Multicolor tunable emission through energy transfer in Dy³⁺/Ho³⁺ co-doped CaTiO₃ phosphors with high thermal stability for solid state lighting applications

Abstract

The exploration of multicolor emitting phosphors with single phase is extremely important for n-UV chip excited LED/WLED’s and multicolor display devices. In this paper, Dy³⁺, Ho³⁺ singly doped and Dy³⁺/Ho³⁺ co-doped CaTiO₃ phosphor materials have been synthesized by solid state reaction method at 1473 K. The synthesized materials were characterized by XRD, FE-SEM, EDX, FTIR, PL and lifetime measurements. The PL emission spectra of Dy³⁺ doped CaTiO₃ phosphors give intense blue and yellow emissions under UV excitation, while the PL emission spectra of Ho³⁺ doped CaTiO₃ phosphor show intense green emission under UV/blue excitations. Further, to get the multicolor emission including white light, Dy³⁺ and Ho³⁺ were co-doped simultaneously in CaTiO₃ host. It is found that alongwith colored and white light emissions, it also shows energy transfer from Dy³⁺ to Ho³⁺ with 367 nm and from Ho³⁺ to Dy³⁺ under 362 nm excitations. The energy transfer efficiency is found to be 67.76% and 69.39% for CaTiO₃:4Dy³⁺/3Ho³⁺ and CaTiO₃:3Ho³⁺/5Dy³⁺ phosphors, respectively. The CIE color coordinates, CCT and color purity of the phosphors have been calculated, which show color tunability from whitish to deep green via greenish yellow color. The lifetime of ⁴F_9/2 level of Dy³⁺ ion and ⁵S₂ level of Ho³⁺ ion is decreased in presence of Ho³⁺ and Dy³⁺ ions, respectively. This is due to energy transfer from Dy³⁺ to Ho³⁺ ions and vice versa. A temperature dependent photoluminescence studied of CaTiO₃:4Dy³⁺/2Ho³⁺ phosphor show a high thermal stability (82% at 423 K of initial temperature 303 K) in the temperature range 303–483 K with activation energy 0.17 eV. The PLQY are 30%, 33% and 35% for CaTiO₃:4Dy³⁺, CaTiO₃:4Dy³⁺/2Ho³⁺ and CaTiO₃:3Ho³⁺ phosphors, respectively. Hence, Dy³⁺, Ho³⁺ singly doped and Dy³⁺/Ho³⁺ co-doped CaTiO₃ phosphor materials can be used in the field of single matrix perovskite color tunable phosphors which may be used in multicolor display devices, n-UV chip excited LED/WLED’s and photodynamic therapy for the cancer treatment.

Introduction

Lanthanide ions doped phosphor materials produce multicolor tunable emissions under UV, visible or NIR excitations^1,2,3, which have various applications in multicolor display devices, plasma displays panels (PDP) and light emitting diodes (LED) for solid-state lighting devices^4,5,6. Lanthanides are also used in several other areas such as induced optical heating, optical thermometry, bio-imaging, lasers, solar cells and in photodynamic therapy to destroy cancer cells etc^7,8,9,10,11.

The color tunable emission is very interesting property of lanthanide doped/co-doped phosphors. It can be observed by several ways, viz. by varying the concentration of dopants for same excitation wavelength or fixed dopants concentration and varying the excitation wavelength or through energy transfer from sensitizer to activator^12,13,14. Chen et al. prepared Sm³⁺ doped Ca₂NaZn₂V₃O₁₂ phosphor by solid state reaction method and found the color tunabillity by tuning the Sm³⁺ ion concentration under 365 nm excitation¹⁵. Lohia et al. reported Eu³⁺ doped BaZnO₂ phosphor and studied their photoluminescence emission under different excitation wavelengths (275, 370, 395 & 467 nm) in which the color was found to tune from whitish to red via orange one¹⁶. The multicolor tunable emission has been seen by many researchers via energy transfer from sensitizer to activator^17,18. Förster’s and Dexter’s theories help to understand the energy transfer process from sensitizer (donor) to activator (acceptor) ions in organic and inorganic materials^19,20. Qu et al. studied the color tunability in Tb³⁺ and Eu³⁺ co-doped Ca₂YZr₂Al₃O₁₂ phosphor via energy transfer from Tb³⁺ to Eu³⁺ ion²⁰. They found that the color changes from green to yellow and then red by varying the concentration of Eu³⁺ ion. Li et al. have reported the color tunability from greenish yellow to red in KBaY(MoO₄)₃:Dy³⁺,Eu³⁺ phosphor through energy transfer from Dy³⁺ to Eu³⁺ ion²¹. Dwivedi et al. observed the multicolor tunable emission through energy transfer in Ho³⁺, Eu³⁺ co-doped Ca_0.05Y_1.93-xO₂ nanophosphors⁴. Recently, Rai et al. have also observed color tunability from green to red via orange in LaVO₄:Tb³⁺:Eu³⁺ phosphors on varying the concentration of Eu³⁺ ion²².

Among the lanthanide ions, Dy³⁺ and Ho³⁺ ions emit effective blue, yellow and green emissions, respectively in almost all hosts^3,6. The emission from Dy³⁺ and Ho³⁺ ions individually under UV and blue excitations have been studied by several researchers in different hosts^{23,24,25,26,27}. The blue emission of Dy³⁺ and green emission of Ho³⁺ may be used in photodynamic therapy for the cancer treatment^28,29,30. The Dy³⁺ ions have been also used as an excellent sensitizer in different hosts^31,32. Fu et al. successfully prepared the K₃YB₆O₁₂:Dy³⁺, Eu³⁺ phosphor by solid state reaction method and have reported Dy³⁺ ion to behave as a sensitizer and transfer a part of its excitation energy to the Eu³⁺ ions³³. Zhang et al. synthesized rare earth and transition metal combination (Ho³⁺/Bi³⁺) co-doped LaNbTiO₆ phosphor via a facile sol–gel as well as combustion methods³⁴. They found an energy transfer from Bi³⁺ to Ho³⁺ ions and blue to green color tunability. Recently, Guan et al. reported the Dy³⁺, Ho³⁺ co-doped NaGdF₄ nanophosphor and studied energy transfer from Dy³⁺ to Ho³⁺ ion under 360 nm excitations³⁵. They found color tunable emission in narrow region from blue to weak green with color coordinates to vary from (0.26, 0.33) to (0.27, 0.36). Therefore, we need to broad the color tunable region, which lie from whitish to deep green via greenish yellow. For this we choose Dy³⁺/Ho³⁺ co-doped CaTiO₃ and studied the energy transfer, color tunability and temperature dependent photoluminescence in detail, which has not been studied till now to the best of our knowledge.

In the present work, we have prepared Dy³⁺, Ho³⁺ singly doped and Dy³⁺/Ho³⁺ co-doped CaTiO₃ phosphors by solid state reaction method at 1473 K. The structural and morphological properties were studies by XRD, FE-SEM and EDX measurements. The vibrational bands due to different groups present in phosphor were analyzed by FTIR measurements. It is found that whereas Ho³⁺ emits intense green emission and weak red, Dy³⁺ gives intense blue and yellow emissions on UV excitation. We optimized the concentrations of Dy³⁺ and Ho³⁺ ions separately in this host to get maximum photoluminescence intensities. All the colors are emitted in Dy³⁺/Ho³⁺ co-doped CaTiO₃ on UV excitations, which are color tunable with concentrations. It is also found that an energy transfer takes place from Dy³⁺ to Ho³⁺ on excitation with 367 nm in CaTiO₃:4Dy³⁺/yHo³⁺ phosphors and from Ho³⁺ to Dy³⁺ on excitation with 362 nm in CaTiO₃:3Ho³⁺/xDy³⁺ phosphors. The energy transfer efficiency and interaction involved have been studied in the two cases. The CIE coordinates calculations show that this phosphor emits white light and tunable greenish yellow to deep green color light with the variation in the concentration of Dy³⁺ and Ho³⁺ ions. The lifetime of ⁴F_9/2 level of Dy³⁺ and ⁵S₂ level of Ho³⁺ ions have been also measured in the absence and presence of Ho³⁺ and Dy³⁺ ions, respectively. The temperature dependent photoluminescence studies have been carried out to check the thermal stability behavior of the CaTiO₃:4Dy³⁺/2Ho³⁺ phosphor. Thus, Dy³⁺, Ho³⁺ singly doped and Dy³⁺/Ho³⁺ co-doped CaTiO₃ phosphor materials may be useful in achieving multicolor display devices, n-UV chip excited LED/WLED’s and in photodynamic therapy for the cancer treatments.

Results and discussion

Structural and morphological characterization

XRD measurements

The X-ray diffraction patterns of the pure CaTiO₃, CaTiO₃:4Dy³⁺, CaTiO₃:3Ho³⁺ and CaTiO₃:4Dy³⁺/2Ho³⁺ phosphor samples were monitored in the 2θ range of 20-80º and it is shown in Fig. 1a–d along with its JCPDS file no—220153. The CaTiO₃ host has orthorhombic phase and its space group is Pnma(62). All the doped phosphors show sharp diffraction peaks which are confirm highly crystalline nature of the phosphors. There is no additional diffraction peaks found by doping of 4Dy³⁺, 3Ho³⁺ and 4Dy³⁺/2Ho³⁺ in pure CaTiO₃. Therefore, addition of 4Dy³⁺, 3Ho³⁺ and 4Dy³⁺/2Ho³⁺ ions in CaTiO₃ donot affects the phase of the CaTiO₃. Though doping of these ions at Ca²⁺ site, shift the diffraction peaks slightly towards higher 2θ angle side. This can be understood on the basis of their ionic radii. The ionic radii of Dy³⁺, Ho³⁺ and Ca²⁺ ions are 0.091, 0.090 and 0.100 nm, respectively. Therefore, on doping of Dy³⁺, Ho³⁺ ions at Ca²⁺ sites, crystal cell shrinks slightly due to which XRD peaks are shifted towards higher 2θ angle side. The effect of these shift for (121) peak is shown in Fig. 1e.

Figure 1

XRD patterns of (a) CaTiO₃, (b) CaTiO₃:4Dy³⁺, (c) CaTiO₃:3Ho³⁺ and (d) CaTiO₃: 4Dy³⁺/2Ho³⁺ phosphors (e) Zoomed XRD patterns in 32.4–33.8º range.

The value of average crystallite size (D) of pure CaTiO₃, CaTiO₃:4Dy³⁺, CaTiO₃:3Ho³⁺ and CaTiO₃:4Dy³⁺/2Ho³⁺ phosphors were calculated with the help of Debye–Scherrer (D–S) equation³⁶:

$$\mathrm{D }= \frac{\mathrm{k \lambda }}{\mathrm{\beta cos\theta }},$$

(1)

where D is the crystallite size, k is the shape factor (0.89), λ is the X-ray wavelength, β is the full width at half maximum (FWHM) and θ is the diffraction angle. The average crystallite size values were found to be 30.0, 31.42, 32.12 and 31.70 nm for the pure CaTiO₃, CaTiO₃:4Dy³⁺, CaTiO₃:3Ho³⁺ and CaTiO₃:4Dy³⁺/2Ho³⁺ phosphors, respectively.

The micro-strain (e) has been also calculated for pure CaTiO₃, CaTiO₃:4Dy³⁺, CaTiO₃:3Ho³⁺ and CaTiO₃:4Dy³⁺/2Ho³⁺ phosphors by using the relation:

$$\mathrm{e }= \frac{\upbeta }{4\mathrm{ tan\theta }},$$

(2)

where the terms have their usual meaning. The micro-strain values for pure CaTiO₃, CaTiO₃:4Dy³⁺, CaTiO₃:3Ho³⁺ and CaTiO₃:4Dy³⁺/2Ho³⁺ phosphors were found to be 6.1 $\times$ 10^–5, 5.6 $\times$ 10^–5, 5.3 $\times$ 10^–5 and 5.9 $\times$ 10^–5, respectively.

FE-SEM and EDX measurements

The surface morphology of CaTiO₃:4Dy³⁺, CaTiO₃:3Ho³⁺ and CaTiO₃:4Dy³⁺/2Ho³⁺ phosphors are given in Fig. 2a–c, respectively. From the figure it is clear that particles are nearly spherical in shape and some particles are agglomerated with each other.

Figure 2

FE-SEM images of (a) CaTiO₃:4Dy³⁺, (b) CaTiO₃:3Ho³⁺, (c) CaTiO₃:4Dy³⁺/2Ho³⁺ and histogram plots for particles size distribution of (d) CaTiO₃:4Dy³⁺, (e) CaTiO₃:3Ho³⁺ and (f) CaTiO₃: 4Dy³⁺/2Ho³⁺ phosphors.

The average particles size of the CaTiO₃:4Dy³⁺, CaTiO₃:3Ho³⁺ and CaTiO₃:4Dy³⁺/2Ho³⁺ phosphors were calculated by histogram plots using image j software [see Fig. 2d–f)] and the values obtained are 0.35, 0.41 and 0.39 µm, respectively. Chauhan et al. have calculated the average particles size by SEM images in which some particles are agglomerated. They have used image j software and the histogram plot method for calculate average particles size³⁷. Our group have also used image j software and histogram plot method for calculate average particles size from the SEM images³⁸.

The elements present in the prepared phosphors were verified by energy dispersive X-ray spectroscopic (EDX) measurements. Figure 3a–c shows the EDX patterns of CaTiO₃:4Dy³⁺, CaTiO₃:3Ho³⁺ and CaTiO₃:4Dy³⁺/2Ho³⁺ phosphors, respectively. The EDX spectra show the presence of Ca, Ti, O, Dy and Ho elements in the phosphors. The Dy and Ho peaks are very weak appear in the EDX spectra due to its low concentrations. Several researchers also reported the EDX spectra in which the concentration of do-pants are low and due to this its peak appear very weak in EDX spectra^38,39,40.

Figure 3

EDX spectra of (a) CaTiO₃:4Dy³⁺, (b) CaTiO₃:3Ho³⁺ and (c) CaTiO₃: 4Dy³⁺/2Ho³⁺ phosphor samples.

Optical characterization

Fourier transform infrared spectra (FTIR)

The FTIR spectra of the samples were used to categorize the vibrational bands due to different molecular groups present in the phosphor samples. The FTIR spectra of the different samples in the spectral range 400–3000 cm⁻¹ are shown in the Fig. 4. The vibrational bands are observed at 433 and 541 cm⁻¹, which corresponds to Ca-O and Ti–O groups^2,3.

Figure 4

FTIR spectra of CaTiO₃:4Dy³⁺, CaTiO₃:3Ho³⁺ and CaTiO₃:4Dy³⁺/2Ho³⁺ phosphor samples.

Photoluminescence excitation and emission spectra of CaTiO₃:xDy³⁺ phosphors

The PL excitation spectra of CaTiO₃:xDy³⁺ (where x = 3.0, 4.0 & 5.0 mol %) phosphor samples in the spectral region 300–475 nm with λ_em = 573 nm are shown in Fig. 5a. The PL excitation spectra show a number of peaks due to different transitions of Dy³⁺. The peaks of Dy³⁺ ions situated at 352, 367, 388, 430 and 460 nm, which could be assigned to arise due to ⁶H_15/2 → ⁶P_7/2, ⁶H_15/2 → ⁶P_5/2, ⁶H_15/2 → ⁴I_13/2, ⁶H_15/2 → ⁴G_11/2 and ⁶H_15/2 → ⁴I_15/2 transitions, respectively. The peaks at 352, 367 and 388 nm are intense compared to other peaks^1,41,42,43. All these peaks wavelength lie in the emission range of n-UV LED chip. These peaks are therefore useful for LEDs applications.

Figure 5

(a) Photoluminescence excitation (PLE) spectra of the xDy³⁺ doped CaTiO₃ phosphors by monitoring at λ_em = 573 nm. PL emission spectra of the xDy³⁺ doped CaTiO₃ phosphors with λ_ex = (b) 352 and (c) 367 nm.

The PL emission spectra of CaTiO₃:xDy³⁺ phosphors on excitation with 352 and 367 nm wavelengths in the spectral region 450–625 nm are shown in Fig. 5b,c, respectively. The emission spectra contain two intense peaks at 480 and 573 nm due to ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 transitions, respectively^43,44,45. The 573 nm peak is more intense than 480 nm peak. The intensity of emission is highest for 4 mol % concentration of Dy³⁺ ion (see inset in Fig. 5b). The intensity of the peaks decreases for higher concentrations due to concentration quenching. The emission intensity is larger for 352 nm excitation.

Photoluminescence excitation and emission spectra of CaTiO₃:yHo³⁺ phosphors

Figure 6a shows the photoluminescence excitation (PLE) spectra of the yHo³⁺ (where y = 1.0, 3.0 & 5.0 mol %) doped CaTiO₃ phosphors in the spectral region of 300–500 nm with λ_em fixed at 547 nm (corresponding to the ⁵S₂ → ⁵I₈ transition of Ho³⁺ ion). The spectra contain intense excitation peaks at 362, 419, 452 and 486 nm and they are assigned due to ⁵I₈ → ³H_5, ⁵I₈ → ⁵G₅, ⁵I₈ → ⁵G₆ and ⁵I₈ → ³F₃ transitions of Ho³⁺ ions, respectively^3,46. The excitation peak at 452 nm due to ⁵I₈ → ⁵G₆ transition appears with maximum intensity. The intensity of excitation peaks is optimum for 3 mol % of Ho³⁺ ions. Figure 6b–d show the PL emission spectra of the yHo³⁺ doped CaTiO₃ phosphors on excitations with 362, 419 and 452 nm in the spectral region 500–675 and 500–800 nm, respectively. The PL emission spectra contain an intense green alongwith weak red and very weak NIR bands at 539/547, 652 and 756 nm corresponding to ⁵F₄/⁵S₂ → ⁵I₈, ⁵F₅ → ⁵I₈ and ⁵S₂ → ⁵I₇ transitions of Ho³⁺ ions, respectively^3,46,47. The intensity of green emission is dominated over the red and NIR emissions. The photoluminescence emission intensity is maximum at 3 mol % of Ho³⁺ ions concentration. For higher doping concentrations the emission intensity decrease due to concentration quenching effect (see inset of Fig. 6b). The PL emission intensity is largest for 452 nm excitation.

Figure 6

(a) Photoluminescence excitation (PLE) spectra of the yHo³⁺ doped CaTiO₃ phosphors by monitoring λ_em = 547 nm. PL emission spectra of the yHo³⁺ doped CaTiO₃ phosphors with (b) λ_ex = 362 nm, (c) λ_ex = 419 nm and (d) λ_ex = 452 nm.

Photoluminesence excitation and emission spectra of CaTiO₃:4Dy³⁺/yHo³⁺ phosphors

Photoluminescence excitation and emission spectra of CaTiO₃:4Dy³⁺ and CaTiO₃:3Ho³⁺ phosphors are shown in Fig. 7a,b, respectively. The transitions involved in the excitation and emission spectra are discussed earlier. Figure 7c shows the photoluminescence excitation and the emission spectra of CaTiO₃:4Dy³⁺/2Ho³⁺ phosphor at λ_em = 547, 573 nm and λ_ex = 367 nm, respectively. The excitation peaks of Dy³⁺ and Ho³⁺ observed in mixed case (CaTiO₃:4Dy³⁺/2Ho³⁺ phosphor) are exactly reproduced as in their individual cases. The 367 nm excitation in the mixed case gives blue and yellow as well as green emissions. This wavelength matches well with the level of Dy³⁺. The appearance of emission from Ho³⁺ ion under 367 nm is partially due to excitation of Ho³⁺ ion (because Ho³⁺ ion also weakly excited by 367 nm) and the rest due to the energy transfer from Dy³⁺ to Ho³⁺ ions. A similar type spectra are also seen on excitation with 362 nm (a wavelength which match exactly with Ho³⁺ level) where the Dy³⁺ emission also appears with the Ho³⁺ emission [see Fig. 8b]. This shows that color emitted by the phosphor sample can be tuned by appropriate doping ratio of Dy³⁺/Ho³⁺ ions in CaTiO₃ and selecting proper excitation wavelength.

Figure 7

Photoluminescence excitation and emission spectra of (a) CaTiO₃:4Dy³⁺, (b) CaTiO₃:3Ho³⁺ and (c) CaTiO₃:4Dy³⁺/2Ho³⁺ phosphors.

Figure 8

Photoluminescence emission (PL) spectra of (a) CaTiO₃:4Dy³⁺/yHo³⁺ (where y = 0, 0.5, 1.0, 1.5, 2.0 & 3.0 mol %) phosphors with λ_ex = 367 nm and (b) CaTiO₃:3Ho³⁺/xDy³⁺ (where x = 0, 1.0, 3.0 & 5.0 mol %) phosphors with λ_ex = 362 nm.

The energy transfer from sensitizer to activator ions can occur when the excitation spectrum of the activator and emission spectrum of the sensitizer overlap on each other⁴⁸. Figure 7a,b clearly shows that the exication peak due to ⁵I₈ → ⁵F₃ transition (486 nm) of Ho³⁺ ions overlap with the emission peak due to ⁴F_9/2 → ⁶H_15/2 transition (480 nm) of Dy³⁺ ions. This clearly indicates that an energy transfer is possible from Dy³⁺ to Ho³⁺ ions.

In order to understand the effect of Ho³⁺ ion concentrations on the photoluminescence intensity of Dy³⁺, energy transfer efficiency and color tunability, we synthesized the CaTiO₃:4Dy³⁺/yHo³⁺ (where y = 0, 0.5, 1.0, 1.5, 2.0 & 3.0 mol%) phosphors, and monitored their photoluminescence emission spectra with λ_ex = 367 nm. The spectra obtained are shown in Fig. 8a. It can be seen that the PL emission intensity of Dy³⁺ ions decreased while that of Ho³⁺ ions increases with increasing the concentrations of Ho³⁺ ions and this is due to energy transfer from Dy³⁺ to Ho³⁺ ions. A reverse energy transfer (i.e. from Ho³⁺ to Dy³⁺ ions) is also possible for 362 nm excitation (due to overlapping of 362 nm band of Ho³⁺ and 367 nm band of Dy³⁺ [see Fig. 7c)]. For this we co-doped xDy³⁺ (where x = 0, 1.0, 3.0 & 5.0 mol %) in CaTiO₃:3Ho³⁺ phosphor and this is shown in Fig. 8b. The emission intensity of Ho³⁺ ions decrease with the increase of the concentration of Dy³⁺ ions. This clearly shows the energy transfer from Ho³⁺ to Dy³⁺ ions.

To understand the variation in emission intensity, the emission intensity of Ho³⁺ and Dy³⁺ ions with different concentrations of Ho³⁺ ions and fixed concentration of Dy³⁺ [i.e. 4Dy³⁺/yHo³⁺ (where y = 0, 0.5, 1.0, 1.5, 2.0 & 3.0 mol%) co-doped CaTiO₃ phosphors] are shown in Fig. 9a,b). From the figure it is obvious that the emission intensity of Ho³⁺ ions increase while that of Dy³⁺ ion decrease with increasing the concentration of Ho³⁺ ions in 4Dy³⁺/yHo³⁺ co-doped CaTiO₃ phosphors and it is maximum for 2 mol% of Ho³⁺ ions.

Figure 9

(a,b) Variation in emission intensities of Ho³⁺ and Dy³⁺ ions bands with the variation of concentrations of Ho³⁺ ion in CaTiO₃:4Dy³⁺/yHo³⁺ phosphors under 367 nm excitation.

Energy transfer efficiency from Dy³⁺ to Ho³⁺ and Ho³⁺ to Dy³⁺ ions and nature of interaction

The energy transfer efficiency from Dy³⁺ to Ho³⁺ and Ho³⁺ to Dy³⁺ ions under 367 nm and 362 nm excitations can be calculated using the equation⁴:

$${\eta }_{T}=\left(1-\frac{I}{{I}_{0}}\right)\times 100\%,$$

(3)

where I and I₀ are the emission intensities of sensitizer in the presence and absence of activator ion, respectively. Here, ${\eta }_{T}$ represents the energy transfer efficiency. Under the 367 nm excitation the energy transfer efficiency with different Ho³⁺ ion concentrations is shown in Fig. 10. The value of energy transfer efficiency is 0, 13.55, 23.66, 36.32, 44.6 and 67.76% for CaTiO₃:4Dy³⁺/yHo³⁺ (where y = 0, 0.5, 1.0, 1.5, 2.0 & 3.0 mol%) phosphors.

Figure 10

Energy transfer efficiency from Dy³⁺ to Ho³⁺ ion in CaTiO₃:4Dy³⁺/yHo³⁺ (where y = 0.0, 0.5, 1.0, 1.5, 2.0 & 3.0 mol %) phosphors.

The energy transfer efficiency has been also calculated in the case of CaTiO₃:3Ho³⁺/xDy³⁺ (where x = 0, 1.0, 3.0 & 5.0 mol%) phosphors under 362 nm excitation and the values are 0, 36.42, 59.56 and 69.39%, respectively.

Generally, the energy transfer from sensitizer (donor) to activator (acceptor) ions can be studied by the Förster’s (FRET)/Dexter’s theories in organic and inorganic materials^19,20. These studied have many pros and cons in the advanced material design for different applications. The FRET usually observed in organic materials design and use in cell biology, medicine, WLED etc^19,49,50. The energy transfer efficiency in FRET depends upon refractive index of solvent, dipole orientation and quantum yield of the donor etc¹⁹. However, Dexter’s theory usually used to understand the non-radiative energy transfer due to exchange or multipolar interaction in oxide phosphors²¹. If the value of critical distance less than 5 Å, it is exchange interaction. However, if the value of critical distance is greater than 5 Å, it is multipolar interaction. In order, to understand the energy transfer interaction from sensitizer (Dy³⁺) to activator (Ho³⁺) ions in the CaTiO₃:4Dy³⁺/yHo³⁺ phosphors, the value of critical distance needs to be evaluated. The value of critical distance between Dy³⁺-Ho³⁺ ions could be calculated by using the relation⁵¹:

$${\mathrm{R}}_{\mathrm{c}} = 2\left[\frac{3V}{4\pi \mathrm{x}N}\right]1/3,$$

(4)

where V is the volume of the unit cell, N is the number of Ca site. The critical concentration x is defined as the total concentration of Dy³⁺ and Ho³⁺ ions, when the emission intensity of Dy³⁺ with Ho³⁺ is half of that without Ho³⁺ doping. Here in, x is approximately ~ 0.063, V = 224.0342 ų and N = 4. The calculated value of R_c is found to be 11.44 Å. This value is greater than 5 Å which indicates that the energy transfers from sensitizer (Dy³⁺) to activator (Ho³⁺) ion is due to multipolar interaction. Dwivedi et al. have also studied the energy transfer from Ho³⁺ to Eu³⁺ in Ca_0.05Y_1.93−xO₂ host. They have reported the value of critical distance is 15.14 Å and have calculated the multipolar interaction by Dexter’s formula and Reisfield’s approximation⁴.

The multipolar interaction can be determined by Dexter’s formula for energy transfer and Reisfield’s approximation⁵²:

$$\frac{I}{{I}_{0}}= {\mathrm{C}}^{\mathrm{n}/3},$$

(5)

where I and I₀ are the emission intensities of Dy³⁺ peak in the absence and presence of Ho³⁺ ions. C is the critical concentrations of Dy³⁺ and Ho³⁺ ions and n = 6, 8 and 10 for dipole–dipole (d–d), dipole–quadrupole (d–q) and quadrupole–quadrupole (q–q) interactions, respectively. The plot of I/I₀ versus concentration are given in Fig. 11. A better fitting is observed for n = 10. Hence, the energy transfers from Dy³⁺ to Ho³⁺ ions is due to the quadrupole–quadrupole (q-q) interactions. This interaction in the case of energy transfer from Ho³⁺ to Dy³⁺ is found to be due to dipole–dipole.

Figure 11

Plots of I/I₀ vs C^n/3 (where n = 6, 8 & 10) for CaTiO₃:4Dy³⁺/yHo³⁺ phosphors.

Energy level diagram of Dy³⁺, Ho³⁺ ions and the mechanism of energy transfer from Dy³⁺ to Ho³⁺ ions/Ho³⁺ to Dy³⁺ ions

The excitation and emission spectra of Dy³⁺ and Ho³⁺ ions in Figs. 5 and 6 can be understood easily on the basis of energy level diagram shown in Fig. 12. The Dy³⁺ ions present in the ground state (⁶H_15/2), on excitation with 352 and 367 nm it promoted to the ⁶P_7/2 and ⁶P_5/2 excited states, respectively. The excited Dy³⁺ ions from ⁶P_7/2 and ⁶P_5/2 state populate the lowest excited ⁴F_9/2 state through several non-radiative relaxation processes. From ⁴F_9/2 state, Dy³⁺ ions give emissions at 480 (blue) and 573 nm (yellow) by ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 transitions, respectively.

Figure 12

Schematic energy level diagrams of Dy³⁺, Ho³⁺ ions and energy transfer mechanism involved from Dy³⁺ to Ho³⁺ and Ho³⁺ to Dy³⁺ ions in photoluminescence process.

Similarly, the Ho³⁺ ions in ground state (⁵I₈), on excitation with 362, 419 and 452 nm are promoted to the ³H_5, ⁵G₅ and ⁵G₆ excited states, respectively. The excited Ho³⁺ ions from these states relax to the ⁵F₄/⁵S₂ and ⁵F₅ states. The Ho³⁺ ions from ⁵F₄/⁵S₂ states give strong green emissions at 539 and 547 nm through ⁵F₄/⁵S₂ → ⁵I₈ transitions. The ions in ⁵S₂ state also give emission in NIR region at 756 nm due to ⁵S₂ → ⁵I₇ transition. The ⁵F₅ state also populated through non-radiative relaxation from ⁵F₄/⁵S₂ states gives relatively a weak red emission at 652 nm due to ⁵F₅ → ⁵I₈ transition of Ho³⁺ ion.

However, when Dy³⁺ and Ho³⁺ both ions are present in the sample simultaneously, an energy transfer takes place from Dy³⁺ to Ho³⁺ ions on 367 nm excitation due to which emission intensity of Ho³⁺ bands increased and that of Dy³⁺ decreased. The energy transfer mechanism from Dy³⁺ to Ho³⁺ ions can be understood though the energy level diagram shown in the Fig. 12. Actually when Ho³⁺ ions are co-doped in CaTiO₃:Dy³⁺ phosphor, a part of energy is transferred from ⁶P_5/2 state of Dy³⁺ to ⁵G₄ state of Ho³⁺ ions under 367 nm excitations. The Ho³⁺ ions from ⁵G₄ excited state relax non radiatively to the ⁵F₄/⁵S₂ excited states and give green emission at 539/547 nm due to ⁵F₄/⁵S₂ → ⁵I₈ transitions. Another possibility of energy transfers from Dy³⁺ to Ho³⁺ may be that the excited Dy³⁺ ions from ⁶P_5/2 state relax non radiatively to ⁴F_9/2 (lowest excited state) and transfer a part of its excitation energy from this state to the ⁵F₃ state of Ho³⁺ ion and loose another part radiatively to give radiative transitions ⁴F_9/2 → ⁶H_15/2 (blue) and ⁴F_9/2 → ⁶H_13/2 (yellow) with smaller intensity. The excited Ho³⁺ ions in ⁵F₃ state relax non-radiatively to the ⁵F₄/⁵S₂ states which finally gives green emission.

As we have mentioned earlier, on excitation with 362 nm, a reverse energy transfer (i.e. from Ho³⁺ to Dy³⁺ ions) occurs and the mechanism involved can be understood again on the basis of energy level diagram shown in Fig. 12. Ho³⁺ ions transfer a part of their excitation energy from ³H₅ state (Ho³⁺) to ⁶P_5/2 state (Dy³⁺) under 362 nm excitations in CaTiO₃:3Ho³⁺/xDy³⁺ phosphors. The Dy³⁺ ions from this state relax non-radiatively to the ⁴F_9/2 excited state and give blue and yellow emissions. Another possibility of energy transfer from Ho³⁺ to Dy³⁺ is that the excited Ho³⁺ ions from ³H₅ state relax non radiatively to ⁵F₂ state and from there Ho³⁺ ions transfer a part of their excitation energy to the ⁴F_9/2 state of Dy³⁺ ion. Thus, on excitation with 367/362 nm, we get bands due to Dy³⁺ as well as Ho³⁺ both ions via energy transfer from Dy³⁺ to Ho³⁺ ions and vice versa.

Color coordinates, CCT and color purity calculations

As we have seen that the Ho³⁺ doped CaTiO₃ emits intense green, weak red and NIR emissions. Whereas, Dy³⁺ doped CaTiO₃ sample emits blue and yellow emissions. So if both the rare earth ions are doped together in CaTiO₃ host, it gives multicolor emission on excitation with 362/367 nm wavelengths. The CIE (commission Internationale de I’Eclairage) diagram is an excellent tool to verify the multicolor emitted from the doped/co-doped phosphors². When only Dy³⁺ ions are present in the CaTiO_3, the CIE coordinates lie in whitish region on excitation with 367 nm. However, if yHo³⁺ co-doped in CaTiO₃:4Dy³⁺ phosphor, the color coordinates vary from whitish to greenish yellow region. On the other hand, CaTiO₃:3Ho³⁺ phosphor gives intense green emission. The color coordinates vary from deep green to greenish yellow in CaTiO₃:3Ho³⁺/xDy³⁺ phosphors under the 362 nm excitation. Figure 13a,b shows the CIE diagram of CaTiO₃:4Dy³⁺/yHo³⁺ and CaTiO₃:3Ho³⁺/xDy³⁺ phosphors under 367 and 362 nm excitation wavelengths, respectively.

Figure 13

CIE diagram of (a) CaTiO₃:4Dy³⁺/yHo³⁺ phosphors under 367 nm excitations and (b) CaTiO₃:3Ho³⁺/xDy³⁺ phosphors under 362 nm excitation.

It is interesting to observe that CaTiO₃:4Dy³⁺/yHo³⁺ phosphors show excellent color tunability from whitish to green through greenish yellow under 362 nm excitation. Figure 14a,b shows the PL spectra and the CIE diagram for CaTiO₃:4Dy³⁺/yHo³⁺ phosphors with λ_ex = 362 nm. The green emission intensity increased with increasing the Ho³⁺ ion concentration and the intensity of blue and yellow (Dy³⁺ ions) decreased. This occurs due to sharing of excitation energy in between Ho³⁺ and Dy³⁺ both ions. Thus for 0 mol% doping of Ho³⁺ in CaTiO₃:4Dy³⁺/yHo³⁺ phosphors, the emission color lies in whitish region (due to pure Dy³⁺) with CIE coordinates (0.33, 0.38). When the Ho³⁺ concentrations in CaTiO₃:4Dy³⁺/yHo³⁺ phosphors is increased from 0 to 3 mol %, CIE coordinates of the phosphors is changed from whitish (0.33, 0.38) to green (0.28, 0.60). This is also obvious from emission spectra in Fig. 14a.

Figure 14

(a) Photoluminescence emission spectra and (b) CIE diagram of CaTiO₃:4Dy³⁺/yHo³⁺ phosphors under 362 nm excitations.

The values of CIE coordinates for different concentrations of Ho³⁺ are given in Table 1. A similar thing is observed in CaTiO₃:3Ho³⁺/xDy³⁺ phosphors and the color coordinates are given in Table 1. The variation in CIE coordinates clearly show the possibility of achieving multicolor tunability by adjusting the Dy³⁺/Ho³⁺ doping concentrations. Hence, Ho³⁺/Dy³⁺ co-doped CaTiO₃ phosphors can be used in the field of single matrix perovskite tunable phosphor materials which are suitable for display devices, LED/WLEDs for solid state lighting applications.

Table 1 CIE coordinates and CCT values of Dy³⁺, Ho³⁺ and Dy³⁺/Ho³⁺ co-doped CaTiO₃ phosphors under 362 and 367 nm excitation wavelengths.

The correlated color temperature (CCT) is used to evaluate the nature of emitted light from the phosphors i.e. whether it is warm, natural or cool white light. The CCT values are calculated using McCammys’ equation^{2, 52}:

$${\text{T }} = \, - { 449 }\left( {{\text{n}}^{{3}} } \right) \, + { 3525 }\left( {{\text{n}}^{{2}} } \right) \, {-}{ 6823}.{3 }\left( {\text{n}} \right) \, + { 552}0.{33,}$$

(6)

where n = (x – 0.3320)/(y – 0.1858) and (x, y) refers the CIE coordinates. The calculated CCT values are given in Table 1. The CCT values lie between natural and cool white light range.

The color purity of the phosphor materials is another important parameter to recognize it as a good source of light for a particular color for solid state lighting applications. The color purity of the light source can be calculated by following relation⁵²:

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

(7)

where (x, y) are the CIE coordinates of the phosphor, (x_d, y_d) are the CIE coordinates of dominant wavelength and (x_i, y_i) are the CIE coordinates for standard white light source. The value of color purity is mentioned in the Table 1. The value of color purity varies from 13.2 to 66.3 for CaTiO₃:4Dy³⁺/yHo³⁺ phosphors under 362 nm excitations.

Lifetime measurements

Lifetime study helps to understand the fundamental issues such as dopant location, surface defect etc. in the phosphor materials⁵³. We have measured the lifetime of ⁴F_9/2 level of Dy³⁺ in the absence and presence of Ho³⁺ ions with λ_ex = 367 nm and λ_em = 480 nm and the decay curves are shown in Fig. 15a–d. We have also measured the lifetime of ⁵S₂ level of Ho³⁺ ion in the absence and presence of Dy³⁺ ion under λ_ex = 362 nm and λ_em = 547 nm and the decay curves are shown in Fig. 16a–d. It is found that all the decay curves fit well by single exponential formula⁵⁴:

$${\text{I}}\left( {\text{t}} \right) \, = {\text{ I}}_{0} {\text{exp}}\left( { - {\text{t}}/\tau } \right),$$

(8)

where I₀ and I(t) are the PL emission intensities at the time zero and at t seconds, respectively and ‘τ’ is the lifetime. It is interesting to note that even in co-doped cases the decay curves fit well with single exponential. If the decay curve needs multiexponential fitting, then probably there is defect also involved which must affect the lifetime of activator⁵⁵. However, in our case even after the co-doping the decay curve fits well by single exponential. Therefore, we have concluded that there is no defect present in the materials. The values of lifetime are found to be 209, 197, 173 and 165 µs for 4Dy³⁺ doped and 4Dy³⁺/1Ho³⁺, 4Dy³⁺/2Ho³⁺, 4Dy³⁺/3Ho³⁺ co-doped CaTiO₃ phosphors, respectively. However, the values of lifetime are found to be 27.11, 26.53, 25.40 and 24.60 µs for 3Ho³⁺ doped and 3Ho³⁺/1Dy³⁺, 3Ho³⁺/3Dy³⁺, 3Ho³⁺/5Dy³⁺ co-doped CaTiO₃ phosphors, respectively. Thus, the lifetime for ⁴F_9/2 level of Dy³⁺ ion decreased in presence of Ho³⁺ ions, due to energy transfer from Dy³⁺ to Ho³⁺ ions and the lifetime for ⁵S₂ level of Ho³⁺ ion also decreased in presence of Dy³⁺ ions, due to energy transfer from Ho³⁺ to Dy³⁺ ions. Dwivedi et al. studied the downshifting and upconversion in Ho³⁺/Yb³⁺ co-doped GdNbO₄ phosphor. They have also reported the lifetime of Ho³⁺ in microsecond (µs) under 449 nm excitation and 540 nm emission wavelengths⁵⁶. Mahata et al. also reported the lifetime of Ho³⁺ ions in microsecond⁵⁷.

Figure 15

Lifetime of ⁴F_9/2 level of Dy³⁺ in (a) 4Dy³⁺, (b) 4Dy³⁺/1Ho³⁺, (c) 4Dy³⁺/2Ho³⁺ and (d) 4Dy³⁺/3Ho³⁺ co-doped CaTiO₃ with λ_ex = 367 nm and λ_em = 480 nm.

Figure 16

Lifetime of ⁵S₂ level of Ho³⁺ in (a) 3Ho³⁺, (b) 3Ho³⁺/1Dy³⁺, (c) 3Ho³⁺/3Dy³⁺ and (d) 3Ho³⁺/5Dy³⁺ co-doped CaTiO₃ with λ_ex = 362 nm and λ_em = 547 nm.

Temperature dependent photoluminescence and photoluminescence quantum yield (PLQY) measurements

To know the thermal stability of the phosphor samples, we studied the temperature dependent photoluminescence emission spectra (TDPL) of CaTiO₃:4Dy³⁺/2Ho³⁺ phosphor in the temperature range 303–483 K with 367 nm excitation. The thermal stability is very important for the applications of phosphor in industrial fields. The temperature dependent normalized PL emission intensity for CaTiO₃:4Dy³⁺/2Ho³⁺ phosphor is depicted in Fig. 17a. As can be seen, the PL emission intensity decreased with the increase in temperature. This decrease in PL intensity is due to thermal quenching, which occurs due to non-radiative relaxation of phonons from higher excited states⁴. From Fig. 17a we found that the emission intensity at 423 K is 82% of 303 K for CaTiO₃:4Dy³⁺/2Ho³⁺ phosphor. Hence the loss in emission intensity is only 18% at 423 K. This clearly shows that this phosphor material is highly stable. We have compared the thermal stability of our materials with the thermal stability of other Dy³⁺ doped/co-doped materials and it is given in the Table 2. The obtained value of thermal stability in present study has been higher than the other reported values [see Table 2]. Hence, CaTiO₃:4Dy³⁺/2Ho³⁺ phosphor shows good thermal stability and can be used for LED applications.

Figure 17

(a) An intensity plot as a function of temperature and (b) ln[(I₀/I_T)-1] vs 1/kT plot for CaTiO₃:4Dy³⁺/2Ho³⁺ phosphor.

Table 2 A comparison of the thermal stability for CaTiO₃:4Dy³⁺/2Ho³⁺ phosphor with the thermal stability of other Dy³⁺ doped/co-doped phosphors.

The activation energy for thermal quenching process is calculated by Arrhenius equation^63,64:

$$\frac{{I}_{T}}{{I}_{0}}= \frac{1}{1+{Aexp}^{\left(-\frac{\Delta E}{kT}\right)}},$$

(9)

where I₀ is the initial emission intensity at 303 K, I_T is the emission intensity at temperature T K, ∆E is the activation energy, k is Boltzman constant (8.629 $\times$ 10^–5 eV/K) and A is the constant frequency factor. The value of activation energy could be calculated from ln[(I₀/I_T) − 1] versus 1/kT plot and it is shown in Fig. 17b. The slope of the fitted line gives the value of activation energy. In the present case its value is found to be 0.17 eV for CaTiO₃:4Dy³⁺/2Ho³⁺ phosphor. This clearly shows that this phosphor material is highly stable for various applications.

The photoluminescence quantum yield (PLQY) is an important parameter for the phosphor material to know its photoluminescence efficiency. The quantum yield is mathematically defined as the ratio of total number of photon emitted to the total number of photon absorbed under certain excitation^65,66.

The absolute photoluminescence quantum yield of CaTiO₃:4Dy³⁺, CaTiO₃:4Dy³⁺/2Ho³⁺ and CaTiO₃:3Ho³⁺ phosphors are monitored by using an integrating sphere with λ_ex = 367 and 452 nm, respectively. The value of PLQY is found to be 30%, 33% and 35% for CaTiO₃:4Dy³⁺, CaTiO₃:4Dy³⁺/2Ho³⁺ and CaTiO₃:3Ho³⁺ phosphors, respectively. We have compared the PLQY of our materials with PLQY of other Dy³⁺ doped/co-doped phosphors and it is given in the Table 3. The obtained value of PLQY in present study has been higher than the PLQY of other Dy³⁺ doped/co-doped phosphors [see Table 3]. Hence, CaTiO₃:4Dy³⁺, CaTiO₃:4Dy³⁺/2Ho³⁺ and CaTiO₃:3Ho³⁺ phosphors can be used for LED/WLED’s applications.

Table 3 A comparison of the PLQY values for CaTiO₃:4Dy³⁺, CaTiO₃:4Dy³⁺/2Ho³⁺ and CaTiO₃:3Ho³⁺ phosphors with the PLQY of other Dy³⁺ doped/co-doped phosphors.

Conclusions

In this study, Dy³⁺, Ho³⁺ singly doped and Dy³⁺/Ho³⁺ co-doped CaTiO₃ phosphor materials have been prepared by solid state reaction method at 1473 K. The prepared materials are characterized using structural and optical techniques. The phosphor samples show orthorhombic structure with Pnma(62) space group. The PL emission spectra of Dy³⁺ doped CaTiO₃ gives intense blue and yellow emissions, while the Ho³⁺ doped CaTiO₃ shows intense green emissions under UV LED wavelengths. A co-doping of the two ions together show an energy transfer from Dy³⁺ to Ho³⁺ as well as from Ho³⁺ to Dy³⁺ for CaTiO₃:4Dy³⁺/yHo³⁺ and CaTiO₃:3Ho³⁺/xDy³⁺ phosphors on λ_ex = 367 and 362 nm excitations, respectively. It is found that the energy transfers from Dy³⁺ to Ho³⁺ ion are due to quadruple-quadruple interaction whereas for Ho³⁺ to Dy³⁺ ion it is due to dipole–dipole. The energy transfer efficiency is found to be maximum 67.76% and 69.39% for CaTiO₃:4Dy³⁺/3Ho³⁺ and CaTiO₃:3Ho³⁺/5Dy³⁺ phosphors, respectively. The CIE color coordinates and the correlated color temperature (CCT) of the phosphors have been calculated, which show color tunability from whitish to green via greenish yellow color. The lifetime of ⁴F_9/2 level of Dy³⁺ and ⁵S₂ level of Ho³⁺ ions are decrease in presence of Ho³⁺ and Dy³⁺ ions, respectively. This is due to energy transfer from Dy³⁺ to Ho³⁺ ions and vice versa. Temperature dependent photoluminescence spectra measurement has been carried out to know the thermal stability of phosphor materials for various applications. The temperature dependent photoluminescence spectra of CaTiO₃:4Dy³⁺/2Ho³⁺ phosphor shows high thermal stability (at 423 K is 82% of initial temperature 303 K) with activation energy 0.17 eV. We have also carried out the photoluminescence quantum yield (PLQY) measurements and it value in different cases are found to be 35% for CaTiO₃:3Ho³⁺, 30% for CaTiO₃:4Dy³⁺ and 33% for CaTiO₃:4Dy³⁺/2Ho³⁺ phosphors. Hence, Dy³⁺ and Ho³⁺ singly doped and Dy³⁺/Ho³⁺ co-doped CaTiO₃ phosphor materials can be used in field of single matrix perovskite color tunable phosphors which may be suitable for multicolor display devices, near UV chip excited LED/WLED’s for the solid state lighting applications and photodynamic therapy for cancer treatment.

Experimental section

Synthesis of materials

The phosphor samples were prepared by solid state reaction method. The starting chemicals were calcium carbonate (CaCO₃, 99.9%), titanium dioxide (TiO₂, 99.9%), dysprosium oxide (Dy₂O₃, 99.9%) and holmium oxide (Ho₂O₃, 99.9%). We prepared a series of samples of xDy³⁺ (x = 3.0, 4.0 & 5.0 mol %) and of yHo³⁺ (where y = 1.0, 3.0 & 5.0 mol %) doped CaTiO₃ phosphors to get the particular concentration of Dy³⁺ and Ho³⁺ ions, respectively for optimum PL emission. At the next step mixed samples of CaTiO₃:xDy³⁺:yHo³⁺ were synthesized to study the energy transfer and color tunability. For this we fixed the concentration of Dy³⁺ at 4 mol % (the concentration at which the emission intensity of Dy³⁺ was optimum) and varied the concentration of Ho³⁺ in CaTiO₃:4Dy³⁺/yHo³⁺ (where y = 0.5, 1.0, 1.5, 2.0 & 3.0 mol %) phosphors and also by fixing the concentration of Ho³⁺ at 3 mol % and varying the concentration of Dy³⁺ in CaTiO₃:3Ho³⁺/xDy³⁺ (where x = 1.0, 3.0 & 5.0 mol %) phosphors.

The pure chemicals were weighed carefully and mixed in an agate mortar using acetone as mixing medium for one hour. The samples were heated at 1473 K for 4 h in a programmable electrical furnace in air environment. The heated samples were cooled and crushed further to get fine powder. These powder samples were used for various characterizations.

Instrumentations

The crystallization and phase detection of phosphors were made by XRD measurements using CuK_α radiation (λ = 0.15406 nm) with MiniFlex600 (Rigaku, Japan). The morphology of phosphors were studied using FE-SEM with the help of Zeiss, Evo 18 Research system. The energy dispersive X-ray spectroscopic (EDX) measurements were carried out to verify the elements present in the phosphor materials. The Fourier transform infrared (FTIR) measurements were carried out to know the vibrational groups present in phosphor samples using PerkinElmer I-Frontier system in the 400–3000 cm⁻¹ region. The photoluminescence excitation and emission spectra of the materials were monitored by using Fluorolog-3 spectrophotometer with 450W Xenon lamp as source (Horiba Jobin Yvon). We have also measured the lifetime of ⁴F_9/2 level of Dy³⁺ ion and ⁵S₂ level of Ho³⁺ ion in absence and presence of Ho³⁺ and Dy³⁺ ion, respectively using 25W pulsed xenon lamp attached with the same unit (Fluorolog-3 spectrophotometer). The absolute PLQY measurement was done with the help of Horiba PTI QuantaMaster-400 fluorescence spectrometer equipped with an integrating sphere.