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Luminescent studies of Eu doped ZnAl2O4 spinels synthesized by low-temperature combustion route
Optik - International Journal for Light and Electron Optics 204 (2020) 164173
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Original research article
Luminescent studies of Eu doped ZnAl2O4 spinels synthesized by low-temperature combustion route
T
Ruby Priyaa, Astha Negib, Shivani Singlaa, O.P. Pandeya,* a b
School of Physics and Materials Science, Thapar Institute of Engineering & Technology, Patiala, 147004, India Department of Nanoscience and Technology, Mount Carmel College, Bengaluru, 560052, India
A R T IC LE I N F O
ABS TRA CT
Keywords: Luminescence Europium Spinel Judd-Ofelt Decay curves
Eu3+- doped ZnAl2O4 phosphors were synthesized via low-temperature combustion synthesis route using urea as a fuel. The concentration of the Eu3+ was varied from 1 to 6 mol% in the host lattice. The structural, morphological, optical and luminescent properties were studied using Xray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy-dispersive Xray spectroscopy (EDS), Fourier transformed infrared spectroscopy (FT-IR), UV/visible spectroscopy (UV–vis) and photoluminescence (PL) and decay curves. The effect of the europium concentration has been studied in detail in respect of excitation spectra, emission spectra, and decay curves. The as-synthesized phosphors emit intense red emission spectra around 613 nm corresponding to the 5D0 → 7F2 transitions under excitation at different wavelengths. The luminescence mechanism and type of energy transfer were studied using Dexter and Blasse’s theory. The photometric parameters such as CIE coordinates, CCT values, and color purity were also calculated. In order to probe the behavior of the Eu3+ ions in ZnAl2O4 spinel, Judd Ofelt, and other radiative parameters have been calculated from the emission spectra. These phosphors are expected to have vibrant applications in the optical devices, white light-emitting diodes, and solidstate lighting applications.
1. Introduction Increasing demand of luminescent materials in the emerging fields of optics specifically for display devices, solid-state laser, white light-emitting diode, and plasma display panels is encouraging the scientific community to develop more efficient materials [1–5]. Rare-earth doped phosphors are seeking great attention due to its enhanced optical, electronic and chemical properties resulting from 4f transitions. Moreover, the selection of the host matrix plays a crucial role in improving luminescent behavior. Spinel structured zinc aluminate (ZnAl2O4) also known as Gahnite, from the family of metal aluminates, is one of the representative members of inorganic materials that finds several research interests as ceramic, electronic and catalytic materials [6–10]. Due to its remarkable chemical and thermal stability, photocatalytic activity, transparency, hydrophobic behavior and low acidity, Gahnite finds applications in high-temperature materials, optical coating, catalyst and dielectrics [11–18]. Attempts have been made to synthesize ZnAl2O4 phosphors via different chemical routes [7,10,19–21]. Among the various synthesis routes, combustion method is facile, economical, and less time consuming with high yield. In addition to this, it also facilitates to produce highly porous products of nano range with larger surface area [22,23]. Recently, Gahnite has been proven to be a fascinating phosphor host material to design various optoelectronic components [7].
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Corresponding author. E-mail address: [email protected] (O.P. Pandey).
https://doi.org/10.1016/j.ijleo.2020.164173 Received 13 November 2019; Received in revised form 19 December 2019; Accepted 3 January 2020 0030-4026/ © 2020 Published by Elsevier GmbH.
Optik - International Journal for Light and Electron Optics 204 (2020) 164173
R. Priya, et al.
There are many reports available in literature dealing with doping of several transition metal (TM) ions (Mn2+ and Co2+) and rareearth (RE) ions (Dy3+, Eu3+, Tb3+) in Gahnite making it excellent phosphor materials for various optical applications [6,8,24]. RE and TM ions containing zinc aluminate spinels are found to emit efficient emission spectra in the visible region. In addition, RE containing zinc aluminates are more interesting to investigate because of its exceptional stability and large emission quantum yield. Among the various rare-earth elements, Eu3+ doped phosphors exhibit sharp, intense and narrow emission spectra due to its 5D0 → 7 FJ (J = 0, 1, 2, 3 and 4) transitions under UV irradiation. Till now, many researchers have attempted to synthesize Eu dopedZnAl2O4 phosphors by different chemical routes. However, systematic studies related to the effect of Eu doping in zinc aluminate spinel on the structural, optical, luminescent and decay curve properties are not studied in detail. To the best of our knowledge, there is no report on the pure phase Eu doped ZnAl2O4 phosphors synthesized via combustion method. In the present report, an attempt has been made to synthesize a series of Eu3+ (1−6 mol%) doped ZnAl2O4 phosphors via lowtemperature solution combustion route using metal nitrates precursors and urea as fuel. The detailed study related to the effect of doping concentration on the structural, optical and luminescent properties has been carried out here. To gain deeper insight into the luminescent behavior of Eu3+ ions in ZnAl2O4 lattice, Judd-Ofelt parameters and other radiative parameters have been calculated from emission spectra. All the results have been discussed in detail. 2. Experimental details A series of Eu doped (1−6 mol%) ZnAl2O4 phosphors were synthesized via solution combustion route. In a typical synthesis procedure, stoichiometric amount of Zn(NO3)2.4H2O (Sigma Aldrich, 99.9 %), Al(NO3)3.9H2O (Sigma Aldrich) and Eu(NO3)3.6H2O (Sigma Aldrich, 99.9 %) were dissolved in 10 mL of distilled water under constant magnetic stirring. To this solution, an appropriate amount of urea (Sigma Aldrich) was added as fuel according to propellant chemistry [25]. The solution was allowed to stir for half an hour at 90 °C. Then, the mixed solution was transferred to the silica crucible and placed in a preheated programmable furnace at 550 ± 5 °C. After few minutes, the solution started boiling with the liberation of enormous gases and underwent ignition with flame. The whole combustion process took place within few minutes and white, fluffy and voluminous ash was obtained. The obtained ash was ground in agate mortar and pestle. To improve the crystallization and removal of the last traces of organic pollutants, the samples were further annealed at 800 °C for 2 h. The obtained phosphors were again ground and used for further characterizations. The sample ids were assigned to the obtained samples as ZA1, ZA2, ZA3, ZA4, ZA5 and ZA6 (for Eu doped 1–6 mol%), respectively. X-ray diffraction (XRD) studies were carried out using Panalytical X'Pert Pro XRD diffractometer using Cu-Kα radiation with an inbuilt Ni filter operating at 45 kV. The XRD data was recorded in a wide range of Bragg angles 10° ≤ θ ≤ 70° with a step size of 0.0131°. Field emission scanning electron microscopy (FE-SEM) micrographs were recorded using HITACHI-SU8010 operating at 15 kV. The elemental analysis was performed using scanning electron microscope coupled with energy dispersive spectroscopy (EDS) model JEOL (JSMIT100). The Fourier transform infrared (FT-IR) spectra were recorded in the wavenumber range 4000–400 cm−1 using Perkin Elmer-Spectrum RX-IFTIR spectrometer using KBr pellet technique. Optical absorption spectra were recorded with double beam UV–vis spectrometer using Hitachi- U3900H within the range 200–800 nm. Photoluminescent (PL) spectra and decay curves of all the powdered samples were recorded with fluorescent spectrometer (Agilent Technologies- Model Cary Eclipse) equipped with Xenon lamp. The emission and the excitation slit width were set as 2.5 nm. All the measurements were performed at room temperature. 3. Results and discussion The phase purity, crystallinity, and average crystallite size are determined from the XRD spectra of synthesized phosphors. The as-
Fig. 1. (a) XRD patterns of the Eu-doped ZnAl2O4 phosphors and (b) variation in the (220) plane of all the doped ZnAl2O4:Eu phosphors. 2
Optik - International Journal for Light and Electron Optics 204 (2020) 164173
R. Priya, et al.
Table 1 Crystallite size calculated using Scherrer and WeH method and optical band of the Eu doped ZnAl2O4 phosphors. Sample Id
Dhkl (nm) by Scherrer
Dhkl (nm) by W-H
Optical band gap (in eV)
ZA1 ZA2 ZA3 ZA4 ZA5 ZA6
12.66–13.22 12.76–13.22 12.31–13.05 10.77–11.34 20.65–25.79 16.52– 18.76
13.61 13.83 13.60 12.11 30.39 20.20
3.077 3.128 3.131 3.051 3.137 3.089
obtained XRD spectra of Eu doped (1–6 mol%) ZnAl2O4 are shown in Fig. 1 (a). The obtained XRD peaks are well-matched with the standard ICDD card number 01-071-0968 confirming the formation of pure cubic ZnAl2O4 phase with Fd 3¯ m (227) symmetry. The main XRD peaks in the diffractograms appear at 31.2°, 36.8°, 44.8°, 49.1°, 55.6°, 59.3°, 65.2°, 74.1° and 77.3° corresponding to the XRD reflections at (111), (220), (311), (400), (331), (422), (511), (440), (620) and (533) of zinc spinels, respectively. No other impurity phase peaks corresponding to initial nitrate precursors and fuel are found. The sharp and intense peaks confirm the incorporation of Eu ions in the host lattice. However, it is observed that for samples containing Eu3+ ions concentration more than 4 mol%, a weak impurity phase appears centered around 34.2°. This impurity phase corresponds to the intermediate EuAlO3 phase (ICDD card No. 24-0394). This depicts that the optimum concentration of the Eu ions which can be incorporated in the host lattice is 4 mol%. A small shift in the XRD spectra is observed with the increase in the Eu ions concentrations indicating the lattice distortion of ZnAl2O4 spinel. Fig. 1 (b) represents the variation in the peak corresponding to the (220) plane around 36.8̊. The crystallite size (Dhkl) of the synthesized phosphors is calculated using Scherrer formula and Williamson-Hall Method [26–28]. The calculated average crystallite sizes of all the samples are given in Table 1. It can be seen from the results that up to 4 mol%, small variation in the crystallite size is observed and thereafter crystallite size increases due to large size of Eu3+ ions in comparison to Zn2+ ions and Al3+ ions. To affirm the cubic structure of the as-synthesized phosphor, Rietveld refinement was performed using FullProf program. The results are refined with Pseudo Voigt function profile. Fig. 2 represents the structural refinement results of the representative sample ZA4. The results are in agreement with the observed and calculated values. The refinement confirmed the pure cubic structure with Fd 3¯ m symmetry. The refined parameters are a = b=c=8.077104 nm, α=β=γ = 90̊, and χ2 = 1.26. The quality of structural refinement data is checked by determining goodness of fit parameter (GOF) which is given as the ratio of Rwp to Rexp, where Rwp is weighted profile residual and Rexp is expected profile residual. The GOF value for sample ZA4 is found to be 1.13 which is close to unity [29]. Fig. 3 represents the FTIR spectra of the Eu doped ZnAl2O4 phosphors in the range 4000–400 cm−1. In Fig. 3, small bands observed at 3460 and 1620 cm−1, corresponding to the free OeH stretching vibrations and HeOeH bending vibrations in surface absorbed water molecules (not displayed here) [9]. Since all the metal–oxygen vibration bands appear in the 1000–400 cm−1 range, so the spectra are represented in this region only. The three prominent bands are observed around 660, 552 and 492 cm−1 peak positions. These bands are attributed to stretching vibrations in AlO6 octahedral groups. More precisely, the bands appearing at 670 and 552 cm−1, are due to Al-O stretching vibrations. The band positioned at 492 cm−1 is attributed to O-Al-O bending vibrations. The absence of any peak in the 700–850 cm−1 range confirms that the host structure consists of the AlO6 octahedral phase only [30,31]. No significant variation in the spectra has been observed with the increase in concentrations of Eu3+. Fig. 4 (a–b) represents the FESEM micrographs of ZnAl2O4 sample containing 4 mol% of Eu3+ ions at different magnifications. It can be depicted from the micrographs that the particles are having irregular spherical to faceted morphology. The average particle size is found to be 35 nm falling within the range of 20–65 nm. The inset Fig. 4 (b) represents the particle size distribution. Fig. 4 (c)
Fig. 2. Rietveld refinement of ZA4 sample. 3
Optik - International Journal for Light and Electron Optics 204 (2020) 164173
R. Priya, et al.
Fig. 3. FTIR spectra of Eu-doped ZnAl2O4 samples.
Fig. 4. (a–b) FESEM images of sample ZA4 and the inset showing the particle size distribution and (c) EDX spectra of the same sample.
represents the EDX spectra of ZA4 and the spectra represents Al, Zn, O, and Eu elements. No other element is found which represents the formation of pure phase. This is consistent with the XRD results. Fig. 5 represents the absorbance spectra of the synthesized Eu-doped zinc aluminate phosphors. The band gap of all the phosphors is calculated by the theory proposed by Tauc [32]. Fig. 6 represents the Tauc Plots for all the samples and the calculated bandgap values are summarized in Table 1. It can be observed that no significant variation in the bandgap is observed with the Eu3+ ions concentration. This can be attributed to the insignificant variation observed in the crystallite size for all the samples (Table 1) [33]. Fig. 7 represents the excitation spectra of ZA4 samples monitored under λemi=613 nm in the range 220–500 nm. The peaks between 240–270 nm represent the charge transfer band (CT). The other small peaks are observed at 393 and 465 nm corresponding to 7F0 → 5D0 and 7F0 → 5D1 transitions, respectively [7,21]. The emission spectra are observed under different excitation wavelengths at 240, 258, 266, 393 and 465 nm, respectively as shown in Fig. 8. The highest peak intensity is found for excitation wavelength at 260 nm for all the samples. Irrespective of excitation under different wavelengths, the peaks position and shape of the emission spectra are found to be same for all the samples. The peaks observed around 591, 612, 653 and 708 nm correspond to the 5D0 → 7F1, 7 F2, 7F3 and 7F4 transitions, respectively [34]. The broad peak at 475 nm is absent in the spectra which is due to the host lattice. Chen et al. [35] synthesized ZnAl2O4:Eu phosphors, but there exists broadband around 475 in the emission spectra. However, in the present 4
Optik - International Journal for Light and Electron Optics 204 (2020) 164173
R. Priya, et al.
Fig. 5. Absorption spectra of Eu-doped ZnAl2O4 samples.
Fig. 6. Tauc plots of Eu-doped ZnAl2O4 samples.
work, the peaks observed are typical characteristics of only Eu3+ ions. The ratio of the integrated intensity of the electric dipole ∫I transition to magnetic dipole transitions known as asymmetry ratio ( ed ) is an evidence of the asymmetric nature of Eu ions in the ∫ Imd
host lattice (Fig. 9). The value of the asymmetric ratio is found to be more than 1, which exhibits highly non-symmetric environment of the Eu ions in the zinc aluminate spinel. The transitions corresponding to the Eu ion substitution are generally sharp and narrow [36,37]. But in case of ZnAl2O4:Eu, the observed transitions are broad. This might be due to the multisite distribution of the Eu ions in the host lattice. There are two possibilities that Eu ions can either replace Zn2+ ions or Al3+ cationic sites. In ZnAl2O4 spinel, Zn2+ cations occupy tetrahedral sites and Al3+ ions octahedral sites. The size of the Eu ions (ionic radius = 95 pm) is close to the size of the Zn2+ ions (ionic radius = 88 pm) and much larger than the Al3+ ions (ionic radius = 54 pm), so Eu ions are likely to replace Zn2+ ions. The other possibility is that Eu ions due to their larger size prefer to occupy octahedral site and thus entering into the Al3+ ions. The splitting of the emission bands clearly shows the occupancy of Eu3+ ions at more than one site. Thus, the Eu3+ ions have replaced Zn and Al ions both in the host lattice. The doping concentration of the Eu is varied from 1 to 6 mol% in the host lattice. The maximum PL intensity is found for 4 mol% and beyond 4 mol% concentration quenching is observed. One of the possible reasons for concentration quenching is the formation of impure phases when excessive doping of Eu3+ ions is done. The other reason for the concentration quenching is the non-radiative 5
Optik - International Journal for Light and Electron Optics 204 (2020) 164173
R. Priya, et al.
Fig. 7. Excitation spectra of ZA4 sample at λexc=258 nm and the inset showing the emission spectra corresponding to λemi=613 nm.
Fig. 8. (a–e) PL emission spectra of ZnAl2O4:Eu (1–6 mol%) under different excitation wavelengths, and f) representing the emission spectra of ZA4 sample under different excitation wavelengths for comparison.
energy transfer among the Eu ions due to decrease in the distance between the ions. The decrease in the distance results in the 3V
increase in the non-radiative energy transfer. The minimum distance {Rc = 2 ⎡ 4 π X N ⎤ 1/3} known as critical distance can be found by c ⎦ ⎣ Blasse’s equation as given below [38]. For the present system, N = 8, Xc = 0.04, V = 526.947 Å3 and the value of Rc is found to be 14.65 Å. Consequently, the main cause of the non-radiative energy transfer is multipole- multipole interactions. The nature of the transitions can be found by Dexter’s theory [39]. According to this, the relation between the luminescence intensity and concentration of dopants is related by the following equation:
6
Optik - International Journal for Light and Electron Optics 204 (2020) 164173
R. Priya, et al.
Fig. 9. Variation of asymmetry ratio and PL intensity for λexc=258 nm.
I = K [1 + β x θ/3]−1 x
(1)
Here, x is the concentration of the dopant ions, I is the emission intensity. K and β are the constants for the same excitation conditions for a given host lattice. The value of θ is equal to 6, 8 and 10 for electric dipole-dipole (d-d), electric quadrupole- dipole (qd) and electric quadrupole- quadrupole (e- d) transitions, respectively. To obtain θ, graph is plotted between log(I/x) (along Y-axis) and log(x) (along X-axis) shown in Fig. 10. The graph is found to be linear and the slope of the graph is −2.22. The value of the θ is found to be 6.64 Å which is approximately close to 6. This indicates that the main cause of concentration quenching is dipole-dipole interaction. The behavior of the Eu3+ ions in the host lattice is studied by Judd-Ofelt analysis. The Judd-Ofelt and other radiative parameters are calculated from the emission spectra utilizing standard calculations which are reported in the literature [40–43]. The calculated Judd-Ofelt and other radiative parameters are tabulated in Table 2a, Table 2b. It can be observed from the table that the value of Ω2 is greater than Ω4. The parameter Ω2 is highly sensitive to the local environment of Eu ions in the host lattice and represent the asymmetric nature of these ions. The branching ratio and radiative cross-sectional area are also affected by the variation in the Eu ions and found maximum for the 5D0 → 7F2 transitions. The branching ratio for all the samples is found to be more than 70 % for the 5D0 → 7F2 transitions which make them useful for solid-state device applications. The calculated CIE (Commission Internationale de l’Eclairage) coordinates and CCT (correlated color temperature) values and color purity of the phosphors are summarized in Table 3. The emission color coordinates of the phosphors are determined using CIE calculator through standard MATLAB program {MATLAB 9.1 (version R2016B)}. The calculated values of the CIE coordinates lie in the red region which is close to the values of ideal red coordinates (0.67, 0.33) as standardized by the National Television Standard Committee (NTSC). Fig. 11 represents the CIE coordinate diagram of all the synthesized phosphors. It can be observed that with increasing Eu ion concentrations, the coordinates fall in the deep-red region. The CCT values are calculated using the McCamy empirical formula [44]. The calculated values of the CCT lie in the range from 1033 to 1052 K, which represents warm light and significantly make them suitable for solid-state devices. The quality of the red light emitted by the phosphors is investigated by the color purity of the prepared samples [45]. The color purity of the synthesized phosphors is found to be nearly 90 %. Fig. 12 illustrates the decay curves of the Eu doped ZnAl2O4 samples (1–6 mol%) from the energy levels of 5D0 of the Eu ions. The decay curves of all the samples are monitored under 258 nm excitation wavelength and 613 nm emission wavelength. In the present case, the decay curves are fitted using bi-exponential function given by:
Fig. 10. Variation of log(I/x) versus log(x). 7
Optik - International Journal for Light and Electron Optics 204 (2020) 164173
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Table 2a Spectral parameters of Eu-doped ZnAl2O4 samples. Judd- Ofelt Intensity Parameters Sample ID
Ω2 (10−20 cm2)
Ω4(10−20 cm2)
A0-1(sec−1)
A0-2(sec−1)
A0-4(sec−1)
ZA1 ZA2 ZA3 ZA4 ZA5 ZA6
3.365 3.586 3.489 3.659 3.634 3.565
0.899 0.103 0.918 0.905 0.932 0.935
50 50 50 50 50 50
175.33 186.87 181.79 190.66 189.35 185.75
22.32 25.67 22.80 22.46 23.14 23.21
Table 2b Spectral parameters of Eu-doped ZnAl2O4 samples. Sample ID
τrad (ms)
AT (Ψj) (s−1)
Branching ratio β0-2
β0-1
ZA1 ZA2 ZA3 ZA4 ZA5 ZA6
247.64 262.54 254.59 263.12 262.48 258.96
4.04 3.81 3.92 3.80 3.81 3.86
Asymmetry ratio
20.2 19.0 19.6 19.0 19.0 19.3
% % % % % %
70.8 71.2 71.4 72.5 72.1 71.7
β0-4
% % % % % %
9.0 9.8 9.0 8.5 8.8 9.0
% % % % % %
3.99 4.13 4.18 4.23 4.12 4.11
Stimulated emission cross-section (cm2) σ0-1 (10−22)
σ0-2(10 −21
σ0-4(10 22
1.34 1.45 1.38 1.41 1.42 1.40
7.42 8.15 7.91 8.47 8.19 8.05
1.46 1.95 1.72 1.65 1.66 1.71
)
−
)
Table 3 CIE, CCT and color purity of Eu doped ZnAl2O4 phosphors. Phosphor
CIE Coordinates
ZA1 ZA2 ZA3 ZA4 ZA5 ZA6
X
Y
0.6472 0.6484 0.6489 0.6494 0.6497 0.6497
0.3524 0.3512 0.3507 0.3503 0.3499 0.3500
CCT (K)
Color Purity (%)
1052 1043 1039 1035 1033 1033
89.57 89.96 90.07 90.20 90.28 90.26
Fig. 11. CIE diagram for ZnAl2O4:Eu (1–6 mol%).
t t I(t) = I0 + A1 exp ⎛− ⎞ + A2 exp ⎛− ⎞ τ τ 1 ⎝ ⎠ ⎝ 2⎠ ⎜
⎟
⎜
⎟
(2)
where, I(t) is the luminescent intensity at any time t, I0 is the intensity at infinity time, A1 and A2 are the amplitude corresponding to their decay rates, τ1 and τ2 are the fluorescence decay times contributing to the average lifetime. The average lifetime can be calculated using the following equation: 8
Optik - International Journal for Light and Electron Optics 204 (2020) 164173
R. Priya, et al.
Fig. 12. PL decay curves of Eu doped ZnAl2O4 samples.
Average lifetime (τavg) =
A1 τ12 + A2 τ22 A1 τ1 + A2 τ2
(3)
The two lifetime of the Eu ions suggests the two environments of these ions in the host lattice. The percentage of the Eu ions corresponding to the specific lifetime is calculated using following formula [46]:
N% ( τn ) =
An τn × 100% ∑n = 1, 2 An τn
(4)
The average lifetime of the doped samples and their occupancy in a particular lifetime are summarized in Table 4. The variation in lifetime with Eu ions doping concentration is shown in Fig. 13. It can be depicted from the figure that lifetime decreases with increase in Eu doping. This type of behavior is observed when there is decrease in the distance between the dopant ions which lead to increase in the non-radiative transitions [47]. As a result, the lifetime decreases with dopant ion concentration. 4. Conclusion Eu (1–6 mol%) doped ZnAl2O4 spinel were synthesized via combustion synthesis route at 550 ℃. XRD spectra confirmed the formation of pure cubic ZnAl2O4 phase and formation of intermediate EuAlO3 phase for Eu ions concentration more than 4 mol%. The FESEM micrographs confirmed the spherical to faceted morphology of the as-synthesized phosphors in the nanometer range. A small variation in the bandgap is observed with increase in concentration due to the lower content of the doping in the host matrix. The phosphors are found to emit intense red color emission spectra. The maximum PL intensity was found for 4 mol% Eu ions in the host lattice and thereafter concentration quenching was observed. The reason for the non-radiative transfer was found to be multipolemultipole transitions among the Eu ions. The CIE coordinates of the all the synthesized phosphors lie in the deep-red region with Table 4 Average lifetime and occupancy of Eu ions in specific lifetime. Sample Id
τavg
N (τ1) (%)
N (τ2 ) (%)
Z1 Z2 Z3 Z4 Z5 Z6
1.647 1.534 1.383 1.337 1.036 .999
17.5 12.0 8.2 12.6 12.8 13.1
82.5 88.0 91.8 87.4 87.2 86.9
9
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Fig. 13. Variation in lifetime with Eu ion concentration.
color purity around 90 %. The Judd-Ofelt and other radiative parameters exhibit the highly asymmetric nature of the Eu ions in the ZnAl2O4 spinel. The lifetime decreases with the increase in the Eu ion concentration suggesting an increase in the non-radiative radiations. It can be concluded from the results that these phosphors can be effectively used as red light emitter in potential optoelectronic device applications. Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment The authors highly acknowledge SAIF, PU, Chandigarh for XRD, FTIR and FESEM facilities. One of the authors (Ruby Priya) acknowledges Ms. Harmanpreet Kaur, Assistant Professor, Chandigarh University, Mohali for her continuous motivation and efforts. References [1] H. Daicho, T. Iwasaki, K. Enomoto, Y. Sasaki, Y. Maeno, Y. Shinomiya, S. Aoyagi, E. Nishibori, M. Sakata, H. Sawa, S. Matsuishi, H. Hosono, A novel phosphor for glareless white light-emitting diodes, Nat. Commun. 3 (2012) 1132–1138, https://doi.org/10.1038/ncomms2138. [2] Q. Dai, M.E. Foley, C.J. Breshike, A. Lita, F. Strouse, Ligand-Passivated Eu:Y2O3 Nanocrystals As a Phosphor for White Light Emitting Diodes, (2011), pp. 15475–15486. [3] G. Zhu, Z. Ci, Q. Wang, Y. Wen, S. Han, Y. Shi, S. Xin, Y. Wang, A new type of color tunable composite phosphor Y2SiO5:Ce/Y3Al5O12:Ce for field emission displays, J. Mater. Chem. C Mater. Opt. Electron. Devices 1 (2013) 4490–4496, https://doi.org/10.1039/c3tc30601a. [4] Y.H. Song, M.J. Lee, Y.L. Song, G.S. Han, K. Senthil, M.K. Jung, H.S. Jung, D.H. Yoon, Preparation and luminescence characteristics of single-phase rod-like BaSi2O2N2:Eu2+ phosphor with new synthetic route for white light generation, Mater. Lett. 129 (2014) 178–181, https://doi.org/10.1016/j.matlet.2014.05.012. [5] Z. Lei, D. Meng, Z. Gao, X. Zhang, Q. Yang, Effects of Eu3+ concentration and heat-treatment on photoluminescence properties of Zn1-xEuxAl2O4 phosphors, J. Mater. Sci. Mater. Electron. 27 (2016) 1840–1846, https://doi.org/10.1007/s10854-015-3962-7. [6] S.V. Motloung, K.G. Tshabalala, R.E. Kroon, T.T. Hlatshwayo, M. Mlambo, S. Mpelane, Effect of Tb3+ concentration on the structure and optical properties of triply doped ZnAl2O4:1% Ce3+,1% Eu3+,x% Tb3+ nano-phosphors synthesized via citrate sol-gel method, J. Mol. Struct. 1175 (2019) 241–252, https://doi.org/ 10.1016/j.molstruc.2018.08.002. [7] B. Cheng, S. Qu, H. Zhou, Z. Wang, Porous ZnAl2O4 spinel nanorods doped with Eu3+: synthesis and photoluminescence, Nanotechnology 17 (2006) 2982–2987, https://doi.org/10.1088/0957-4484/17/12/027. [8] S.F. Wang, F. Gu, M.K. Lü, X.F. Cheng, W.G. Zou, G.J. Zhou, S.M. Wang, Y.Y. Zhou, Synthesis and photoluminescence characteristics of Dy3+-doped ZnAl2O4 nanocrystals via a combustion process, J. Alloys. Compd. 394 (2005) 255–258, https://doi.org/10.1016/j.jallcom.2004.07.088. [9] C. Ragupathi, J. Judith Vijaya, A. Manikandan, L. John Kennedy, Phytosynthesis of nanoscale ZnAl2O4 by using sesamum (sesamum indicum L.) optical and catalytic properties, J. Nanosci. Nanotechnol. 13 (2013) 8298–8306, https://doi.org/10.1166/jnn.2013.7922. [10] S.V. Motloung, F.B. Dejene, H.C. Swart, O.M. Ntwaeaborwa, Effects of Cr3+ mol% on the structure and optical properties of the ZnAl2O4:Cr3+ nanocrystals synthesized using sol-gel process, Ceram. Int. 41 (2015) 6776–6783, https://doi.org/10.1016/j.ceramint.2015.01.124. [11] L. Chen, X. Sun, Y. Liu, K. Zhou, Y. Li, Porous ZnAl2O4 synthesized by a modified citrate technique, J. Alloys. Compd. 376 (2004) 257–261, https://doi.org/10. 1016/j.jallcom.2004.01.013. [12] N.J. Van Der Laag, M.D. Snel, P.C.M.M. Magusin, G. De With, Structural, elastic, thermophysical and dielectric properties of zinc aluminate (ZnAl2O4), J. Eur. Ceram. Soc. 24 (2004) 2417–2424, https://doi.org/10.1016/j.jeurceramsoc.2003.06.001. [13] W.S. Tzing, W.H. Tuan, The strength of duplex Al2O3- ZnAl2O4 composite, J. Mater. Sci. Lett. 15 (1996) 1395–1396, https://doi.org/10.1007/BF00275286. [14] M. García-Hipólito, J. Guzmán-Mendoza, E. Martínez, O. Alvarez-Fregoso, C. Falcony, Growth and cathodoluminescent characteristics of blue emitting ceriumdoped zinc aluminate layers synthesized by spray pyrolysis technique, Phys. Status Solidi Appl. Res. 201 (2004) 1510–1517, https://doi.org/10.1002/pssa. 200306796. [15] M.C. Marion, E. Garbowski, M. Primet, Catalytic properties of copper oxide supported on zinc aluminate in methane combustion, J. Chem. Soc. Faraday Trans. 87 (1991) 1795–1800, https://doi.org/10.1039/FT9918701795. [16] N. Guilhaume, M. Primet, Catalytic combustion of methane : copper oxide supported on high-specific-area spinels synthesized by a sol-gel process, J. Chem. Soc. Faraday Trans. 90 (1994) 1541–1545.
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Original research article
Luminescent studies of Eu doped ZnAl2O4 spinels synthesized by low-temperature combustion route
T
Ruby Priyaa, Astha Negib, Shivani Singlaa, O.P. Pandeya,* a b
School of Physics and Materials Science, Thapar Institute of Engineering & Technology, Patiala, 147004, India Department of Nanoscience and Technology, Mount Carmel College, Bengaluru, 560052, India
A R T IC LE I N F O
ABS TRA CT
Keywords: Luminescence Europium Spinel Judd-Ofelt Decay curves
Eu3+- doped ZnAl2O4 phosphors were synthesized via low-temperature combustion synthesis route using urea as a fuel. The concentration of the Eu3+ was varied from 1 to 6 mol% in the host lattice. The structural, morphological, optical and luminescent properties were studied using Xray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy-dispersive Xray spectroscopy (EDS), Fourier transformed infrared spectroscopy (FT-IR), UV/visible spectroscopy (UV–vis) and photoluminescence (PL) and decay curves. The effect of the europium concentration has been studied in detail in respect of excitation spectra, emission spectra, and decay curves. The as-synthesized phosphors emit intense red emission spectra around 613 nm corresponding to the 5D0 → 7F2 transitions under excitation at different wavelengths. The luminescence mechanism and type of energy transfer were studied using Dexter and Blasse’s theory. The photometric parameters such as CIE coordinates, CCT values, and color purity were also calculated. In order to probe the behavior of the Eu3+ ions in ZnAl2O4 spinel, Judd Ofelt, and other radiative parameters have been calculated from the emission spectra. These phosphors are expected to have vibrant applications in the optical devices, white light-emitting diodes, and solidstate lighting applications.
1. Introduction Increasing demand of luminescent materials in the emerging fields of optics specifically for display devices, solid-state laser, white light-emitting diode, and plasma display panels is encouraging the scientific community to develop more efficient materials [1–5]. Rare-earth doped phosphors are seeking great attention due to its enhanced optical, electronic and chemical properties resulting from 4f transitions. Moreover, the selection of the host matrix plays a crucial role in improving luminescent behavior. Spinel structured zinc aluminate (ZnAl2O4) also known as Gahnite, from the family of metal aluminates, is one of the representative members of inorganic materials that finds several research interests as ceramic, electronic and catalytic materials [6–10]. Due to its remarkable chemical and thermal stability, photocatalytic activity, transparency, hydrophobic behavior and low acidity, Gahnite finds applications in high-temperature materials, optical coating, catalyst and dielectrics [11–18]. Attempts have been made to synthesize ZnAl2O4 phosphors via different chemical routes [7,10,19–21]. Among the various synthesis routes, combustion method is facile, economical, and less time consuming with high yield. In addition to this, it also facilitates to produce highly porous products of nano range with larger surface area [22,23]. Recently, Gahnite has been proven to be a fascinating phosphor host material to design various optoelectronic components [7].
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Corresponding author. E-mail address: [email protected] (O.P. Pandey).
https://doi.org/10.1016/j.ijleo.2020.164173 Received 13 November 2019; Received in revised form 19 December 2019; Accepted 3 January 2020 0030-4026/ © 2020 Published by Elsevier GmbH.
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There are many reports available in literature dealing with doping of several transition metal (TM) ions (Mn2+ and Co2+) and rareearth (RE) ions (Dy3+, Eu3+, Tb3+) in Gahnite making it excellent phosphor materials for various optical applications [6,8,24]. RE and TM ions containing zinc aluminate spinels are found to emit efficient emission spectra in the visible region. In addition, RE containing zinc aluminates are more interesting to investigate because of its exceptional stability and large emission quantum yield. Among the various rare-earth elements, Eu3+ doped phosphors exhibit sharp, intense and narrow emission spectra due to its 5D0 → 7 FJ (J = 0, 1, 2, 3 and 4) transitions under UV irradiation. Till now, many researchers have attempted to synthesize Eu dopedZnAl2O4 phosphors by different chemical routes. However, systematic studies related to the effect of Eu doping in zinc aluminate spinel on the structural, optical, luminescent and decay curve properties are not studied in detail. To the best of our knowledge, there is no report on the pure phase Eu doped ZnAl2O4 phosphors synthesized via combustion method. In the present report, an attempt has been made to synthesize a series of Eu3+ (1−6 mol%) doped ZnAl2O4 phosphors via lowtemperature solution combustion route using metal nitrates precursors and urea as fuel. The detailed study related to the effect of doping concentration on the structural, optical and luminescent properties has been carried out here. To gain deeper insight into the luminescent behavior of Eu3+ ions in ZnAl2O4 lattice, Judd-Ofelt parameters and other radiative parameters have been calculated from emission spectra. All the results have been discussed in detail. 2. Experimental details A series of Eu doped (1−6 mol%) ZnAl2O4 phosphors were synthesized via solution combustion route. In a typical synthesis procedure, stoichiometric amount of Zn(NO3)2.4H2O (Sigma Aldrich, 99.9 %), Al(NO3)3.9H2O (Sigma Aldrich) and Eu(NO3)3.6H2O (Sigma Aldrich, 99.9 %) were dissolved in 10 mL of distilled water under constant magnetic stirring. To this solution, an appropriate amount of urea (Sigma Aldrich) was added as fuel according to propellant chemistry [25]. The solution was allowed to stir for half an hour at 90 °C. Then, the mixed solution was transferred to the silica crucible and placed in a preheated programmable furnace at 550 ± 5 °C. After few minutes, the solution started boiling with the liberation of enormous gases and underwent ignition with flame. The whole combustion process took place within few minutes and white, fluffy and voluminous ash was obtained. The obtained ash was ground in agate mortar and pestle. To improve the crystallization and removal of the last traces of organic pollutants, the samples were further annealed at 800 °C for 2 h. The obtained phosphors were again ground and used for further characterizations. The sample ids were assigned to the obtained samples as ZA1, ZA2, ZA3, ZA4, ZA5 and ZA6 (for Eu doped 1–6 mol%), respectively. X-ray diffraction (XRD) studies were carried out using Panalytical X'Pert Pro XRD diffractometer using Cu-Kα radiation with an inbuilt Ni filter operating at 45 kV. The XRD data was recorded in a wide range of Bragg angles 10° ≤ θ ≤ 70° with a step size of 0.0131°. Field emission scanning electron microscopy (FE-SEM) micrographs were recorded using HITACHI-SU8010 operating at 15 kV. The elemental analysis was performed using scanning electron microscope coupled with energy dispersive spectroscopy (EDS) model JEOL (JSMIT100). The Fourier transform infrared (FT-IR) spectra were recorded in the wavenumber range 4000–400 cm−1 using Perkin Elmer-Spectrum RX-IFTIR spectrometer using KBr pellet technique. Optical absorption spectra were recorded with double beam UV–vis spectrometer using Hitachi- U3900H within the range 200–800 nm. Photoluminescent (PL) spectra and decay curves of all the powdered samples were recorded with fluorescent spectrometer (Agilent Technologies- Model Cary Eclipse) equipped with Xenon lamp. The emission and the excitation slit width were set as 2.5 nm. All the measurements were performed at room temperature. 3. Results and discussion The phase purity, crystallinity, and average crystallite size are determined from the XRD spectra of synthesized phosphors. The as-
Fig. 1. (a) XRD patterns of the Eu-doped ZnAl2O4 phosphors and (b) variation in the (220) plane of all the doped ZnAl2O4:Eu phosphors. 2
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Table 1 Crystallite size calculated using Scherrer and WeH method and optical band of the Eu doped ZnAl2O4 phosphors. Sample Id
Dhkl (nm) by Scherrer
Dhkl (nm) by W-H
Optical band gap (in eV)
ZA1 ZA2 ZA3 ZA4 ZA5 ZA6
12.66–13.22 12.76–13.22 12.31–13.05 10.77–11.34 20.65–25.79 16.52– 18.76
13.61 13.83 13.60 12.11 30.39 20.20
3.077 3.128 3.131 3.051 3.137 3.089
obtained XRD spectra of Eu doped (1–6 mol%) ZnAl2O4 are shown in Fig. 1 (a). The obtained XRD peaks are well-matched with the standard ICDD card number 01-071-0968 confirming the formation of pure cubic ZnAl2O4 phase with Fd 3¯ m (227) symmetry. The main XRD peaks in the diffractograms appear at 31.2°, 36.8°, 44.8°, 49.1°, 55.6°, 59.3°, 65.2°, 74.1° and 77.3° corresponding to the XRD reflections at (111), (220), (311), (400), (331), (422), (511), (440), (620) and (533) of zinc spinels, respectively. No other impurity phase peaks corresponding to initial nitrate precursors and fuel are found. The sharp and intense peaks confirm the incorporation of Eu ions in the host lattice. However, it is observed that for samples containing Eu3+ ions concentration more than 4 mol%, a weak impurity phase appears centered around 34.2°. This impurity phase corresponds to the intermediate EuAlO3 phase (ICDD card No. 24-0394). This depicts that the optimum concentration of the Eu ions which can be incorporated in the host lattice is 4 mol%. A small shift in the XRD spectra is observed with the increase in the Eu ions concentrations indicating the lattice distortion of ZnAl2O4 spinel. Fig. 1 (b) represents the variation in the peak corresponding to the (220) plane around 36.8̊. The crystallite size (Dhkl) of the synthesized phosphors is calculated using Scherrer formula and Williamson-Hall Method [26–28]. The calculated average crystallite sizes of all the samples are given in Table 1. It can be seen from the results that up to 4 mol%, small variation in the crystallite size is observed and thereafter crystallite size increases due to large size of Eu3+ ions in comparison to Zn2+ ions and Al3+ ions. To affirm the cubic structure of the as-synthesized phosphor, Rietveld refinement was performed using FullProf program. The results are refined with Pseudo Voigt function profile. Fig. 2 represents the structural refinement results of the representative sample ZA4. The results are in agreement with the observed and calculated values. The refinement confirmed the pure cubic structure with Fd 3¯ m symmetry. The refined parameters are a = b=c=8.077104 nm, α=β=γ = 90̊, and χ2 = 1.26. The quality of structural refinement data is checked by determining goodness of fit parameter (GOF) which is given as the ratio of Rwp to Rexp, where Rwp is weighted profile residual and Rexp is expected profile residual. The GOF value for sample ZA4 is found to be 1.13 which is close to unity [29]. Fig. 3 represents the FTIR spectra of the Eu doped ZnAl2O4 phosphors in the range 4000–400 cm−1. In Fig. 3, small bands observed at 3460 and 1620 cm−1, corresponding to the free OeH stretching vibrations and HeOeH bending vibrations in surface absorbed water molecules (not displayed here) [9]. Since all the metal–oxygen vibration bands appear in the 1000–400 cm−1 range, so the spectra are represented in this region only. The three prominent bands are observed around 660, 552 and 492 cm−1 peak positions. These bands are attributed to stretching vibrations in AlO6 octahedral groups. More precisely, the bands appearing at 670 and 552 cm−1, are due to Al-O stretching vibrations. The band positioned at 492 cm−1 is attributed to O-Al-O bending vibrations. The absence of any peak in the 700–850 cm−1 range confirms that the host structure consists of the AlO6 octahedral phase only [30,31]. No significant variation in the spectra has been observed with the increase in concentrations of Eu3+. Fig. 4 (a–b) represents the FESEM micrographs of ZnAl2O4 sample containing 4 mol% of Eu3+ ions at different magnifications. It can be depicted from the micrographs that the particles are having irregular spherical to faceted morphology. The average particle size is found to be 35 nm falling within the range of 20–65 nm. The inset Fig. 4 (b) represents the particle size distribution. Fig. 4 (c)
Fig. 2. Rietveld refinement of ZA4 sample. 3
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Fig. 3. FTIR spectra of Eu-doped ZnAl2O4 samples.
Fig. 4. (a–b) FESEM images of sample ZA4 and the inset showing the particle size distribution and (c) EDX spectra of the same sample.
represents the EDX spectra of ZA4 and the spectra represents Al, Zn, O, and Eu elements. No other element is found which represents the formation of pure phase. This is consistent with the XRD results. Fig. 5 represents the absorbance spectra of the synthesized Eu-doped zinc aluminate phosphors. The band gap of all the phosphors is calculated by the theory proposed by Tauc [32]. Fig. 6 represents the Tauc Plots for all the samples and the calculated bandgap values are summarized in Table 1. It can be observed that no significant variation in the bandgap is observed with the Eu3+ ions concentration. This can be attributed to the insignificant variation observed in the crystallite size for all the samples (Table 1) [33]. Fig. 7 represents the excitation spectra of ZA4 samples monitored under λemi=613 nm in the range 220–500 nm. The peaks between 240–270 nm represent the charge transfer band (CT). The other small peaks are observed at 393 and 465 nm corresponding to 7F0 → 5D0 and 7F0 → 5D1 transitions, respectively [7,21]. The emission spectra are observed under different excitation wavelengths at 240, 258, 266, 393 and 465 nm, respectively as shown in Fig. 8. The highest peak intensity is found for excitation wavelength at 260 nm for all the samples. Irrespective of excitation under different wavelengths, the peaks position and shape of the emission spectra are found to be same for all the samples. The peaks observed around 591, 612, 653 and 708 nm correspond to the 5D0 → 7F1, 7 F2, 7F3 and 7F4 transitions, respectively [34]. The broad peak at 475 nm is absent in the spectra which is due to the host lattice. Chen et al. [35] synthesized ZnAl2O4:Eu phosphors, but there exists broadband around 475 in the emission spectra. However, in the present 4
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Fig. 5. Absorption spectra of Eu-doped ZnAl2O4 samples.
Fig. 6. Tauc plots of Eu-doped ZnAl2O4 samples.
work, the peaks observed are typical characteristics of only Eu3+ ions. The ratio of the integrated intensity of the electric dipole ∫I transition to magnetic dipole transitions known as asymmetry ratio ( ed ) is an evidence of the asymmetric nature of Eu ions in the ∫ Imd
host lattice (Fig. 9). The value of the asymmetric ratio is found to be more than 1, which exhibits highly non-symmetric environment of the Eu ions in the zinc aluminate spinel. The transitions corresponding to the Eu ion substitution are generally sharp and narrow [36,37]. But in case of ZnAl2O4:Eu, the observed transitions are broad. This might be due to the multisite distribution of the Eu ions in the host lattice. There are two possibilities that Eu ions can either replace Zn2+ ions or Al3+ cationic sites. In ZnAl2O4 spinel, Zn2+ cations occupy tetrahedral sites and Al3+ ions octahedral sites. The size of the Eu ions (ionic radius = 95 pm) is close to the size of the Zn2+ ions (ionic radius = 88 pm) and much larger than the Al3+ ions (ionic radius = 54 pm), so Eu ions are likely to replace Zn2+ ions. The other possibility is that Eu ions due to their larger size prefer to occupy octahedral site and thus entering into the Al3+ ions. The splitting of the emission bands clearly shows the occupancy of Eu3+ ions at more than one site. Thus, the Eu3+ ions have replaced Zn and Al ions both in the host lattice. The doping concentration of the Eu is varied from 1 to 6 mol% in the host lattice. The maximum PL intensity is found for 4 mol% and beyond 4 mol% concentration quenching is observed. One of the possible reasons for concentration quenching is the formation of impure phases when excessive doping of Eu3+ ions is done. The other reason for the concentration quenching is the non-radiative 5
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Fig. 7. Excitation spectra of ZA4 sample at λexc=258 nm and the inset showing the emission spectra corresponding to λemi=613 nm.
Fig. 8. (a–e) PL emission spectra of ZnAl2O4:Eu (1–6 mol%) under different excitation wavelengths, and f) representing the emission spectra of ZA4 sample under different excitation wavelengths for comparison.
energy transfer among the Eu ions due to decrease in the distance between the ions. The decrease in the distance results in the 3V
increase in the non-radiative energy transfer. The minimum distance {Rc = 2 ⎡ 4 π X N ⎤ 1/3} known as critical distance can be found by c ⎦ ⎣ Blasse’s equation as given below [38]. For the present system, N = 8, Xc = 0.04, V = 526.947 Å3 and the value of Rc is found to be 14.65 Å. Consequently, the main cause of the non-radiative energy transfer is multipole- multipole interactions. The nature of the transitions can be found by Dexter’s theory [39]. According to this, the relation between the luminescence intensity and concentration of dopants is related by the following equation:
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Fig. 9. Variation of asymmetry ratio and PL intensity for λexc=258 nm.
I = K [1 + β x θ/3]−1 x
(1)
Here, x is the concentration of the dopant ions, I is the emission intensity. K and β are the constants for the same excitation conditions for a given host lattice. The value of θ is equal to 6, 8 and 10 for electric dipole-dipole (d-d), electric quadrupole- dipole (qd) and electric quadrupole- quadrupole (e- d) transitions, respectively. To obtain θ, graph is plotted between log(I/x) (along Y-axis) and log(x) (along X-axis) shown in Fig. 10. The graph is found to be linear and the slope of the graph is −2.22. The value of the θ is found to be 6.64 Å which is approximately close to 6. This indicates that the main cause of concentration quenching is dipole-dipole interaction. The behavior of the Eu3+ ions in the host lattice is studied by Judd-Ofelt analysis. The Judd-Ofelt and other radiative parameters are calculated from the emission spectra utilizing standard calculations which are reported in the literature [40–43]. The calculated Judd-Ofelt and other radiative parameters are tabulated in Table 2a, Table 2b. It can be observed from the table that the value of Ω2 is greater than Ω4. The parameter Ω2 is highly sensitive to the local environment of Eu ions in the host lattice and represent the asymmetric nature of these ions. The branching ratio and radiative cross-sectional area are also affected by the variation in the Eu ions and found maximum for the 5D0 → 7F2 transitions. The branching ratio for all the samples is found to be more than 70 % for the 5D0 → 7F2 transitions which make them useful for solid-state device applications. The calculated CIE (Commission Internationale de l’Eclairage) coordinates and CCT (correlated color temperature) values and color purity of the phosphors are summarized in Table 3. The emission color coordinates of the phosphors are determined using CIE calculator through standard MATLAB program {MATLAB 9.1 (version R2016B)}. The calculated values of the CIE coordinates lie in the red region which is close to the values of ideal red coordinates (0.67, 0.33) as standardized by the National Television Standard Committee (NTSC). Fig. 11 represents the CIE coordinate diagram of all the synthesized phosphors. It can be observed that with increasing Eu ion concentrations, the coordinates fall in the deep-red region. The CCT values are calculated using the McCamy empirical formula [44]. The calculated values of the CCT lie in the range from 1033 to 1052 K, which represents warm light and significantly make them suitable for solid-state devices. The quality of the red light emitted by the phosphors is investigated by the color purity of the prepared samples [45]. The color purity of the synthesized phosphors is found to be nearly 90 %. Fig. 12 illustrates the decay curves of the Eu doped ZnAl2O4 samples (1–6 mol%) from the energy levels of 5D0 of the Eu ions. The decay curves of all the samples are monitored under 258 nm excitation wavelength and 613 nm emission wavelength. In the present case, the decay curves are fitted using bi-exponential function given by:
Fig. 10. Variation of log(I/x) versus log(x). 7
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Table 2a Spectral parameters of Eu-doped ZnAl2O4 samples. Judd- Ofelt Intensity Parameters Sample ID
Ω2 (10−20 cm2)
Ω4(10−20 cm2)
A0-1(sec−1)
A0-2(sec−1)
A0-4(sec−1)
ZA1 ZA2 ZA3 ZA4 ZA5 ZA6
3.365 3.586 3.489 3.659 3.634 3.565
0.899 0.103 0.918 0.905 0.932 0.935
50 50 50 50 50 50
175.33 186.87 181.79 190.66 189.35 185.75
22.32 25.67 22.80 22.46 23.14 23.21
Table 2b Spectral parameters of Eu-doped ZnAl2O4 samples. Sample ID
τrad (ms)
AT (Ψj) (s−1)
Branching ratio β0-2
β0-1
ZA1 ZA2 ZA3 ZA4 ZA5 ZA6
247.64 262.54 254.59 263.12 262.48 258.96
4.04 3.81 3.92 3.80 3.81 3.86
Asymmetry ratio
20.2 19.0 19.6 19.0 19.0 19.3
% % % % % %
70.8 71.2 71.4 72.5 72.1 71.7
β0-4
% % % % % %
9.0 9.8 9.0 8.5 8.8 9.0
% % % % % %
3.99 4.13 4.18 4.23 4.12 4.11
Stimulated emission cross-section (cm2) σ0-1 (10−22)
σ0-2(10 −21
σ0-4(10 22
1.34 1.45 1.38 1.41 1.42 1.40
7.42 8.15 7.91 8.47 8.19 8.05
1.46 1.95 1.72 1.65 1.66 1.71
)
−
)
Table 3 CIE, CCT and color purity of Eu doped ZnAl2O4 phosphors. Phosphor
CIE Coordinates
ZA1 ZA2 ZA3 ZA4 ZA5 ZA6
X
Y
0.6472 0.6484 0.6489 0.6494 0.6497 0.6497
0.3524 0.3512 0.3507 0.3503 0.3499 0.3500
CCT (K)
Color Purity (%)
1052 1043 1039 1035 1033 1033
89.57 89.96 90.07 90.20 90.28 90.26
Fig. 11. CIE diagram for ZnAl2O4:Eu (1–6 mol%).
t t I(t) = I0 + A1 exp ⎛− ⎞ + A2 exp ⎛− ⎞ τ τ 1 ⎝ ⎠ ⎝ 2⎠ ⎜
⎟
⎜
⎟
(2)
where, I(t) is the luminescent intensity at any time t, I0 is the intensity at infinity time, A1 and A2 are the amplitude corresponding to their decay rates, τ1 and τ2 are the fluorescence decay times contributing to the average lifetime. The average lifetime can be calculated using the following equation: 8
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Fig. 12. PL decay curves of Eu doped ZnAl2O4 samples.
Average lifetime (τavg) =
A1 τ12 + A2 τ22 A1 τ1 + A2 τ2
(3)
The two lifetime of the Eu ions suggests the two environments of these ions in the host lattice. The percentage of the Eu ions corresponding to the specific lifetime is calculated using following formula [46]:
N% ( τn ) =
An τn × 100% ∑n = 1, 2 An τn
(4)
The average lifetime of the doped samples and their occupancy in a particular lifetime are summarized in Table 4. The variation in lifetime with Eu ions doping concentration is shown in Fig. 13. It can be depicted from the figure that lifetime decreases with increase in Eu doping. This type of behavior is observed when there is decrease in the distance between the dopant ions which lead to increase in the non-radiative transitions [47]. As a result, the lifetime decreases with dopant ion concentration. 4. Conclusion Eu (1–6 mol%) doped ZnAl2O4 spinel were synthesized via combustion synthesis route at 550 ℃. XRD spectra confirmed the formation of pure cubic ZnAl2O4 phase and formation of intermediate EuAlO3 phase for Eu ions concentration more than 4 mol%. The FESEM micrographs confirmed the spherical to faceted morphology of the as-synthesized phosphors in the nanometer range. A small variation in the bandgap is observed with increase in concentration due to the lower content of the doping in the host matrix. The phosphors are found to emit intense red color emission spectra. The maximum PL intensity was found for 4 mol% Eu ions in the host lattice and thereafter concentration quenching was observed. The reason for the non-radiative transfer was found to be multipolemultipole transitions among the Eu ions. The CIE coordinates of the all the synthesized phosphors lie in the deep-red region with Table 4 Average lifetime and occupancy of Eu ions in specific lifetime. Sample Id
τavg
N (τ1) (%)
N (τ2 ) (%)
Z1 Z2 Z3 Z4 Z5 Z6
1.647 1.534 1.383 1.337 1.036 .999
17.5 12.0 8.2 12.6 12.8 13.1
82.5 88.0 91.8 87.4 87.2 86.9
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Fig. 13. Variation in lifetime with Eu ion concentration.
color purity around 90 %. The Judd-Ofelt and other radiative parameters exhibit the highly asymmetric nature of the Eu ions in the ZnAl2O4 spinel. The lifetime decreases with the increase in the Eu ion concentration suggesting an increase in the non-radiative radiations. It can be concluded from the results that these phosphors can be effectively used as red light emitter in potential optoelectronic device applications. Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment The authors highly acknowledge SAIF, PU, Chandigarh for XRD, FTIR and FESEM facilities. One of the authors (Ruby Priya) acknowledges Ms. Harmanpreet Kaur, Assistant Professor, Chandigarh University, Mohali for her continuous motivation and efforts. References [1] H. Daicho, T. Iwasaki, K. Enomoto, Y. Sasaki, Y. Maeno, Y. Shinomiya, S. Aoyagi, E. Nishibori, M. Sakata, H. Sawa, S. Matsuishi, H. Hosono, A novel phosphor for glareless white light-emitting diodes, Nat. Commun. 3 (2012) 1132–1138, https://doi.org/10.1038/ncomms2138. [2] Q. Dai, M.E. Foley, C.J. Breshike, A. Lita, F. Strouse, Ligand-Passivated Eu:Y2O3 Nanocrystals As a Phosphor for White Light Emitting Diodes, (2011), pp. 15475–15486. [3] G. Zhu, Z. Ci, Q. Wang, Y. Wen, S. Han, Y. Shi, S. Xin, Y. Wang, A new type of color tunable composite phosphor Y2SiO5:Ce/Y3Al5O12:Ce for field emission displays, J. Mater. Chem. C Mater. Opt. Electron. Devices 1 (2013) 4490–4496, https://doi.org/10.1039/c3tc30601a. [4] Y.H. Song, M.J. Lee, Y.L. Song, G.S. Han, K. Senthil, M.K. Jung, H.S. Jung, D.H. Yoon, Preparation and luminescence characteristics of single-phase rod-like BaSi2O2N2:Eu2+ phosphor with new synthetic route for white light generation, Mater. Lett. 129 (2014) 178–181, https://doi.org/10.1016/j.matlet.2014.05.012. [5] Z. Lei, D. Meng, Z. Gao, X. Zhang, Q. Yang, Effects of Eu3+ concentration and heat-treatment on photoluminescence properties of Zn1-xEuxAl2O4 phosphors, J. Mater. Sci. Mater. Electron. 27 (2016) 1840–1846, https://doi.org/10.1007/s10854-015-3962-7. [6] S.V. Motloung, K.G. Tshabalala, R.E. Kroon, T.T. Hlatshwayo, M. Mlambo, S. Mpelane, Effect of Tb3+ concentration on the structure and optical properties of triply doped ZnAl2O4:1% Ce3+,1% Eu3+,x% Tb3+ nano-phosphors synthesized via citrate sol-gel method, J. Mol. Struct. 1175 (2019) 241–252, https://doi.org/ 10.1016/j.molstruc.2018.08.002. [7] B. Cheng, S. Qu, H. Zhou, Z. Wang, Porous ZnAl2O4 spinel nanorods doped with Eu3+: synthesis and photoluminescence, Nanotechnology 17 (2006) 2982–2987, https://doi.org/10.1088/0957-4484/17/12/027. [8] S.F. Wang, F. Gu, M.K. Lü, X.F. Cheng, W.G. Zou, G.J. Zhou, S.M. Wang, Y.Y. Zhou, Synthesis and photoluminescence characteristics of Dy3+-doped ZnAl2O4 nanocrystals via a combustion process, J. Alloys. Compd. 394 (2005) 255–258, https://doi.org/10.1016/j.jallcom.2004.07.088. [9] C. Ragupathi, J. Judith Vijaya, A. Manikandan, L. John Kennedy, Phytosynthesis of nanoscale ZnAl2O4 by using sesamum (sesamum indicum L.) optical and catalytic properties, J. Nanosci. Nanotechnol. 13 (2013) 8298–8306, https://doi.org/10.1166/jnn.2013.7922. [10] S.V. Motloung, F.B. Dejene, H.C. Swart, O.M. Ntwaeaborwa, Effects of Cr3+ mol% on the structure and optical properties of the ZnAl2O4:Cr3+ nanocrystals synthesized using sol-gel process, Ceram. Int. 41 (2015) 6776–6783, https://doi.org/10.1016/j.ceramint.2015.01.124. [11] L. Chen, X. Sun, Y. Liu, K. Zhou, Y. Li, Porous ZnAl2O4 synthesized by a modified citrate technique, J. Alloys. Compd. 376 (2004) 257–261, https://doi.org/10. 1016/j.jallcom.2004.01.013. [12] N.J. Van Der Laag, M.D. Snel, P.C.M.M. Magusin, G. De With, Structural, elastic, thermophysical and dielectric properties of zinc aluminate (ZnAl2O4), J. Eur. Ceram. Soc. 24 (2004) 2417–2424, https://doi.org/10.1016/j.jeurceramsoc.2003.06.001. [13] W.S. Tzing, W.H. Tuan, The strength of duplex Al2O3- ZnAl2O4 composite, J. Mater. Sci. Lett. 15 (1996) 1395–1396, https://doi.org/10.1007/BF00275286. [14] M. García-Hipólito, J. Guzmán-Mendoza, E. Martínez, O. Alvarez-Fregoso, C. Falcony, Growth and cathodoluminescent characteristics of blue emitting ceriumdoped zinc aluminate layers synthesized by spray pyrolysis technique, Phys. Status Solidi Appl. Res. 201 (2004) 1510–1517, https://doi.org/10.1002/pssa. 200306796. [15] M.C. Marion, E. Garbowski, M. Primet, Catalytic properties of copper oxide supported on zinc aluminate in methane combustion, J. Chem. Soc. Faraday Trans. 87 (1991) 1795–1800, https://doi.org/10.1039/FT9918701795. [16] N. Guilhaume, M. Primet, Catalytic combustion of methane : copper oxide supported on high-specific-area spinels synthesized by a sol-gel process, J. Chem. Soc. Faraday Trans. 90 (1994) 1541–1545.
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