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In the present work, we report a novel luminescent characteristic of the ZnS ceramics. ZnS undoped nanopowders have been synthesized by a wet chemical precipitation method using Na2S as the source of sulfur. Spark plasma sintering (SPS) has been applied to the nanopowders to fabricate dense ZnS ceramics in the pure phase of zinc blende. Photoluminescence (PL) and fluorescence lifetime spectra have been utilized to characterize the luminescent properties of the ZnS ceramics, indicating that these materials exhibit green phosphorescence. In addition, elemental analysis has also been adopted to determine the elemental composition and valency of elements within the ceramic samples. It is concluded that the green phosphorescence results from the presence of elemental sulfur species and Na impurities.
© 2014 Optical Society of America
1. Introduction
It is well-known that zinc sulfide is an II-VI semiconductor compound with a wide band gap, which is desirable for optical applications in the field of luminescence. The addition of dopants as activators into ZnS crystals, such as transition metals, rare earth elements, and halogen elements, can make ZnS-based materials produce photoluminescent characteristics. These ZnS-based materials are called ZnS-based phosphors [1–8]. Due to their prominent capability of being doped with various elements to bring out luminescence with a diversity of colors, ZnS-type phosphors are widely used in cathode ray tubes (CRT), radar screens, and lamps [9, 10].
Early in 1866, the French chemist Théodore Sidot by chance developed green phosphorescent ZnS by adding a small amount of copper (Cu) into ZnS [11]. Since then, ZnS: Cu has become one of the most commonly-used phosphors, and the phosphorescence mechanism has been extensively studied. Randall et al. asserted that the delay for the phosphorescence was due to the time that the electrons spend in the trap, resulting from Cu+ doping in ZnS crystals [12]. This has become the most widely accepted mechanism for the phosphorescence. In addition, a study by Clabau et al. indicated that the point defects in the ZnS crystals might affect the phosphorescence as well [13].
In regard to the green luminescence of ZnS, dopants such as Ni2+ and Cu2+ both can act as the luminescent centers in the material [14–16]. In recent years, it has been reported that undoped ZnS nanocrystals, nanotubes, nanobelts, and nanoribbons can exhibit green luminescence due to their various imperfections. Tsuruoka et al. investigated the green emission peak of ZnS nanobelts by applying scanning near-field optical microscopy and ascribed it to the line and planar defects, without further explanation on the specific defects [17]. Ye et al. revealed that the green photoluminescence band at 535 nm was related to elemental sulfur species on the surface of ZnS nanobelts [18, 19]. According to the investigations by Chen et al., the green luminescence was derived from the electronic states determined by zinc vacancies [20, 21]. Nevertheless, Wang et al. claimed that the green luminescence was attributed to the electron transfer from sulfur vacancies to interstitial sulfur states [21]. Lee et al. concluded that radiative transition from the level of sulfur vacancy to the valence band for charge compensation of Zn vacancy would lead to green emission in both pure sphalerite, and mixture of sphalerite and wurtzite [22].
Although previous researchers have conducted tremendous investigations on the green luminescence, including phosphorescence of ZnS, there are few studies regarding the long lifetime green phosphorescent characteristic of undoped ZnS ceramics. Different from single crystals, optical ceramics include the merits of cost effectiveness, higher laser performance, and probability of productions in large volumes [23]. In the present study, powders synthesized by adopting Na2S as the source of sulfur were sintered by spark plasma sintering (SPS), and the ZnS ceramics with zinc blende crystal structure were obtained. As the investigation continued, these ceramics emitted green phosphorescence. Furthermore, photoluminescence spectra and fluorescence lifetime spectra were collected to analyze and confirm the special optical property.
2. Experimental
2.1 Synthesis
The ZnS powders were synthesized through a wet chemical precipitation method, which was similar to the one mentioned in previous literature [24]. In the first step, 250 mL 0.2 mol/L zinc nitrate hexahydrate (Zn(NO3)2·6H2O, purum p.a., crystallized, ≥99.0% (KT)) and 250 mL 0.2 mol/L sodium sulfite nonahydrate (Na2S·9H2O, ACS reagent, ≥98.0%) were dissolved into deionized water separately. Nitric acid (HNO3, ACS reagent, 70%) was then added into Zn(NO3)2·6H2O solution to adjust the pH value to 2.0. Na2S solution was then slowly added dropwise into Zn(NO3)2 solution while being stirred. As the two solutions were mixed together, the solution turned to a milky suspension with white color, indicating the production of ZnS particles. The mixed suspension was mildly stirred for 6 hours, and subsequently heated up to 65-68 °C. After being held at 65-68 °C for 30 min to enable adequate growth of the ZnS particles, the suspension was cooled down to 0 °C in an ice water bath to prevent further particle growth. Finally, the powders were washed with deionized water (Millipore, USA) and ethyl alcohol (C2H5OH, 200 proof, 100%), centrifuged at a speed of 3500 rpm/min for 30 min (Allegra X-12 Centrifuge, Beckman Coulter, USA), and then dried in a drying oven at 55 °C. In order to obtain ceramic pellet samples, the synthesized ZnS powders were sintered in graphite dies using SPS under the vacuum condition (HP D 25, FCT Systeme GmbH, Germany). A pyrometer was applied to measure the temperature during the sintering process. The samples were sintered at 820 °C for 20 min under the uniaxial pressure of 50 MPa at a heating rate of 50 °C /min, and a cooling rate of 10 °C /min. The pressure was added gradually during the heating process, and was released in the end of the dwell process.
2.2 Characterization
The crystalline phases and microstructure of the ZnS powders, and the sintered ceramics, were determined using XRD (Bruker D2 PHASER, Germany) equipped with Cu radiation (λ = 0.154 nm), scanning electron microscopy (SEM) (FEI Quanta 200, USA) and transmission electron microscopy (TEM) (JEOL2100F equipped with EDS at 200kV, Japan). MDI Jade 9.0 was utilized to investigate the phase composition of the samples. To investigate the specific surface area, single point Brunauer–Emmett–Teller (BET) method (Tristar II 3020 system, Micromeritics, USA) was used on nitrogen adsorption at 77 K after degassing at 523 K for 1 hour. The chemical elemental composition and valency of the ceramics were examined with WDS (JEOL-JXA-8200, Japan), XPS (ESCALAB 250, Thermo Fisher, USA) and XRF (Bruker S4 PIONEER, Germany). The characterization of photoluminescence was recorded using PL spectra (Jobin Yvon Fluorolog-3 spectrofluorometer, Horiba, Japan), and the phosphorescence property was confirmed by a compact fluorescence lifetime spectrometer (Quantaurus-Tau C11367, Hamamatsu Photonics, Japan) and a streak camera (C4434, Hamamatsu Photonics, Japan). All characterization of optical properties was at 300 K in air, with an excitation wavelength of 365 nm.
3. Results and discussion
3.1 Phase and microstructure
The XRD patterns of the synthesized ZnS powders and sintered ceramic pellets are shown in Fig. 1. All of the peaks in both the synthesized powders, and the as-sintered ceramics patterns, are well indexed as sphalerite (JCPDS card no. 65-9585, a = b = c = 5.4093 Å). Upon observation of Fig. 1, it can be seen that for the synthesized powders, there are obvious peak broadenings in the XRD pattern, which is attributable to the small crystallite size of the ZnS synthesized powders. According to the Debye-Scherrer equation:
where L is the size of the crystallite size, λ is the wavelength of the X-ray, B is the full width at half maximum (FWHM) of peak in the unit of radian, and θ is the Bragg angle [25]. The crystallite size of the synthesized powders can be calculated to be 12-36 nm. The average crystallite size is ~23 nm, suggesting the formation of ZnS nanoparticles.For the sintered ceramics, it is noteworthy that hexagonal wurtzite, the most commonly reported second phase detrimental to the optical properties of ZnS ceramics [26], is not observed within the sample, due to the relatively low sintering temperature of SPS. In addition, the lattice constants (a = b = c = 5.40839 Å) are very close to the standard constants after unit cell refinement. Compared with the XRD pattern for powders, the peaks in the pattern generated for ceramic pellets are sharper and narrower, indicating the growth of the crystallites. This crystallite growth can be explained by grain growth during sintering, including both the heating up process and dwelling process.
Figure 2 exhibits the SEM images of the ZnS powders and ZnS sintered ceramics. As can be seen from Fig. 2(a), obvious micron-scale agglomerates are present within the powders, consisting of small nanoparticles. This is in accordance with the aforementioned XRD analysis, which suggested that the crystallite size of the ZnS powders is on the nanometer scale. The nanoscale particles are responsible for large specific surface area (176.77 m2/g) measured by the BET method, which also enables the particles to have a tendency to agglomerate [27]. In comparison with the powders, the grain size of the sintered ceramic pellets is larger, which is consistent with the comparison between XRD spectra. Figure 2(b) reveals that the ceramics are nearly densified, but some residual pores can still be observed. The density of the sample is measured to be 97.5% by adopting the Archimedes method. Bearing in mind that the hot pressing sintering temperature for ZnS is much higher (around 1000 °C) and the grain size is larger [26], we confirm that SPS with a high heating rate and a short dwell time can aid in lowering the sintering temperature, and suppressing the grain growth during sintering, to some extent. However, as mentioned above, there are some pores in the ceramics sintered by SPS since the short dwell time will be insufficient to allow the complete removal of the pores. In previous work, hot pressing (HP) and SPS were applied to consolidate the ZnS nanopowders. Our preliminary results reveal that the HP method with the longer dwell time will be a more effective method for densifying the ZnS ceramics. Figure 3 shows the TEM images of the ZnS ceramics sintered by SPS and HP. As Fig. 3 indicates, the ZnS ceramics densified by SPS (Fig. 3(a)) exhibit some porosity, while the ZnS ceramics sintered by HP method (Fig. 3(b)) exhibit a highly dense microstructure.
3.2 Elemental analysis
The qualitative elemental analysis of WDS for the ZnS ceramics is shown in Fig. 4. The analysis was conducted by choosing six different areas, in order to eliminate as many errors as possible. As can be seen from the figure, three main elements exist within the sample. Apart from the elements zinc and sulfur, a slight amount of carbon is detected in the ZnS ceramics, resulting from the carbon contamination during the SPS sintering process [28]. According to Table 1, showing the quantitative elemental WDS analysis of the sample (for Zn and S), there is a higher atomic (%) of S present (50.83), than Zn (49.17). It can thus be concluded that nonstoichiometry exists within the ZnS samples, which may be responsible for some imperfections in the crystallite. The excess of sulfur could result in zinc vacancies or sulfur interstitials in the ZnS crystals. Meanwhile, it may also lead to the excess of sulfur elemental species in the ZnS ceramics.
Fig. 4 Qualitative elemental WDS analysis for the sintered ZnS ceramics((a) corresponds to the whole spectrum, (b) to zinc, and (c) to sulfur).
Table 1. Quantitative elemental WDS analysis of the sintered ZnS ceramics (for S and Zn).
Meanwhile, the elemental composition and valences of the elements zinc and sulfur within the sample were detected using XPS. It is shown that the major elements are sulfur and zinc, in accordance with the WDS results. Figure 5 indicates the valences of S and Zn investigated by XPS. It reveals that two peaks are present in the zinc XPS spectrum. The low-energy peak at 1021.3 eV is due to Zn (2p 3/2) and the high-energy peak at 1044.3 eV corresponds to Zn (2p 1/2). Both peaks indicate that the valence of Zn in the sample is divalent and no other valences exist. In the sulfur spectrum, the peak at 161.1 eV is due to the presence of (S2-), and the higher energy peak at 162.1 eV correlates with the elemental sulfur species on the surface of the sample [29]. This supports the assumption that the excess of sulfur results in the excess of elemental S species.
3.3 Optical properties
Figure 6 displays photos of the sintered ZnS ceramics. The pellet is in the diameter of 18.75 mm and in the thickness of 0.83 mm. UV lamp (ENF-280C, Spectroline, USA) was used to irradiate the ZnS ceramic pellet. It should be noted that the ceramic pellet exhibits the property of green phosphorescence in darkness without UV light irradiation, and it exhibits stronger green phosphorescence under UV light of wavelength 365nm because of a higher energy of UV light than that of visible light. Moreover, the green phosphorescence can still be clearly observed in darkness for more than 30 min.
The PL spectra of the ZnS ceramics with green phosphorescence are illustrated in Fig. 7. It can be observed that two emission peaks are detected within the sample. The weak peak at 460 nm is the indication of blue light and the strong peak at 530 nm is the indication of green light. The blue light may result from the self-activation of cation vacancies in the ZnS sample [22, 30].The strong green light, responsible for the green phosphorescence, is consistent with the ceramics observed in the aforementioned photos.
Fig. 7 PL spectra of the ZnS ceramics measured at 300 K in air, with an excitation wavelength of 365 nm (before UV light illumination and after 30 min UV light illumination).
Figure 8 displays the transient PL curve of the ZnS ceramics measured at 300 K in air with an excitation wavelength of 365 nm. The detection range of the fluorescence lifetime spectrometer is 50 μs-50 ms, with a time resolution of 1 ns. The curve is fitted by adopting 3rd order fitting. Table 2 shows the fluorescence lifetime obtained by fitting of the transient PL curve in Fig. 8. τ1 corresponds to the fitted prompt lifetime; τ2 and τ3 correspond to the fitted delayed lifetimes. As it indicates, prompt fluorescence and delayed fluorescence coexist in the sample. Compared with the prompt fluorescence, it can be clearly seen that the lifetime of delayed fluorescence corresponding to phosphorescence is much longer. In addition, the delayed fluorescence ratio (DR), determined by calculating the ratio for the addition of delayed lifetime multiplication with the corresponding decay constants and the total addition of lifetime multiplication with the corresponding decay constants, is calculated to be 97.8%, accounting for most of the luminescence of the sample. The delayed ratio is calculated by the equation:
where τ1 corresponds to the fitted prompt lifetime; τ2 and τ3 correspond to the fitted delayed lifetimes; A1, A2 and A3 are the corresponding decay constants for the lifetimes, respectively.Fig. 8 Transient PL curve of the ZnS ceramics measured at 300 K in air, with an excitation wavelength of 365 nm.
Table 2. Fluorescence lifetime of the ZnS ceramics.
Moreover, the phosphorescence lifetime, in the unit of ms, exactly stands for the long afterglow property. The total photoluminescence quantum efficiency (PLQE) in the ZnS ceramic sample is determined by the ratio of the number of photons emitted, to the number of photons absorbed directly by the instrument. The total PLQE is measured to be 20%. According to the delayed ratio, the PLQE of the prompt component (Φp), and the PLQE of the delayed component (Φd), are calculated to be 19.56% and 0.44%, respectively.
The streak camera with a detection range of 0-10 ms, was employed in further confirming the green phosphorescence of the ZnS ceramics. Figure 9 shows the streak images and PL spectra of the ZnS ceramics for different time ranges of 10 μs, 500 μs, and 10 ms. The measurement was taken with an excitation wavelength of 365 nm at 300 K. In the streak images, the spots correspond to the intensity of photoluminescence: red, yellow, and green signal strong, intermediate and weak emission, respectively [31]. It can be seen from the images that the ZnS ceramics exhibit blue fluorescence (the peak at 460 nm) in the range of 10 μs. For 500 μs, both prompt blue fluorescence and green fluorescence (the peak at 530 nm) exist within the sample. As the time goes on to 10 ms, the green fluorescence is dominant and the peak for the blue fluorescence is weak. It should be noted that background of the streak image of 10 ms is attributed to delayed fluorescence of the ZnS ceramics. The PL spectra of the ZnS ceramics with different observation times are resolved and combined in Fig. 10.These spectra further suggest that the blue fluorescence belongs to the prompt component, and the green fluorescence belongs to the delayed component in the ZnS ceramics. Moreover, it can be seen that the PL spectrum with the time of 10 ms is very similar to the PL spectrum in Fig. 7. Combined with all these aforementioned results, it can be concluded that the as-sintered ZnS ceramics have the characteristic of green phosphorescence.
Fig. 9 Streak images and PL spectra of the ZnS ceramics for different time ranges ((a) 10 μs, (b) 500 μs, (c) 10 ms.)
Fig. 10 PL spectra of the ZnS ceramics with different observation times (10 µs, 500µs, and 10 ms) with an excitation wavelength of 365 nm.
The elemental analysis based on WDS and XPS mentioned above, reveals and confirms the excess of elemental S species. It has been asserted in several references that the main reason for the green luminescence is the elemental sulfur species on the surface [18, 19, 32]. To confirm that the green luminescence is attributed to the excess of elemental sulfur species, the PL spectrum of the ZnS ceramics was measured after 30 min UV light illumination. Figure 7 shows the comparison of the PL spectra before and after UV light illumination. It reveals that the PL intensity for the peak at around 530 nm after UV light illumination is much weaker than the previous one. However, the peak for the blue light does not exhibit a drastic change. It is considered to be derived from the fact that UV light (almost 3-4 eV) is high enough to initiate the photo-oxidation process on the surface of ZnS sample and to transform elemental sulfur species to sulfur dioxides [18, 32], thus leading to the decrease of the PL intensity. The amount of decrease of the PL intensity might be consistent with the amount of transformation from the elemental sulfur species to sulfur dioxides. However, further investigations are needed to confirm the assumption. In addition, PL intensity does not recover after the irradiation of UV light because of the irreversible transformation of the elemental sulfur species to sulfur dioxides. Therefore, it is suggested that the elemental sulfur species is responsible for the green luminescence. Given that WDS and XPS have the limitation of detection of the subsurface elements, XRF is adopted to detect the elemental composition for the whole ZnS ceramic pellets. From the semi-quantitative analysis of XRF, an additional small amount (less than 1%) of Na exists within the sample, in addition to the main elements zinc and sulfur. Based on these results, we assume that it is the co-action of the elemental sulfur species and the imperfections in the ZnS crystals resulting from slight amounts of Na impurities from raw materials that facilitate the release of electromagnetic energy from triplet state to ground state, and prolong the time for the green luminescence [33–36]. Further studies are needed to confirm the more detailed reasons of the phosphorescence.
4. Conclusions
In the present study, ZnS nanopowders were synthesized by a wet chemical precipitation method using Na2S as the source of sulfur. Sintered by spark plasma sintering, the ZnS ceramics with the sphalerite phase were firstly found exhibiting long lifetime (long lasting) green phosphorescence. We confirmed that the ZnS ceramics had the characteristic of green phosphorescence using PL spectra and fluorescence lifetime spectra. Our study reveals that the ZnS ceramics contain an excess of sulfur, which is considered to result from the excess of elemental sulfur species on the surface of the sample. It is concluded that the green luminescence is derived from the elemental sulfur species on the surface, and the green phosphorescence is a result of the co-action of the elemental sulfur species and a slight amount of Na impurities in the ZnS crystals. Further study and analysis will be needed to confirm the source of the phosphorescence.
Acknowledgments
TEM experiments were carried out at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886.
References and links
1. J. M. Huang, Y. Yang, S. H. Xue, B. Yang, S. Y. Liu, and J. C. Shen, “Photoluminescence and electroluminescence of ZnS: Cu nanocrystals in polymeric networks,” Appl. Phys. Lett. 70(18), 2335–2337 (1997). [CrossRef]
2. G. Roussos and H. J. Schulz, “A new emission of ZnS:Ni2+ arising from the 3T1(P) → 3T1(F) transition,” Solid State Commun. 51(9), 663–664 (1984). [CrossRef]
3. W. Chen, J. O. Malm, V. Zwiller, Y. N. Huang, S. M. Liu, R. Wallenberg, J. O. Bovin, and L. Samuelson, “Energy structure and fluorescence of Eu2+ in ZnS: Eu nanoparticles,” Phys. Rev. B 61(16), 11021–11024 (2000). [CrossRef]
4. J. F. Suyver, S. F. Wuister, J. J. Kelly, and A. Meijerink, “Synthesis and photoluminescence of nanocrystalline ZnS: Mn2+,” Nano Lett. 1(8), 429–433 (2001). [CrossRef]
5. R. Sarkar, C. S. Tiwary, P. Kumbhakar, and A. K. Mitra, “Enhanced visible light emission from Co2+ doped ZnS nanoparticles,” Physica B 404(21), 3855–3858 (2009). [CrossRef]
6. D. Amaranatha Reddy, G. Murali, R. P. Vijayalakshmi, B. K. Reddy, and B. Sreedhar, “Effect of Cr doping on the structural and optical properties of ZnS nanoparticles,” Cryst. Res. Technol. 46(7), 731–736 (2011). [CrossRef]
7. S. Shionoya, T. Koda, K. Era, and H. Fujiwara, “Nature of luminescence transitions in ZnS crystals,” J. Phys. Soc. Jpn. 19(7), 1157–1167 (1964). [CrossRef]
8. Y. Li, S. Zhou, Z. Chen, Y. Yang, N. Chen, and G. P. Du, “Luminescence properties of Br-doped ZnS nanoparticles synthesized by a low temperature solid-state reaction method,” Ceram. Int. 39(5), 5521–5525 (2013). [CrossRef]
9. C. Feldmann, T. Jüstel, C. R. Ronda, and P. J. Schmidt, “Inorganic luminescent materials: 100 years of research and application,” Adv. Funct. Mater. 13(7), 511–516 (2003). [CrossRef]
10. D. A. Davies, J. Silver, A. Vecht, P. J. Marsh, and J. A. Rose, “A novel method for the synthesis of ZnS for use in the preparation of phosphors for CRT devices,” J. Electrochem. Soc. 148(10), H143–H148 (2001). [CrossRef]
11. S. Shionoya, W. M. Yen, and T. Hase, Phosphor Handbook (CRC press, 1999).
12. J. T. Randall and M. H. F. Wilkins, “Phosphorescence and electron traps. ii. the interpretation of long-period phosphorescence,” Proc. R. Soc. A 184(999), 390–407 (1945). [CrossRef]
13. F. Clabau, X. Rocquefelte, T. Le Mercier, P. Deniard, S. Jobic, and M. H. Whangbo, “Formulation of phosphorescence mechanisms in inorganic solids based on a new model of defect conglomeration,” Chem. Mater. 18(14), 3212–3220 (2006). [CrossRef]
14. M. W. Wang, L. D. Sun, X. F. Fu, C. S. Liao, and C. H. Yan, “Synthesis and optical properties of ZnS: Cu (II) nanoparticles,” Solid State Commun. 115(9), 493–496 (2000). [CrossRef]
15. A. A. Bol, J. Ferwerda, J. A. Bergwerff, and A. Meijerink, “Luminescence of nanocrystalline ZnS: Cu2+,” J. Lumin. 99(4), 325–334 (2002). [CrossRef]
16. P. Yang, M. Lü, D. Xü, D. Yuan, J. Chang, G. Zhou, and M. Pan, “Strong green luminescence of Ni2+-doped ZnS nanocrystals,” Appl. Phys., A Mater. Sci. Process. 74(2), 257–259 (2002). [CrossRef]
17. T. Tsuruoka, C. H. Liang, K. Terabe, and T. Hasegawa, “Origin of green emission from ZnS nanobelts as revealed by scanning near-field optical microscopy,” Appl. Phys. Lett. 92(9), 091908 (2008). [CrossRef]
18. C. H. Ye, X. S. Fang, G. H. Li, and L. D. Zhang, “Origin of the green photoluminescence from zinc sulfide nanobelts,” Appl. Phys. Lett. 85(15), 3035–3037 (2004). [CrossRef]
19. C. H. Ye, X. S. Fang, M. Wang, and L. D. Zhang, “Temperature-dependent photoluminescence from elemental sulfur species on ZnS nanobelts,” J. Appl. Phys. 99(6), 063504 (2006). [CrossRef]
20. H. T. Chen, Y. P. Hu, and X. H. Zeng, “Green photoluminescence mechanism in ZnS nanostructures,” J. Mater. Sci. 46(8), 2715–2719 (2011). [CrossRef]
21. X. L. Wang, J. Y. Shi, Z. C. Feng, M. R. Li, and C. Li, “Visible emission characteristics from different defects of ZnS nanocrystals,” Phys. Chem. Chem. Phys. 13(10), 4715–4723 (2011). [CrossRef] [PubMed]
22. J. C. Lee and D. H. Park, “Self-defects properties of ZnS with sintering temperature,” Mater. Lett. 57(19), 2872–2878 (2003). [CrossRef]
23. A. Ikesue and Y. L. Aung, “Ceramic laser materials,” Nature Photon. 2(12), 721–727 (2008). [CrossRef]
24. H. Hu and W. H. Zhang, “Synthesis and properties of transition metals and rare-earth metals doped ZnS nanoparticles,” Opt. Mater. 28(5), 536–550 (2006). [CrossRef]
25. H. P. Klug and L. E. Alexander, X-ray Diffraction Procedures: For Polycrystalline and Amorphous Materials (Wiley-Interscience, 1974).
26. L. A. Xue and R. Raj, “Effect of hot‐pressing temperature on the optical transmission of zinc sulfide,” Appl. Phys. Lett. 58(5), 441–443 (1991). [CrossRef]
27. C. Chlique, O. Merdrignac-Conanec, N. Hakmeh, X. H. Zhang, and J. L. Adam, “Transparent ZnS ceramics by sintering of high purity monodisperse nanopowders,” J. Am. Ceram. Soc. 96(10), 3070–3074 (2013).
28. C. Chlique, G. Delaizir, O. Merdrignac-Conanec, C. Roucau, M. Dollé, P. Rozier, V. Bouquet, and X. H. Zhang, “A comparative study of ZnS powders sintering by Hot Uniaxial Pressing (HUP) and Spark Plasma Sintering (SPS),” Opt. Mater. 33(5), 706–712 (2011). [CrossRef]
29. D. E. Dunstan, A. Hagfeldt, M. Almgren, H. O. Siegbahn, and E. Mukhtar, “Importance of surface reactions in the photochemistry of zinc sulfide colloids,” J. Phys. Chem. 94(17), 6797–6804 (1990). [CrossRef]
30. F. A. Kröger and H. J. Vink, “The origin of the fluorescence in self‐activated ZnS, CdS, and ZnO,” J. Chem. Phys. 22(2), 250–252 (1954). [CrossRef]
31. A. Endo, K. Sato, K. Yoshimura, T. Kai, A. Kawada, H. Miyazaki, and C. Adachi, “Efficient up-conversion of triplet excitons into a singlet state and its application for organic light emitting diodes,” Appl. Phys. Lett. 98(8), 083302 (2011). [CrossRef]
32. N. Kumbhojkar, V. V. Nikesh, A. Kshirsagar, and S. Mahamuni, “Photophysical properties of ZnS nanoclusters,” J. Appl. Phys. 88(11), 6260–6264 (2000). [CrossRef]
33. K. Osada, “Phosphorescence of sodium-and potassium-acetates,” J. Phys. Soc. Jpn. 11(4), 425–429 (1956). [CrossRef]
34. K. Osada, “Phosphorescence of sodium acetate,” J. Chem. Phys. 30(5), 1363–1364 (1959). [CrossRef]
35. R. M. A. Von Wandruszka and R. J. Hurtubise, “Room-temperature phosphorescence of compounds adsorbed on sodium acetate,” Anal. Chem. 49(14), 2164–2169 (1977). [CrossRef]
36. G. V. Colibaba, D. D. Nedeoglo, and V. V. Ursaki, “Impurity centers in ZnSe: Na crystals,” J. Lumin. 131(9), 1966–1970 (2011). [CrossRef]
