Journal of Alloys and Compounds 588 (2014) 488–495

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Journal of Alloys and Compounds

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Characterization and optical properties of Dy3+ doped nanocrystallineSrAl2O4: Eu2+ phosphor

0925-8388/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jallcom.2013.11.045

⇑ Corresponding author. Tel.: +91 9425213445, +91 7714052486.E-mail address: akhare.phy@nitrr.ac.in (A. Khare).

D.S. Kshatri, Ayush Khare ⇑Department of Physics, National Institute of Technology, Raipur 492 010, Chhattisgarh, India

a r t i c l e i n f o

Article history:Received 21 August 2013Received in revised form 26 October 2013Accepted 8 November 2013Available online 20 November 2013

Keywords:NanocrystallineOptical propertiesPhotoluminescenceAfterglow

a b s t r a c t

Nanocrystalline SrAl2O4: Eu2+, Dy3+ phosphors are synthesized by a reliable low temperature combustionsynthesis technique (CST). Multiple techniques including X-ray diffraction (XRD), scanning electronmicroscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) are used to examine the surface mor-phology and structural properties of SrAl2O4: Eu, Dy phosphors. The average crystal size calculated usingXRD lies in nano-range. The optical properties are presented and discussed in terms of photolumines-cence (PL), thermoluminescence (TL) and afterglow decay spectra. The as-obtained nanocrystallineSrAl2O4: Eu2+, Dy3+ phosphors show higher PL emission intensity (at 515 nm). After being irradiated withultraviolet (UV) light, the phosphor samples emit green long lasting phosphorescence (LLP) with excita-tion and emission peaks at 324 nm and 515 nm respectively. Using TL glow curves, the trap-depth valuecalculated for most intense sample is found to be 0.69 eV. Furthermore, the decay curves contain a rapidand slow-decaying portion sustaining for longer time, suggesting potential applications in many fields.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction are considered as one of the first produced LLPs. This type of

As a kind of energy-saving materials, the long-lasting phos-phors absorb sunlight, store energy, and then gradually releasethe solar energy as visible light, which leads to a long persistentafterglow in the darkness [1,2]. In recent years, the demand to de-velop efficient luminescent materials has attracted researchersbecause of its possible photonic applications, good luminescentcharacteristics, stability in high vacuum and absence of corrosivegas emission under electron bombardment compared to currentlyused sulfide based phosphors. Recently, nanomaterials and nano-technology have attracted several researchers from differentfields, especially the luminescence field. Nanophosphors differfrom existing bulk phosphors in terms of their electrical, opticaland structural features. The quantum size effect generated by in-crease in the band-gap value due to a decrease in the quantumallowed state existing in small particles plays an important rolein tailoring the electrical and optical characteristics of very smallparticles. This improves the surface and interface effects. Rareearth (RE) and non RE doped inorganic phosphors are widely usedin a variety of applications such as in lamp industries, radiationdosimetry, color display etc. [3,4]. Deposition of energy in a mate-rial by ionizing radiation results in the generation of charge car-riers (electrons or holes). These charge carriers, localized in thelattice are subsequently trapped at vacancies and interstitials[5]. Copper-doped metal sulfides (e.g., ZnS: Cu, CaS: Cu, SrS: Cu)

phosphor maintains phosphorescence for no more than a fewhours and breaks down easily when exposed to UV radiation ormoisture [6]. Furthermore, the use of radioactive elements (e.g.,Pm3+) as an auxiliary excitation source results in environmentalpollution. Thus, the conventional LLPs have been gradually re-placed with innovative phosphors such as RE metal ion-dopedalkaline earth aluminates, alumino silicates, alkaline earth titan-ates etc.

In the SrO–Al2O3 system, six phases of strontium aluminatedoped with Eu2+ ions that yield afterglow have been reported.These phases are SrAl2O4 [7,8], Sr2Al6O11 [9], SrAl4O7 [10],Sr4Al14O25 [11], SrAl12O19 [12] and Sr3Al2O6 [13]. Among variousLLPs reported, SrAl2O4: Eu2+, Dy3+ has drawn much attention be-cause of its high luminescent intensity, long-lasting time, chemicalstability and environmental capability. This green-emitting phos-phor is a useful material that applies in an unexpectedly large fieldof applications like the tri-color low-pressured mercuryfluorescence lamps, luminous paints in highways, ceramic prod-ucts, display devices etc. [14–17]. In addition, it is equally usefulin textiles, the dial plates of glow watches, warning signs and fordetection of damage in bridges and big buildings [14,15]. Manyresearchers have reported the influence of Eu2+ concentration onluminescent properties [18–20]. The emission with Eu2+ ions asemission centers in SrAl2O4: Eu2+, Dy3+ phosphors is very stronglydependent on the host lattice and occurs from ultraviolet to redregion. This is because the excited 4f65d1 configuration of Eu2+ isextremely sensitive to the change in the lattice environment ofhost structure [21–25]. In addition to a higher chemical stability,

D.S. Kshatri, A. Khare / Journal of Alloys and Compounds 588 (2014) 488–495 489

the intensity and the duration of the phosphorescence of strontiumaluminates (SrAl2O4: Eu2+, Dy3+) makes it possible to envisage acontinuous light emission during a whole night (10 h), hencegreatly renewing interests in the phosphorescence phenomenon.

In recent years, several techniques have been employed to syn-thesize SrAl2O4: Eu2+, Dy3+ phosphors. These include solid-statereaction [6], sol–gel [26], co-precipitation [21], combustion syn-thesis [27] and reverse microemulsion method [28]. Among thesemethods, the combustion process is an efficient technique for thesynthesis of phosphors due to good mixing of starting materialsand relatively low reaction temperature resulting in more homoge-neous product than those obtained by other methods. There is acomparative lack of information related to optical properties ofthe present phosphors in the scientific literature. A need of infor-mation on the results with varying concentration of Dy3+ dopinghas thus motivated the present work, where the SrAl2O4 employedas the base material is doped with rare earth elements Eu2+ andDy3+ as activator and co-activator respectively. We have succeededin developing SrxAl2O4: Eu2þ

0:01;Dy3þy (x = 0.99, y = 0; x = 0.98,

y = 0.01; x = 0.97, y = 0.02; x = 0.96, y = 0.03; x = 0.95, y = 0.04 andx = 0.94, y = 0.05) nanophosphors by using CST. The object of thepresent paper is to report the results of luminescent studies, interms of PL, TL and afterglow properties of SrxAl2O4: Eu2þ

0:01;Dy3þy

phosphors and to investigate possible mechanism of long lastingphosphorescence. To the best of our knowledge and on the basisof what literature survey suggests, it is claimed that this paper at-tempts to address the effect of varying concentration of Dy3+ onSrAl2O4 host for the first time.

2. Experimental

2.1. Sample preparation

For preparing powder samples, stoichiometric amounts of strontium nitrate [Sr(NO3)2], aluminum nitrate [Al (NO3)3�9H2O] and urea [CO (NH2)2] are used as rawmaterials with proportionate amount of distilled water. In addition to it, europiumoxide (Eu2O3) and dysprosium oxide (Dy2O3) taken as co-activators are dissolved inconcentrated nitric acid (HNO3) before transferring them to crucible. The smallamount of boric acid (H3BO3) is used as the flux while the urea [CO (NH2)2] is usedas fuel. All chemicals used are of analytical reagent grade (99.9% pure). After thesolution is transferred into the crucible, it is placed into a furnace already main-tained at a temperature of 600 ± 5 �C. Within 5 min, the furnace reaches the desiredtemperature and reaction starts giving yellowish flame. The mixture froths andswells forming foam, which ruptures with a flame and glows to incandescence. Thiscontinues for next few seconds and as it is over, crucible is taken out of the furnaceand kept in open to allow cooling. Upon cooling, we get fluffy form of material,which is then crushed for 1 h using agate pestle mortar.

2.2. Measuring instruments

The materials were weighed using Shimadzu ATX 224 single pan analytical bal-ance and the samples were prepared in a digital furnace already maintained a tem-perature of 600 ± 5 �C. The crystalline structure, size and phase composition of thesamples were examined by Bruker D8 Advance X-ray diffractometer using Co Karadiation (k = 1.790 ÅA

0

). The morphology of prepared samples was studied usingGerman make ZEISS-EVO 60 scanning electron microscope while energy dispersiveX-ray spectroscopy was carried out with Oxford Inca EDX System. The excitationand emission spectra are measured at RT using Hitachi F-2500 fluorescence spectro-photometer. The afterglow measurements are made with an indigenous experi-mental set-up comprising of an RCA-931 photomultiplier tube (PMT) and adigital nanoammeter (model DNM-121). The TL spectra at different temperaturesare recorded using Nucleonics make TLD reader (model I 1009 h), where sampleswere excited with UV radiation at 365 nm wavelength for 5 min.

3. Results and discussion

3.1. Characterization studies

3.1.1. X-ray diffraction (XRD)The typical XRD patterns of SrxAl2O4: Eu2þ

0:01;Dy3þy (x = 0.99,

y = 0; x = 0.98, y = 0.01; x = 0.97, y = 0.02; x = 0.96, y = 0.03;

x = 0.95, y = 0.04 and x = 0.94, y = 0.05) phosphor samples are pre-sented in Fig. 1(a–f). When compared with standard JCPDS card(No. 34–0379), these patterns are found to be characterized bypeaks at 2h values of 23o, 33o, 34o, 35o and 41� corresponding tothe planes (020), (�211), (220), (211) and (031). All these peakssignify the presence of monoclinic SrAl2O4 [29]. There are not ob-served signs of any other products or starting materials. This im-plies that the little amount of doped rare earth ions has almostno effect on the SrAl2O4 phase composition [30]. The same was re-ported by Wu et al. also [31]. The corresponding XRD data for(020), (�211), (220), (211) and (031) planes are summarizedin Table 1(a–f).

The crystalline sizes (D) of various samples are estimated usingScherrer’s Formula [32]:

D ¼ Kkb cos h

ð1Þ

where K is a constant having different values for different grainshapes, k the wavelength of X-rays, b-the full width at half maxima(FWHM) and h is the Bragg’s angle. The average crystallite sizes fordifferent SrxAl2O4: Eu2þ

0:01;Dy3þy (x = 0.99, y = 0; x = 0.98, y = 0.01;

x = 0.97, y = 0.02; x = 0.96, y = 0.03; x = 0.95, y = 0.04 and x = 0.94,y = 0.05) phosphor samples are calculated to be as 86.0328 nm,100.6871 nm, 95.2057 nm, 95.7787 nm, 97.5996 nm and71.0787 nm respectively, which support nanocrystalline nature ofprepared phosphors.

The dislocation density (d) is defined as the length of the dislo-cation lines per unit volume and is determined using following for-mula [33]:

d ¼ 1D2 ð2Þ

The strain values (e) are calculated using the relation [34]:

b ¼ kD cos h

� e tan h ð3Þ

The values of FWHM (b), crystal size (D), dislocation density (d)and strain (e) for different SrAl2O4: Eu2+, Dy3+ phosphors are alsosummarized in Table 1(a–f). It is observed that with decreasingvalues of b, the crystallite sizes increase and correspondingly thedislocation density and strain values decrease. The decrease in dis-location density and strain suggests that phosphors have becomemore crystalline.

3.1.2. Energy dispersive X-ray spectroscopy (EDX)The EDX spectra are further used as a tool to determine the

chemical compositions of the final products. The EDX spectra ofSrxAl2O4: Eu2þ

0:01;Dy3þy (x = 0.99, y = 0; x = 0.98, y = 0.01; x = 0.97,

y = 0.02; x = 0.96, y = 0.03; x = 0.95, y = 0.04 and x = 0.94, y = 0.05)phosphor samples are shown in Fig. 2(a–f). The results of EDX stud-ies confirm the presence of aluminum (Al), strontium (Sr), oxygen(O), europium (Eu) and dysprosium (Dy) in the prepared samples.It is observed that for each of the respective phosphor concentra-tion, the distribution of the elements is fairly uniform. On the basisof calculations made for Sr0.97Al2O4: Eu0.01 Dy0.02 phosphor, whichshows maximum intensity, the ratio of Sr: Al: O: Eu: Dy is11.84:21.13:65.32:0.57:1.14 indicating that Eu2+ with Dy3+ ionsare completely doped into SrAl2O4 host matrix [35].

3.1.3. Scanning electron microscopy (SEM)The SEM studies were carried out to investigate the surface

morphology and the crystalline size of the synthesized phosphors.The SEM micrographs of SrxAl2O4: Eu2þ

0:01;Dy3þy (x = 0.99, y = 0;

x = 0.98, y = 0.01; x = 0.97, y = 0.02; x = 0.96, y = 0.03; x = 0.95,y = 0.04 and x = 0.94, y = 0.05) phosphor samples are shown inFig. 3(a–f). As observed, all the samples have similar irregular

Fig. 1. X-ray diffractograms of different SrxAl2O4: Eu2þ0:01;Dy3þ

y (x = 0.99, y = 0; x = 0.98, y = 0.01; x = 0.97, y = 0.02; x = 0.96, y = 0.03; x = 0.95, y = 0.04 and x = 0.94, y = 0.05)phosphor samples.

490 D.S. Kshatri, A. Khare / Journal of Alloys and Compounds 588 (2014) 488–495

morphology with angularity and corners with varying grain sizes.It is further noticed that the crystalline sizes are nearly same irre-spective of the composition of the sample. This leads to the conclu-sion that the doping process does not make significant changes tothe morphology and size of the nanostructures. This was alreadyestablished by the finding of Ayvacikli et al. [36]. They reportedthat the phosphor powders were irregular particles with differentshapes and sizes. In the present case also, the surfaces of the foams

show a lot of cracks, voids and pores formed by the gases escapingduring combustion reaction. In fact, the large amount of escapinggases dissipates heat and thereby prevents the material from sin-tering and thus provides conditions for formation of nanocrystal-line phase. Apart from above, it is also reported that nanorodcrystallites have larger surface area than flower-like crystallites[37]. The large surface area has the serious drawback that the lumi-nescent intensity is impaired due to the introduction of a large

Table 1XRD data of different SrAl2O4: Eu, Dy phosphors.

2h (�) FWHM (b) (radian) Lattice spacing (d) (Å) Intensity (I) (%) hkl Crystallite size (D) (nm) Dislocation density (d) (lin/m2) Strain (e) (lin�2 m�4)

(a) Sr0.99Al2O4: Eu0.01

23.3899 0.0018 4.41284 62.68 020 95.4594 1.09 � 1014 5.53 � 10�4

33.2444 0.0022 3.12692 80.47 �211 79.8172 1.56 � 1014 4.70 � 10�4

34.2549 0.0018 3.03732 100.00 220 97.8156 1.04 � 1014 3.72 � 10�4

35.0108 0.0019 2.97373 62.90 211 92.8582 1.15 � 1014 3.84 � 10�4

41.0638 0.0028 2.55036 71.83 031 64.1690 2.42 � 1014 4.77 � 10�4

(b) Sr0.98Al2O4: Eu0.01, Dy0.01

23.3725 0.0013 4.41607 63.99 020 132.1704 5.72 � 1013 4.01 � 10�4

33.2379 0.0018 3.12751 80.97 �211 97.5527 1.05 � 1014 3.31 � 10�4

34.2480 0.0016 3.03791 100.00 220 110.0406 8.25 � 1013 3.31 � 10�4

35.0098 0.0017 2.97381 61.14 211 103.7825 9.28 � 1013 3.44 � 10�4

41.0560 0.0030 2.55082 68.01 031 59.8896 2.71 � 1014 5.11 � 10�4

(c) Sr0.97Al2O4: Eu0.01, Dy0.02

23.3570 0.0014 4.41896 64.29 020 122.7262 6.63 � 1013 4.32 � 10�4

33.2194 0.0019 3.12920 82.52 �211 92.4139 1.17 � 1014 4.06 � 10�4

34.2309 0.0016 3.03938 100.00 220 110.0355 8.25 � 1013 3.31 � 10�4

34.9905 0.0018 2.97540 61.97 211 98.0115 1.04 � 1014 3.64 � 10�4

41.0445 0.0034 2.55151 60.78 031 52.8417 3.54 � 1014 5.79 � 10�4

(d) Sr0.96Al2O4: Eu0.01, Dy0.03

23.3736 0.0018 4.41586 67.33 020 95.4566 1.09 � 1014 5.55 � 10�4

33.2415 0.0020 3.12719 75.82 �211 87.7982 1.29 � 1014 4.207 � 10�4

34.2400 0.0018 3.03860 100.00 220 97.8117 1.04 � 1014 3.74 � 10�4

34.9976 0.0018 2.97482 56.28 211 98.0135 1.04 � 1014 3.64 � 10�4

41.0493 0.0018 2.55122 97.56 031 99.8137 1.003 � 1014 3.06 � 10�4

(e) Sr0.95Al2O4: Eu0.01, Dy0.04

23.3170 0.0017 4.42643 61.79 020 101.0614 9.79 � 1013 5.25 � 10�4

33.1851 0.0018 3.13235 81.75 �211 97.5393 1.05 � 1014 3.85 � 10�4

34.1947 0.0015 3.04250 100.00 220 117.3598 7.26 � 1013 3.11 � 10�4

34.9558 0.0015 2.97826 64.42 211 117.6027 7.23 � 1013 3.04 � 10�4

40.9983 0.0033 2.55426 61.92 031 54.4348 3.37 � 1014 5.63 � 10�4

(f) Sr0.94Al2O4: Eu0.01, Dy0.05

23.3965 0.0017 4.41160 72.68 020 101.0758 9.78 � 1013 5.24 � 10�4

33.2607 0.0030 3.12543 71.96 �211 58.5351 2.91 � 1014 6.41 � 10�4

34.2510 0.0025 3.03766 100.00 220 70.4265 2.01 � 1014 5.14 � 10�4

35.0093 0.0030 2.97385 52.21 211 58.8099 2.89 � 1014 6.07 � 10�4

41.0675 0.0027 2.55014 88.08 031 66.5464 2.25 � 1014 4.60 � 10�4

D.S. Kshatri, A. Khare / Journal of Alloys and Compounds 588 (2014) 488–495 491

number of defects into the phosphor crystal. The defects haveserious implications on luminescence materials as they providenon-radiative recombination routes for electrons and holes. Theincreasing percentage of Dy in the original sample results in thegrains with poor boundaries, which is supported by optical studieswhere luminescence intensity drops after a particular Dyconcentration.

3.2. Optical studies

3.2.1. PL studiesFig. 4(a) shows the emission spectra of SrAl2O4 doped with

0.01 M concentration of Eu2+ and 0.01, 0.02, 0.03, 0.04, 0.05 thatof Dy3+ ions at an exciting wavelength of 365 nm. These spectra ex-hibit a broad band emission accompanied by the peak at 515 nm,which is ascribed to the typical 4f65d1–4f7 transitions of Eu2+.There are no special emissions of Dy3+ and Eu3+ in the spectra,which imply that Eu3+ ions have been changed to Eu2+ completely[21]. However, the position of the emission peak in the phospho-rescence curve shows negligible change, regardless of the variedamount of the Dy3+ ions doping. Here the role of Dy3+ lies in induc-ing the formation of hole trap levels and prolonging the afterglow.Thus, in SrAl2O4: Eu2+ samples with higher concentrations of Dy3+,creation of more and more hole-trap levels takes place leading to

greater PL intensities [38]. Probably this is corresponding to4f65d1 to 4f7 transition of Eu2+ ions. Since the excited 4f65d1 config-urations of Eu2+ ions are extremely sensitive to the change in thelattice environment, the 5d electron may couple strongly to the lat-tice [39]. Hence the mixed states of 4f and 5d configurations will besplitted by the crystal, which may lead to the blue shift of its emis-sion peak [40,16]. It is already reported that the emission maximaof the phosphor prepared by combustion synthesis method shiftsto shorter wavelength (520–516 nm) [41]. This may be attributedto the changes of crystal lattice around Eu2+ and quantum sizeeffect.

The doping of Dy3+ ions generate deeper trapping energy levelsin the crystal matrix, which trap and store electrons. It disturbs theprocess of relaxation transitions of excited electrons of Eu2+ ionsfrom excited state returning to ground state [42]. When the molarconcentration of Eu2+ and Dy3+ ions reaches a certain value, thebest synergistic effect between Eu2+ and Dy3+ ions is achieved.Mean while both the density and depth of trapping and theamounts of electrons stored are appropriate and the best emissionintensity is obtained corresponding to this value. As shown inFig. 4(a), when the molar concentration of Eu2+ and Dy3+ ions are0.01 and 0.02 respectively, the emission intensity of RE strontiumaluminate is found to be the highest. This observation also is ingood agreement with corresponding XRD results (for same

Sr0.99Al2O4: Eu0.01 Sr 0.98Al2O4: Eu0.01Dy0.01

Sr0.97Al2O4: Eu0.01Dy0.02 Sr0.96Al2O4: Eu0.01Dy0.03

Sr0.95Al2O4: Eu0.01Dy0.04 Sr0.94Al2O4: Eu0.01Dy0.05

(a) (b)

(c) (d)

(e) (f)

Fig. 2. EDX spectra of different SrxAl2O4: Eu2þ0:01;Dy3þ

y (x = 0.99, y = 0; x = 0.98, y = 0.01; x = 0.97, y = 0.02; x = 0.96, y = 0.03; x = 0.95, y = 0.04 and x = 0.94, y = 0.05) phosphorsamples.

492 D.S. Kshatri, A. Khare / Journal of Alloys and Compounds 588 (2014) 488–495

concentration). However, for any higher molar concentration(y > 0.02 in SrxAl2O4: Eu0.01 Dyy), the phenomenon of concentrationquenching occurs, which results in decrease in emission intensity[43].

The excitation spectrum of Sr0.97Al2O4: Eu0.01 Dy0.02 phosphor(most intense) recorded at RT is shown in Fig. 4(b). The excitationspectrum displays a broad band (300–350) with a maxima cen-tered at 324 nm, leading to the green luminescence of the materi-als. The bandwidths of the observed peaks are quite large, whichare due to transitions between the 4f65d1 and 4f7 electron config-urations of Eu2+ [44]. The excitation spectrum indicates that as-prepared SrAl2O4: Eu2+, Dy3+ phosphor can be excited by a broad

range of light including visible light suggesting its appropriatenessfor applications discussed in introduction section.

3.2.2. TL studiesTL is the thermally stimulated emission of light following the

absorption of energy from radiations. The radiations cause dis-placement of electrons within the crystal lattice of the substance.Upon heating, the trapped electrons return to their normal low-er-energy positions, releasing energy in the process. TL is an effec-tive tool for various applications such as dosimetry, in biologicalapplications, age determination, geology or solid state defect struc-ture analysis, etc. The TL properties measured in a short period are

Sr(a) (b)

(c) (d)

(e) (f)

0.99Al2O4: Eu0.01 Sr 0.98Al2O4: Eu0.01Dy0.01

Sr 0.97Al2O4: Eu0.01Dy0.02 Sr0.96Al2O4: Eu0.01Dy0.03

Sr 0.95Al2O4: Eu0.01Dy0.04 Sr0.94Al2O4: Eu0.01Dy0.05

Fig. 3. SEM micrographs of different SrxAl2O4: Eu2þ0:01;Dy3þ

y (x = 0.99, y = 0; x = 0.98, y = 0.01; x = 0.97, y = 0.02; x = 0.96, y = 0.03; x = 0.95, y = 0.04 and x = 0.94, y = 0.05)phosphor samples.

D.S. Kshatri, A. Khare / Journal of Alloys and Compounds 588 (2014) 488–495 493

indicative of emission intensity and afterglow lifetime, which arehelpful to estimate longtime luminescence. The SrAl2O4: Eu, Dyphosphor is known to exhibit photoconductivity under ultravioletand X-ray irradiation [45]. Actually, the Eu2+ 4f7 ground energystate is 3.2 eV below the bottom of the conduction band and theDy2+ 4f10 ground state is 0.9 eV below it. Following this concept,it is assumed that on exposure to photons of sufficient energy(hm > 3.2 eV; k < 390 nm), the electron from Eu2+ state is excited

to the conduction band and Eu3+ is formed. Since the Dy2+ groundstate is 1 eV below the conduction band, Dy3+ may trap such elec-tron with about 1 eV binding energy [46]. Thermal release of thetrapped electron and recombination with Eu3+ then yields the5d–4f emission of Eu2+ as the persistent luminescence.

Fig. 5(a) shows TL glow curves for SrxAl2O4: Eu2þ0:01;Dy3þ

y

(x = 0.99, y = 0; x = 0.98, y = 0.01; x = 0.97, y = 0.02; x = 0.96,y = 0.03; x = 0.95, y = 0.04 and x = 0.94, y = 0.05) phosphor samples.

Fig. 4. (a) PL emission spectra of different SrxAl2O4: Eu2þ0:01;Dy3þ

y (x = 0.99, y = 0;x = 0.98, y = 0.01; x = 0.97, y = 0.02; x = 0.96, y = 0.03; x = 0.95, y = 0.04 and x = 0.94,y = 0.05) phosphor samples. (b) PL excitation spectrum of Sr0.97Al2O4: Eu0.01 Dy0.02

phosphor.

Fig. 5. (a) TL glow curves of different SrxAl2O4: Eu2þ0:01;Dy3þ

y (x = 0.99, y = 0; x = 0.98,y = 0.01; x = 0.97, y = 0.02; x = 0.96, y = 0.03; x = 0.95, y = 0.04 and x = 0.94, y = 0.05)phosphor samples. (b) Graph for calculation of trap-depth value by initial risemethod (for Sr0.97Al2O4: Eu0.01 Dy0.02 sample showing maximum TL intensity inFig. 5(a)).

494 D.S. Kshatri, A. Khare / Journal of Alloys and Compounds 588 (2014) 488–495

The general nature of TL curves is observed to be similar in differ-ent cases. Each curve peaks in a temperature range 100–130 �C andphosphor Sr0.97Al2O4: Eu0.01, Dy0.02 shows maximum TL intensity at111 �C. It is also observed that varying concentration of Dy3+ doesnot affect the peak position much. However, the emission intensityis found to increase with increasing Dy concentration and for a par-ticular molar concentration (y = 0.02), the intensity gets quenched[47] and decreases for further concentrations (y = 0.03, 0.04 and0.05). The probable reason for such quenching may be the increasein probability of non-radiative transitions of the luminescent mol-ecules from the excited state to the ground state in comparison tothe probability of radiative transitions. The temperature depen-dence of TL intensity is generally represented in the followingway [48]:

I ¼ Apt0

st0exp

�Et

kT

� �exp �

Z T

T0

1bst0

exp�Et

kT

� �dT

� �ð4Þ

where ‘A’ is a constant related to emission efficiency, pt0-the initialdensity of trapped holes, st0-the oscillation factor, Et-the activationenergy (trap-depth), k-the Boltzmann constant, T0-the initial tem-perature and b is the temperature rise rate. Fig. 5(b) depicts the var-iation in trap depth values with increasing temperature. The valueof trap depth resembling the activation energy [38], is calculatedto be 0.69 eV using initial rise method [49].

3.2.3. Afterglow decay studiesFig. 6(a) presents the luminescent decay curves of SrAl2O4: Eu

phosphors with increasing Dy concentration and irradiated by UVlight (k = 365 nm) for 5 min at RT. Theoretically, afterglow phe-nomenon is witnessed only when appropriate trap energy level ex-ists. In case the trap energy level is shallow, electrons in the trap

Fig. 6. (a) Afterglow decay curves of different SrxAl2O4: Eu2þ0:01;Dy3þ

y (x = 0.99, y = 0;x = 0.98, y = 0.01; x = 0.97, y = 0.02; x = 0.96, y = 0.03; x = 0.95, y = 0.04 and x = 0.94,y = 0.05) phosphor samples. (b) Logarithmic graph for SrAl2O4: Eu0.01, Dy 0.02

phosphor (1) the fast (2) intermediate and (3) slow exponential components.

Table 2Decay times fitted for exponential behavior of SrAl2O4: Eu2+ phosphors withincreasing Dy3+ concentration.

System s1 (s) s2 (s) s3 (s)

Sr0.99Al2O4: Eu0.01 51.53 141.27 521.89Sr0.98Al2O4: Eu0.01, Dy0.01 69.43 185.21 733.58Sr0.97Al2O4: Eu0.01, Dy0.02 129.80 344.82 1411.76Sr0.96Al2O4: Eu0.01, Dy0.03 118.56 312.52 1208.67Sr0.95Al2O4: Eu0.01, Dy0.04 106.72 264.84 1102.42Sr0.94Al2O4: Eu0.01, Dy0.05 75.32 212.32 780.43

D.S. Kshatri, A. Khare / Journal of Alloys and Compounds 588 (2014) 488–495 495

are excited easily and come back to the excited state, resulting inshort afterglow time. If the trap energy level is deeper, higher en-ergy is needed when excited electrons in the trap come back to theexcited state, so electrons are stored only in trap energy level andafterglow phenomenon is not observed. From figure, it is evidentthat all the afterglow decay curves are composed of three regimes;the fast, intermediate and the subsequent slow-decaying process.The fast-decaying process is due to the shore survival time of elec-trons in Eu2+ state while the slow-decaying process owes to thedeep trap energy center of Dy3+ [50]. The Eu2+ and Dy3+ ions in alu-minates phosphors are the luminescent centers and the trapsrespectively. The long afterglow property usually results from thetrap energy level produced by doping of Eu2+ and Dy3+ ions inthe crystals. In fact, SrAl2O4: Eu2+, Dy3+ phosphor is monoclinicstructure, which gives rise to the formation of appropriate trapfor producing afterglow [51]. All the SrxAl2O4: Eu2þ

0:01;Dy3þy

(x = 0.99, y = 0; x = 0.98, y = 0.01; x = 0.97, y = 0.02; x = 0.96,y = 0.03; x = 0.95, y = 0.04 and x = 0.94, y = 0.05) phosphor samplesshow a rapid decay followed by long-lasting phosphorescence. Thedecay curves corresponding to different phosphor samples indicatethe initial luminescent intensity and afterglow decay rate to be dif-ferent from each other, which are mainly influenced by the Dy3+

concentration and host structure. Decay times are calculated by acurve fitting technique (Fig. 6(b)) based on the following third or-der exponential decay equation [52]:

I ¼ A1 exp�ts1

� �þ A2 exp

�ts2

� �þ A3 exp

�ts3

� �ð5Þ

where I is phosphorescence intensity; A1, A2 and A3 are constants; tis time and s1, s2 and s3 are decay times for the fast, intermediateand slow exponential components respectively. The data summa-rized in Table 2 indicate that these phosphors exhibit different de-cay times. The decay times calculated for Sr0.97Al2O4: Eu0.01 Dy0.02

phosphor comprise of three exponential components (Fig. 6(b))with different decay times and the maximum value reaching upto1411.76 s.

4. Conclusions

The SrAl2O4: Eu2+, Dy3+ nanocrystalline powders are investi-gated as new green phosphor with good emission color puritydue to the non-centrosymmetric site for the europium ion. TheXRD patterns of the synthesized phosphors belong to alpha phasemonoclinic structure with average crystal size falling in nanorange.The EDX results confirm the doping of Eu2+ and Dy3+ in the SrAl2O4

host matrix. The SEM micrographs reveal that SrAl2O4: Eu2+ phos-phors with different Dy3+ concentrations show similar irregularmorphology. The excited transition centered at 515 nm is foundto be sensitive in nature resulting in green emission. However,the afterglow characteristics present highest decay time for mostintense phosphor. The PL behavior is supported by the TL charac-teristics. TL measurements show that the trap generated by Dy3+

ions in monoclinic structure phosphor is deeper. Further, the excel-

lent emission properties of this phosphor suggest that it may beused for display applications.

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