Structural and luminescence characterization of terbium doped siliconitride phosphors for afterglow applications

Journal Pre-proof Structural and luminescence characterization of terbium doped siliconitride phosphors for afterglow applications Chung-Hsin Lu, Che-Yuan Yang, Sudipta Som, Subrata Das

PII:

S0030-4026(19)31929-1

DOI:

https://doi.org/10.1016/j.ijleo.2019.164030

Reference:

IJLEO 164030

To appear in:

Optik

Received Date:

31 October 2019

Accepted Date:

10 December 2019

Please cite this article as: Lu C-Hsin, Yang C-Yuan, Som S, Das S, Structural and luminescence characterization of terbium doped siliconitride phosphors for afterglow applications, Optik (2019), doi: https://doi.org/10.1016/j.ijleo.2019.164030

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Structural and luminescence characterization of terbium doped siliconitride phosphors for afterglow applications

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Chung-Hsin Lua,b,c,*, Che-Yuan Yanga, Sudipta Soma, and Subrata Dasa,d

Department of Chemical Engineering, National Taiwan University Taipei, Taiwan,

Department of Chemical Engineering, National Taiwan University of Science and

Technology, Taipei, Taiwan, ROC

Advanced Research Center of Green Materials Science & Technology, Taipei

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10617, Taiwan

Materials Science and Technology Division, CSIR National Institute for

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Interdisciplinary Science and Technology, Thiruvananthapuram, Kerala,

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695019,India

------------------------*Corresponding author: [email protected] Tel: +886-2-23651428, Fax: +886-2-23623040 1

Abstract Terbium ions-activated Sr2Si5N8 phosphors were synthesized via solid state reaction route for dark glow applications. The luminescence properties and the corresponding ligand environment were controlled with the synthesis temperatures. The phosphors prepared at various heating temperatures exhibited the typical Tb3+ emission lines around 545 nm upon UVC excitation. As the concentration of

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terbium ions increased, the emission intensity of Sr2-xTbxSi5N8 reached to a maximum at x = 0.06. The present phosphors exhibited long afterglow after UVC excitation and the afterglow intensity of phosphors

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increased sharply as the excitation wavelength decreased from 420 nm to 265 nm. The decay time of phosphors changed with the activator concentration. The present study demonstrated a series of

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Sr2-xTbxSi5N8 phosphors for the application in persistent luminescence devices.

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Keyword: Silico-nitride phosphor; Focused ion beam; Elemental mapping; concentration quenching;

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Persistent luminescence

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Introduction

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Nitridosilicate-based phosphors have received extensive attention owing to the high covalency, large crystal field effect, high chemical stability, and reliable thermal stability [1- 2]. The phosphors usually exhibit relatively long excitation and emission wavelengths because of the strong crystal field effect. Y2Si4N6C: Ce3+ exhibits a green emission band at around 540-550 nm under blue excitation [3]. MSiN2: Eu2+ (Sr, Ba) shows a red emission band in 600-670 nm upon blue excitation [4], and CaAlSiN3: Eu2+ 2

displays a red emission at around 650-660 nm under blue excitation [5]. Among the nitridosilicate-based phosphors, M2Si5N8: Eu2+ (M = Ca, Sr, Ba) materials have been reported as promising orange-red emitting phosphors for white light-emitting diodes due to the high quantum efficiency under blue excitation and high thermal stability [6]. In addition, M2Si5N8 hosts can be synthesized under ambient pressure, leading to the relatively low cost in production than other nitridosilicate-based phosphors such

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as MAlSiN3 (M=Ca, Sr), LaSi3N5, and α-sialon [7]. However, M2Si5N8 hosts are hardly ever used in long lasting phosphors for dark glow applications.

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Long lasting phosphorescence materials exhibiting impending applications in luminous paints, safety indication, in vivo bio-imaging, and watch dials draws recent research attention [8] though the long

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persistent materials are still limited to the efficient alkaline-earth aluminates. Moreover, terbium

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ions-doped luminescent materials have been widely investigated for application in displays and

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fluorescent lamps [9- 10]. So far, the literature on terbium ions-activated M2Si5N8 hosts and efficient trap distribution for long lasting phosphors is rare.

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In the present work, terbium ions were deliberately doped into the Sr2Si5N8 host matrix. The

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structure of Sr2-xTbxSi5N8 was refined and the corresponding structural evaluations were presented. The variation of luminescence properties for the prepared phosphors with the synthesized temperatures was investigated. The decay time and afterglow properties of the prepared phosphors after various excitation wavelengths and related long persistency were discussed in details.

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Materials and methods Sr2-xTbxSi5N8 phosphors were prepared via the solid-state reaction process. Analytical-grade Sr3N2, Si3N4, and Tb4O7 were mixed according to the chemical formula of Sr2-xTbxSi5N8 (x = 0.02-0.10). The mixtures were ball-milled using zirconia balls for 2 h in an argon-filled glove box. Then the mixed

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powders were placed in molybdenum crucibles and calcined at 1300oC-1600oC for 8 h in a reducing atmosphere (10 vol. % H2 and 90 vol. % N2).

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The structural analysis of the obtained samples was carried out using X-ray diffractometer (Rigaku, Ultima IV). The PDXL program was used to refine the crystal structure. The morphology, particle sizes,

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and elemental distribution were investigated using a field effect scanning electron microscope (FESEM,

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JEOL JSM- 7610F). TEM Rigaku, D/max 2550, JEOL-2010 transmission electron microscopy was used

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to investigate the microstructures of the synthesized phosphors. For TEM measurement, the specimens were prepared via the double-beam focused ion beam (FIB). The indexing of the TEM diffraction spot

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and corresponding characterizations were carried out via CrysTBox software [11]. The

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photoluminescence characteristics of the prepared phosphors were investigated using a fluorescence spectrophotometer (Hitachi, F-4500). X-ray photoelectron spectroscopy (XPS) measurement was carried out with a theta probe angle-resolved XPS System (Thermo Scientific).

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Results and Discussions Sr1.94Tb0.06Si5N8 phosphors were prepared at different temperatures varying from 1300°C to 1600°C in a reducing atmosphere. Figure 1(a) presents the XRD patterns of Sr1.94Tb0.06Si5N8 phosphors synthesized at different temperatures. All the diffraction patterns matched well and indexed with the

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standard pattern of Sr2Si5N8 (ICDD No. 85-0101), and no impurity phase was found. The increase in the annealing temperature led to an increase of the diffraction intensity owing to the enhancement in the

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crystallinity of phosphors. Figure 1(b) presents the XRD patterns and the refinement results of

Sr1.94Tb0.06Si5N8 synthesized at 1600oC. The solid curve indicates the simulated diffraction data, the “×”

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marks represent the experimental diffraction data, the straight bars show the positions of simulated

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diffraction patterns, and the dotted line denotes the deviation between the simulated and experimental

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values. The calculated Rp and wRp parameters were converged to reliable values of 0.0482 and 0.0715, respectively. The refinement results confirmed that Sr1.94Tb0.06Si5N8 possessed an orthorhombic structure

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with the space group of Pmn21 (no. 31). The calculated lattice parameters were a = 5.7168 Å, b =6.8301 Figure 1(c)

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Å and c = 9.3371 Å. The inset of Fig. 1(b) shows the crystal structure of Sr1.94Tb0.06Si5N8.

shows the elemental mapping of the Sr1.94Tb0.06Si5N8 phosphor. The figure indicated the presence of agglomerated particles with uniform distribution. The elemental distribution of Sr, Si, N, and Tb over the whole particle range was clearly observed from the figure and it revealed the uniform distribution of various elements. 5

Figure 2(a) displays the picked out laminar specimen for TEM using FIB. TEM images and corresponding diffraction pattern of the specimen is shown in Fig. 2(b). The diffraction pattern was indexed with the help of CrysTBox software [11] and is shown in Fig. 2 (c). The structure of the selected zone is orthorhombic, with a zone axis of [0 0 1]. Figure 2(d) shows the high-resolution transmission electron microscopy pattern. The d spacing value was estimated as 0.68 nm which corresponded to the (0

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1 0) plane of the synthesized Sr1.94Tb0.06Si5N8 phosphor. The HRTEM analysis indicates the formation of the pure phase of the present phosphor materials.

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Figure 3(a) presents the photoluminescence emission spectra of Sr1.94Tb0.06Si5N8 phosphors under UV excitation at 276 nm. The emission spectrum of Sr1.94Tb0.06Si5N8 prepared at 1300°C displayed

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several sharp emission peaks at 488 nm, 545 nm, 587 nm, and 622 nm due to the 5D4→7FJ (J=6, 5, 4 and 3)

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transitions of Tb3+ ions, respectively [10]. When the annealing temperature was increased from 1300°C to 1400°C, the emission intensity increased due to the enhanced crystallinity. With further increasing the

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annealing temperatures from 1400oC to 1600oC, the emission intensity of Tb3+ ions was observed to

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decrease significantly and an additional broad emission band at approximately 600 nm became visible.

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The variation of color emission with the increase in annealing temperature is shown in CIE color coordinate diagram (Fig. 3(b)). The emitted color was seen to shift from green region to yellow region with the increase in annealing temperature. The inset of Fig. 3(a) illustrates the excitation spectra of Sr1.94Tb0.06Si5N8 phosphors. The excitation spectra obtained at 544 nm revealed a broadband in the UV region from 200 nm to 300 nm due to the 6

4f8 → 4f75d1 transition of Tb3+ ions [13]. For phosphors synthesized at 1300°C, the position of the excitation band was centered at approximately 256 nm. Following an increase in the heating temperatures from 1300 to 1600°C, a red-shift of the excitation band to 276 nm was observed. The red-shift of the excitation band indicates the increase of nephelauxetic effect and crystal-field splitting in the host lattice [1]. These phenomena are

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supposed to be due to the change of coordinate environment for Tb3+ ions at high temperatures. The nephelauxetic effect is owing to the highly covalent chemical bonding between rare earth ions and N3-

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ions, while the large crystal-field splitting results from the large electronic charge of N3- ions [1].

XPS analysis was used to investigate the coordinate environment and chemical states of terbium

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ions in Sr1.94Tb0.06Si5N8. Inset of Fig. 3(a) presents the XPS spectra of Sr1.94Tb0.06Si5N8 synthesized at

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various temperatures ranging from 1300°C to 1600°C. At the calcination temperature of 1300°C, one peak at 1276 eV due to 3d3/2 photoelectrons of terbium ions was observed. The spectrum indicates the

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existence of Tb3+ ions. After raising the annealing temperatures, another peak at 1284 eV due to the

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shake-up satellite of terbium ions was observed [14]. The ratios of the satellite to parent photoelectron

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peaks were found to increase with the heating temperatures, indicating the enhancement of covalency for chemical bonds surrounding terbium ions [15]. These results supported the red-shift of Tb3+ excitation peaks with the annealing temperatures, as shown in the inset of Fig. 3(a). The relationship between the concentration of terbium ions and the relative emission intensity of Sr2-xTbxSi5N8 (x = 0.02-0.10) phosphors is illustrated in Fig. 3(c). Increasing the doping amount of terbium ions to x = 0.06 led to an 7

increase in the emission intensity of Sr2-xTbxSi5N8. However, further increasing the concentration of terbium ions reduced the emission intensity owing to the concentration quenching effects [16]. Concentration quenching effects are usually caused by the transfer of energy from one activator to another until an energy sink in the lattice is reached [16]. The critical distance for energy transfer, Rc, can be calculated using the following equation [17] Rc ≈ 2(3V/4πxcN)1/3

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(1)

where xc is the critical concentration, N is the number of metal ions in the unit cell and V is the unit cell

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volume. From the appropriate V (363.99 Å3), N (2) and xc values (0.03), the value of Rc for Sr2-xTbxSi5N8 was calculated to be 22.63 Å.

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Fig. 3(d) shows the variation of the afterglow intensity for Sr1.94Tb0.06Si5N8 phosphors as a function

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of different excitation wavelengths for investigating the trap level of Sr2-xTbxSi5N8 phosphors. The

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afterglow intensity of phosphors decreased sharply as the excitation wavelength increased from 265 nm to 420 nm. Following UVC excitation (<280 nm), Sr1.94Tb0.06Si5N8 exhibited pronounced afterglow

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properties. However, after blue excitation, no afterglow properties were observed for phosphors. The

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observed persistent luminescence in the present phosphors can be attributed due to the difference in ionic radius between strontium and terbium ions. Because of the ionic radius mismatch, different trap levels are created during the substitution of terbium ions into the strontium ionic sites in Sr2Si5N8 and results in the afterglow phenomena [18]. Fig. 4(a) depicts a comparison of the fluorescence and afterglow spectra for Sr1.94Tb0.06Si5N8. Upon 8

UV excitation at 276 nm, the emitting color of Sr1.94Tb0.06Si5N8 was yellow, and the associated CIE coordinate was (0.48, 0.47). After turning off the excitation shutter, the associated CIE coordinate shifted to (0.58, 0.41) in the red region as shown in Fig. 4(b). The emitting light of the phosphors was still visible after 30 sec without any obvious change in emitting color. The decay curves of Sr2-xTbxSi5N8 (x = 0.02-0.10) phosphors were monitored for 602 nm emission band under UV excitation of 276 nm, and the

according to the following equation [19]

(2)

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I (t) = A1 exp(−t/τ1) + A2 exp(−t/τ2)

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results are depicted in Fig. 4(c). The received decay curves were fitted into two exponential components

where I is the luminescence intensity, A1 and A2 are constants, t is time, and τ1 and τ2 are the decay times

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of the exponential components. The average decay time (τav) can be calculated from Eq. 3 as mentioned

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below [19]

τav = (A1τ12 + A2τ22) /(A1τ1 + A2τ2)

(3)

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According to Eq. 2 and Eq. 3, the relation between the average decay time (τav) and the concentration of

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terbium ions is plotted in the inset of Fig. 4(c). At x = 0.2, τav was calculated to be approximately 1.5 s.

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Raising the concentration of terbium ions to x = 0.4 increased τav to a maximum of 2.4 s. A further increase in the concentration of terbium ions decreased the decay time of phosphors due to the non-radiative decay between the activator ions. Figure 4(d) shows the afterglow spectra of Sr1.94Tb0.06Si5N8 after closing the excitation shutter from 3 to 30 sec. The inset of Fig. 4(d) depicts the energy level diagram of Sr2-xTbxSi5N8 phosphors. After 9

turning off the excitation shutter, only the red emission band can be observed. These results indicated that terbium ions were responsible for the persistent luminescence properties. Such emission properties revealed the suitability of the present phosphors as afterglow phosphors. As a consequence of the present work, Sr2-xTbxSi5N8 phosphors with a broad emission band in red region were prepared. The potential of

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the present phosphors for persistent luminescence devices was demonstrated.

Conclusions

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Terbium ions-activated Sr2Si5N8 phosphors with efficient afterglow emission were developed in this study. The ligand behavior of terbium ions and the emission colors of phosphors were controlled via

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the variation of synthesized temperatures. The Rietveld refinement indicated that the prepared phosphors

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exhibited a hexagonal structure with the space group of Pmn21 (no. 31). The phosphors prepared at various heating temperatures exhibited the typical Tb3+ emission lines around 488 nm, 545 nm, 587 nm,

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and 622 nm due to the 5D4→7FJ (J=6, 5, 4 and 3) transitions of Tb3+ ions, respectively. The emission

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intensity of Sr2-xTbxSi5N8 attained to a maximum at x = 0.06 with the increase in the concentration of

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terbium ions. The present phosphors also exhibited long afterglow which augmented precisely as the excitation wavelength decreased from 420 nm to 265 nm. The decay time of phosphors varied with the activator concentration however the associated CIE coordinate was at (0.58, 0.41) in the red region. The present study demonstrated a series of Sr2-xTbxSi5N8 phosphors for the application in persistent luminescence devices. 10

Competing Interest. The authors declare no competing interests

Acknowledgment This work was financially supported by the “Advanced Research Center For Green Materials Science and Technology” from The Featured Area Research Center Program within the framework of the Higher

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Education Sprout Project by the Ministry of Education (107L9006) and the Ministry of Science and

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Technology in Taiwan (MOST 107-3017-F-002-001 and MOST 107-2218-E-002-022).

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[8] W. Q. Yang, H. G. Liu, M. Gao, Y. Bai, J.T. Zhao, X. D. Xu, B. Wu, W. C. Zheng, G. K. Liu, Y. Lin, Dual-luminescence-center single-component white-light Sr2V2O7:Eu3+ phosphors for white LEDs, Acta Materialia 61 (2013) 5096–5104. [9] C. H. Hsu, B. M. Cheng, C. H. Lu, Structure and novel optical characteristics of SrSi2O2N2:Ce3+/Tb3+ oxynitride phosphors, J. Am. Ceram. soc. 94 (2011) 3256-3260.

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(RE = Lu, Y, and Gd) phosphors, Chem. Mater. 23 (2011) 1851-1861. [14] J. F. Moulder, W. F. Stickle, P. E. Sobol, K. D. Bomben, Handbook of X-ray photoelectron spectroscopy 1-261 (Physical Electronics, United States of America, 1995). [15] M. A. Brisk, A. D. Baker, Shake-up satellites in X-ray photoelectron spectroscopy, J. Electron. Spectrosc. Relat. Phenom. 7 (1975) 197-213. 13

[16] G. Blasse, B. C. Grabmaier, Luminescent Materials Ch. 3, 40 (Springer-Verlag, 1994). [17] X. Jin, M. Götz, S. Wille, Y. K. Mishra, R. Adelung, C. Zollfrank, A Novel Concept for Self-Reporting Materials: Stress Sensitive Photoluminescence in ZnO Tetrapod Filled Elastomers, Adv. Mater. 25 (2013) 1342–1347. [18] J. Xu, S. Tanabe, Persistent luminescence instead of phosphorescence : History , mechanism , and

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Figure captions Figure 1 (a) X-ray diffraction patterns of Sr1.94Tb0.06Si5N8 phosphors prepared at 1300oC, 1400oC, 1500oC and 1600oC. (b) Refinement pattern of observed (×) and calculated (solid line) X-ray diffraction patterns, difference profile (dot line), and positions of all the reflections (vertical bars) for Sr1.94Tb0.06Si5N8. Inset: crystal structure of Sr1.94Tb0.06Si5N8. (c) Elemental mapping of Sr1.94Tb0.06Si5N8

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phosphor.

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Figure 2 (a) The picked out laminar specimen for TEM, view of picked out thin foils TEM using FIB, (b) diffraction pattern of Sr1.94Tb0.06Si5N8 phosphor, (c) indexing of TEM Diffraction spot {Zone axis= [001]},

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and (d) HRTEM pattern

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Figure 3 (a) Emission spectra of Sr1.94Tb0.06Si5N8 phosphors synthesized at different temperatures ranging from 1300°C to 1600°C upon UVC excitation at 276 nm. Left Inset: excitation spectra monitored at the

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green emission of 544 nm. Right Inset: XPS spectra of Sr1.94Tb0.06Si5N8 synthesized at various synthesis

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temperatures from 1300°C to 1600°C, (b) relation between concentration of terbium ions and relative PL emission intensity of Sr2-xTbxSi5N8 phosphors, (c) variation of the afterglow intensity for Sr1.94Tb0.06Si5N8 phosphors with the excitation wavelengths after closing the excitation shutter for 4 sec, and (d) CIE color coordinate diagram of Sr1.94Tb0.06Si5N8 phosphors synthesized at different temperatures ranging from 1300°C to 1600°C upon UVC excitation at 276 nm. 15

Figure 4 (a) The comparison of the fluorescence and afterglow spectra, and (b) CIE coordinates of Sr1.94Tb0.06Si5N8 phosphors upon UVC excitation at 276 nm. (c) Photoluminescence decay curves of Sr2-xTbxSi5N8 (x = 0.02-0.10) phosphors monitored at 602 nm emission band after the excitation at 276 nm. Inset: dependence of τav on the concentration of terbium ions. (d) Afterglow spectra of Sr1.94Tb0.06Si5N8 phosphors after closing the excitation shutter from 3 to 30 sec. Inset shows the energy

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level diagram of Tb3+ in Sr1.94Tb0.06Si5N8 phosphors.

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(b) 1600 °C

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N

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Zone axis= [001]

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Figure 2

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Intensity (a.u.)

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(d)

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Excitation wavelength (nm)

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Figure 4

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