Superlattices and Microstructures 70 (2014) 24–32

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Superlattices and Microstructures

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . co m / l o c a t e / s u p e r l a t t i c e s

Effect of manganese concentrationon photoluminescence properties ofZn2SiO4:Mn nanophosphor material

http://dx.doi.org/10.1016/j.spmi.2014.02.0220749-6036/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: Laboratory of Physics of Materials and Nanomaterials Applied at Environment (LaPhyMNUniversity, Faculty of Sciences in Gabes, Gabes, Tunisia. Tel.: +216 97 40 87 56; fax: +216 75 39 24 21.

E-mail address: Lassaad.ElMir@fsg.rnu.tn (L. El Mir).

K. Omri a, L. El Mir a,b,⇑a Laboratory of Physics of Materials and Nanomaterials Applied at Environment (LaPhyMNE), Gabes University, Faculty of Sciences inGabes, Gabes, Tunisiab Al Imam Mohammad Ibn Saud Islamic University (IMSIU), College of Sciences, Department of Physics, Riyadh 11623, Saudi Arabia

a r t i c l e i n f o a b s t r a c t

Article history:Received 1 January 2014Accepted 24 February 2014Available online 14 March 2014

Keywords:Zn2SiO4

NanophosphorsSol–gelLuminescenceOptical materials

Nanophosphor b-Zn2SiO4:Mn with bright yellow light emissionwere synthesized by a sol–gel process. These samples were pre-pared by a simple solid-phase reaction under natural atmosphereat 1500 �C after the incorporation of ZnO:Mn nanoparticles, in sil-ica monolith. X-ray diffraction (XRD) and transmission electronmicroscopy (TEM) were used to characterize the phase purity, par-ticle size and morphology. In addition photoluminescence (PL) wasused for optical study. The PL spectrum for the b-Zn2SiO4:Mn nano-phosphors showed a dominant peak at 574 nm, which originatedfrom the 4T1 ?

6A1 transitions of Mn2+ ions. The level of manganesedoping did not greatly affect the crystallinity, but did affect theluminescence of nanophosphors. Upon 255 nm excitation, theluminescence decay time of the yellow emission of b-Zn2SiO4 witha Mn doping concentration of 2 at.% around 574 nm is 13 ms. Thecharacteristics of crystallinity, morphology and luminescenceproperty of the obtained nanophosphors were investigated.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

In recent years, the market for white light emitting diodes (LED) is expanding rapidly. In the light ofniche applications, like the field emission displays, plasma display panels (PDPs), and X-ray imaging

E), Gabes

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scintillators have excited a lot of studies on the luminescence properties of inorganic phosphors [1]such as oxides, phosphates, silicates, and aluminates. Among all the known inorganic phosphors,the silicate such as manganese doped zinc silicate (Zn2SiO4:Mn) is an efficient and potentialgreen-emitting phosphor and has been widely used in the PDPs, cathode ray tubes, and fluorescentlamps because of its high saturated color, high luminescence efficiency, and chemical stability [2,3].Up to now, the traditional solid-state reaction process is mainly employed to produce the commerciala-Zn2SiO4:Mn green phosphors with high crystallinity and efficient luminescence [4]. Traditionally,solid state diffusion method has been employed for the synthesis of Zn2SiO4, which involves crushing,grinding, ball milling, and sintering of source materials at very high temperatures. But, nowadays,sol–gel, forced precipitation, pulsed laser deposition (PLD), organometallic complex route, combustionmethods, dry reaction, spray-pyrolysis, polymer assisted methods, etc. [5–12] are widely used to syn-thesize Zn2SiO4 nanophosphor powders or thin films. Zeng et al. [13] synthesized Zn2SiO4 using hydro-thermal method with possible lowest crystallization temperature. Lukic et al. [14] synthesizedpolymer assisted Zn2SiO4 phosphor powders using the sol–gel method, where polymer polyethyleneglycol is utilized to produce gel. Recently, Lee et al. have reported the nanofabrication of the patternedlayers of Zn2SiO4:Mn2+ phosphor using biogenic silica [15,16].

However, a post-treatment is still necessary in these preparation methods for obtaining high crys-tallinity and high purity phase. Such a post-treatment strongly affects the photoluminescence proper-ties of Zn2SiO4:Mn phosphors. Therefore, it is of importance to develop a new approach for preparingZn2SiO4:Mn phosphor without any post-treatment.

The long decay time of Zn2SiO4:Mn is an obstacle for plasma display panels (PDP) application. Thus,lot of efforts have been dedicated to shorten the decay time of Zn2SiO4:Mn phosphor particles withoutlosing the much efficiency [17]. A more conspicuous feature is that some manganese ions make Mn–Mn pairs as the Mn concentration increases [18]. Barthou et al. [19] identified experimentally the de-cay process of both isolated Mn ions and Mn–Mn pairs by measuring the extremely high and low con-centration of Mn. Based on the above findings, several research groups [19–21] have investigatedthese materials in terms of manganese doping content and developed some smart understandingsboth theoretically and experimentally. There is, however, still a need for a systematic understandingof the decay process in moderate Mn concentration regime (0.05–0.12) in which favorableintensity and decay time are expected. On account of very complicated interactions between Mn ions,however, the decay behavior is too complicated to be interpreted with ease in this Mn concentrationrange [19].

Therefore, in this paper, we pose a new and well-defined question that how the photoluminescenceproperties of b-Zn2SiO4 are going to be changed for different Mn contents, which may lead to a mul-tiplied change in the PL properties? Characteristics of nanophosphors such as phase purity, crystallin-ity, mean size, morphology, and luminescence properties were investigated.

2. Experimental procedure

2.1. Preparation of b-Zn2SiO4:Mn nanophosphors

The preparation of colloid suspension particles in silicate host matrix has been done in three steps.In the first one, nanocrystalline ZnO:Mn aerogels were prepared by a sol–gel method under supercrit-ical conditions of ethyl alcohol (EtOH) based on El Mir et al. protocol [3], where the water for hydro-lysis was slowly released by esterification reaction to control the size of the formed nanoparticles. Inthe second step, we have prepared ZnO:Mn confined in silica aerogel according to the following pro-cess: 0.5 ml of TEOS was first dissolved in EtOH. Then, with constant stirring of the mixture of TEOSand EtOH, 0.44 ml of water and 30 mg of nanoparticles powder prepared in the first step were added.The whole solution was stirred for about 30 min, resulting in the formation of a uniform sol. The solswere transferred to tubes in ultrasonic bath where 100 ml of fluoride acid was added. The wet gelformed in few seconds. Monolithic and white aerogel was obtained by supercritical drying in EtOHas described in the first step. Finally, silica glasses containing b-Zn2SiO4:Mn particles were obtainedafter firing aerogel at 1500 �C for 2 h.

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2.2. Characterization

The crystalline phases of our samples were identified by X-ray diffraction (XRD) using a BrukerD5005 powder X-ray diffractometer using a Cu Ka source (1.5418 Å radiation). Crystallite sizes(G, in Å) were estimated from the Scherrer’s equation [21]:

G ¼ 0:9kB cos hB

ð1Þ

where k is the X-ray wavelength (1. 5418 Å), hB is the maximum of the Bragg diffraction peak (in radi-ans) and B is the linewidth at half maximum. Transmission electron microscopy (TEM, JEM-200CX)were used to study the morphology and particle size of the phosphor powders. The specimens forTEM were prepared by putting the as-grown products in EtOH and immersing them in an ultrasonicbath for 15 min, then dropping a few drops of the resulting suspension containing the synthesizedmaterials onto TEM grid. For photoluminescence (PL) measurements, 450-W Xenon lamp was usedas an excitation source. The emitted light from the sample collected by an optical fiber on the sameside as the excitation was analyzed by a Jobin–Yvon Spectrometer HR460 and a multichannel CCDdetector (2000 pixels). The photoluminescence excitation (PLE) measurements were performed on aJobin–Yvon Fluorolog 3-2 spectrometer. The decays were analyzed by a PM Hamamatsu R928 and ascope Nicolet 400 with a time constant on the order of 1 ns. The low temperature experiments werecarried out in a Janis VPF-600 Dewar with variable temperature controlled between 78 and 300 K.

3. Results and discussion

3.1. Structural studies

The X-ray diffraction spectra obtained from the b-Zn2SiO4 nanophosphors doped with various Mnconcentrations are presented in Fig. 1. These spectra show that, a new zincic (b-Zn2SiO4) compoundwas formed. In our results the pattern resembles to that of b-Zn2SiO4, slightly different in the peakposition and the number of peaks to the results reported by Rooksby and McKeag [22]. Theseb-Zn2SiO4:Mn nanophosphors with various concentrations crystallizes in triclinic structure (JCPDSCard 19-1479) [7,22]. The above results imply that the solid reaction between ZnO and SiO2 occurredand formed b-Zn2SiO4 phase during heat treatment at 1500 �C. It is clear that the crystalline phase isthe most dominant one corresponding to the b-phase Zn2SiO4, in parallel we note the appearance of

Fig. 1. X-ray diffraction pattern of the b-Zn2SiO4 nanophosphors with different Mn concentrations.

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two other phases of silica [22]. However, at high temperature, Zn2+ and Si2+ species on the surface ap-pear to be mobile enough to move and diffuse inside the porous body and contribute to the formationof b-Zn2SiO4 phase. There is no marked difference in all the diffraction curves, excepting increase ofthe diffraction peaks positions with Mn2+ ion concentrations. Results indicating that all the sampleshave the same triclinic system structure, the Mn2+ doping ion makes lattice decreases, the latticeparameters reduced when the Mn2+ concentration increased. However, we do not find any diffractionpeak corresponding to Mn2+ ion compound, which indicates that Mn2+ ions entered into the lattice ofb-Zn2SiO4. The average grain size of the crystallites Zn2SiO4 in our samples varies from 60 nm to 80 nm[12], has been estimated using Scherrer’s formula (1).

We investigated the size and morphology of our samples by transmission electron microscopy.Fig. 2a and b shows the TEM micrographs of the undoped and Mn doped b-Zn2SiO4 phase indicatingthat the crystallized material is nanostructured. At high temperature (1500 �C), Zn and Si species,move and diffuse inside the porous body to form b-Zn2SiO4 phase with a particle size is about80 nm. Energy dispersive spectroscopy (EDX) analysis, shown in Fig. 2c, confirms the presence of man-ganese, in good agreement with XRD results.

Fig. 2. (a) TEM photograph of the undoped sample, (b) TEM photograph of 2 at.% Mn doped sample and (c) EDX analysis of2 at.% Mn doped sample.

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3.2. Photoluminescence properties

For further characterizations of the optical responses of our samples, we present in the following,the different luminescence properties of the obtained nanophosphors in a silica matrix. In the presentinvestigation, the emission spectra of b-Zn2SiO4 nanophosphors doped with various Mn concentra-tions are shown in Fig. 3. All light emissions of our samples have peaks at around 574 nm, and canbe ascribed to the 4T1 ? 6A1 transition of the Mn2+ ions in b-Zn2SiO4. The high internal surface areaof porous material might make the reaction take place faster than the reaction in traditional solid stateannealing method [23]. The main yellow emission peak of Zn2SiO4:Mn nanoparticles was shifted to-wards longer wavelength as manganese content increases, which means the shift of color coordinates.The b-Zn2SiO4 nanophosphors with a Mn doping concentration of 2 at.%, presented the maximum PLintensity. This result of PL intensities of particles is well coincided with the results of XRD. In the XRDspectra, the particles with a Mn doping concentration of 2 at.% had the highest triclinic crystallinity.Generally, particles with large crystallite size have higher brightness than the smaller ones, becauseof low concentration of defects, which act as sites for the non-radiative recombination of electron–hole pairs.

In the case of nanophosphors with a Mn doping concentration of 0.1 at.%, the spectrum shows twobroadband emissions, the first one centered at 574 nm and the second at 760 nm. It is clear that theband centered at about 574 nm attributed to Mn2+ in Zn2SiO4, which originated from the 4T1 ? 6A1

transitions of Mn2+ ions. While the band centered at about 760 nm is attributed to energy transferfrom Zn2SiO4 particles to NBOHs interface defects [5]. However, the doping concentration usuallyhas prominent influence on the luminescence properties of phosphor [2,7,8]; taking the advantagesof this method, exact doping of Mn concentration can be adjusted easily. When the Mn dopingconcentration is less than 0.01, a new luminescence peak at 570 nm can be observed. According tothe Tanabe–Sugano diagram [24], such shift reflects the increase in crystal field. Marco de Lucaset al. [25] obtained a useful experimental result elucidating the dependence of emission band positionon Mn–O distance for several manganese doped compounds that have a octahedrally coordinated Mnsite. As a result, it turns out that the peak energy of manganese emission increases linearly with theMn–O distance. The red shift observed in the present investigation should be due to the shortening ofMn–O distance in the samples treated at high temperature at (1500 �C).

The photoluminescence (PL) spectra of b-Zn2SiO4 nanophosphors with a Mn doping concentrationof 2 at.%, at different measurement temperatures of are shown in Fig. 4. These spectra depict the pres-ence of intense bands in the visible range at 574 nm [12,26]. With 255 nm excitation wavelength, we

Fig. 3. PL spectra of the b-Zn2SiO4 nanophosphors doped with different Mn2+ concentrations.

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also note the change of the PL intensity with temperature, which is an increase of PL intensity whenthe measurement temperature decreases. This is due to d–d transition on Mn2+ as the relevant lumi-nescent center. All emission is driven by 4T1 ?

6A1 relaxation on Mn2+ [12,26]. The color of emissionstrongly depends on the relevant crystal field, and bond distances of the Mn2+ ion. To this concern,emission can vary in principle from green to red [12,23]. Increasing the crystal field reduces the energydifference of the ground and first excited state, resulting in red-shift of the luminescence [23]. Theappearance of yellow luminescence is basically an effect of the higher crystal field in the b-Zn2SiO4

phase host [19,23]. In addition, the area of sub yellow-emission band as a function of measurementtemperatures is presented in Fig. 5. The area declined markedly when the measurement temperatureincreases.

3.3. Photoluminescence excitation properties

On the other hand, the excitation spectra of b-Zn2SiO4 nanophosphors doped with various Mn con-centrations consist of a strong excitation band ranging from 220 to 300 nm with a maximum at255 nm, which corresponds to excitation of Mn2+ charge transfer transition [12] (Fig. 6). The spectrashow an intense and a broadband peak, which is ascribed to the charge transfer (CT) band of Mn2+ inthe Zn2SiO4 system as reported by Mishra et al. [27]. In addition, to the CT band, other bands of Mn2+

(d–d) transition are also observed at higher wavelengths; these are caused by the splitting of the 4Dand 4G levels due to the crystal field, as shown by the Orgel diagram for Mn2+ [19,20,28]. The electronsat the 6A1(6S) ground state of Mn2+ ions, which originate from the photoexcited ionization of Mn2+, areexcited to the conduction band of Zn2SiO4 by photons, and the free electrons in the conduction bandrelax back to the 4T1(4G) excited state through a non-radiative process [29]. Finally, this is followed byradiative transition from the 4T1(4G) excited state to the 6A1(6S) ground state, emitting yellow light(574 nm). This happens when Mn ions occupy the tetrahedral sites of the triclinic structure. It can alsoexplain the reason why the thermal treatment of samples at high temperature (1500 �C) is usuallynecessary in the formation of b-Zn2SiO4:Mn nanophosphors, since Zn2+/Mn2+ ions are likely to moveto larger interstices more than tetrahedral ones, to fix them in such larger interstices rather than goback to tetrahedral interstices [19]. Furthermore, the difference in the coordination environmentmay be the origin of the red shift of Mn emission in Zn2SiO4:Mn [23].

The maximum intensity at shorter wavelengths cannot be detected because of limitations of theused instrument (Fig. 7). The other bands associated with the manganese transitions were also ob-served around 354, 382, 421, 437, and 472 nm, which were assigned to the 6A1(6S)–4E1(4D),

Fig. 4. PL spectra of 2 at.% Mn doped b-Zn2SiO4 nanophosphor at different measurement temperatures.

Fig. 5. Area of the sub yellow-emission band as function of the measurement temperature.

Fig. 6. PLE spectra of the b-Zn2SiO4:Mn2+ nanophosphors at different Mn concentrations.

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6A1(6S)–4T2(4D), 6A1(6S)–4E1(4G), 6A1(6S)–4T2(4G) and 6A1(6S)–4T1(4G) transitions, respectively [30].According to the Orgel diagram [28], these transitions are ascribed to the splitting of the 4D and 4Glevels of divalent manganese, similar results have been presented elsewhere [31].

3.4. Time decay

Generally it is possible to reduce the time decay of the Zn2SiO4:Mn phosphor material by increasingthe manganese dopant content [21,23,32]. The decay curves of 574 nm emission were measured underthe UV excitation. Fig. 8 shows the 2% decay time at 78 K and 300 K. The sample has exponential PLdecay with a lifetime of 13 ms, characteristic for the forbidden Mn2+ d–d transitions in Zn2SiO4, whichis consistent with the value of 8–16 ms as reported elsewhere [32]. Morell and Khiati [20] argued thatsuch behavior is closely related only with the concentration quenching. But more recently Barthouet al. [19] found the presence of two different activation centers with different time decays and the

Fig. 7. Enlarged PLE spectra of the b-Zn2SiO4:Mn nanophosphors.

Fig. 8. Decay curve of the b-Zn2SiO4 nanophosphors with 2 at.% Mn doping concentration at different measurementtemperatures.

K. Omri, L. El Mir / Superlattices and Microstructures 70 (2014) 24–32 31

faster color center is predominant at high Mn concentration, which is believed to be Mn–Mn pairs.Ronda and Amrein [26] suggested that the exchange interaction between Mn ions indeed results inallowed optical transitions on Mn ion pairs and hence gives rise to shortening of time decay in rela-tively high Mn concentration. In addition, there are two in-equivalent Mn sites substituting Zn siteswith slightly distorted tetrahedral configurations in the Zn2SiO4:Mn which can influence the radiativedecay time [32].

4. Conclusion

In summary, b-Zn2SiO4 nanophosphors doped with various Mn concentrations embedded in silicahost matrix have synthesized via sol–gel method. This procedure enables low temperature synthesis,pure phase of well-crystallized, as shown by X-ray and TEM analyses. The photoluminescence of the

32 K. Omri, L. El Mir / Superlattices and Microstructures 70 (2014) 24–32

nanophosphors shows an intense yellow emission at 574 nm wavelength, which comes from the Mn2+

and corresponds to the 4T1 ?6A1 energy transition. The optimum concentration of Mn2+ ions showing

the maximum of photoluminescence intensity was 2 at.%. Besides, given the excitation spectrum,emission spectrum, and decay curve, the as-prepared Zn2SiO4:Mn nanophosphors reveal excellentoptical properties in nature. The findings reported in this work may open up new avenue for preparingphosphors with controlled structures and optical properties.

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