Structural and photoluminescence characteristics of the single-host green-light-emitting T-phase Ba1.3Ca0.7SiO4: Tb3+ phosphors for LEDs

Journal Pre-proofs Research paper Structural and photoluminescence characteristics of the single-host greenlight-emitting T-phase Ba1.3Ca0.7SiO4: Tb3+ phosphors for LEDs D.R. Golja, F.B. Dejene PII: DOI: Reference:

S0009-2614(20)30037-3 https://doi.org/10.1016/j.cplett.2020.137122 CPLETT 137122

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Chemical Physics Letters

Received Date: Revised Date: Accepted Date:

17 December 2019 6 January 2020 12 January 2020

Please cite this article as: D.R. Golja, F.B. Dejene, Structural and photoluminescence characteristics of the singlehost green-light-emitting T-phase Ba1.3Ca0.7SiO4: Tb3+ phosphors for LEDs, Chemical Physics Letters (2020), doi: https://doi.org/10.1016/j.cplett.2020.137122

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Structural and photoluminescence characteristics of the single-host greenlight-emitting T-phase Ba1.3Ca0.7SiO4: Tb3+ phosphors for LEDs D.R. Golja1*, F. B. Dejene1 Department of physics, University of the Free State (QwaQwa Campus), Private Bag X13, Phuthaditjhaba, 9866, South Africa. * Corresponding author: e-mail: [email protected]

Abstract In this report, Ba1.3Ca0.7SiO4: Tb3+ phosphors were prepared by the low-temperature solution-combustion method. The structural properties of the Ba1.3Ca0.7SiO4: Tb3+ phosphors were examined by XRD and SEM. The phosphorescence spectra of low Tb3+ doped ceramics have revealed prominent blue and green emissions from 5D3 and 5D4 exciting levels to 7Fj ground state respectively. The calculated CIE color coordinates confirmed that varying the content of Tb3+ makes the luminescence color of Ba1.3Ca0.7SiO4: Tb3+ to be tuned from blue to green region. The highest emission intensity at 231 nm excitation wavelengths signifies its potential applicability in the fields of lighting and displays. Keywords: - Green luminescence; multipolar interaction; crystal structure; nanophosphors.

1. Introduction The rare-earth ions (Ren+) doped phosphors received attractive attention because of their excellent photo-stability and long luminescence lifetime, low cost, large-volume production possibility. These phosphors behave easy shaping of elements for applications in high energy physics, x-ray, and computed tomography for industrial and medical imaging, advanced lighting, displays, and detection systems [1-4]. A recent study reveals alkaline earth silicates doped with rare earth have been increasing paid attention for application in white light-emitting diode due to promising luminescent properties [4], long-lasting phosphors [5], multicolor phosphors [6], and highefficiency Si-solar cells [7], display, and solid-state lighting [8]. Moreover, rare-earth (RE) based host materials have been successfully devoted to compact solid state lighting and display devices [9-11]. Silicates based (Ba, Ca) materials have been widely used as host matrix due to several advantages such as simple in synthesis, stable crystal structure, excellent long -term stability, excellent thermal stability, and chemical stability, easy preparation, strong absorption properties

in the near-UV region [12, 13]. The unique optical properties of RE ions are attributed to electronic transitions occurring within the partially filled 4f energy shell, which makes these ions a favorite activator for different host materials [14]. Among different trivalent lanthanides, Tb3+ received significant attention due to its green emission via 5D4→7F5 transition. Recently, Wang et al. [15] synthesized the blue phosphor Na2CaSiO4: Eu3+ by high-temperature solid-state reaction and the white light could be generated by mixing it with the yellow phosphor Li2SrSiO4: Eu2+. Xie et al. [16] synthesized deep red-emitting phosphors Na (2x) Ca (1-x) EuxSiO4 with chromaticity coordinates (0.639, 0.359) by high temperature solid-state reaction. In general, rare earth (RE3+) alkaline earthbased silicate phosphors are synthesized by solid-state reactions at high temperatures [17]. However, this method requires long process time, high processing temperature, repeated milling and washing with chemicals. This process tends to degrade the luminescence efficiency of the phosphors and yield irregularly shaped particles. The most important feature of the trivalent rareearth ions is to provide intense visible light emission as a consequence of 4f→4f transitions. These transitions are barely sensitive to the ion's surroundings due to the shielding effect of outer 5s and 5p shell electrons [18]. Moreover, terbium ion Tb3+ is mainly interesting for green emission, which is one of three primary colors. Furthermore, its sharp green emission around 544 nm arising from the 5D4→ 7F5 transition is close to the optimal values required for the green component of the tricolor [19, 20]. Besides the green emission lines, the blue emissions from higher-level 5D3 are also observed, which depends on the host phonon frequency, crystal structure, and Tb3+ concentrations [21]. To observe the blue emissions, host lattice with low phonon frequency and low doping concentration are needed to avoid the multi-phonon relaxation and cross-relaxation occurring among Tb3+ ions [22]. Therefore, Tb3+ activated phosphors with various emission colors can be achieved by choosing a suitable host and an appropriate concentration of Tb3+. The synthesis of oxide phosphors has been achieved by a variety of methods. The solution-combustion technique can offer a unique synthesis route via a highly exothermic redox reaction between metal nitrate and organic fuel to produce the desired product. This technique has the advantage of a choice of a wide variety of fuels, rapid cooling leads to nucleation of crystallites without any growth. Moreover, the resulting product is very fine particulates of friable agglomerates that can be easily be ground to obtain a much finer particle size [23]. In our previous investigation, we reported synthesis and photoluminescence properties of T- phase Ba1.31Ca0.69SiO4:0.02Dy3+ silicate-based ceramic phosphors for white lighting application [24].

This study is the continuation of our previous study to a broad understanding of the material properties when varying concentration of doped (Tb3+) ions. This present work was presented to develop a new single-phase Tb3+ doped silicate phosphor. The compound Ba1.3Ca0.7SiO4 chosen since the crystal structure is similar and homotypic to that of K3NaS2O8. The Ba, Ca, and Si atoms substitute lattice sites of K, Na, and S, atoms, respectively. Various characterization techniques such as x-ray diffraction (XRD), scanning electron microscopy (SEM) imaging and energy dispersive spectroscopy (EDS) as well as photoluminescence spectroscopy (PL) have been used to investigate the structural and persistence luminescence properties of prepared nanocrystals.

2. Experimental Method Single-phase Ba1.3Ca0.7SiO4: Tb3+ was synthesized by low-temperature solution- combustion. Ba (NO3)2(99.9%), Ca(NO3)2.4H2O, Tb (NO3)3(99.9%), (CH4N2O) (99.9%), and TEOS (Si (OC2H5)4) (99.9) were used as precursor materials. The detail synthesis method had been explained in our previous work [24]. Different molar concentrations (1, 2, 3, 4, and 5 mol. %) of Tb3+ ions were selected and added to the solution. The Stoichiometric quantities of Ba(NO3)2, Ca(NO3)2.4H2O and urea were measured and dissolved in 10 ml deionized water based on total oxidizing and reducing valences of oxidizer to fuel (urea) molar ratios. Next, the estimated quantity of Tb(NO3)3 was added to the solution as doping metal ions. Then, a small amount of TEOS dissolved into 10ml of ethanol and added in dropwise to the above solution while vigorously stirring. The mixtures were continuously stirred at a constant temperature of 800C until transparent gel formed. At the final stage, the mixed solution quickly transferred to alumina crucible and put it into a preheated furnace at 5500C. The whole process had taken 5min and white foamy like powder formed. Ba1.3Ca0.7SiO4: Tb3+ nanocrystal obtained after annealing at 1000 0C for 2hr and crusher to fine powders for further characterization. The prepared samples were characterized by x-ray diffractometer (XRD)-Advance Bruker diffractometer with CuKɑ used to obtain crystallite size and phase of nanoparticles. The surface morphology of synthesized powders was performed by Shimadzu Super scan ZU SSX- 550 electron microscope (SEM) and composition of the samples examined by Aztec software for energy-dispersive Spectroscopy (EDS). Varian Cary Eclipse spectrofluorometer used to measure photoluminescence (PL) using xenon lamp as an excitation source.

3. Results and discussions 3.1 Structure Fig. 1a demonstrates the XRD diffraction patterns of Ba1.3Ca0.7SiO4: Tb3+ nanopowder that agree with those of the standard patterns of hexagonal T-phase Ba1.31Ca0.69SiO4 spinal (JCPDS card No: 36-1449) and a single phase formed without impurities. But, a few impurities that exhibited in Fig. 1a may came from contamination during preparation process of the samples. The diffraction peaks are observed from directions planes (102), (104), (110), (202), and (204). The crystal structure of Ba1.3Ca0.7SiO4 is depicted in Fig. 1 (a). It belongs to the space group (p3m1) with hexagonal unit cell parameters 'a' = 0.5827 nm, 'c' = 1.434 nm, 'v' = 0.4867(nm)3 strongly agree with parameters of the standard data [24, 25]. Despite increasing molar concentrations of Tb3+ ions up to 5%, no extra peaks of impurity were observed in the patterns. Therefore, it can be deduced that Tb3+ ions have been successfully incorporated and the doping of Tb3+ ions has no significant influence on the structure. The XRD peaks patterns become weak and slightly broad with an increase of Tb3+ molar concentration. The slightly broaden in XRD peaks might be due to the smaller crystallite size obtained when Tb3+ concentration increases in synthesized phosphors. Fig.1 (b) shows the analysis of the most intense peaks at (110) planes. It can be noticed that the (110) peak position for all Tb3+ doped samples slightly shifts toward higher 2θ angles. The peak shifts occur because of the difference in the size of the atoms and this may cause the distances in the crystal structure to expand or contract depending on whether the doped atom is larger or smaller than the host atom [24]. It is well known that the substitution of host ions with dopants normally meets the requirement of energy minimization and the matching of geometrical size. As a result, it is estimated that the shift to a higher angle was caused by partial replacements of Ba2+ and Ca2+ ions by Tb3+ ions in the host lattice which causes stress and defects in the crystal lattice [25].

3000

(a)

x=5

(b) x=0 x=1 x=2 x=3 x=4 x=5

Intensity(arb.units)

2500

x=3 x=2 x=1

x=0

Intensity(arb.units)

x=4 2000 1500 1000 500

JCPDS #:36-1449

0

25

30

35

40

45 50 2 Theta()

55

60

65

30.6

70

0.0060

30.8 31.0 2 theta()

31.2

31.4

(d)

38 B D

36 Av. crystallite size(Ds)

20

34

FWHM()

0.0055

32

0.0050

30 28

0.0045

26 24

0.0040 0

1 2 3 4 3+ Molar concentration of Tb (mole)

5

Fig. 1. (a) XRD diffraction pattern (b) Peak shift analysis (c) Rietveld refinement of the XRD profile of Ba1.31Ca0.69SiO4 (host matrix) (d) FWHM and crystallite size vs. molar concentrations of Tb3+ in Ba1.3Ca0.7SiO4: x mol%Tb3+ (0 ≤ x ≤ 5). Compared to the host, evidently, there is a slight shifting to the higher 2 theta angle as the dopant mol% of Tb3+ is varied (Fig 1(b)). A shifting of diffraction angles to the higher side indicates either lattice parameters are slightly lower than those of un-doped Ba1.3Ca0.7SiO4 or the small atom partially substitutes larger atom [26]. These phenomena can be explained in terms of the smaller dopants ionic radius Tb3+ (1.0Å) substituting the larger Ba3+ (1.35Å) [26] ions. Note that when the Tb3+ ions singly doped into the Ba1.3Ca0.7SiO4 matrix, it results in the shrinking of the unit-cell volume of the Ba1.3Ca0.7SiO4 nanocrystals. This result indicates the dopant ions were well incorporated into the host matrix [27].

The average crystallite size of synthesized phosphors was calculated using Scherrer’s equation by considering the five most prominent diffraction peaks [24];

DS 

K  cos 

(1)

Where Ds is average crystallite size, k is constant having the value of 0.92, β is FWHM, 𝜃hkl is the diffraction angle, and λ is the wavelength of x-ray having λ = 0.154 nm. The crystallite size and their full width at half maximum (FWHM) were calculated and shown in Fig. 1(c) in order to observe the effect of varying the molar concentration of Tb3+ ions on the size of crystallite and FWHM. The FWHM was calculated from the prominent (110) peak for all samples. It is clear from Fig. 1(c) that as the doping concentrations rise, the FWHM decreases and the crystallite size increases, and vis-versa. This shows that there is an enhancement in the quality of the synthesized phosphors as the doping concentrations increase. The average crystallite size is estimated to be 34.18 nm. The lattice parameters of synthesized Ba1.3Ca0.7SiO4: Tb3+ nanocrystal were calculated by using Eq.2; 1 4  h 2  hk  k 2   2 d hkl 3 a2

 l2  2  c

(2)

Where (hkl) represents the miller indices, ‘a’, and ‘c’ are lattice parameters, and ‘dhkl’ is the interplane spacing. Lattice parameters (a, b, c), full width and half maxima (FWHM), and average crystallite size values for all of the samples are organized in Table 1. Calculated parameters such as symmetry, Bragg’s reflections including lattice parameters, space groups were analyzed by the Rietveld refinement results of B1.3Ca0.7SiO4 (host matrix) as shown in Fig.1c. The calculated structural parameters are space group (p3m1), crystal system (hexagonal), lattice parameters (a = 5.780 Å, b = 5.780 Å, and c = 14.714Å), cell volume (V= 422.54 Å3), and other fitted parameters such as residual factors (Rp = 5.89%, Rwp = 8.30%). It was observed that the calculated parameters and experimental parameters are almost comparable and this indicates the desired nanomaterial is formed successfully.

Table 1: Crystallite size, strain (𝜀) and other structural parameters of Ba1.3Ca0.7SiO4: x mol%Tb3+ (0, 1, 2, 3, 4, and 5). Samples

FWHM(rad)

a(Å)

c(Å)

Ds(nm)

composition

DW-

Strain

H(nm)

(𝜖)𝑥 10 ―3

Ba1.3Ca0.7SiO4

0.0042

5.82

14.82

36.97

38.63

2.66

Ba1.3Ca0.7SiO4:

0.0043

5.80

14.83

34.64

36.42

-0.38

0.0042

5.80

14.83

35.31

37.12

0.757

0.0042

5.79

14.83

35.72

39.65

0.13

0.0049

5.870

14.89

37.70

41.85

-3.20

0.0060

5.80

14.80

24.75

27.47

0.23

0.01Tb3+ Ba1.3Ca0.7SiO4: 0.02Tb3+ Ba1.3Ca0.7SiO4: 0.03Tb3+ Ba1.3Ca0.7SiO4: 0.04Tb3+ Ba1.3Ca0.7SiO4: 0.05Tb3+

The distance between the plane (𝑑ℎ𝑘𝑙) can be calculated using Bragg’s law [nλ = 2dsinθ] where ‘n’ is an integer (n = 1). According to Bragg's low, decreasing in a d-spacing causes XRD peaks to shift towards higher diffraction angles, and vice versa. Usually, when a crystal is strained in a certain direction, the appropriated d-spacing is decreased and all peaks shift slightly toward the higher 2𝜃 than in the non-strained state, and vice versa. This could be associated with the presence of stress in the materials. The lattice strain (ε) induced and the average grain-size were calculated from the Williamson and Hall (W-H) [28];

hkl cos hkl  

k  4 sin hkl  DW  H

(3)

Where ‘𝛽ℎ𝑘𝑙’ represents FWHM of a different plane, ‘DW-H’ is Williamson and Hall crystallite size.

0.0060 x=0 x=1 x=2 x=3 x=4 x=5

0.0055 0.0050 cos

0.0045 0.0040 0.0035 0.0030 0.6

0.8

1.0 4*sin 1.2

1.4

1.6

Fig. 2 Williamson-Hall (W-H) plots of Ba1.3Ca0.7SiO4: x mol%Tb3+ (x = 0, 1, 2, 3, 4, and 5) The negative and positive values of micro-strain show compressive and expansive properties appeared in the crystal lattice. Most of the time, lattice strain is caused by an increasing volume of grain boundaries due to defect, dislocation, and lattice distortion in the crystal [29]. Table 1 represents the organized average crystallite size calculated by Scherrer's and Williamson-Hall method. The crystallite size calculated from Williamson-Hall method slightly greater than the value obtained from Scherer's. The values obtained by using both methods support each other. The calculated values of strain can be negative or positive depending on whether the strain is expansive or compressive, respectively. It was observed that the lattice strain of Ba1.3Ca0.7SiO4: Tb3+ nanopowder changes with the amount of Tb3+ ions concentrations [24, 29]. The plots for samples doped with 1 mol% and 4 mol% Tb3+ display a negative slope which confirms the presence of compressive strains in the lattice [30]. The samples doped with 0, 1, 3, and 5 mol % of Tb3+ ions show positive strain developed in the material which indicates expansive strain exist and this may due to increasing the lattice constant. The slight increase in crystallite size with the presence of negative strain for sample doped with Tb3+ mol% of 1.0 and 4.0 may due to non-uniform strain established during the grain development or due to the presence of native structural disorder in this material at the time of formation.

3.2 . SEM/EDS Fig. 3 shows the SEM images of Ba1.3Ca0.7SiO4: xTb3+(x= 1, 2, 3, 4, and 5 mol. %). The micrographs of the synthesized phosphors were taken from the host, lower, the middle and maximum doped concentration of Tb3+ ions to relate and find out morphologically better nanophosphors for prospective applications in the field of display technology. Some reported works on simultaneous control over the size, shape, and phase of nano-particles explained in ref. [31-34]. The SEM morphology of the host Ba1.3Ca0.7SiO4 exhibits sponge-like agglomerated nanocrystalline powder as shown in Fig. 3a. After doping the specific percentage Tb3+ ions into Ba1.3Ca0.7SiO4, the agglomeration of smaller and the larger particles having regular shapes were formed from adhesion as shown in Fig. 3b. The images display that the phosphors have a uniform size distribution with tightly bonded granular shaped particles. The crystallized nanostructure that forms fine and regular hexagonal structure exhibiting granular surface and well developed face. The exothermic reaction during the solution combustion process is characterized by decomposition and removal of nitric oxides significantly varies depending on the precursor ingredient and the ratio of metal nitrate to urea [35, 36]. Consequently, the surfaces of samples reveal pores and voids, which may be attributed to the evolved gases during combustion. The granular surface with welldeveloped faces seems to dominate the morphology of the Tb3+ doped samples as shown in Fig. 3c and d. The phosphors with regular morphology are well known to improve packing density, slurry properties, and improve luminescence properties [37]. This explains the high luminescence intensity obtained by sample doped with 2 mol%Tb3+.

(a)

(b)

(c)

(d)

Fig. 3 SEM images of Ba1.3Ca0.7SiO4: x mol %Tb3+ (a) host (b) x=1 (c) x=3 (d) x=5 To confirm the inclusion of composition elements EDS analysis was performed. Fig. 4(a-d) illustrated the EDS measurement of both host and doped Ba1.3Ca0.7SiO4 phosphors. It was found that the host Ba1.3Ca0.7SiO4 consists of the only Ba, Ca, Si, O, and C as shown in Fig. 4(a), while Ba1.31Ca0.69SiO4: Tb3+ samples describe the existence of Ba, Ca, Si, O, Tb, and C peaks. The peak equivalent to C for all samples found in the spectra because of conductive adhesive. The atomic percent of the host and Tb3+ doped Ba1.3Ca0.7SiO4 of nanocrystals are shown in Table 2. It proves that Tb3+ has been successfully doped into Ba1.3Ca0.7SiO4 host.

(a)

(b)

(c)

(d)

Fig. 4. EDS spectra of Ba1.3Ca0.7SiO4: x mol%Tb3+ at (a) x=0 (b) x=1 (c) x=3 and (d) x=5

Table 2: The weight and atomic percentage of Ba1.3Ca0.7SiO4: xTb3+ samples with x= 0, 1, 3 and 5mol %. Tb3+ (mole %) Elements Weight (%) Atomic (%) 0

1

3

5

Ba

29.1

10.82

Ca

11.1

14.14

Si

4.4

8.0

O

21

67.03

Ba

30.5

8.04

Ca

20.2

18.24

Si

13.0

16.75

O

25.0

56.56

Tb

1.8

0.41

Ba

30.8

9.82

Ca

11.4

12.46

Si

10.3

16.06

O

22.3

61.05

Tb

2.2

0.61

Ba

30.2

8.62

Ca

9.9

9.68

Si

13.0

18.14

O

25.5

62.47

Tb

4.4

1.09

3.3 Excitation and emission Fig. 5 shows the PL spectra of the Ba1.3Ca0.7SiO4: x mol%Tb3+ (1 ≤ x ≤5). The PL excitation spectra were monitored at 544 nm revealed that the maximum excitation peak approximately appeared at 231 nm as shown in Fig. 5a. The observed excitation band at ~231 nm is due to the spin allowed 4f8 →4f75d1 transition of the Tb3+ [37]. This transition has been detected as single broadband with clear stark components as has been shown in Fig. 5a. All doped samples show that host emissions are completely suppressed may be due to energy transfer from the Ba1.3Ca0.7SiO4 (host) to the Tb3+ (activator ions). The PL emission spectra for the series of the Ba1.3Ca0.7SiO4: x mol%Tb3+ (1 ≤ x ≤5) phosphors were shown in Fig. 5b. The most prominent emission band at 544 nm is due to the 5D4→7F5 transition that shows a bright green, and it comprises the strongest intensity and high probability of the electric dipole transition [37]. All of the prepared samples with the different Tb3+ concentrations exhibited similar emission peaks except for the intensity values. The transition of 5D3 or 5D4 to 7FJ produces a wide coverage emission between 400 nm and 650 nm. The peaks that are centered at 416 nm and 438 nm are from 5D3 →7FJ (J = 5, 4) transition [38], while the peaks that are centered at 489 nm, 544 nm, 585 nm, and 621 nm are due to 4f→4f transitions from 5D4 →7FJ (J=6, 5, 4, 3) [39]. The Ba1.3Ca0.7SiO4: x mol%Tb3+ phosphors emit green light under a UV source. The relative emission intensities show a trend that is similar to the increasing of the Tb3+ doping concentration. The emission intensity of the Ba1.3Ca0.7SiO4: x mol%Tb3+ (1 ≤ x ≤ 5) phosphors initially increased and reached the maximum at x = 2 mol%Tb3+, but then decreased due to concentration quenching as shown in Fig. 5c. The energy transfer mainly causes the concentration quenching from one activator to another i.e., in the concentration of Ba1.3Ca0.7SiO4: Tb3+ multipolar interaction [40].

Intensity(arb.units)

4000 3000 2000

0 200

250

300 Wavelength(nm)

350

x=1 x=2 x=3 x=4 x=5

_ex=231nm

5000 4000 3000 2000 1000

1000

(b)

544nm

5000

6000

489nm

x=1 x=2 x=3 x=4 x=5

400

0 400

438nm

_em =544 nm

416nm

(a)

Intensity(arb. units)

6000

585nm

450

621nm

500 550 600 Wavelength(nm)

650

700

6000 (c)

Intensity(arb.units)

5500 5000 4500 4000 3500 3000 2500 1

2 3 4 3+ Tb concentration (mol%)

5

Fig. 5 Excitation and emission spectra of silicate phosphor doped with different Tb3+ concentrations (c) variation in the intensity of strong emission (544 nm) vs the Tb3+ ions concentrations of Ba1.3Ca0.7SiO4: x mol%Tb3+ (1 ≤ x ≤ 5) In the present study, the reduction of green emission (5D4→7Fj) intensity at higher concentrations may be caused by the cooperative energy transfer to upper levels. For the 5D3→5D4 crossrelaxation, it is possible to obtain the critical distance R0 from the concentration quenching data. R0 is the critical separation between donor and acceptor, at which the non-radiative rate equals to that of the radiative rate for the internal single ion relaxation [41]. Concentration quenching mechanism can be described using Blasse equation [42];  3V  R0  2    4 x0 N 

1 3

(4)

Where R0 is the critical distance between Tb3+, V is the volume of a unit cell, N is the number of cations in the unit cell and x0 is the critical concentration of the activator ion. The synthesized Ba1.3Ca0.7SiO4 has hexagonal spinal structure having N =10 [43], x0 = 0.02, and V = 486.7(Å)3. The critical distance (R0) between neighboring Tb3+ ions was calculated using Eq. 4 and the obtained value was 16.69Å. Since the resulted value of R0 is not less than 5Å, the exchange interaction becomes useless and only multipolar interaction is the main cause for concentration quenching in the Ba1.3Ca0.7SiO4: Tb3+ phosphors [44]. The reduction in the rate of increase of green emission from 5D4 level may be mainly due to the second type of quenching mechanism through cooperative energy transfer to upper 5d levels, resulting in the non-linearity of decay curves and decrease in the lifetime of 5D4 at higher concentrations of terbium. Phosphorescence decay analysis is very useful for understanding the energy transfer mechanism and quenching behavior of luminescence of Tb3+ ions. Fig. 6 shows the phosphorescence decay curves for 5D4→7F5 (at 544 nm) emission transition. Thus, the afterglow characteristics of the Ba1.3Ca0.7SiO4: x mol%Tb3+ phosphors were summarized in Table 3. The phosphors exhibit a single exponential decay feature with fitting parameters shown in the graph. The the obtained decay curve was calculated using the single exponential function using Eq. 5 [45]. t

I (t )  I 0  Ae 

(5)

Where ‘I’ is phosphorescent intensity, ‘I0’ and ‘A’ are constant value, ‘𝜏’ decay time. Table 3: parameters for fitted decay curves of the Ba1.3Ca0.7SiO4: x mol%Tb3+ (1 ≤ x ≤ 5) Tb3+ concentration

A

𝜏(ms)

1

3.25

21.26

2

4.56

32.72

3

4.40

29.33

4

3.95

25.76

5

4.25

31.18

(mole %)

These phosphorescence lifetimes (short) was calculated from decay profiles at 544 nm emission lines for sample excited at 231 nm. The decay lifetimes of electrons varied with the Tb3+ concentrations were explained in Table 3. It was noted that the decay time t1 is measured in millisecond (ms). Relative work that has been done on the doping RE- ions in some oxides for LEDs applications [22] indicates Tb3+ acts as a supplier of traps forming defects when it replaces Ba in the crystal. The longer lifetime and low decay rate were obtained at 2 mol. % of Tb3+ ions. The calculated value of lifetime (32.72ms) of the phosphors reveals the 2 mol. % of the Tb3+ ions is the optimum doping concentration to obtain the best phosphor for the green light in LEDs applications. 2500

Intensity(arb.units)

2000

1500

3.5 3.0 2.5 Log Intensity

x=1 x=2 x=3 x=4 x=5

2.0 1.5 1.0 0.5

1000

0.0 0

5

10 Decay time(ms)

15

20

500

0

5

10 Decay time(ms)

15

20

Fig. 6 Decay time and the inset is fitted decay curves of Ba1.3Ca0.7SiO4: x mol%Tb3+ (1 ≤ x ≤ 5). The values of CIE color coordinate for samples Ba1.3Ca0.7SiO4 doped with different concentration of Tb3+ ions would be calculated as (x, y) = (0.2392, 0.4770), (0.2547, 0.5720), (0.2563, 0.0.5513), (0.2677, 0.5561), and (0.2423, 0.5046) as shown in Fig 7 (a). It has been observed that an adjustment of the Tb3+- doping concentration in the phosphors can significantly tune a variety of emission colors from blue to green. In the present case, green emission was achieved since the emission line of the Tb3+ in the Ba1.3Ca0.7SiO4 phosphors slightly covers the visible region. The increased Tb3+ concentration induced the emission colors to shift from the blue to the green region. This fundamental work might be important in the ongoing development of new luminescence devices, making it applicable for the green phosphors of the lamp industry. Furthermore, it could

be helpful in the development of the phosphor-based generator that produces white light with a particular ratio.

Fig. 7 (a) CIE chromaticity coordinates and their corresponding color band display of Ba1.3Ca0.7SiO4: x mol%Tb3+ (1 ≤ x ≤ 5) phosphors (b) Energy level diagram of Tb3+ ions and visible emission transitions in Tb3+ doped alkaline-earth silicate ceramics. (c) Energy transfer mechanism between Tb3+ ions is indicated.

Fig. 7b shows the energy level diagram and visible emission transitions in the Tb3+-doped alkalineearth silicate ceramics. It was observed that there are two non-radiative processes from 5D3 → 5D4 states: a fast cross relaxation due to resonance energy transfer and multi-phonons assisted nonradiation due to the slow energy gap between radiative 5D3 and 5D4 [41]. Since the energy difference between 5D3 and 5D4 very close to that between 7F6 and 7F0 level, the cross relaxation [Tb3+(5D3) +Tb3+(7F6)] →[Tb3+(5D4) +Tb3+(7F0)] between two Tb3+ ions could occur when Tb3+ ion is very close to another due to high concentration of Tb3+ [41]. Thus, the population of 5D4 level increases. It results in intensities of the emissions originated the transitions from 5D3 level to lower 7F levels decrease and the intensities of the emissions from 5D4 level to lower 7F levels increase.

4. Conclusion

Single-phased Ba1.3Ca0.7SiO4: Tb3+ phosphors were successfully synthesized by the low temperature solution-combustion method. The crystal structure, morphology, and luminescence properties of prepared samples were characterized by XRD, SEM, EDS, and PL respectively. The XRD measurement confirmed that prepared phosphors were a T-phase hexagonal structure. SEM image displays an agglomerated nanoparticles having granular like shape and it was investigated that surface morphology is changed with doping Tb3+ ions. Some of the bigger particles were formed by the adhesion of the smaller particles. The phosphors with single excitation band at 231 nm, could be efficiently excited by UV-LEDs and it is realized that it could be promising green emitting phosphors for the lamp industry. The weaker emissions characteristics bands at 416 nm and 438 nm are from 5D3 →7FJ (J = 5, 4) transitions, while the stronger emissions that are centered at 489 nm, 544 nm, 585 nm, and 621 are due to 4f→4f transitions from 5D4 →7FJ (J=6, 5, 4, 3). The strongest emission peak that is centered at 544 nm is the most favorable for fulfillment phosphor with high color purity. The emission and excitation are changed with Tb3+ concentrations. The optimum molar concentration of Tb3+ ions to synthesis Ba1.3Ca0.7SiO4: Tb3+ phosphors was determined to be 2 mol. %, i.e., the intermediate concentration of Tb3+ at which the concentration quenching mechanism occurred and controlled by multipolar interaction. The CIE color coordinate diagram for the emission spectrum of Ba1.3Ca0.7SiO4: Tb3+ phosphors fall in the

green region. Combining all results, it can indicate that Ba1.3Ca0.7SiO4: Tb3+ serve as green lightemitting phosphors for the light-emitting diode.

Acknowledgments: The authors would like to acknowledge the financial support given from the ministry of science and higher education (Ethiopia) and national research foundation (NRF), South Africa.

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