Comparative study of optical and structural properties of micro- and nanocrystalline SrAl 2 O 4 : Eu 2 þ , Dy 3 þ phosphors D.S. Kshatri, Ayush Khare n Department of Physics, National Institute of Technology, Raipur 492010, Chhattisgarh, India article info Article history: Received 25 February 2014 Received in revised form 31 May 2014 Accepted 12 June 2014 Available online 6 July 2014 Keywords: Nanocrystalline Optical properties Photoluminescence Afterglow SEM XRD abstract Micro- (MC) and nanocrystalline (NC) strontium aluminate (SrAl 2 O 4 : Eu 2 þ , Dy 3 þ ) phosphor powders are synthesized separately by solid state reaction (SSRT) and combustion synthesis technique (CST) respectively. The characterization features of both types of samples (MC and NC) are compared using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) while the optical properties are discussed in terms of photoluminescene (PL), after glow (AG) and thermoluminescence (TL) measurements. The SEM results show agglomeration of particles in both the cases and signify the presence of comparatively smaller particles in NC phase. The results of XRD studies indicate the presence of monoclinic SrAl 2 O 4 exhibiting wider diffraction peaks in NC samples as compared to their MC counterpart. The EDX profiles are used to confirm the presence of different starting materials. The absorption spectra and corresponding Tauc's plots confirm the wide band-gap of prepared samples. The emission spectra for MC samples are found to be more intense as compared to NC samples. For the MC samples the spectra exhibit a broad band emission accompanied by a peak at 520 nm, which in the case of NC samples it shifts to 515 nm. The broad emission band peaking is attributed to the typical 4f 6 5d 1 –4f 7 transitions of Eu 2 þ . For MC samples the different TL peaks fall in a temperature range 139–171 1C while for NC samples the curves peak in temperature range 95–111 1C, indicating existence of only one trap center. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Nanophase materials are being vigorously explored as most of their physical properties are size dependent and are markedly affected as the particle sizes tend to nanometer level. Especially the optical properties and useful aspects of nanomaterials have been fascinating mankind for long. These optical properties offer numer- ous applications, including optical detectors, sensors, lasers, phos- phor displays, solid state lighting, solar cells, photoelectrochemistry, photocatalysis and biomedicine [1–5]. The optical properties of nanomaterials are dependent on various parameters like size, shape, surface characteristics, and other variables, including doping and interaction with the surrounding environment or other nanostruc- tures [6]. One of the examples is the well-known blue-shift of absorption and photoluminescence spectra of semiconductor nano- particles with decreasing particle size. Especially for semiconductors, size is a critical parameter affecting optical properties [7]. Phosphors are one of the materials that show promising behavior when synthesized in nanophase. As luminescent materials, the phosphorescence properties are greatly affected by the grain size; when the grain size reaches nanoscale, many new properties can be obtained [8]. Plenty of phosphors have been made in nanophase by employing different techniques [9,10], including the sol–gel method, hydrothermal synthesis, chemical precipitation, etc. In most of the luminescent materials, the decay of the light emission lasts no longer than a few milliseconds after the end of the excitation. On the contrary, persistent phosphors can continue emitting light for minutes or even hours. This phenomenon is used not only in common applications like safety signage, dials, displays and decora- tion [11] but also in less obvious applications, such as night-vision surveillance [12] or in vivo medical imaging [13]. Rare earth (RE) and alkaline earth aluminates have attracted so much attention because they have been widely used as hosts in preparation of dosimeters as well as PDP phosphors [14]. Doping is an effective and useful way to alter the optical, electronic, and magnetic properties of nano- materials [15]. Divalent europium (Eu 2 þ ) activated alkaline earth aluminates are known to be efficient long persisting phosphors for their excellent properties [16] and high quantum efficiency in the visible region. These are essentially interesting, as they do not involve any radioactive isotope [17]. Particularly, Eu 2 þ and Dy 3 þ ions doped green emitting SrAl 2 O 4 (SAO) phosphor has been considered as one of the best and long lasting phosphorescent materials ( 450 h) with emission wavelength of 530–540 nm [9]. As compared to classical sulfide phosphorescent phosphors, aluminates have several valuable Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jlumin Journal of Luminescence http://dx.doi.org/10.1016/j.jlumin.2014.06.028 0022-2313/& 2014 Elsevier B.V. All rights reserved. n Corresponding author: Tel.: þ91 9425213445, þ91 7714052486. E-mail address: [email protected](A. Khare). Journal of Luminescence 155 (2014) 257–268
Transcript
Comparative study of optical and structural propertiesof micro- and nanocrystalline SrAl2O4: Eu2þ , Dy3þ phosphors
D.S. Kshatri, Ayush Khare n
Department of Physics, National Institute of Technology, Raipur 492010, Chhattisgarh, India
a r t i c l e i n f o
Article history:Received 25 February 2014Received in revised form31 May 2014Accepted 12 June 2014Available online 6 July 2014
Micro- (MC) and nanocrystalline (NC) strontium aluminate (SrAl2O4: Eu2þ , Dy3þ) phosphor powders aresynthesized separately by solid state reaction (SSRT) and combustion synthesis technique (CST)respectively. The characterization features of both types of samples (MC and NC) are compared usingX-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy(EDX) while the optical properties are discussed in terms of photoluminescene (PL), after glow (AG) andthermoluminescence (TL) measurements. The SEM results show agglomeration of particles in both thecases and signify the presence of comparatively smaller particles in NC phase. The results of XRD studiesindicate the presence of monoclinic SrAl2O4 exhibiting wider diffraction peaks in NC samples ascompared to their MC counterpart. The EDX profiles are used to confirm the presence of differentstarting materials. The absorption spectra and corresponding Tauc's plots confirm the wide band-gap ofprepared samples. The emission spectra for MC samples are found to be more intense as compared to NCsamples. For the MC samples the spectra exhibit a broad band emission accompanied by a peak at520 nm, which in the case of NC samples it shifts to 515 nm. The broad emission band peaking isattributed to the typical 4f65d1–4f7 transitions of Eu2þ . For MC samples the different TL peaks fall in atemperature range 139–171 1C while for NC samples the curves peak in temperature range 95–111 1C,indicating existence of only one trap center.
& 2014 Elsevier B.V. All rights reserved.
1. Introduction
Nanophase materials are being vigorously explored as most oftheir physical properties are size dependent and are markedlyaffected as the particle sizes tend to nanometer level. Especially theoptical properties and useful aspects of nanomaterials have beenfascinating mankind for long. These optical properties offer numer-ous applications, including optical detectors, sensors, lasers, phos-phor displays, solid state lighting, solar cells, photoelectrochemistry,photocatalysis and biomedicine [1–5]. The optical properties ofnanomaterials are dependent on various parameters like size, shape,surface characteristics, and other variables, including doping andinteraction with the surrounding environment or other nanostruc-tures [6]. One of the examples is the well-known blue-shift ofabsorption and photoluminescence spectra of semiconductor nano-particles with decreasing particle size. Especially for semiconductors,size is a critical parameter affecting optical properties [7].
Phosphors are one of the materials that show promising behaviorwhen synthesized in nanophase. As luminescent materials, thephosphorescence properties are greatly affected by the grain size;
when the grain size reaches nanoscale, many new properties can beobtained [8]. Plenty of phosphors have been made in nanophase byemploying different techniques [9,10], including the sol–gel method,hydrothermal synthesis, chemical precipitation, etc. In most of theluminescent materials, the decay of the light emission lasts no longerthan a few milliseconds after the end of the excitation. On thecontrary, persistent phosphors can continue emitting light forminutes or even hours. This phenomenon is used not only incommon applications like safety signage, dials, displays and decora-tion [11] but also in less obvious applications, such as night-visionsurveillance [12] or in vivo medical imaging [13]. Rare earth (RE) andalkaline earth aluminates have attracted so much attention becausethey have been widely used as hosts in preparation of dosimeters aswell as PDP phosphors [14]. Doping is an effective and useful wayto alter the optical, electronic, and magnetic properties of nano-materials [15]. Divalent europium (Eu2þ) activated alkaline earthaluminates are known to be efficient long persisting phosphors fortheir excellent properties [16] and high quantum efficiency in thevisible region. These are essentially interesting, as they do not involveany radioactive isotope [17]. Particularly, Eu2þ and Dy3þ ions dopedgreen emitting SrAl2O4 (SAO) phosphor has been considered as oneof the best and long lasting phosphorescent materials (450 h) withemission wavelength of 530–540 nm [9]. As compared to classicalsulfide phosphorescent phosphors, aluminates have several valuable
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/jlumin
Journal of Luminescence
http://dx.doi.org/10.1016/j.jlumin.2014.06.0280022-2313/& 2014 Elsevier B.V. All rights reserved.
properties [18,19] such as high radiation intensity, color purity, longerafterglow, chemically stabilization, safety and non-radioactivity.
After the discovery of SrAl2O4: Eu2þ , Dy3þ in 1996 [20],researchers and publications involved in the field of persistentluminescent materials have focused on divalent europium (Eu2þ)as the activating ion. The synthesis of these materials for displayapplications with considerably good initial brightness and longafterglow has been a major goal of many research groups [10] allover the world both in industry and academia for well over a decade.The development of new efficient afterglow phosphors is currentlyhampered by a limited understanding of the persistent luminescencemechanism. Commercial microcrystalline SAO powders with largesize particles are usually prepared by a solid-state reaction technique(SSRT) [21] and smaller particles are obtained by grinding the larger
phosphor particles, which can easily induce additional defects andgreatly reduce the luminescence efficiency. The SSRT is a techniquein which appropriate oxides/carbonates along with the dopants andfluxes are mixed and fired at temperatures around 1200–1500 1C fora few hours. This treatment results in a highly sintered, dense andhard mass of phosphor, which is difficult to crush and grind. Xiaoet al. [22] developed a new mixing method for solid-state reactionsynthesis of SrAl2O4: Eu2þ , Dy3þ long afterglow phosphors. Theyused a ball mill and wet-mixing machine in their preparation andobtained a kind of thick slurry form of phosphor. Recently, Nazidaet al. [23] synthesized double activated SrAl2O4: Eu2þ , Dy3þ phos-phor with improved properties through a solid state route atdifferent firing atmospheres (reduction, vacuum, oxygen and CO2
wet gas) and heating times (1–8 h). The combustion synthesis
Fig. 1. SEM images of different microcrystalline (MC) and nanocrystalline (NC) SrxAl2O4: Eu2þ0.01, Dy3þy (x¼0.99, y¼0; x¼0.98, y¼0.01; x¼0.97, y¼0.02; x¼0.96, y¼0.03;x¼0.95, y¼ 0.04 and x¼0.94, y¼0.05) phosphor samples.
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technique (CST) or self-propagating high-temperature synthesis(SHS) on the other hand is an effective, low-cost method forproducing various industrially useful nanocrystalline materials. Thecombustion process to prepare powder samples, however, is veryfacile and takes only a few minutes, which has been extensivelyapplied to the preparation of various nanosized oxide materials. TheSrAl2O4: Eu2þ , Dy3þ phosphor resulting from CST has opticalproperties similar to those of the phosphor resulting from the formermethod but the sintering temperature of the sample is much lowerthan that prepared by solid state reaction or sol–gel method. Shafiaet al. [24] reported the spectroscopic and host phase properties ofSrAl2O4: Eu2þ , Dy3þ phosphors with a series of different initiatingcombustion temperatures, urea concentrations as a fuel and criticalpH of precursor solution. The SrAl2O4: Eu2þ , Dy3þ nanoparticle
pigments were obtained by an exothermic combustion processwithin less than 5 min. Today CST has become a very popularapproach for preparation of nanomaterials and is practiced in 65countries. Recently a number of important breakthroughs in this fieldhave been made, notably for development of new catalysts andnanocarriers with properties better than those for similar traditionalmaterials [25]. The combustion synthesis route gives a fluffy massreducible to quite fine particles with almost no effort. Conditionsprevailing during the processing should favor formation of fineparticles in sub-micron region. An oxidizing atmosphere prevails inthe combustion process [26]. Incorporation of europium in bivalentstate invariably requires a reducing atmosphere [27]. With a view todevelop a process for the instant synthesis of nanophase particles ofthe phosphor, we employed a combustion route [28–30].
Fig. 1. (continued)
D.S. Kshatri, A. Khare / Journal of Luminescence 155 (2014) 257–268 259
In the present paper, we present and discuss the results ofsynthesis, characterization and optical studies of micro- andnanophase SrAl2O4: Eu2þ , Dy3þ phosphors prepared by solid statereaction and combustion synthesis techniques.
2. Experimental
In order to prepare microcrystalline powder samples, stron-tium carbonate (SrCO3), aluminum oxide (Al2O3), europium oxide(Eu2O3) and dysprosium oxide (Dy2O3) (all 99.9 pure, supplied byMerk) were taken as starting materials. Apart from this, appro-priate amount of boric acid (H3BO3) was also used as flux.
First, stoichiometric raw materials were weighted and thor-oughly ground with an agate mortar and a pestle by manualoperation. For preparing MC samples, the mixture was put into analumina crucible with a cover and sintered at 1200 1C for 2 h in adigitally controlled electronic furnace in open air. After coolingdown to room temperature, samples were taken out of furnaceand ground. Finally, the hard body of fired samples was brokenand ground to obtain the final products. The pre-sintered sampleswere ground and sintered again at 1350 1C for 5 h in a reducingatmosphere (95% Arþ5% H2).
Nanocrystalline powder samples were synthesized from thesources of strontium nitrate [Sr(NO3)], aluminum nitrate [Al(NO3)3 �9H2O], europium oxide (Eu2O3), dysprosium oxide (Dy2O3)(all 99.9% pure, supplied by Merk), urea [CO(NH2)2, AR] and boricacid (H3BO3, AR) by the following process. For preparing NCsamples, the crucible comprising the mixture of above compoundswas placed into a furnace already maintained at a temperature of60075 1C. Within 5 min, the furnace reached the desired tempera-ture and combustion started giving yellowish flame. The mixturefrothed and swelled, forming foam that ruptured with a flame andshine to incandescence. This continued for next few seconds andafter it was over, the crucible was pulled out of the furnace and keptin open to allow cooling. Upon cooling a fluffy material wasobtained, which was then crushed for 1 h using the agate pestle
Fig. 2. (a) XRD patterns of different microcrystalline SrAl2O4: Eu2þ , Dy3þ phosphorsamples and (b) XRD patterns of different nanocrystalline SrAl2O4: Eu2þ , Dy3þ
phosphor samples.
Table 1XRD data of different SrAl2O4: Eu2þ , Dy3þ phosphor samples.
D.S. Kshatri, A. Khare / Journal of Luminescence 155 (2014) 257–268260
and mortar. The final products are further heat treated at 1200 1C ina weak reductive atmosphere (95% Arþ5% H2) for 3 h [24]. TheEu2þ doped strontium aluminate may grow according to anucleation-growth-assembly process [31]. For different studies,micro- and nanocrystalline SrAl2O4: Eu2þ , Dy3þ phosphors areprepared with different molar ratios Eu2þ:Dy3þ:Sr2þ:Al3þ¼0.01:0.00:0.99:1, 0.01:0.01:0.98:1, 0.01:0.02:0.97:1, 0.01:0.03:0.96:1, 0.01:0.04:0.95:1, and 0.01:0.05:0.94:1.
The materials were weighed using a Shimadzu ATX 224 singlepan analytical balance and the samples were fired in a digitalfurnace. The morphologies and size of particles were examinedusing a ZEISS-EVO 60m German make scanning electron micro-scope (SEM) coupled with an Oxford Inca energy dispersive X-rayspectrometer (EDX), and energy dispersive X-ray (EDX) spectro-scopy was carried out with an Oxford Inca EDX System. Thepowder XRD profiles of as-synthesized samples were recordedon a Bruker Advance D8 X-ray diffractometer with Co-Kα 1.790 Åline operated at 40 kV voltage and 40 mA anode current. Datawere collected at 2θ values ranging from 201 to 801. The UV–visible absorption spectra of the NC powder samples wererecorded with a Shimadzu UV–VIS spectrophotometer (Model:UV-1700). The excitation and emission spectra were recorded atRT on a Hitachi fluorescence spectrophotometer (Model: F-2500)with a spectral resolution of 2.5 nm. The afterglow measurementswere made with an indigenous experimental setup comprising aphotomultiplier tube (RCA-931) and a digital nanoammeter(Model: DNM-121). After exciting the samples with UV radiationat 365 nm wavelength for 5 min, the TL spectra at differenttemperatures were recorded using a Nucleonics make Thermo-luminescence (TL) Reader (Model: 1009I).
3. Results and discussion
All the following studies were conducted on a set of samples ofcomposition SrxAl2O4: Eu2þ0.01, Dy3þy (x¼0.99, y¼0; x¼0.98, y¼0.01;x¼0.97, y¼0.02; x¼0.96, y¼0.03; x¼0.95, y¼ 0.04 and x¼0.94,y¼0.05). In order to investigate the surface morphologies and thecrystalline size of the synthesized phosphors, the SEM studies werecarried out. Fig. 1(a)–(f) presents comparison of SEMmicrographs ofabove mentioned samples in MC and NC phases. As is evident fromthe figures, in MC samples, as they result from SSRT, harder form ofparticles is seenwhich need to be ground for further studies. On theother hand, the NC samples produced at lower temperatures
resemble those extracted from combustile ash and fluffy form ofmaterial. Secondly, in both the cases, agglomeration in particles iswitnessed and comparatively smaller particles are observed in NC
D.S. Kshatri, A. Khare / Journal of Luminescence 155 (2014) 257–268 261
samples. Nearly all the samples possess irregular morphology withangularity and corners. It is further noticed that the crystalline sizesare nearly the same irrespective of the composition of the sample[32]. This leads to the understanding that the doping process doesnot make significant changes to the morphology and size of thenanostructures. This has already been established by Ayvacikli et al.[33], who reported the phosphor powders to be irregular particleswith different shapes and sizes. In the present case also, the surfacesof the foams in both categories (MC and NC) show a lot of cracks,voids and pores created by the gases escaping during combustionreaction. Actually in CST (to synthesize NC samples) large amountsof escaping gases dissipate heat, thereby prevent the sintering ofmaterial and thus support the formation of NC phase. Apart from
the above, another feature of nanorod crystallites is that they havelarger surface area as compared to micro- or flower-like crystallites[34]. However, the large surface area has the serious drawback thatit may introduce various defects causing non-radiative recombina-tion routes for electrons and holes and thus resulting in thelowering of luminescence intensity [35]. The increasing percentageof Dy in the original sample results in grains with poor boundaries,which is supported by optical studies where luminescence intensitygets quenched after a particular Dy concentration.
The monoclinic phase of as prepared SrAl2O4: Eu, Dy samples(MC and NC) was confirmed by the powder X-ray diffraction asshown in Fig. 2(a) and (b). The patterns were indexed to JCPDScard no 34-0379 and the parameters were a¼8.44 Å, b¼8.82 Å,
Fig. 3. EDX spectra of different SrAl2O4: Eu2þ , Dy3þ phosphor samples.
D.S. Kshatri, A. Khare / Journal of Luminescence 155 (2014) 257–268262
c¼5.16 Å and β¼93.411. Various diffraction peaks for both MC andNC SrAl2O4 are observed for 2θ¼231, 331, 341, 351 and 411corresponding, respectively, to the planes (0 2 0), (�2 1 1),(2 2 0), (2 1 1) and (0 3 1). All these peaks signify the presence ofα-phase monoclinic SrAl2O4 [36]. The well established relationshipbetween lattice plane spacing and lattice parameters for a mono-clinic lattice is [37]
1
d2¼ 1
sin 2β
h2
a2þk2 sin 2β
b2þ l2
c2�2hl cos β
ac
!ð1Þ
Also, in both the MC and NC samples, no signs of other productsor starting materials are observed. From this, it is concluded that thesmall amount of dopant has almost no effect on the SrAl2O4 phasecomposition [37]. The same was reported by Wu et al. also [38]. Thecorresponding XRD data for (0 2 0), (�2 1 1), (2 2 0), (2 1 1) and(0 3 1) planes are summarized in Table 1(a)–(f). Comparison of datain these tables advocates that in comparison to MC samples, asnoticed from full width at half maxima (FWHM) values, the widthof XRD peaks of NC samples is more, which signifies that thesamples in the latter case truly belong to nanophase [39]. However,a slight change in intensities is observed. The crystalline sizes (D) of
Fig. 3. (continued)
D.S. Kshatri, A. Khare / Journal of Luminescence 155 (2014) 257–268 263
various samples are estimated using Scherrer's formula [40]:
D¼ Kλβ cos θ
ð2Þ
where K is a constant having different values for different grainshapes, λ the wavelength of X-rays used, β the FWHM and θ theBragg angle. The average crystallite sizes calculated for different MCand NC SrAl2O4: Eu2þ , Dy3þ phosphor samples are found to be0.1823 mm and 91.06 nm respectively.
The dislocation density (δ) defined as the length of the dislocationlines per unit volume is determined using the formula [41]
δ¼ 1
D2 ð3Þ
The strain values (ε) are calculated using the relation [42]
β¼ λD cos θ
�ε tan θ ð4Þ
The values of dislocation density (δ) and strain (ε) for differentmicro- and nano-SrAl2O4: Eu2þ , Dy3þ phosphors are compared inTable 2(a)–(f). It is observed that with decreasing values of β, thecrystallite sizes increase and correspondingly the dislocationdensity and strain values decrease. The decrease in dislocationdensity and strain is an indicative of phosphors becoming morecrystalline [43].
The EDX spectra provide further information about the chemi-cal compositions of the final products. The EDX profiles of differentMC and NC SrAl2O4: Eu2þ , Dy3þ phosphor samples are comparedin Fig. 3(a)�(f). These measurements confirm the presence ofmain constituents aluminum, strontium, oxygen, europium anddysprosium in the powder samples. It is observed that EDXprofiles of different doped samples in both micro- and nanophasesare almost similar, and for each of the respective phosphorconcentration the distribution of the elements is fairly uniform(Tables 3 and 4). No compositional variation is witnessed uponprobing different locations within each powder sample, indicatingthat they are homogenous. On the basis of calculations made forSr0.97Al2O4: Eu0.01 Dy0.02 phosphor, which shows the maximumintensity in both cases, the ratio of Sr:Al:O:Eu:Dy is12.62:23.96:61.41:0.67:1.34 (MC) and 11.84:21.13:65.32:0.57:1.14(NC), indicating that Eu2þ with Dy3þ ions are completely dopedinto SrAl2O4 host matrix [44].
In order to find out band-gap value, absorption spectra ofdifferent NC SrAl2O4: Eu2þ , Dy3þ phosphor samples were recordedin the spectral region from 190 to 790 nm at RT (Fig. 4). Theabsorption profiles for all the samples exhibit broad bands in the UVregion (below 290 nm). The fundamental absorption band forSrAl2O4 structure as already reported by Dorenbos [45,46] has beenobserved at about 200 nm. However, for SrAl2O4: Eu structure it isreported to lie at 217 nm [31]. In the present case the absorptionbands for different doped NC samples are found to be locatedbetween 220 and 230 nm. The relation between absorption coeffi-cient (α) and the band-gap (Eg) is expressed as [47]
α¼ ðhν�EgÞ1=2hν
ð5Þ
where ‘Eg’ is the optical band-gap and ‘ν’ the frequency of emittedlight. Thus, as shown in Fig. 5, the experimental band-gap values ofdifferent NC phosphor samples determined from Tauc's plots [48]are found to be 5.4, 5.44, 5.49, 5.50, 5.51 and 5.62 eV. However,Holsa et al. [49] reported experimental band-gap energy (Eg) valueof 6.6 eV for undoped SrAl2O4: Eu2þ material. In practice, it isdifficult to derive an accurate Eg value from the experimental curvedue to the temperature effect and the presence of possible excitonicfeatures at the bottom of the conduction band. The lack of sharpstructures in the spectrum presents difficulties in deriving an
accurate Eg value, too. In conclusion, the materials of present studyare of wide band-gap nature and with increase in Dy doping theband-gap is also found to increase [50].
Fig. 6(a) and (b) depicts the RT emission spectra of MC and NCSrAl2O4; Eu phosphor at varied concentrations of Dy3þ ions (0.01,0.02, 0.03, 0.04, 0.05) excited at 365 nm wavelength. The generalnature of curves in both the cases seems to be similar except forthe MC curves being more intense than the NC ones. For the MCsamples the spectra exhibit a broad band emission accompaniedby the peak at 520 nm, which in the case of NC samples shifts to515 nm. This shift is possibly due to the changes of crystal latticearound Eu2þ and quantum size effect (QSE) of the phosphornanoparticles [51]. The significantly large luminescence intensityand the broad single emission signify homogenous and singlephase material obtained by optimization of growth conditions. It isalready reported that the emission maxima of the phosphorprepared by combustion synthesis method shift to shorter wave-length (520–516 nm) [52]. The broad emission band peaking is
Table 3Atomic percents of elements in different microcrystalline SrAl2O4: Eu2þ , Dy3þ
Sr 12.74 Sr 12.24Al 21.30 Al 21.23O 65.39 O 65.39Eu 0.57 Eu 0.57Dy 0.00 Dy 0.57
(c) Sr0.97Al2O4: Eu0.01, Dy0.02 (d) Sr0.96Al2O4: Eu0.01, Dy0.03Sr 11.84 Sr 11.44Al 21.13 Al 21.01O 65.32 O 65.27Eu 0.57 Eu 0.57Dy 1.14 Dy 1.71
(e) Sr0.95Al2O4: Eu0.01, Dy0.04 (f) Sr0.94Al2O4: Eu0.01, Dy0.05Sr 10.92 Sr 10.50Al 20.98 Al 20.91O 65.25 O 65.17Eu 0.57 Eu 0.57Dy 2.28 Dy 2.85
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attributed to the typical 4f65d1–4f7 transitions of Eu2þ , where thelowest excited state of 4f levels of Eu2þ is located higher than the4f 65d1 level in most crystals, so that Eu2þ usually gives broad-band emission due to f–d transitions [53–55]. The Eu2þ ionspresumably occupy cation (Sr2þ) site, because A13þ site is toosmall to accommodate them. The 5d orbits lie outside and aretherefore strongly affected by the environment. Consequently, thepositioning of the various associated energy levels may varyconsiderably [56]. Thermal vibrations of the surrounding ions
and local vibrations in the lattice structure may result in lumines-cence spectra with no sharp lines within a relatively broad band.There are no special emissions of Dy3þ and Eu3þ in the spectra,which imply that Eu3þ ions have been changed to Eu2þ comple-tely [57]. However the position of the emission peak in thephosphorescence curve shows negligible change, regardless ofthe varied amounts of the Dy3þ ions doping. The role of co-doped Dy3þ is to increase the number of cation vacancies and toincrease the depth of the existing vacancies [58]. The 4f electronsin Eu2þ state are well shielded by the outer shell, but the 5delectrons are viable to splitting by the action of crystal fieldstrength. Emission from a certain energy level can occur whenthe energy gap to the next lower level is more than four or fivetimes the maximum phonon energy of the host lattice [59]. In casethe energy gap is smaller, multi-phonon relaxation occurs and theemission seizes [60]. Characteristic lines in the emission spectrumare observed in the visible spectrum covering green light at515 nm. The featured emission band in the spectrum is relatedto the transition between 4f65d1 and the 4f7 (8S7/2) levels of theEu2þ ion. In SrAl2O4 the crystal field is strong enough to move thelowest state of the 4f65d1 excited electronic configuration belowthe 6P7/2 level due to which a broad band luminescence related tothe parity allowed 4f65d1–4f7 (8S7/2) transition is observed [61].The already reported mechanism [59] states the transfer of anelectron from the valence band (VB) creating a hole in the vicinityof the VB and getting trapped by Sr ion in vacancy level VSr. TheSr2þ sites are easily substituted by Eu3þ because of similar radius.However, in order to maintain charge neutrality, two Eu3þ ionsshould be needed to substitute for three Sr2þ ions. In this way theelectrons in the vacancy defects of Sr2þ would be transferred toEu3þ sites and Eu3þ reduced to Eu2þ . As a result of thermalstimulation the VSr vacancy might be released and swim in thehost lattice [31]. The energy released is then transferred to a Eu2þ
ion, which is excited and then de-excited instantaneously, leadingto emission of visible light [62].
Fig. 7 presents the excitation spectra of MC and NC Sr0.97Al2O4:Eu0.01 Dy0.02 phosphor (the most intense sample) recorded at RT.The maxima in two curves showing green luminescence lie at 365and 324 nm, correspondingly. The large bandwidths of theobserved peaks are attributed to transitions between the 4f65d1
and 4f7 electron configurations of Eu2þ [63]. The excitation spectra
Fig. 6. The room temperature emission spectra of different SrAl2O4: Eu2þ , Dy3þ phosphor samples: (a) MC and (b) NC.
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indicate the possibility of SrAl2O4: Eu2þ , Dy3þ being excited by abroad range of light including visible light.
TL is an important phenomenon since the study of thetemperature dependence of the afterglow intensity and spectrumpermits one to draw conclusions about the depths at whichimpurity levels lie in solids. The SrAl2O4: Eu, Dy phosphor isknown to exhibit photoconductivity under ultraviolet and X-rayirradiation [64]. The TL mechanism relies on the fact that the 4f7
ground energy state of Eu2þ is 3.2 eV below the bottom of theconduction band (CB) and the 4f10 ground state of Dy2þ is 0.9 eVbelow it. Upon exposure to photons of sufficient energy(hν43.2 eV), the electron from Eu2þ state is excited to the CBand Eu3þ is formed. Since the Dy2þ ground state is 1 eV below theCB, Dy3þ may trap such electron with about 1 eV binding energy.Thermal release of the trapped electron and recombination withEu3þ then yields the 5d–4f emission of Eu2þ as persistentluminescence [65].
Fig. 8(a) and (b) shows temperature dependence of TL intensity(TL glow curves) for different MC and NC SrAl2O4: Eu2þ , Dy3þ
phosphor samples. These samples were exposed to UV light(λ¼365 nm) for 5 min and spectra recorded at a heating rate ofβ¼3 1C/s. As is apparent from the figure, all the curves behavesimilarly. For MC samples, different peaks fall in the temperaturerange 139–171 1C while for NC samples they peak in temperaturerange 95–111 1C. However, in both the cases the most intense(Sr0.97Al2O4: Eu0.01, Dy0.02) phosphorescence sample shows themaximum TL intensity at 171 1C and 111 1C, correspondingly. Theappearance of a single glow peak for all the samples indicates that
there is only one trap center that contributes to afterglow proper-ties in this host. The temperature dependence of TL intensity isgenerally presented in the following way [66]:
I¼ Apt0τt0
exp�EtkT
� �exp �
Z T
T0
1βτt0
exp�EtkT
� �dT
� �ð6Þ
where ‘A’ is a constant related to emission efficiency, pt0 is theinitial density of trapped holes, τt0 the oscillation factor, Et theactivation energy (trap-depth), k the Boltzmann constant, T0 theinitial temperature and β the temperature rise rate. The value oftrap depth that resembles the activation energy [58] is anotherparameter playing vital role in choosing a material for lumines-cence applications. The dominating trapping centers of a phosphorwith appropriate trap depths in the range of 0.4–0.7 eV showexcellent phosphorescence property [67]. In the present case,using the initial rise method, it is calculated to be 0.61 and0.69 eV for the most intense phosphorescence MC and NC speci-mens respectively.
Another observation is that the peak positions in differentcurves remain almost unchanged with varying concentration ofDy3þ . The emission intensity is found to increase with increasingDy3þ concentration and for a particular molar concentration ofDy3þ (y¼0.02), the intensity gets quenched [68] and decreases forfurther concentrations (y¼0.03, 0.04 and 0.05). The plausiblereason for such quenching may be the increase in probability ofnon-radiative transitions of the luminescent molecules from theexcited state to the ground state in comparison to the probabilityof radiative transitions (Fig. 9). Fig. 10(a) and (b) shows thevariation of TL intensity for most intense MC and NC samples atdifferent exposure times. The TL intensities of MC and NCSr0.97Al2O4: Eu0.01, Dy0.02 samples were recorded after irradiatingthem with UV light for different exposure times. In the two casesthe intensity peaks at 171 1C and 111 1C, correspondingly, suggest-ing existence of deeper trapping levels. This observation is inaccordance with the findings of Choubey et al. [69]. The TLintensity increases with increase in UV exposure time and ismaximum for irradiation time of 25 min.
Fig. 11(a) and (b) presents, respectively, comparative decaycurves of different MC and NC SrAl2O4: Eu2þ , Dy3þ phosphorsamples irradiated by UV light (λ¼365 nm) for 5 min at RT. All thesamples show the same decay and long afterglow property. Fromthe figure it is noticed that all the afterglow decay curves arecomposed of three regions: the fast, intermediate and the sub-sequent slow-decaying process. The fast-decaying process is dueto the shore survival time of electrons in Eu2þ state while theslow-decaying process is due to the deep trap energy center ofDy3þ [70]. The Eu2þ and Dy3þ ions in aluminate phosphors are
Fig. 7. Comparison of room temperature excitation spectra of Sr0.97Al2O4: Eu0.01Dy0.02 phosphor (the most intense sample).
Fig. 8. TL glow curves of different SrAl2O4: Eu2þ , Dy3þ phosphor samples: (a) MC and (b) NC.
D.S. Kshatri, A. Khare / Journal of Luminescence 155 (2014) 257–268266
the luminescent centers and the traps respectively. The longafterglow property usually results from the trap energy levelproduced by doping of Eu2þ and Dy3þ ions in the crystal. Incomparison to NC samples, initial intensity of MC samples is muchlarger showing rapid decay at higher rate but their total decaytimes are more than 2 h. The decay behavior can be analyzed by acurve fitting procedure relying on the following triple exponential
equation [56]:
I¼ C1exp�tτ1
� �þC2exp
�tτ2
� �þC3exp
�tτ3
� �ð7Þ
where I represents the phosphorescence intensity; C1, C2 and C3are constants; t is time and τ1, τ2 and τ3 are decay times for thefast, intermediate and slow exponential components respectively.The fitting results of parameters τ1, τ2 and τ3 are listed in Table 5. Itis well known that the recombination between the excitedelectrons and trapped holes is greatly influenced by trap depths.As a consequence, the decay components are determined by thecarriers trapped at different trapping levels (Table 5).
4. Conclusions
SSRT and CST prove to be viable methods for synthesizingmicro- and nanocrystaiiline SrAl2O4: Eu2þ , Dy3þ powder samples.The results of XRD studies confirm the α-phase monoclinicstructure of SrAl2O4 and help in assigning (0 2 0), (�2 1 1),(2 2 0), (2 1 1) and (0 3 1) planes to different peaks. In both MCand NC samples, signs of dopant materials have not beenobserved. Comparison of FWHM values support the formation ofNC particles. The EDX results confirm the doping of Eu2þ andDy3þ in the SrAl2O4 host matrix. Tauc's plots help in determiningwide band-gap nature of the prepared NC samples. The emissionintensity increases with greater concentration of Dy followed byquenching at still higher concentrations. In comparison to MCsamples, the emission spectra for NC samples exhibit a blue shift.The position of the emission peak in the phosphorescence curve
Fig. 10. TL glow curves of Sr0.97Al2O4: Eu0.01Dy0.02 for different exposure times (a) MC and (b) NC.
Fig. 11. Afterglow decay curves of different SrAl2O4: Eu2þ , Dy3þ phosphor samples: (a) MC and (b) NC.
0 1 2 3 4 5 6
0
1000
2000
3000
4000
5000
x = 0.94, y = 0.05x = 0.95, y = 0.04
x = 0.94, y = 0.05
x = 0.95, y = 0.04
x = 0.96, y = 0.03
x = 0.96, y = 0.03
x = 0.97, y = 0.02
x = 0.97, y = 0.02
x = 0.98, y = 0.01
x = 0.98, y = 0.01
x = 0.99, Eu = 0.01, y = 0
Inte
grat
ed T
L In
tens
ity (A
rb. U
nits
)
Dopant Concentrations (Mol%)
MC SrxAl2O4: Eu0.01, Dyy NC SrxAl2O4: Eu0.01, Dyy
x = 0.99, Eu = 0.01, y = 0
Fig. 9. Variation of TL integrated intensity with dopant concentration.
D.S. Kshatri, A. Khare / Journal of Luminescence 155 (2014) 257–268 267
shows negligible change, regardless of the varied concentration ofDy3þ . For MC samples, different TL peaks fall in a temperaturerange 139–171 1C while for NC samples they peak in temperaturerange 95–111 1C indicating existence of only one trap center. Incomparison to NC samples, initial intensity of MC samples is muchmore showing rapid decay at higher rate.
Acknowledgment
Authors are thankful to Prof. Rahul Mitra of Central ResearchFacility, IIT Kharagpur, India, for their help in performing char-acterization studies at their center.
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Table 5Decay times fitted for exponential behavior of SrAl2O4: Eu2þ phosphors with increasing Dy3þ concentration.
