Transcript

Sol–gel synthesis and luminescence of undoped and Mn-doped zincorthosilicate phosphor nanocomposites

J. El Ghoul a,n, L. El Mir a,b

a Laboratoire de Physique des Matériaux et des Nanomatériaux Appliquée à l’Environnement, Faculté des Sciences de Gabès,Cité Erriadh Manara Zrig, 6072 Gabès, Tunisieb Al Imam Mohammad Ibn Saud Islamic University (IMSIU), College of Sciences, Departement of Physics, Riyadh 11623, Saudi Arabia

a r t i c l e i n f o

Article history:Received 19 July 2013Received in revised form20 October 2013Accepted 30 November 2013Available online 7 December 2013

Keywords:Zn2SiO4

PhotoluminescenceNanoparticlesSol–gelOptical materials

a b s t r a c t

Zn2SiO4 and Zn2SiO4:Mn particles embedded in SiO2 host matrix prepared by sol gel method undersupercritical conditions of ethyl alcohol in two steps. Were prepared by a simple solid-phase reactionunder natural atmosphere at 1200 1C after the incorporation of ZnO and ZnO:Mn nanoparticles,respectively, in silica monolith. In the case of SiO2/Zn2SiO4 nanocomposite, the powder with an averageparticle size of 80 nm shows a strong luminescence band centred at around 760 nm in the visible range.In addition, the PL spectrum for the SiO2/Zn2SiO4:Mn nanocomposite showed that a dominant peak at525 nm appeared, which originated from the 4T1–6A1 transitions of Mn2þ ions. The luminescenceproperties of nanocomposites were characterized by emission and excitation spectra as well theirdependencies of upon temperature and power excitation density.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Over the last decade, the luminescent properties of silicate basedinorganic phosphors have been extensively investigated [1–4].Among the silicates, one of the promising candidates so far ismanganese doped zinc silicate (Zn2SiO4:Mn) phosphor [5–8], whichhas been used in cathode ray tubes, fluorescent lamps, and plasmadisplay panels due to its high luminescence efficiency and chemicalstability. Although researches about the effect of Mn2þ ions onluminescent properties have been performed, most of the studiesconcerning the amount of Mn2þ ions substituting for Zn2þ ionshave chemically focused on the quantities of Mn2þ ions in Zn2SiO4.Since the amount of Mn2þ ions doped into the crystal lattice ofZn2SiO4 is closely related to the performance of Zn2SiO4:Mn2þ

phosphor such as decay time, color shift and concentration quench-ing, the determination of the quantities of Mn2þ ions in the crystallattice is more essential. Furthermore, the site preference of Mn2þ

ions is indispensible to advance the luminescent properties of greenor yellow color-emitting phosphor.

It is well known that the luminescence properties of phosphordepend on its synthesis process. Usually, Zn2SiO4:Mn2þ phosphorsare prepared by solid-state reactions [9]. In solid-state reaction,

it requires high-temperature process with long period of time.Therefore, agglomerated particles with irregular shape and largesize are produced by the solid-state reaction method. Accordingly,grinding and milling process, which is harmful to the luminescentproperties, are necessary to reduce the particle size desired for theapplication [10]. In recent years, many new preparation approacheshave been proposed, including spray pyrolysis route, sol–gel method,chemical vapor synthesis and hydrothermalmethod [11–14]. Amongthese methods, the hydrothermal synthesis has great advantage,such as better distribution of metal ions, controllable morphologyand lower cost [15–18]. Therefore, it is imperative to develop asimple synthesis route in order to potentially overcome most ofthese difficulties in commercialization [19]. It is in this context thatare situated the work of our team. It comes to new protocols findnanoparticle synthesis by the sol–gel technique for various chemicaland physical applications [20,21]. Our objective in this work is, in afirst step the in-situ synthesis of luminescent nanoparticles ofZn2SiO4:Mn in a silica matrix whose objective is to increase the lifeof components and prevent contamination caused by the externalenvironment. The sol–gel method has been confirmed to have moreadvantages in lowering the firing temperature, distributing theactivator ions homogeneously and improving the emission efficiencyfor the powder phosphors [22,23].

In this study, the method is applied to prepare Zn2SiO4 andZn2SiO4:Mn particles embedded in silica monolith by the sameprotocol of sol–gel method combined with a furnace firing [3,9] butusing for the first time, manganese doped zinc oxide nanoparticles

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Journal of Luminescence

0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jlumin.2013.11.090

n Corresponding author. Tel.: þ216 97 86 35 99; fax: þ216 75 39 24 21.E-mail address: [email protected] (J. El Ghoul).

Journal of Luminescence 148 (2014) 82–88

and studied the structural and optical properties of the obtainednanocomposites.

2. Experimental

2.1. Synthesis

The preparation of colloid suspension particles in silicate hostmatrix has been done in three steps. In the first one, nanocrystallineZnO and ZnO:Mn aerogels were prepared by a sol–gel methodunder supercritical conditions of ethyl alcohol (EtOH) based onOmri et al. protocol [9,24], where the water for hydrolysis wasslowly released by esterification reaction to control the size of theformed nanoparticles. In the second step, we have prepared ZnOand ZnO:Mn confined in silica aerogel according to the followingprocess: 0.5 ml of TEOS was first dissolved in EtOH. Then, withconstant stirring of the mixture of TEOS and EtOH, 0.44 ml of waterand 30 mg of nanoparticles powder prepared in the first step wereadded. The whole solution was stirred for about 30 min, resulting inthe formation of a uniform sol. The sols were transferred to tubes inultrasonic bath where 100 ml of fluoride acid was added. The wetgel formed in few seconds. Monolithic and white aerogel wasobtained by supercritical drying in EtOH as described in the firststep. Finally, silica glasses containing Zn2SiO4 and Zn2SiO4:Mnparticles were obtained after firing aerogel at 1200 1C for 2 h.

2.2. Characterizations

The crystalline phases of annealed samples were identified byX-ray diffraction (XRD) using a Bruker D5005 powder X-raydiffractometer using a CuKα source (1.5418 Å radiation). Crystallitesizes were estimated from the Scherrer's equation [25].

G¼ 0:9λB cos θB

ð1Þ

where λ is the X-ray wavelength (λ¼1.5418 Å), θB is the maximumof the Bragg diffraction peak (rad) and Β is the line width at halfmaximum.

Transmission electron microscopy (TEM, JEM-200CX) was used tostudy the morphology and particle size of the phosphor powders.The specimens for TEM were prepared by putting the as-grownproducts in EtOH and immersing them in an ultrasonic bath for15 min, then dropping a few drops of the resulting suspensioncontaining the synthesized materials onto TEM grid. The opticalabsorbance of the powders was determined using a Schimadzu UV-3101 PC spectrophotometer with integrating sphere in the wave-length range 200–2000 nm. For photoluminescence (PL) measure-ments, the 450-W Xenon lamp was used as an excitation source. Theemitted light from the sample collected by an optical fiber on thesame side as that of excitation was analyzed with a Jobin-YvonSpectrometer HR460 and a multichannel CCD detector (2000 pixel).The photoluminescence excitation (PLE) measurements were per-formed on a Jobin-Yvon Fluorolog 3-2 spectrometer. The lowtemperature experiments were carried out in a Janis VPF-600 Dewarwith variable temperature controlled between 78 and 300 K.

3. Results and discussion

3.1. Structural studies

The XRD spectra of undoped and Mn doped ZnO nanopwderhave been presented in Fig. 1. In the case of undoped ZnO, wenoticed the appearance of nine pronounced diffraction peaks at2θ¼31.501, 34.191, 36.151, 47.391, 56.441, 62.791, 67.931, 68.911,

72.331, 76.971, 89.411 and 95.311 which can be attributed to the(1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (0 1 3), (1 1 2), (2 0 1),(0 2 2), (0 2 3) and (2 1 1) planes of ZnO, respectively [26]. Theobtained XRD spectra matched well with the space group P63mc(1 8 6) (No. 36-1451) of wurtzite ZnO structure [24,26]. Afterdoping of ZnO with manganese, in addition to the peaks corre-sponding to ZnO, one secondary additional phase were detectedwhich can be attributed to ZnOMn3, (JCPDS Card 37-1485). Thelattice constants calculated from the XRD pattern, which are veryclose to ZnO ones, i.e., a¼ 3:2498Å and c¼ 5:2066Å. These resultsare in a good agreement with those obtained by El Ghoul et al.[26–28]. Due to the small size of the crystallites in the aerogel, thediffraction lines are broadened and are further found to depend onthe Miller indices of the corresponding sets of crystal planes. Forour samples, the (0 0 2) diffraction line is always narrower thanthe (1 0 1) line and the latter is narrower than the (1 0 0) line.After a correction for the instrumental broadening, an averagevalue of the basal diameter of the cylinder-shape crystallites wasfound to be 14–22 nm, whereas the height of the crystallites was23–30 nm.

Fig. 2 shows the X-ray diffraction patterns of SiO2/Zn2SiO4 andSiO2/Zn2SiO4:Mn nanocomposites treated at 1200 1C for 2 h in air.It is clear that all the diffraction peaks are in good agreement with

Fig. 2. X-ray diffraction pattern of the (a) SiO2/Zn2SiO4 and (b) SiO2/Zn2SiO4:Mnnanocomposite.

Fig. 1. X-ray diffraction pattern of the (a) undoped ZnO and (b) ZnO:Mn nanopowder.

J. El Ghoul, L. El Mir / Journal of Luminescence 148 (2014) 82–88 83

those of the standard pattern reported by the Joint Committee onPower Diffraction Standard (JCPDS No. 08-492 and 37-1485)[21,28]. The peak signatures of hexagonal wurtzite ZnO were alsoobserved. Therefore, the hexagonal ZnO and willemite Zn2SiO4

may coexist in the composite [6]. After incorporation of thesenanoparticles ZnO:Mn in SiO2 and thermal treatment, a new zincic(α-Zn2SiO4) compound was formed (Fig. 2b). The willemite(α-Zn2SiO4) crystals were well developed. This analysis showsthat the peaks of these samples are indexed to the α-Zn2SiO4

(JCPDS Card 37-1485) [28–30]. The difference in the peak intensityand width shows that the seeded sample has a higher degree ofcrystallinity. In fact the presence of manganese or vanadium inthese materials improves their crystallinity and this was observedin our previous works [9,20,21]. The lattice constants calculatedfrom the XRD pattern are a¼ 13:944Å and c¼ 9:314Å, which arevery close to willemite α-Zn2SiO4 ones, i.e., [29]. This resultindicates that α-Zn2SiO4 has a rhombohedral structure [30]. It isclear that the crystalline phase is the most dominant one corre-sponding to the α-phase Zn2SiO4, in parallel we note the appear-ance two other phases of silica that correspond to cristobalite andquartz [28]. Average grain size (G) of the crystallites Zn2SiO4:Mnvaries from 70 to 90 nm [30], which has been estimated usingScherrer's formula (1). However, Jiang et al. [29] predicted thatthe optimum particle size for the Zn2SiO4:Mn nanopowder was100 nm.

We investigated the size and morphology of our samples byTEM. The TEM images of undoped and Mn-doped ZnO nanopar-ticles (Fig. 3) show that very small ZnO particles are present in theas-prepared aerogel powder. The size of the majority of ZnOparticles in this powder varied between 15 and 30 nm [6]. Takinginto account the results of crystallite size measurements by XRD, it

can be concluded that the crystallite size is approximately equal tothe particle size in ZnO and ZnO:Mn powder prepared in thepresent work. Fig. 4 shows the TEM micrographs of the samplesZn2SiO4 and Zn2SiO4:Mn indicating that the well-crystallizedZn2SiO4 particles. At high temperature at 1200 1C, Zn and Sispecies, move and diffuse inside the porous body to form Zn2SiO4

phase with a particle size greater than 80 nm (Fig. 4a). In the caseof Zn2SiO4:Mn colloid suspension is formed in silica host matrixwith a particle size of about 70 nm (Fig. 4b). Energy dispersivespectroscopy (EDX) analysis during the TEM observation, areshown in Fig. 4c and d. The concentrations of Zn, Si, and O areshown in the corresponding tables, which correspond to thestoichiometric composition of Zn2SiO4. The excess Zn ions overthe stoichiometric ratio of Zn2SiO4 crystals exist at the near-surface side due to low Si concentrations at a distance far fromthe quartz surface, and they are also texturized along with somepreferred orientations of Zn2SiO4:Mn2þ crystals as seen in theXRD results. The other peaks such as Cu and C come from coppergrid and contamination.

3.2. Optical studies

Study of the optical absorption spectra provides a very usefultool for the investigation of optically induced transition andinsight in the energy gap and band structure of crystalline andnon crystalline materials. However, the absorption spectrum ofα-Zn2SiO4:Mn nanoparticles in the silica matrix at room tempera-ture are presented in Fig. 5. For the purpose of comparison, Zn2SiO4

nanoparticles without Mn doping are presented in this figure aswell. A strong absorption band with a maximum at 215 nm isobserved in both α-Zn2SiO4:Mn and Zn2SiO4 nanoparticles,

Fig. 3. Typical TEM photograph showing the general morphology (a) ZnO and (b) ZnO:Mn nanopowder and its EDX analysis.

J. El Ghoul, L. El Mir / Journal of Luminescence 148 (2014) 82–8884

indicating that this band originates from the absorption of Zn2SiO4

host lattice. A typical value of absorption edge at 220 nm(�5.63 eV), which was the absorption edge of Zn2SiO4 [31].Furthermore, by a comparison of the absorption curve of Zn2SiO4

nanoparticles with that of Zn2SiO4:Mn nanoparticles, it can beclearly seen that there is more absorption in the latter than in theformer in the range 220–280 nm. Obviously, this is attributed to theabsorption of Mn2þ charge transfer transition in Zn2SiO4 host lattice.Due to the low concentration of Mn2þ , the Mn2þ absorption is notstrong [31]. The values obtained for the band gap energies of theZn2SiO4 pur is about 5.493 eV, whereas the α-Zn2SiO4:Mn is around5.347 eV (Fig. 5, inset) [32]. We suggest that this decrease is due tothe reduction of the bandwidth of ZnO after doping with Mn. In fact,it was shown that the band gap energies of the ZnO materials show

a decrease with the increase of doping concentration [24,33]. For thelow Mn concentration the reduction in the band gap has beentheoretically explained as a consequence of exchange interactionbetween d electrons of the transition metal ions (Mn) and the s andp electron of the host band.

3.3. Photoluminescence property

In the present investigation, the emission spectra of SiO2/Zn2SiO4 nanocomposite at different temperatures, recorded inthe spectral range 365–1000 nm are shown in Fig. 6. The samplehas broad PL bands centered at different positions. The strikingfeature is the absence of almost any of the usually reported visibleemission bands in the range 400–650 nm (2.4–2.7 eV), and theappearance of a strong and wide near infrared (NIR) emission

Fig. 4. TEM photographs showing the general morphology of the (a) SiO2/Zn2SiO4 and (b) SiO2/Zn2SiO4:Mn nanocomposite and its EDX analysis.

Fig. 5. Absorption spectra of (a) pure SiO2/Zn2SiO4 and (b) SiO2/α-Zn2SiO4:Mn. Theinset shows the absorption of Mn2þ .

Fig. 6. PL spectra of the SiO2/Zn2SiO4 nanocomposite at different temperatures.

J. El Ghoul, L. El Mir / Journal of Luminescence 148 (2014) 82–88 85

band centered around 760 nm, besides a near band edge emissionincluding the bound exciton line. The observed UV–visible emis-sion band is also quite different from what is usually seen [6].

Fig. 7 shows the emission intensities at different temperaturesof Zn2SiO4:Mn nanocomposites synthesized by sol–gel method.The green emission has been assigned to an electronic transition of4T1(4G)-6A1(6S) peaking at the wavelength 525 nm and which is aparity forbidden emission transition of Mn2þ ions [34]. The graphshows that the relative PL intensity of the phosphor increases asthe at increases measure temperatures. This emission centered at522 nm, corresponds to the energy transfer in the Mn ions [35].With Mn2þ occupies part of the Zn2þ sites, which is coordinatedby four oxygen atoms [34]. The weak crystal field around Mn2þ

results in the low splitting width of its 3d energy levels, inaccordance with the observations of Stevels et al. [36]. As a result,an emission at high energy (green) is observed. The red-shift ofthe emission band with the increasing temperature is due to theexchange interactions between ions with the Mn2þ ions [37].In willemite the Mn2þ ions are situated at the slightly distortedtetrahedral sites with four oxygen neighbors [37]. The appliedexcitation photon energy of 4.86 eV (255 nm) is smaller than theband gap of Zn2SiO4. The excitation of Mn2þ ions follows theirionization (transition from the ground state to the conductionband) and the non-radiative relaxation of the electrons to theexcited state 4T1 of Mn2þ [38]. From the emission spectra, a slightred-shift of the peak maximum is observed while the emissionintensity increases. It is generally recognized that the lumines-cence of the Mn2þ ion depends on the Zn2SiO4 host crystal field.Mn2þ ions in the Zn2SiO4 host with higher crystallinity feel astronger crystal field. Increasing the crystal field reduces theenergy difference of the ground and first excited state, resultingin peak broadening and red-shift of the emission peak [8].

PLE spectra of SiO2/Zn2SiO4 and SiO2/Zn2SiO4:Mn nanocompo-sites at the measurement temperature T¼78 K have been pre-sented in Fig. 8. In the case of SiO2/Zn2SiO4, the PLE spectrumdetected at 760 nm which shows the appearance of a very weakpeak at 375 nm (3.3 eV) relative to its value at higher energy(Fig. 8a). The low energy excitation band is due to carrier excita-tion in the near band edge of ZnO nanoparticles [6]. Indeed, as ithas been shown by Chakrabarti et al. [39], a high annealingtemperature (1073 K) results in a rapid grain growth and whenthe radii of the nanoparticles increases to 8.2 nm, a bulk ZnO likeband gap is obtained. However, the most efficient excitationprocess is with photon energies of about 5.4 eV (230 nm), which

are much higher than the ZnO bulk band gap. Unfortunately, thehigh energy peak position of the PLE spectrum cannot be clearlydetermined due to the high energy range limit of our setup. Theshape and the structured nature of the PL emission band, the largeshift between the PL and the PLE energy peaks of the 760 nm PLemission are in principle a signature of a deep level emission withan electron–phonon coupling. However, its full width at halfmaximum (FWHM) dependence with temperature cannot befitted according to this model, which rules out the hypothesis ofan electron–phonon coupling [6].

One should also consider the possible connection of the ZnO/Zn2SiO4 and Zn2SiO4/SiO2 interface states with the observedemission band. Indeed, it has been demonstrated that Si–O–Znbonds can be formed in sol–gel ZnO/SiO2 composites [7], resultingin the creation of interface states. But, as it has been explained byFu et al. [40], the Zn–O–Si interface state is shallow as compared toVO or Zni related states. As a consequence, these interface statescould not be involved in our NIR emission band. The luminescencespectra shown in Fig. 6 show a splitting of the 760 nm peak fortemperatures below 150 K, the origin of this splitting is probablydue to competitive luminescence process between NBOHCs andhydroxyl groups in the interfaces. The absence of the commonlyreported green–yellow emission band can be explained by the factthat, during high temperature annealing in air, oxidation processshould take place. As a result oxygen is incorporated in interfacesites and should therefore induce a large decrease in intensity ofthe OH-related PL emission [6].

Furthermore, the PLE spectra of the band detected at 525 nm(Fig. 8b) show a strong excitation band ranging from 240 to300 nm with a maximum at about 255 nm (4.9 eV) compared tobands in UV–Vis range. The band at 255 nm is considered to beresponsible for the emission at 525 nm. The spectra fully agreewith previously measured excitation spectrum of Zn2SiO4:Mn [40].The broad excitation peak at 255 nm could be attributed to acharge transfer transition (or the ionization of manganese) fromthe divalent manganese ground state (Mn2þ) to the conductionband (CB). This transition will be further discussed later based onthe previous literature reports [41,42]. In addition to the CT band,other bands (inset) of Mn2þ (d–d) transitions are also observed athigher wavelengths (350–500 nm), these are caused by crystalfield splitting of the 4D and 4 G levels as shown by the Orgeldiagram for Mn2þ [43,44]. The electrons in the 6A1 (6S) ground stateof Mn2þ ions, are excited to the conduction band of Zn2SiO4 byphotons, and the free electrons in the conduction band relax back tothe 4T1 (4G) excited state through a non-radiative process [45].

Fig. 7. PL spectra of the SiO2/Zn2SiO4:Mn nanocomposite at different temperatures.(For interpretation of the references to color in this figure legend, the reader isreferred to the web version of this article.)

Fig. 8. PLE spectra of the (a) SiO2/Zn2SiO4 and (b) SiO2/Zn2SiO4:Mn nanocomposite.

J. El Ghoul, L. El Mir / Journal of Luminescence 148 (2014) 82–8886

Finally, this is followed by a radiative transition from the 4T1 (4G)excited state to the 6A1 (6S) ground state, giving rise to a greenemission band (525 nm).

We have studied the intensity and the peak energy dependen-cies of the 760 nm PL band upon power excitation density andtemperature as can be seen in Fig. 9. The emission intensity showsa linear variation with the power excitation (inset of Fig. 9).Furthermore, no change in the PL spectra, neither the shape norposition, was observed with power excitation, even after beingaged for over 90 days, indicating the time stability of thecomposite material. This result strongly suggests that the produc-tion of nanoparticles in SiO2 matrix by sol–gel is a simple way tomaintain the luminescence spectra of the nanoparticles.

On the other hand, exhibited a more significant red-shift andbroadening of the UV photoluminescence peak with increasingexcitation power. We have studied the intensity and the peakenergy dependence of the PL band versus power excitation densityof SiO2/Zn2SiO4:Mn nanocomposites as can be seen in Fig. 8. Theemission intensity shows a similar linear variation with the powerexcitation which suggest that the two emission bands have thesame origin. The green luminescence is the conventional green ofSiO2/α-Zn2SiO4:Mn2þ nanocomposite, occurring at about 525 nm,a well known, it corresponds to an internal transition of Mn2þ in

α-Zn2SiO4 phase and the second bands in the range 560–608 nmfor Mn2þ in the β-Zn2SiO4 phase [6]. Also, the strong emissionspeaks around 525 and 590 nm are observed only under thehighest excitation power density 70 MW/cm2 (Fig. 10). The sharp-ness of the peak can be ascribed as due to the uniformity ofparticle sizes. The emission intensity shows a linear variation withthe power excitation (inset Fig. 10). Furthermore, no change in thePL spectra, neither the shape nor position, was observed withpower excitation, even after being aged for a longue period oftime, indicating the time stability of the composite material. Theratio of the 525/630 nm peaks is quite different in the spectrapresented in Fig. 7 and Fig. 10 for 78 K. In fact in Fig. 7 the power ofexcitation (450 W/cm2) is very low compared to the others used inFig. 10, this confirms again that the two peaks have differentorigins. These result prove that there a origin of the luminescentcentre and the nature of this band has so far not completely beenestablished and calls for further investigations, particularly thestudy of the power excitation effect above 70 MW/cm2.

4. Conclusion

Un-doped and manganese-doped ZnO nanoparticles aerogelwere synthesized by sol–gel method from zinc acetate dihydrateas a precursor. These particles were obtained by slow hydrolysis ofthe precursors using an esterification reaction, followed by asupercritical drying in EtOH. The X-ray diffraction and TEM showa crystalline phase with a particle size ranging between 15 and30 nm. Upon incorporation of ZnO nanoparticles in SiO2 and heattreatment at 1200 1C, Zn2SiO4 phase was formed, and coexist withZnO phase in SiO2 host matrix. PL spectra of the obtainedmonolithe showed a strong near-infrared luminescence bandand the absence of the commonly reported visible emission bands.From the analysis of the PL and PLE spectra, it can be concludedthat the excitation peak near 760 nm can be connected with theformation of NBOHs excited at the spectral region hνZ5.4 eV,such band arises from the absorption of Zn2SiO4 particles. Thissuggests that an energy transfer occurs from Zn2SiO4 particles toNBOHs interfaces defects. In the case of the incorporation of ZnO:Mn nanoparticles in SiO2 and heat treatment at 1200 1C, Zn2SiO4:Mn phase was formed in SiO2 host matrix. From the analysis of thePL and PLE spectra, it can be concluded that the luminescenceband at 525 nm can be attributed to Mn2þ in Zn2SiO4 particle. Theadvantages of this method include simplified procedure, lowannealing temperature, timesaving, controllable size and mor-phology, large-scale production and wide practicality for otherphosphor materials, which have potential applications in display-ing and lighting fields.

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