) [78,79,80] for all synthesized samples. The IQE is always larger than the EQE shown in Table 8.

$$\text{Q}.\text{E}= \frac{{\text{I}}_{\text{S}\text{a}\text{m}.}}{{\text{E}}_{\text{r}\text{e}\text{f}.} -{ \text{E}}_{\text{S}\text{a}\text{m}.}}$$

(19)

$$\text{I}\text{Q}\text{E}= \frac{ {\gamma }_{ab}}{{\gamma }_{ex}-{\gamma }_{r} }$$

(20)

$$\text{E}\text{Q}\text{E}= \frac{ {\gamma }_{ab}}{{\gamma }_{ex} }$$

(21)

Where, γ_ab is the total absorbed photons, γ_ex is the total emitted photons by the excitation, and γ_r is the total reflected photons and not absorbed by the sample. QE, IQE, and EQE were calculated for all samples YAG:Dy, YAG:Eu, YAG:Tb, YAG:DyEu, YAG:DyEuTb_0.01, YAG:DyEuTb_0.02, YAG:DyEuTb_0.03, and YAG:DyEuTb_0.04. In this research work, the maximum quantum is calculated for YAG:DyEuTb_0.02 (93.7%) and YAG:Dy (94.9%). Therefore, this result indicates YAG:DyEuTb_0.02 is suitable and advanced material for making WLED lighting device.

5 Conclusion

In this research work, Dy³⁺, Eu³⁺, Tb³⁺ singly doped, Eu³⁺/Tb³⁺ co- doped, and Dy³⁺/Eu³⁺/Tb³⁺ co-doped Y₃Al₅O₁₂ phosphors were synthesized via combustion method. PXRD and Rietveld refinement plot of the synthesized compounds confirm that the synthesized phosphors present pure cubic structure. The average crystallite size (D_s = 15–60 nm) was observed for the YAG based samples. It is also observed that the average grain size (D_SEM = 50–100 nm) were about 2–3 times higher than the average crystallite size (D_hkl) obtained by the PXRD data of garnet-based phosphor materials. For the formation of advance white phosphor, we were used Y₃Al₅O₁₂ based compound activated Dy, Eu, and Tb rare earth metals. By the PL emission spectra of the Y₃Al₅O₁₂:Dy³⁺ Eu³⁺Tb³⁺ exhibits blue, green, yellow, and red sharp and intense peaks due to excitation of Dy³⁺, Eu³⁺, and Tb³⁺ ions under the 356 nm. The photoluminescence emission spectra, life time decay curve, energy transfer efficiency, and Reisfeld’s approximation reveals the energy transfer from Dy³⁺ to Tb³⁺ and Tb³⁺ to Eu³⁺ions. Finally, Dexter theory is concluded that the dipole-dipole interaction (Q = 5.0784) is responsible for this energy transfer process. The YAG:DyEuTb_0.02 has CIE co-ordinate (0.3369, 0.3312) in white region, higher luminescence color purity (95.68%), CRI (95), CCT (5287 K), and Quantum efficiency (93.7%) indicating that it has the white light emission for the WLED advanced materials for modern lighting system.