Journal of Luminescence 132 (2012) 537–544

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Journal of Luminescence

0022-23

doi:10.1

n Corr

E-m

vsudar@

journal homepage: www.elsevier.com/locate/jlumin

Crystal structure and photoluminescence correlations in white emittingnanocrystalline ZrO2:Eu3þ phosphor: Effect of doping and annealing

S. Dhiren Meetei a, S. Dorendrajit Singh a,n, N. Shanta Singh a, V. Sudarsan b, R.S. Ningthoujam b,M. Tyagi c, S.C. Gadkari c, R. Tewari d, R.K. Vatsa b

a Department of Physics, Manipur University, Canchipur, Imphal 795003, Indiab Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, Indiac Technical Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, Indiad Material Science Division, Bhabha Atomic Research Centre, Mumbai 400085, India

a r t i c l e i n f o

Article history:

Received 28 April 2011

Received in revised form

27 August 2011

Accepted 8 September 2011Available online 16 September 2011

Keywords:

Monoclinic

Tetragonal

Photoluminescence

White emitting nanocrystal

Lifetime

13/$ - see front matter & 2011 Elsevier B.V. A

016/j.jlumin.2011.09.011

esponding author. Tel.: þ91 385 2435080; fa

ail addresses: dorendrajit@yahoo.co.in (S. Dor

barc.gov.in (V. Sudarsan), rsn@barc.gov.in (R

a b s t r a c t

White emitting nanocrystalline ZrO2:Eu3þ phosphors were synthesized by a simple precipitation route

without using a capping agent. X-ray diffraction (XRD) study of ZrO2 and ZrO2:Eu3þsamples revealed

the presence of monoclinic and tetragonal phases. The monoclinic phase increases with increase in the

annealing temperature while the tetragonal phase increases with increase in the concentration of Eu3þ .

This can be attributed to the presence of oxygen vacancy evolved when Zr4þ is replaced by Eu3þ .

Photoluminescence (PL) emission peaks of Eu3þ are observed at 591, 596, 606 and 613 nm on

monitoring excitation wavelengths at 250, 286, 394 and 470 nm. The peaks at 591 and 606 nm were

found to correlate with the tetragonal phase and those at 596 and 613 nm with the monoclinic phase.

Intensities of these peaks are found to change as the crystal structure changes. The lifetime value

corresponding to 591 nm peak increases with Eu3þ concentration at a particular heating temperature

indicating increase of tetragonal phase with respect to monoclinic phase. The CIE co-ordinates of the

doped samples were found to be close to that of white color (0.33, 0.33). The changes in the crystal

structure of the doped samples due to doping and annealing did not affect the white color emission.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

In the past few decades, researchers have been motivated on thestudy of nanomaterials because of novel and interesting physicaland chemical properties shown by the materials in the sizeof nanometer scale [1,2]. Recently, Ariga et al. [2] reported thedevelopments in nanocomposites, which are efficient ways for thedevelopment of new novel nanomaterials because composites adoptsome peculiar characteristics from the components that compose it.Nanocrystalline zirconium dioxide or zirconia (ZrO2) doped withEu3þ is one such material and has aroused great interest due to itsvarious applications [3]. Zirconia is widely used as refractory, catalyst,oxygen sensor, fuel cell [3,4], nucleic acid sensor [5], bone cementadditives [6], super hard materials, etc. [7]. Because of its high meltingpoint, high thermal and mechanical resistance, high thermal expan-sion coefficient, low thermal conductivity, high thermochemicalresistance, high corrosion resistance, high dielectric constant andphotothermal stability, it has extensive applications in photonics andother industries [8–11]. High chemical and photochemical stability

ll rights reserved.

x: þ91 385 2435145.

endrajit Singh),

.S. Ningthoujam).

with high refractive index and low phonon energy makes it an idealmedium for preparation of highly luminescent materials [8,10–12].

Though many reports are found for the existence of a particularphase of zirconia at various temperatures [5,13–16], it exhibits threecrystalline structures under general conditions, that is, P21/c (14)monoclinic (below 1170 1C), P42/nmc (137) tetragonal (1170–2370 1C) and Fm3m (225) cubic (above 2370 1C) [8,9]. It wasreported that the crystalline structures significantly influenced itsphysical properties [3]. Therefore, extensive amount of work hasbeen reported on the mechanical and other physical propertiesof zirconia. However, only few works have been reported onthe photoluminescence of Eu3þ doped zirconia [5,9,10,17,18]. Thevariation in peak intensities of magnetic (5D0-

7F1) and electric(5D0-

7F2) dipole transitions of Eu3þ has been reported in differentcrystalline structures of ZrO2 [5,18]. Moreover, as the performanceof zirconia based devices is dependent on its crystalline structure[3], the crystal structure of zirconia may also influence the lifetimeof the dopant Eu3þ . Some reports of the same are also found inliteratures [18,19]. However, extensive amount of work and data areneeded to clarify the observed phenomena. Therefore, the variationof PL emission and decay lifetime with the crystal structure of ZrO2:Eu3þ is investigated extensively.

In luminescence, white emitting phosphors are of great interestbecause of their tremendous applications in various forms of

S. Dhiren Meetei et al. / Journal of Luminescence 132 (2012) 537–544538

lighting devices. Many reports of white light emission from materi-als containing more than one lanthanide ions are available inliteratures [20–23]. However, such multiple doped materials haveproblems of self absorption, thereby reducing their performance.Therefore, single lanthanide ion doped material will be a potentialcandidate for this purpose. Producing white light simply by doping asingle lanthanide ion is seldom reported. In this work, synthesis ofwhite light emitting phosphor doped with a single lanthanide ionis also reported. Meanwhile, it will be interesting to know whetherthe color emission from this phosphor also changes with crystalstructure. If so happened, the desired color can be tuned for specificpurposes. Therefore, the CIE co-ordinates of the samples are alsocalculated and studied.

Fig. 2. Percentage of the monoclinic and tetragonal phases of Eu3þ (2, 4, 6, 8, 10

and 15 at%) doped ZrO2 vs. annealing temperature.

2. Experimental details

2.1. Synthesis

Pure and Eu3þ (2, 4, 6, 8, 10 and 15 at%) doped nanocrystallinezirconia particles have been synthesized by a simple chemical routetechnique without using a capping agent. For a typical synthesis of2 at% of Eu3þ , 1 g of zirconium oxychloride (ZrOCl2 �8H2O 99%,Aldrich) was dissolved in 25 ml of deionized water. Then 22 mg ofeuropium (III) oxide (Eu2O3 99.99%, Aldrich) was added to thesolution and stirred for 1 h. After dissolving Eu2O3 completely,165 mg of NaOH (98%) dissolved in 25 ml of deionized water wasadded. Upon addition, a white dense precipitate was formed. Thewhole system was then kept on stirring for 1 h. After that, the whiteprecipitate was collected by centrifugation. This precipitate was thenkept in hot air for drying. Similar procedures were followed for thepreparation of other samples. Pure, 2, 4 and 6 at% Eu3þdoped zirconia(ZrO2) were named as D1, D2, D3 and D4, respectively. In order toobserve the effect of crystal structure with Eu3þ concentration,samples of ZrO2 doped with 8, 10 and 15 at% Eu3þ were alsoprepared and named as D5, D6 and D7, respectively. These sampleswere annealed at 800, 1000 and 1300 1C in the ambient atmosphere.For a particular heating of 800, 1000 or 1300 1C, suffix 800, 1000 or1300 was added to Ds. For example, D1-800 denotes D1 sampleannealed at 800 1C.

Fig. 1. XRD patterns of (a) 800, (b) 1000 and (c) 1300 1C heated samples. D1 indicates p

ZrO2 samples, respectively. The labels ‘M’ and ‘T’ indicate the highest peak intensity o

2.2. Characterization

X-ray diffraction patterns were obtained in a PW1729 X-raydiffractometer equipped with a copper anode X-ray tube. Thewavelength used is 1.5405 A. Crystallite size was determined usingthe Scherrer equation: t¼0.9l/b cos y, where 0.9 is the shape factor,l is the x-ray wavelength, b is the line broadening at half themaximum intensity in radians, and y is the Bragg angle [24,25].

Transmission electron microscopy (TEM) images wererecorded using JEM-2000FX microscope (JEOL). Fourier transforminfra-red (FT-IR) spectra were recorded in MB 102 spectrometer(BOMEN). Photoluminescence (PL) emission and excitation spec-tra were obtained from F4500 fluorimeter (Hitachi). The decay ofluminescence is carried out using ms-flash xenon lamp (100 W),FLS920 (EDINBURGH). All the measurements were recorded atroom temperature.

3. Results and discussion

3.1. XRD study

XRD patterns of the pure and Eu3þ (2–15 at%) doped ZrO2

samples annealed at 800, 1000 and 1300 1C are shown in Fig. 1.

ure ZrO2. D2, D3, D4, D5, D6 and D7 indicate 2, 4, 6, 8, 10 and 15 at% Eu3þ doped

f monoclinic and tetragonal phases, respectively.

S. Dhiren Meetei et al. / Journal of Luminescence 132 (2012) 537–544 539

Pure ZrO2 samples (D1) annealed at 800, 1000 and 1300 1C showthe monoclinic phase (JCPDF card no. 86-1451). However, there isa small peak belonging to the tetragonal phase found at the

Fig. 3. TEM images of (a) D1-800 and (b) D4-800. SAED images of (c) D1-800 and

(d) D4-800.

Fig. 4. FT-IR spectra of D1, D2, D3 and D4 annealed at 800 1C.

Fig. 5. (a) Excitation and (b) em

800 1C annealed sample. The lattice parameters of 800 1Cannealed sample based on monoclinic phase are found to bea¼5.27, b¼5.03, c¼5.47 A and b¼95.01. Based on the Scherrer’sequation the crystallite size is found to be 33 nm. The samplesannealed at 1000 and 1300 1C show similar lattice parameterswithin the error bar (0.02 A). The reported values of latticeparameters are a¼5.14, b¼5.21, c¼5.31 A and b¼99.21 (JCPDFcard no. 86-1451).

In the 800 1C annealed samples, the ratio (percentage) of thetetragonal to monoclinic phase increases with the increase inEu3þ concentration (Figs. 1a and 2). Similar observation has beenreported earlier [26,27]. At and above 8 at% Eu3þ the puretetragonal phase is observed. This can be explained on the basisof oxygen vacancies arising from the substitution of Zr4þ by Eu3þ

ions [19,27]. In ZrO2, one oxygen vacancy (O2�) compensates thesubstitution of 2 Zr4þ ions by 2 Eu3þ ions.

In the case of 1000 and 1300 1C annealed Eu3þ doped samples,the amount of tetragonal phase increases as compared to themonoclinic phase (Fig. 2), which is similar to the 800 1C annealedsamples. A complete phase change is observed from monoclinic totetragonal when Eu3þ concentration is Z8 at% in ZrO2 (Fig. 1b, c).However, the amount of the monoclinic phase with respect totetragonal phase increases with the heat treatment temperatureranging from 800 to 1300 1C. Increase of heat treatment does notchange the crystallite size significantly in the case of pure andEu3þ doped samples. It may be due to the arising of variation oftetragonal and monoclinic phases during the doping as well asannealing.

3.2. TEM and FT-IR studies

TEM images of D1-800 and D4-800 are shown in Fig. 3. Theimages show nearly spherical in shape. Crystallinity of the samplesis confirmed from SAED images. From the TEM image, the particlesize is found to be �40 nm.

FT-IR spectra of D1–D4 samples annealed at 800 1C are shown inFig. 4. Since zirconia has low phonon energy [12], the spectra areshown within the fingerprint region of 400–900 cm�1. Vibrationalbands are observed at 418, 455, 502, 579 and 742 cm�1. The bandsat 418, 502 and 742 cm�1 correspond to the monoclinic phase ofzirconia [28,29]. These bands show strong adsorption at low con-centration of Eu3þ , but their intensities decrease at a higherconcentration of Eu3þ . This is consistent with that of XRD where

ission spectra of D1-800.

Fig. 6. Excitation spectra of D3 samples annealed at 800, 1000 and 1300 1C.

Fig. 7. Emission spectra of D2, D3 and D4 samples annealed at (a) 8

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the amount of monoclinic phase with respect to the tetragonalphase decreases at higher Eu3þ concentration. Zr–O bands at 455and 579 cm�1 are related to tetragonal zirconia [29–31].

3.3. Photoluminescence study

Fig. 5a shows the PL excitation spectrum of pure D1-800sample recorded under 486 nm emission. A broad excitation peakis observed at 286 nm, which is assigned to the host absorption.The emission spectrum recorded under 286 nm excitation showsa blue emission at 486 nm, which can be ascribed to hostemission (Fig. 5b). Fig. 6 shows the excitation spectra of D3samples annealed at 800, 1000 and 1300 1C recorded under613 nm emission. The peaks observed at around 245 nm are dueto Eu–O charge transfer (CT) originating from charge transitionfrom 2p states of O2� to excited states of Eu3þ . A hump centeredat 286 nm (4.3 eV) for 800 and 1000 1C annealed samples is due tohost absorption. However, this absorption is redshifted (303 nm)in 1300 1C annealed samples, which may be due to the increase in

00, (b) 1000 and (c) 1300 1C. Excitation wavelength is 250 nm.

S. Dhiren Meetei et al. / Journal of Luminescence 132 (2012) 537–544 541

particle size. The host absorption energy of 4.3 eV is lower thanthe reported optical band gap of 5 eV of bulk ZrO2. This can beattributed to the presence of extrinsic states such as surface trapstates or defect states [32–34]. Other absorption peaks at 366(7F0-

5D4), 387 (7F0,1-5G1, 5L6), 394 (7F0,1-

5L6), 419 (7F0-5D3)

and 470 (7F0,1-5D2) nm are due to f–f transitions of Eu3þ . These

peaks are weak in intensity as compared to host absorption due totheir forbidden nature. The peak at 245 nm dominates the otherpeaks indicating the strong energy transfer from Eu–O CT band tothe excited energy levels of Eu3þ . Also, occurrence of hostabsorption indicates possible energy transfer from host absorp-tion to the excited states of Eu3þ . Similar patterns of excitationspectra are observed for other samples.

Emission spectra of the doped samples (D2–D4) annealed at 800,1000 and 1300 1C recorded under 250 nm excitation are shown inFig. 7. The observed peaks within 550–650 nm are at 579, 591, 596,606, 613, 623 and 629 nm. The peaks at 591 and 596 nm are due tothe magnetic dipole (5D0-

7F1) transition, while the peaks at 606and 613 nm are due to the electric dipole (5D0-

7F2) transition ofEu3þ [29]. The weak Eu3þemission peak at 579 nm is assigned to(5D0-

7F0) [5]. Also, emission spectra of (D2–D4) samples annealedat 800, 1000 and 1300 1C recorded under the excitations of 286, 394and 470 nm show similar patterns of Eu3þ emission characteristics.Fig. 8 shows the integrated area under the curve observed by fitting

Fig. 8. Integrated intensity areas of D4 samples annealed at 800, 1000 and

1300 1C. Excitation wavelengths are 250, 286, 394 and 470 nm.

Fig. 9. PL decay curves for 5D0 level of Eu3þ for D3-800 sample recorded at 613 nm em

of mono and bi-exponential decay fittings to data.

from 582 to 620 nm with excitations of 250, 286, 394 and 470 nm.From this figure it can be clearly observed that the emissionintensity is more when excitation wavelength is 250 nm followedby host absorption at 286 nm. This clearly indicates the efficientenergy transfer from Eu–O CT or host to the excited states of Eu3þ inZrO2:Eu3þ .

In all emission figures the observed four main peaks at 591,596, 606 and 613 nm show variation in intensities with change incrystalline phase (discussed earlier). For all samples annealed at800, 1000 and 1300 1C, intensity of the peak at 591 increasescompared to 596 nm with increase in doping concentration ofEu3þ . Similarly, the intensity of the peak at 606 nm increaseswith respect to 613 nm with increasing Eu3þ concentration. Itwas also observed in XRD pattern that the percentage of tetra-gonal phase with respect to monoclinic phase increases withincrease in the doping concentration of Eu3þ . Therefore, the peaksat 591 and 606 nm may be related to the tetragonal phase andthose at 596 and 613 nm to the monoclinic phase of zirconia [19].Our results are in agreement with Smits et al. [19] who explainedthe presence of tetragonal and cubic phases having highersymmetry with the increase in Eu3þ ZrO2.

The above variation in the emission peak intensities with theEu3þ concentration can be explained according to the structuralsymmetry of tetragonal and monoclinic phases of ZrO2. Thetetragonal phase has a space group of P42/nmc (137), in whichZr4þ ion occupies D4d with inversion symmetry [3,9]. When Eu3þ

ion occupies inversion symmetry site, the intensity of the allowedmagnetic dipole transition dominates over electric dipole. There-fore, the intensity of the peak at 591 nm (magnetic dipoletransition) is more than that at 606 nm (electric dipole transi-tion). On the other hand, the monoclinic phase has a space groupof P21/c (14), in which Zr4þ ion occupies asymmetric environ-ment without inversion symmetry, Cs [9]. When Eu3þ ionoccupies an asymmetry environment, the hypersensitive electricdipole transition (613 nm) becomes dominant over magneticdipole transition (596 nm). Samples with lower doping concen-tration give relatively higher intensity for the electric dipoletransition than that for magnetic dipole transition. This indicatesthat Eu3þ occupies the asymmetry environment since the systemis dominated by monoclinic phase at lower doping concentration.This information obtained from the PL emission is similar to thatobserved from XRD pattern.

3.4. Lifetime study

Fig. 9 shows the typical PL decay curve for 5D0 energy level ofthe D3-800 sample recorded under 250 and 394 nm excitations

ission with excitation (a) 250 and (b) 394 nm. Inset of (a) shows the enlarged view

Table 1Average PL decay lifetimes of emission wavelengths at 591 and 613 nm of samples

heated at 800, 1000 and 1300 1C. Excitation wavelengths are 250, 286 and 394 nm.

Sample lex lem t1 I1 t2 I2 tav w2

S. Dhiren Meetei et al. / Journal of Luminescence 132 (2012) 537–544542

and 613 nm emission. Based on fitting of PL decay curves, it isobserved that the decay of 5D0 energy level is fitted well with bi-exponential equation. Three main possible explanations for thevalidity of this decay are as following [35,36]:

(nm) (nm) (ms) (%) (ms) (%) (ms)

(i)

D2-800 250 591 0.99 60 2.61 40 2.02 3.177

613 0.75 69 1.67 31 1.21 1.126

286 591 0.78 61 2.64 39 2.05 1.043

613 0.73 65 1.54 35 1.16 1.004

394 591 0.49 43 1.88 57 1.65 0.911

Different distributions of Eu3þ ions in two different regionsof the particle, i.e., inner core sphere and outer sphericalshell, thereby creating a difference in probability of non-radiative decay for ions at or near the surface and ions in thecore of the particles.

613 0.65 67 1.48 33 1.09 1.035

(ii)

D2-1000 250 591 0.61 69 1.79 31 1.28 2.703

Inhomogeneous distribution of the doping ions in the hostmaterial leading to variation in the local concentration.

613 0.60 74 1.30 26 0.91 1.456

(iii) 286 591 – – – – – –

The transfer of excitation energy from donor to lanthanideactivators.

613 0.49 54 1.11 46 0.90 1.218

394 591 0.46 46 1.26 54 1.07 1.132

613 0.53 66 1.13 34 0.85 1.311

D2-1300 250 591 0.36 17 0.84 83 0.80 1.035

613 0.57 84 0.99 16 0.68 1.560

286 591 0.29 10 0.80 90 0.78 1.075

613 0.53 63 0.87 37 0.70 1.081

394 591 0.10 14 0.90 86 0.89 1.106

613 0.51 71 0.81 29 0.63 1.121

D3-800 250 591 1.02 57 2.90 43 2.30 1.952

613 0.78 72 1.76 28 1.24 1.753

286 591 0.70 45 1.94 55 1.66 1.365

613 0.57 48 1.37 52 1.15 1.004

394 591 0.47 35 1.96 65 1.79 1.373

613 0.62 64 1.50 36 1.13 1.170

D3-1000 250 591 1.17 51 3.00 49 2.47 1.341

613 0.78 70 1.65 30 1.19 1.126

286 591 0.86 73 2.18 27 1.49 1.170

613 0.66 50 1.37 50 1.14 1.438

394 591 0.94 70 2.76 30 1.95 1.390

613 0.56 70 1.26 30 0.90 1.348

D3-1300 250 591 0.56 58 1.08 42 0.86 1.014

613 0.52 68 0.86 32 0.67 1.282

286 591 0.33 14 0.82 86 0.79 1.143

613 0.51 60 0.85 40 0.69 1.113

394 591 0.22 13 0.92 87 0.90 1.146

613 0.50 73 0.81 27 0.64 1.174

D4-800 250 591 1.12 45 2.97 55 2.54 1.368

613 0.73 67 1.90 33 1.39 1.304

286 591 0.82 48 2.49 52 2.10 1.188

613 0.61 63 1.59 37 1.21 1.558

395 591 0.84 42 2.56 58 2.23 1.167

613 0.60 66 1.66 34 1.22 1.180

D4-1000 250 591 0.92 35 2.74 65 2.46 1.425

613 0.68 60 1.59 40 1.24 1.232

286 591 0.69 65 2.34 35 1.75 1.561

613 0.60 51 1.35 49 1.11 1.111

394 591 0.92 47 2.65 53 2.24 1.204

613 0.61 77 1.57 23 1.03 1.273

D4-1300 250 591 0.64 68 1.75 32 1.27 1.179

613 0.55 77 0.95 23 0.69 1.211

286 591 0.57 64 1.32 36 1.00 1.263

613 0.56 73 0.94 27 0.71 1.178

394 591 0.56 59 1.58 41 1.23 1.233

613 0.52 73 0.90 27 0.67 1.174

Therefore, the experimental decay curves were fitted with bi-exponential equation of the form:

I¼ I1 expf�ðt=t1Þgþ I2 expf�ðt=t2Þg ð1Þ

where I1 and I2 are intensities at two different values of time (t),i.e., at t1 and t2, respectively. In case of 250 nm (indirect throughEu–O CT), the values of t1 and t2 are found to be 0.78 (72%) and1.76 (28%) ms, respectively. Former lifetime is associated with theenergy transfer process from Eu–O CT to Eu3þ and surface Eu3þ .The latter is from core Eu3þ . Since the shape of the particle isspherical, average lifetime tav is calculated using the equation[34–39]:

tav ¼ ðI1t21þ I2t2

2Þ=ðI1t1þ I2t2Þ ð2Þ

The average PL decay lifetime (tav) for D3-800 sample is found tobe 1.23 ms. In case of the excitation (direct Eu3þ absorption) at394 nm, the values of t1 and t2 are found to be 0.62 (64%) and1.50 (36%) ms, respectively. The former lifetime is from thesurface Eu3þ and latter from Eu3þ in the core of the particle.The average lifetime is found to be 1.13 ms, which is less thanthat of 250 nm excitation (given above). This is because ofthe energy transfer process from Eu–O CT to the excited statesof Eu3þ and more excited photons are available at the 5D0

excited level.Table 1 gives the details of average PL lifetime values from 5D0

excited state of samples (D2–D4) annealed at 800, 1000 and1300 1C (emission wavelengths at 591 and 613 nm, and excitationwavelengths at 250, 286 and 394 nm). In case of the 800 1Cannealed samples lifetime values at 591 nm emission are morethan those at 613 nm indicating higher luminescence intensity at591 nm, which is similar to the luminescence study (Fig. 7),where the amount of tetragonal phase is more than that ofmonoclinic phase. For 1000 1C annealed samples the lifetimevalue at 591 nm emission is slightly more than that of 613 nm.However, 1300 1C annealed samples show almost comparablevalues of lifetime at both 591 and 613 nm emissions. It suggeststhe increase of monoclinic phase at 1300 1C as compared to thatat 800 1C. With the increase of Eu3þ concentration at a particularheat treatment the lifetime at 591 nm increases with respect to613 nm indicating the increase of tetragonal phase, which hashigh symmetry [18,19]. Earlier, similar results have been reportedby Smits et al. [19] on the dependence of decay lifetime with thenanocrystal crystal structure, i.e., in Eu3þ decay lifetime in ZrO2 islonger when crystal structure of nanocrystal is tetragonal withhigher symmetry around Eu3þ compared to lower symmetricmonoclinic phase with increasing Eu3þ concentration. Overall,the trend of lifetime is similar to observations from the XRD andluminescence studies.

3.5. CIE chromaticity study

PL emission spectra of the samples of different doping con-centrations and annealing temperatures are shown in Fig. 10. Itshows the presence of both greenish blue and red color. ItsCommission Internationale de l’eclairage (CIE) color space co-ordinate is found to be very close to that of white color.Approximate color space co-ordinate (x, y) corresponding towhite color of the CIE chromaticity diagram is (0.33, 0.33). Thecolor CIE co-ordinate for pure sample, D1-800, recorded under

Fig. 10. PL emission spectra of ZrO2:Eu3þ at different doping concentrations for (a) 800, (b) 1000 and (c) 1300 1C annealed samples. The label D2/250 indicates emission

spectrum of D2 sample recorded under 250 nm excitation. Similarly, D2/286 indicates emission spectrum of D2 sample recorded under 286 nm excitation and so on.

S. Dhiren Meetei et al. / Journal of Luminescence 132 (2012) 537–544 543

250 nm excitation is (0.17, 0.25) and that at 286 nm excitation is(0.16, 0.25). Both the CIE co-ordinates correspond to greenish bluecolor in the chromaticity diagram. However, the co-ordinates ofthe Eu3þ doped samples are found within the region of whitecolor. Calculated CIE co-ordinates (x, y) of the samples are given inTable 2. This clearly shows that the greenish blue color emittedfrom the host zirconia mixed with the red color emitted from thedopant Eu3þ results in white color. Hence, the single lanthanideion, Eu3þ doped ZrO2 shows emission in the white region of CIEco-ordinates. The CIE co-ordinates does not change with Eu3þ (upto 6 at%) as well as annealing up to 1300 1C. Therefore, thesephosphors will be potential candidates for white light emittingdevices.

4. Conclusions

In this work, white emitting nanocrystalline ZrO2:Eu3þ phos-phors were synthesized by a simple precipitation route. Sizes ofthe nanocrystals are in the range of 18–36 nm. The XRD and PLstudies of the samples revealed the following:

(1)

With increase in doping concentration, (a) the tetragonal phaseincreases, (b) the intensity of the PL peak at 591 nm with respectto 596 nm increases and the peak at 606 nm with respect to613 nm increases and (c) the lifetimes increase.

(2)

The changes in the Eu3þ emission peak intensities are due tothe change in the symmetry of ZrO2 crystal structure with the

Table 2CIE color co-ordinates of the samples with corresponding colors.

Sl. no. Sample lexc (nm) x y CIE color

1 D1-800 250 0.17 0.25 Greenish blue

286 0.16 0.25 Greenish blue

2 D2-800 250 0.32 0.32 White

286 0.30 0.30 White

3 D3-800 250 0.32 0.32 White

286 0.31 0.31 White

4 D4-800 250 0.33 0.32 White

286 0.34 0.31 White

5 D2-1000 250 0.32 0.32 White

286 0.27 0.30 White

6 D3-1000 250 0.33 0.32 White

286 0.30 0.31 White

7 D4-1000 250 0.33 0.32 White

286 0.33 0.32 White

8 D2-1300 250 0.28 0.30 White

286 0.22 0.28 White

9 D3-1300 250 0.31 0.31 White

286 0.26 0.29 White

10 D4-1300 250 0.34 0.30 White

286 0.31 0.30 White

S. Dhiren Meetei et al. / Journal of Luminescence 132 (2012) 537–544544

variation in Eu3þ concentration from lower symmetric mono-clinic to higher symmetric tetragonal phase.

(3)

With increase in annealing temperature, (a) the monoclinicphase increases, (b) the intensity of the PL peaks at 596 nmwith respect to 591 nm increases and also the peak intensityat 613 nm with respect to 606 nm increases.

Thus, there are good correlations between crystal structureand photoluminescence of ZrO2:Eu3þ phosphor. PL studies maybe used to predict the crystal phase of a system. The PL emissionand lifetime of Eu3þ change with the crystal structure of ZrO2. Inthis study Eu3þ doped ZrO2 shows white emission according toCIE co-ordinates color space.

Acknowledgments

The authors thank T. Mukherjee, Chemistry Group and D. Das,Chemistry Division, Bhabha Atomic Research Centre, Mumbai,for their encouragement. S. Dhiren Meetei and S. DorendrajitSingh gratefully acknowledged Department of Atomic Energy (DAE),

Government of India, for financial support (Sanction number 2008/37/35/BRNS).

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