Superlattices and Microstructures 75 (2014) 89–98

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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 . c o m / l o ca t e / s u p e r l a t t i c es

Photoconversion from UV-to-yellow in Mn dopedzinc silicate nanophosphor material

http://dx.doi.org/10.1016/j.spmi.2014.07.0210749-6036/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: Al Imam Mohammad Ibn Saud Islamic University (IMSIU), College of Sciences, DeparPhysics, Riyadh 11623, Saudi Arabia.

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

L. El Mir a,b,⇑, K. Omri b

a Al Imam Mohammad Ibn Saud Islamic University (IMSIU), College of Sciences, Department of Physics, Riyadh 11623, Saudi Arabiab Laboratory of Physics of Materials and Nanomaterials Applied at Environment (LaPhyMNE), Gabes University, Faculty of Sciencesin Gabes, Gabes, Tunisia

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

Article history:Received 9 July 2014Accepted 15 July 2014Available online 26 July 2014

Keywords:Sol–gelZn2SiO4:MnNanophosphorsLuminescenceOptical materials

A novel synthesis process of Mn doped b-Zn2SiO4 yellow nano-phosphors based on sol–gel method is presented. These sampleswere prepared by a simple solid-phase reaction under naturalatmosphere at 1500 �C after the incorporation of ZnO:Mn nanopar-ticles, in silica monolith. XRD showed the formation of zinc silicateb-phase, which shows yellow luminescence. In addition, the as-synthesized b-Zn2SiO4:Mn2+ nanophosphor exhibited intensivebroad emission around 574 nm, which was attributed to the4T1–6A1 transition in Mn2+ ions. An exponential fitting analysishas been employed to obtain the lifetime of this yellow emission(574 nm), the average lifetime was found to be 13 ms. The lumi-nescence properties of our nanophosphors were characterized byexcitation and emission spectra as well as luminescence decay.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Recently, much research has been focused on the synthesis and characterization of nanomaterialssuch as sulfides, oxides and nitrides [1–3]. These nanometer-sized inorganic materials can exhibit awide range of optical and electrical properties [4]. Novel properties would be obtained while the crys-tal structure of host materials was changed. However, the synthesis and luminescence properties of

tment of

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nanometer-sized silicate are rarely reported [5]. Traditionally, zinc silicate (Zn2SiO4) as one of themost important family of functional inorganic materials, have numerous applications in the fieldsof phosphors [6], adsorbents [7] and electronic insulators [8] as well as rubber mixtures [9]. Zn2SiO4

is usually synthesized by the high temperature solid-state reaction procedure, involving crushing, ballmilling and sintering at high temperature [10]. However, this method requires a high reaction temper-ature, large thermal energy and long dwell time, and furthermore the morphologies of the resultingproducts are usually irregular microstructures. Over the last decade, many new methods for synthesisof Zn2SiO4 have been reported, including the sol–gel method [11–13], polymer precursor method [14],spray pyrolysis method [15] and hydrothermal method [16]. Among these methods, hydrothermalprocessing has great advantages, such as high purity, controllable morphology and lower cost. Forexample, Hong’s group reports the synthesized of Mn-doped Zn2SiO4 nanowires through a hydrother-mally method and subsequent heat-treatment process [16]. Wang et al. [17] prepared Mn-dopedZn2SiO4 nanorods by a hydrothermal procedure at 280 �C. Zhang et al. [18] prepared Zn2SiO4 rodsby hydrothermal method using the mixed solvent of ethidene diamine and H2O. These Zn2SiO4

samples were exclusively achieved by hydrothermal route at high temperatures above 250 �C for along time, or in the assistance of organic solvent, which not only increased the production cost but alsomade the scale-up synthesis more difficult. However, until now, the way of sol–gel synthesis has usedfor the elaboration of the various nanocomposites or silica behave like a matrix. Our objective in thiswork is, in a first step the in-situ synthesis of luminescent nanoparticles of b-Zn2SiO4:Mn in a silicahost matrix and in second step, the increase of the life time of the components and preventcontamination caused by the external environment.

Used in early luminescent lighting, manganese-doped zinc silicates are well known as green phos-phors [19]. Important for color rendering, a host lattice of Zn2SiO4 containing Eu3+, Mn2+ or Ce3+ dop-ant ions covers the red, green, and blue portions of the visible spectrum, respectively [20–22].Nanophosphors based on the transition metals are receiving considerable attention as potential coat-ings in lamps, cathode ray tubes [19], flat panel displays, electroluminescent and optoelectronicdevices [20] and more recently, radiation detectors in medical imaging systems [23]. The emissionband at 525 nm is due to the spin-forbidden 4T1 ? 6A1 transition of the 3d5 electronic configurationof isolated Mn2+ [21]. The efficiency of a phosphor material is often limited by the dynamics of thedopant ion which depends on crystal nature, concentration, annealing temperature and phonon fre-quency. Recently, was reported that upconversion emission depends on crystal nature [24], concentra-tion and nature of oxide host. Therefore, time-resolved luminescence study is very important tounderstand the effect of concentration and annealing temperature on the excited state of Mn2+ ionin Zn2SiO4 nanocrystals.

Therefore, in this paper the method is applied to prepare ZnO:Mn nanoparticles and b-Zn2SiO4:Mnnanophosphors by the same protocol of sol–gel method combined with a furnace firing [11,12] butusing for the first time, manganese doped zinc oxide nanoparticles with the considered concentrationand studied the structural and optical properties of the obtained nanophosphors.

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, nanoparticles ZnO:Mn aerogels, with a manganese atomic concentration correspond-ing to [Mn]/([Zn] + [Mn]) = 2%, have been prepared using the sol–gel method modified according to ElMir et al. protocol [25–27]. In the second step, we have prepared ZnO:Mn confined in silica aerogelaccording to the following process: 0.5 ml of TEOS was first dissolved in EtOH. Then, with constantstirring of the mixture of TEOS and EtOH, 0.44 ml of water and 30 mg of nanoparticles powderprepared in the first step were added. The whole solution was stirred for about 30 min, resulting inthe formation of a uniform sol. The sols were transferred to tubes in ultrasonic bath where 100 mlof fluoride acid was added. The wet gel formed in few seconds. Monolithic and white aerogel was

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obtained by supercritical drying in EtOH as described in the first step. Finally, silica glasses containingb-Zn2SiO4:Mn particles were obtained after firing aerogel at 1500 �C for 2 h.

2.2. Characterizations

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 by the Scherrer’s equation [28]:

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. The optical absorbance of the powders was determined using SchimadzuUV-3101 PC spectrophotometer with integrating sphere in the wavelength range from 200 to2000 nm.

For photoluminescence (PL) measurements, the 450-W Xenon lamp was used as an excitationsource. The emitted light from the sample collected by an optical fiber on the same side as the exci-tation was analyzed by a Jobin-Yvon Spectrometer HR460 and a multichannel CCD detector (2000 pix-els). The photoluminescence excitation (PLE) measurements were performed on a Jobin-YvonFluorolog 3-2 spectrometer. The time decays were analyzed by a PM Hamamatsu R928 and a scopeNicolet 400 with a time constant on the order of 1 ns. The low temperature experiments were carriedout in a Janis VPF-600 Dewar with 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 nanopowder have been presented in Fig. 1. Wenoticed the appearance of nine pronounced diffraction peaks at 2h = 31.50�, 34.19�, 36.15�, 47.39�,

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

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56.44� and 62.79� which can be attributed to the (100), (002), (101), (102), (110) and (013) planesof ZnO, respectively [29]. The obtained XRD spectra matched well with the space group P63mc (186)(No. 36-1451) of wurtzite ZnO structure [29]. The lattice constants calculated from the XRD patternare very close to ZnO ones, i.e., a = 3.250 Å and c = 5.208 Å. These results are in a good agreement withthose obtained by Omri et al. [30]. This pattern implies the Mn incorporation within the ZnO nano-powder by the means of substitution for Zn. The shift in the lattice constants is supposed to occurdue to the larger radius of Mn2+ ions (0.66 Å) than that of the Zn2+ ions (0.60 Å). According to Vegard’slaw, this peak shift is expected for an Mn-doped ZnO nanopowder. However, in our case, the peak shiftin XRD pattern for the Mn-doped ZnO nanopowder is indiscernible. We propose that it may be due tothe limited resolution of our XRD instrument. Similar results of indiscernible peak shift in XRD pat-terns were also reported in Mn-doped ZnO nanostructures by Guo et al. [31]. Average grain size ofthe crystallites ZnO:Mn varies from 20 nm to 40 nm [18], has been estimated using Scherrer’s formula(1).

After incorporation of these nanoparticles of ZnO:Mn in SiO2 and heat treatment at 1500 �C for 2 hin air, new phase b-Zn2SiO4 was formed as illustrated in Fig. 2, the X-ray diffraction spectra obtainedfrom the Mn doped b-Zn2SiO4 nanophosphors. In our results the b-Zn2SiO4 patterns are slightly differ-ent in the peak position and the number of peaks to the results reported by Rooksby and McKeag [22].These b-Zn2SiO4:Mn nanophosphors with 2 at% concentration crystallizes in triclinic structure (JCPDSCard 19-1479) [19,22]. 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 most dominant crys-talline phase correspond to the b-phase of Zn2SiO4, in parallel we note the appearance two otherphases of silica [19]. However, at high temperature, Zn2+ and Si2+ species on the surface appear tobe mobile enough to move and diffuse inside the porous body and contribute to the formation of b-Zn2SiO4 phase. However, we do not find any diffraction corresponding to Mn2+ ion compound, whichindicates that Mn2+ ions entered into the lattice of b-Zn2SiO4. The average grain size of the crystallitesZn2SiO4 in our samples varies from 50 nm to 80 nm [19], has been estimated using Scherrer’s formula(1).

We investigated the size and morphology of our samples by transmission electron microscopy. TheTEM images of Mn-doped ZnO nanoparticles (Fig. 3a) show that very small ZnO particles are present inthe as-prepared aerogel powder. The size of the majority of ZnO particles in this powder variedbetween 18 and 35 nm equal to the particle size obtained by XRD analysis. The TEM micrographs ofthe b-Zn2SiO4:Mn nanocomposites (Fig. 3b) indicating that the well-crystallized Zn2SiO4 are nano-structured. At high temperature 1500 �C, Zn and Si species, move and diffuse inside the porous body

Fig. 2. X-ray diffraction pattern of the b-Zn2SiO4:Mn nanophosphors.

(b)

100 nm

(a)

Zn2SiO4:Mn (c)

Fig. 3. (a): TEM photograph ZnO:Mn nanopowder, (b): TEM photograph of b-Zn2SiO4:Mn nanophosphors sample and (c): EDXanalysis of 2 at% Mn doped sample.

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to form b-Zn2SiO4 phase. Energy dispersive spectroscopy (EDX) analysis during the TEMobservation, shown in Fig. 3c, confirm the XRD results by showing the formation of Zn2SiO4:Mn. Anestimate of the grain size indicates that they are in the form of nanocrystallites with a dimensionaround 80 nm.

3.2. Optical properties

In general, the UV absorption is related to the electronic transition from filled valence states toempty conduction states. In this study, the absorbance of samples was measured as a function ofwavelength in the range of 200–1800 nm. Fig. 4 shows the UV–vis absorption spectra of ZnO andMn doped-ZnO nanocrystalline particles. Due to the decreased particle sizes of ZnO and Mn doped-ZnO nanocrystalline particles, the excitonic absorption peak is observed at 310 nm. This is lower thanthe band gap wavelength of 388 nm for bulk ZnO [32]. However, the observed absorption edge ofundoped ZnO is less than that of Mn-doped ZnO. This decrease may be attributed to the s–d and p–d exchange interactions between electrons in conduction and valence bands and localized Mn2+ spins,contributing to band gap bowing [33,34], or narrowing effects caused by the high carrier concentra-tion [35]. Based on the absorption spectra, the optical band gap (Eg) for Mn-doped ZnO nanoparticleswas estimated from the theoretical relationship between Eg and absorption coefficient (a) [36]. Thepoint of the extrapolation of the linear part that meets the abscissa will give the value of the bandgap energy (Eg) of the ZnO:Mn materials (Fig. 5). The values of Eg for ZnO and Mn doped-ZnO nano-crystalline particles are found to be 3.33 and 3.19 eV, respectively. Band gap value for Mn doped-ZnO nanocrystalline particles is lower than ZnO. This can be attributed to the magnetic properties

Fig. 4. Absorption spectra of undoped and Mn-doped ZnO nanopowder.

Fig. 5. Plots of (ahm)2 versus photon energy hm of ZnO:Mn nanopowder.

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and consequently, stronger interaction potential of Mn doped-ZnO nanocrystalline particles inducedby doped manganese [37].

3.2.1. Photoluminescence propertiesTo further characterize the optical responses of our samples, we present in the following, the dif-

ferent properties of luminescence of nanophosphors in a silica matrix. In the present investigation, theemission spectra of Mn doped b-Zn2SiO4 are shown in Fig. 6. The yellow emission has been assigned toan electronic transition of 4T1ð4GÞ ! 6A1ð6SÞ peaking at the wavelength 574 nm and which is a parityforbidden emission transition of Mn2+ ions [38,39]. Mn2+ has d5 configuration with high spin (S = 5/2).The emission band is assigned to transitions between the spin–orbit components of the 4T1 excitedstate and the 6A1 ground state [38,39]. The broadness of the luminescence band is intrinsic and is pre-dicted from the different slopes of the energy levels in the Tanabe-Sugano diagram [36]. In general,Zn2SiO4:Mn2+ has the strong excitation band around 255 nm, which is attributed to the charge transfertransition [2,19]. In this study, in order to clarify optically the site preference of Mn2+ ions for two Zn

Fig. 6. PL spectra of the b-Zn2SiO4:Mn nanophosphors at different temperatures.

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sites, the Mn2+ ions in the host lattice was excited by the charge transfer band at 255 nm wavelength[40]. On the basis of the concept associated to the crystallography two different sites of Mn2+ give PLemission spectra at 574 nm. The emission spectra was broad with a maximum at about 574 nm underexcitation at 255 nm, this is due to the splitting of the 4D and 4G levels by crystal field [40,41]. In addi-tion, the area of the sub yellow-emission band (574 nm) as a function of measurement temperatures isreported in Fig. 7. The area declined markedly when the measurement temperatures increases.

Fig. 8 shows an excitation spectrum of Zn2SiO4:Mn2+ nanophosphor, with a strong excitation peakat 255 nm, obtained by monitoring the yellow emission wavelength at 574 nm. The spectrum consistsof a strong excitation band ranging between 220 and 310 nm with a maximum at 255 nm. This is dueto d–d transition on Mn2+ as the relevant luminescent center. The emission is driven by 4T1 to 6A1

relaxation on Mn2+ [40,41]. The broad excitation peak at 255 nm could be attributed to a charge trans-fer transition (or the ionization of manganese) from the divalent manganese ground state (Mn2+) tothe conduction band (CB) [3,24]. By selecting this strong excitation peak at 255 nm, the photolumines-cence spectral measurements on the nanophosphor have been carried out and analyzed. The color of

Fig. 7. The area of the sub yellow-emission band (574 nm) versus the measurement temperatures.

Fig. 8. PLE spectra of the b-Zn2SiO4:Mn nanophosphor.

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emission strongly depends on the relevant crystal field, and locally on the coordination and bond dis-tances of the Mn2+ ion. To this concern, emission can vary in principle from green to red [41,42]. Moreconcrete, yellow to red emission is associated with octahedrally coordinated Mn2+ and a comparablystrong crystal field [40–42]. The observed emission of b-Zn2SiO4:Mn (550–610 nm) is in accordancewith the upper consideration and has been described for bulk materials in detail [40–42]. This hap-pens when Mn ions occupy the octahedral sites of the triclinic structure. It can also explain the reasonwhy the thermal treatment of the sample at high temperature (1500 �C) is usually necessary for theformation of b-Zn2SiO4:Mn nanophosphors, since Zn2+/Mn2+ ions are likely to move to larger inter-stices more than octahedral ones, to fix them in such larger interstices rather than go back to octahe-dral interstices [3]. On the other hand, the maximum intensity and the information at shorterwavelengths cannot be detected because of limitations of the instrument used (inset of Fig. 8). Theother bands associated with the manganese transitions were also observed around 354, 382, 421,437, and 472 nm, which were assigned to the 6A1(6S)–4E1(4D), 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[43], these transitions ascribe the splitting of 4D and 4G levels of divalent manganese. Similar resultshave been presented elsewhere [44].

3.2.2. Time decayThe time decay curve of 574 nm yellow emission of b-Zn2SiO4:Mn2+ nanophosphor were measured

under the UV excitation at 78 K (Fig. 9). The sample has exponential PL decay with a lifetime of 13 ms,characteristic for the forbidden Mn2+ d–d transitions in Zn2SiO4, which is consistent with the value of8–16 ms reported elsewhere [3,42]. The decay time was fitted to be 13 ms, but the decay mechanismis not clear yet. Morell and Elkhiati [10] have reported that the time decay may decrease if there aremore de-excited centers which transit to ground state very quickly. The decay time is shorter com-pared with the films deposited by sol–gel method [45], which is an advantage for opto-electronicdevices, since shorter decay time implies faster switching speed of the device. The long time decayof Zn2SiO4:Mn2+ is an obstacle for the PDP application. Thus, lots of efforts have been dedicated toshorten the decay time of Zn2SiO4:Mn2+ particles without losing much efficiency [3,42]. In general,it is possible to reduce the decay time of a phosphor by increasing the manganese dopant content.However, because of the accompanying energy transfer, nonradiative de-excitation processes willincrease and the nanocomposite efficiency will decrease due to the well-known dopant concentrationeffect. When the dopant content is increased, the competition between a higher number of excitedcenters and lower radiative efficiency will determine the optimal dopant concentration [3].

Fig. 9. Time decay curve of b-Zn2SiO4:Mn nanophosphor with Mn doping concentration of 2 at% at 78 K.

L. El Mir, K. Omri / Superlattices and Microstructures 75 (2014) 89–98 97

4. Conclusion

In conclusion, Mn doped ZnO nanoparticles aerogel and b-Zn2SiO4:Mn2+ nanophosphors were syn-thesized by sol–gel method. The ZnO:Mn nanoparticles were obtained by slow hydrolysis of the pre-cursors using an esterification reaction, followed by a supercritical drying with of ethyl alcohol. Theadvantages of this method include simplified procedure, timesaving, controllable size and morphol-ogy, large-scale production and wide practicality for other phosphor materials, which have potentialapplications in displaying and lighting fields. The X-ray diffraction and TEM show a crystalline phasewith a particle size ranging between 20 and 40 nm. Upon incorporation in SiO2 and heat treatment at1500 �C, b-Zn2SiO4: Mn2+ phase was formed in SiO2 host matrix. Photoluminescence of the synthe-sized nanophosphor shows intense yellow emission, peaking at 574 nm, which comes from the Mndopant and corresponds to the 4T1–6A1 transition. The nanophosphor absorbed excitation energy inthe range from 220 nm to 300 nm, and the maximum excitation wavelength was near 255 nm. Themain emission peak of particles was 574 nm, resulting in a yellow emission. Based on our experimen-tal results, the Mn2+-doped Zn2SiO4 nanophosphor material shows good optical properties. Further-more, this strategy makes it possible to synthesize other silicate nanophosphor.

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