Structural and photoluminescence studies of Eu3þ doped cubicY2O3 nanophosphors

P. Packiyaraj a,b, P. Thangadurai a,n

a Centre for Nanoscience and Technology, Pondicherry University, RV Nagar, Kalapet, Puducherry 605 014, Indiab Nano photonics laboratory, Department of Physics, Indian Institute of Technology-Delhi, New Delhi 110 016, India.

a r t i c l e i n f o

Article history:Received 16 May 2013Received in revised form18 July 2013Accepted 31 July 2013Available online 8 August 2013

Keywords:Eu3þ doped Y2O3

NanophosphorsPhotoluminescencePL lifetimeCIE diagram

a b s t r a c t

Structural and photoluminescence properties of undoped and Eu3þ doped yttrium oxide (Y2O3:Eu3þ)

nanoparticles heat-treated at 600 and 900 1C were reported. Three concentrations of Eu3þ (1, 3 and5 mol%) were doped in Y2O3. The heat-treated Y2O3:Eu3þ nanoparticles were cubic in structure withoutany impurity phase as studied by X-ray diffraction and transmission electron microscopy. The samplesshowed high crystallinity and average particle size was in the range of 10–16 nm and 20–25 nm whenannealed at 600 and 900 1C respectively. The 900 1C annealed Y2O3:Eu3þ exhibited a strong redphotoluminescence due to homogeneously occupied Eu3þ ions in the Y2O3 lattice and high crystallinity.The PL lifetime decreases with the dopant concentration from of 2.26 to 1.77 ms and from 2.35 to 1.81 msin the case of 600 and 900 1C annealing respectively. Emission becomes faster with higher loading ofEu3þ . Strong photoluminescence characteristics at most commonly available UV-blue excitationwavelengths make these phosphors suitable for LEDs and other display applications.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Luminescent nanomaterial have potential applications in thefield of photonics, nanotechnology, electronics, display, lasing,detection, optical amplification, fluorescent sensing to biomedicalengineering, and environmental control [1–3]. And also it provideschallenges to both fundamental research and breakthrough devel-opment of technologies. Yttrium oxide (Y2O3) is an excellent hostmaterial due to its large band gap (5.8 eV), high dielectric constant(14–18) and also optically isotropic with a refractive index of 1.91. Ithas high thermal stability because of high melting point (2450 1C)[2]. The small phonon energy (380 cm�1) is enough for effectiveradiative transitions between electronic energy levels of the rareearth ions in Y2O3 host and it is a good choice of host material [4].

Nanophosphors should have a spherical shape and high lumines-cence efficiency for the lighting applications. The efficiency of lightemission and the brightness of a phosphor screen are improvedbecause of their minimized light scattering on their surfaces [6–8].However, the shape and size of luminescent nanoparticles depend ontheir synthesis method. Nanoparticles synthesized by different meth-ods show variation in their size, shape, and optical properties. TheEu3þ ions doped Y2O3 (Y2O3:Eu3þ) is one of the main oxide basedred-emitting phosphors and is widely used in the lighting industryand in solid-state-laser-based devices [5], was studied for a long time

because of its efficient luminescence under ultraviolet (UV) andcathode-ray excitation. Reduction of sizes to the nanoscale levels,improve the brightness and resolution of displays in devices of presenttechnology. Y2O3:Re3þ nanostructures of different morphologies havebeen synthesized by using different methods such as gas phasecondensation technique [9], sol–gel route [10,11], spray pyrolysis[12], chemical vapor technique [13], combustion synthesis [14],and hydrothermal method [15]. Specially, the hydrolysis assistedco-precipitation method has advantages due to its simplicity in natureover other methods. Co-precipitation synthesis involves dissolution ofcompound salt precursor in aqueous media and subsequent precipita-tion from the solution by pH adjustment. Apart from its simplicity,atomic mixing of the constituents by chemical co-precipitation yields afinal product of near-perfect stoichiometry without high temperaturetreatment [16]. However, it is very important to develop phosphorswith controlled morphology ultra-small sizes [17,18] and in particular,the site-selectivity and relative behavior of the strongest 5D0�7F2 (red)transition from Eu3þ needs to be further investigated for red-phosphor applications[19,20].

In this work, we used hydrolysis assisted co-precipitationmethod to prepare pure and Eu3þ-doped yttrium oxide nano-particles and annealing them at different temperatures. Their struc-tural and emission characteristics were studied in detail and reported.

2. Experimental

Pure and Eu3þ doped yttrium oxide nanoparticles were pre-pared by hydrolysis assisted co-precipitation method for three

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Journal of Luminescence

0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jlumin.2013.07.074

n Corresponding author. Tel.: þ91 413 265 4974; fax: þ91 413 265 6758.E-mail addresses: thangadurai.p@gmail.com,

thangaduraip.nst@pondiuni.edu.in (P. Thangadurai).

Journal of Luminescence 145 (2014) 997–1003

dopant concentrations such as 1, 3 and 5 mol% of Eu3þ . Yttrium(III) nitrate hexahydrate (Y(NO)3 �6H2O, 99.8% pure, SigmaAldrich), europium (III) nitrate pentahydrate (Eu(NO)3 �6H2O,99.9% pure, Sigma Aldrich), and Ammonia solution about 30% GR(NH3, Merck) were used as received without further purificationfor the preparation. In the case of preparation of pure (orundoped) Y2O3, yttrium nitrate hexahydrate was used as precursorand it was dissolved in ultrapure water to form 0.1 N homoge-neous solution. The precursor solution was placed in a 500 mlflask fitted with a reflux condenser and was hydrolyzed for 20 h.After hydrolysis, the reaction mixture was cooled down to roomtemperature. Then ammonium hydroxide [NH4OH] solutionwas added to the hydrolyzed solution to control the pH of thesolution and a precipitate was obtained. Using centrifuge, theprecipitate was separated and washed several times withdistilled water and dried at room temperature. The as-preparedsamples were annealed at 600 and 900 1C for 2 h in air toobtain different grain sizes and good crystallinity. On the otherhand, Y2O3:Eu3þ nanoparticles were prepared by the samemethod using Eu(NO)3d6H2O as Eu3þ source in Y(NO)3d6H2Osolution. Europium ions were added to a doping level of 1, 3, 5 mol% relative to yttrium ions. The precursor solutions have undergonesame procedure as discussed above forthe un-doped Y2O3

preparation.Structural studies were performed by using powder X-ray

diffraction (XRD) method. The XRD patterns were recorded byusing the Cu-Kα1 radiation (λ¼1.5406 Å) in a Rigaku powder X-ray diffractometer at a scanning rate of 0.021 per second in the 2θrange of 151r2θr601. The X-ray diffractometer was operated at40 kV and 30 mA. Crystallite size was estimated from XRD peakbroadening using the Scherrer calculation [21]. The morphologyof the samples were recorded from Carl Zeiss Scanning ElectronMicroscope (SEM) (Model EVO 18) working at the acceleratingvoltage of 15 kV. Carbon coating was given on the Y2O3:Eu3þ

powder to avoid charging under electron beam for SEM measure-ments. Microstructures were studied by transmission electronmicroscopy (TEM) and TEM studies were conducted in a FEITecnai G2 TEM microscope operated at a voltage of 200 kV. Thespecimen for TEM studies were made by dispersing the samplepowder in ethanol by ultrasonication for about 30 min and a dropfrom the dispersed solution was put on the standard carbon filmcoated Cu TEM grids with 300 mesh. The grid was dried underlight for the solvent to evaporate and mounted on the TEMsample holder for measurements. The steady-state and time-resolved photoluminescence (PL) measurements were carried outusing a home-built set-ups with 410 nm (CW laser) and 532 nm(CW laser) lasers as the excitation source. The emission from thesamples was coupled into a monochromator (Andor SP2300) thatis coupled to CCD through the appropriate lenses and filters forrecording the emission spectra. For time-resolved photolumines-cence measurements (PL lifetime studies), a frequency generator(5 Hz), lock-in amplifier, digital storage oscilloscope and a mono-chromator (Acton SP2300) coupled to a photo multiplier tube(PMT) were employed. The PL excitation spectra were acquired ina RF-5301 PC Spectrofluorophotometer. For PL and time resolvedPL measurements, the samples were made as thin layer on theultrasonically cleaned (in acetone) glass slide and loaded in thesample holder. The PL imaging were done by a modified laserscanning confocal microscope (Olympus, BX51) equipped withXY-piezo stage and excited with a 410 nm CW laser. Both PLwhite light and conventional confocal bright and dark fieldimages were recorded using an ALP (All long pass) 4410 nmfilter. For PL imaging, Y2O3:Eu3þ powder was kept on the samplestage which was mounted on the microscope, then laser wasallowed to fall on the sample and images were recorded by usinga CCD detector.

3. Results and discussion

3.1. X-ray diffraction analysis

The phase and crystal structure of the as-prepared andannealed Y2O3:Eu3þ at various temperatures above 600 1C of thepure and doped were analyzed by XRD. Fig. 1 shows the powderXRD patterns of 0, 1, 3 and 5 mol% Y2O3:Eu3þ nanoparticles, asprepared and annealed at 900 1C. All the as-prepared samplesshowed an amorphous nature, with no well-defined crystallinepeaks (except a broad and weak diffraction at 291) and the phasepurity of crystalline nature has been started evolving from theannealing temperatures above 600 1C (supplementary informa-tion). Such a low degree of crystallinity in as-prepared samples isexpected because the precipitate is in the form of unreactedyttrium hydroxide [Y(OH)3] that should be either amorphousor very small crystallites of [Y(OH)3] has a hexagonal crystalstructure, close to amorphous [22–24]. This would only trans-form to Y2O3 until they were subjected to appropriate annealingtemperatures.

When the samples were annealed at and above 600 1C, welldefined diffraction peaks have appeared with a good intensitybecause of its improved crystallinity (Fig. 1b). All the XRD peakpositions have been compared with reported values (JCPDS cardNo. 88-2162) and found that the structural phase of Y2O3 is a cubicphase. The corresponding peaks are marked in the Fig. 1. XRD datafurther suggest no traces of any unreacted constituents and/orother crystalline phases such as monoclinic, hexagonal [25].Similar observations have also been made for the samplesannealed at 600 1C and shown in the supplementary information.

Fig. 1. Powder X-ray diffraction patterns of Eu3þ doped Y2O3 nanoparticles (a) as-synthesized and the [2 2 2] reflection is from hexagonal [Y(OH)3] (JCPDS cardno.83-2042) and (b) annealed at 900 1C with the Eu3þ concentrations of 0, 1, 3 and5 mol%. The diffraction peaks were indexed by comparing with the standard datafor cubic Y2O3 (JCPDS card no. 88-2162).

P. Packiyaraj, P. Thangadurai / Journal of Luminescence 145 (2014) 997–1003998

Since, Eu3þ has a close ionic radius (of 0.109 nm) to that of Y3þ

(0.104 nm) ions, the minimum deviation in the estimated latticeparameters of cubic phase of Y2O3 suggest that Eu3þ ions areincorporated into the Y3þ sites even up to 5 mol% [26]. The FWHMof the peaks is found to decrease in the 900 1C sample comparedto the 600 1C annealed samples. It would be expected that thewidth should get narrower when annealed at higher tempera-tures. This is obviously due to the increase in crystallinity of theY2O3:Eu3þ nanopowders. In addition, heat treatment has also ledto an increase in crystallite size.

The average crystallite size (d) of the Y2O3:Eu3þ phosphorsobtained from XRD data using Scherrer's equation are tabulated inTable 1. The average crystallite sizes are 13 and 23 nm for theannealing temperatures of 600 and 900 1C respectively. Oneanother interesting observation has been made in the trend ofthe particle size increase with doping concentration at a givenannealing temperature. Since Eu3þ is little heavier atom comparedto Y3þ and this mass difference might have played a role in crystalgrowth process by diffusion at the time of synthesis reactions.

Microstructure analysis was conducted by TEM and SEM. Thebright field TEM image of as-prepared and 900 1C annealed 5 mol%Y2O3:Eu3þ nanoparticles is shown in Fig. 2(a,c). The particles arefound to be agglomerates containing many crystallites havingmore or less spherical shape. The sizes determined from TEM(for one particle it is marked as 30 nm in Fig. 2c) matches close tothe calculated size from XRD data. Fig. 2(d) shows the SAED(selected area electron diffraction) pattern of the 900 1C annealedsample. The diffraction rings corresponding to the (222), (440),(622) planes are indicated in the SAED pattern confirming thecrystal structure of Y2O3. The diffused SAED ring pattern (Fig. 2(b))is observed for as-prepared sample indicating the less crystalline/

amorphous nature. The SAED for 900 1C annealed sample(Fig. 2(d)) showed a clear diffraction spots indicating good crystal-linity of the samples. This results corroborates with the XRDresults. It is confirmed that the as-prepared precursor sample isamorphous in nature. The morphology of the all Eu3þ doped Y2O3

nanoparticles was observed in SEM. The SEM images of the 3% and5% Eu3þ doped Y2O3 annealed at 600 and 900 1C are shown inFig. 3. (a,b) and (c,d) respectively. The SEM micrographs show thatthese particles are agglomerated in nature.

3.2. Photoluminescence studies

The room-temperature Photoluminescence (PL) spectra ofY2O3:Eu3þ nanoparticles annealed at different temperatures andwith various Eu3þ concentrations were acquired by using variouscommercially excitation sources 337, 410 and 532 nm. Fig. 4(a, b)present the PL spectra of 900 1C annealed Y2O3:Eu3þ excited at410 and 532 nm respectively. The emission spectra have a series ofemission peaks between 570 and 650 nm, well agree with thereported values of Eu3þ emission transitions [22,27]. The emissionspectral lines of Eu3þ ion are sharp which is due to the screeningof 4f orbital by 5s and 5p orbitals from crystal field of the hostlattice. PL intensity increases with annealing temperature becauseof better crystallinity of the sample with increasing grain size and/or the removal of impurities. Overall, the emission intensities fromEu3þ rare earth ions were found to increase with the increase ofEu3þ doping.

The emission spectral lines observed at 581, 588, 594, 600, 612,and 632 nm are attributed to the 5D0-

7FJ (J¼0, 1, 2, 3, 4)transitions of Eu3þ ions in the Y2O3 host matrix. The PL intensityof 5D0-

7FJ transition and the Stark-crystal field splitting is

Table 1Crystallite size and optical parameters of the Y2O3:Eu3þ nanoparticles.

Y2O3:Eu3þ (xmol%) Average crystalline size (nm) R/O Ratio CIE co-ordinate Life-time (ms)

410 nm laser 532 nm laser

600 1C 900 1C 600 1C 900 1C 600 1C 900 1C 600 1C 900 1C 600 1C 900 1C 600 1C 900 1C

X Y X Y

0 mol% 10 21 – – – – – – – – – –

1 mol% 11 22 8.25 10.44 10.21 10.34 0.579 0.339 0.594 0.337 2.26 2.353 mol% 14 24 9.46 10.51 10.83 10.87 0.618 0.335 0.607 0.337 2.02 2.055 mol% 16 25 9.66 10.91 11.29 11.54 0.622 0.340 0.628 0.342 1.77 1.81

Fig. 2. TEM images with SAED pattern as inset of 5 mol% Y2O3:Eu3þ nanoparticles (a, b) as prepared and (c, d) annealed at 900 1C.

P. Packiyaraj, P. Thangadurai / Journal of Luminescence 145 (2014) 997–1003 999

strongly dependent on the local symmetry of the crystal field ofthe Eu3þ ion [26]. Among all emission spectral lines, 5D0–

7F0,5D0–

7F2 and 5D0–7F3 transitions originate from C2 sites by electric

dipole transition and the 5D0–7F1 by magnetic dipole transition

positioned at both S6 and C2 sites [28–30]. The correspondingsplitting of 5D0–

7F1 electronic transition (at 588, 594, and 600 nm)emission is from the C2 and S6 sites [22–24]. Y2O3 presents a cubicstructure with lattice constant a¼1.0604 Å [19–24]. The primitiveunit cell contains 80 atoms (48 O and 32 Y), Y atoms occupy twosites with the C2 and S6 (C3i) symmetry site. TheY2O3:Eu3þ

nanoparticles that is reported in this work present a clearlydominant typical C2 symmetry site, where almost 75% of Eu3þ

ions occupy the C2 sites and the rest occupy the S6 sites [5]. Theintense peak at 612 nm can be assigned to the 5D0–

7F2, hypersen-sitive forced electric dipole transitions from the C2 site which isfound to be the strongest among all other emission lines. It hasbeen established that there is a strong energy transfer from theEu3þ ions occupying S6 site to C2 site resulting in the increase inthe emission efficiency at 612 nm. The second strongest is theorange emission line at 588 nm, corresponding to the magnetic

dipole transition 5D0-7F1. The intensity of energy transfer

increases with the increase in the Eu3þ concentration which isshown in the Fig. 4(a) and (b). Hence the presence of PL emissionpeaks confirms that the Eu3þ ions occupy the C2 and S6 yttria sites.

In general, the transition probability of the magnetic-dipoletransition 5D0-

7F1 is nearly independent of the host matrix andother electric-dipoles allowed for 5D0-

7FJ (J¼2, 4 and 6) transi-tions are strongly influenced by the local structure and siteasymmetry around the Eu3þ ion [31]. Therefore, the photolumi-nescence intensity ratio of 5D0-

7F2 to 5D0-7F1 (Red to Orange, R/

O) provides valuable information about the symmetry at the siteoccupied by Eu3þ ions and covalent nature of the host matrix[32,33]. Comparatively higher values of R/O suggest the strongcovalent nature of the Eu3þ bonding with the surroundings.Overall, the dominating 5D0-

7F2 transition provides the red-color richness, confirmed from Commission International de I'Eclai-rage (CIE) calculations, which can be very helpful in developing redluminescent optical systems [32,33]. The emission intensity ratiobetween red and orange (R/O) color transitions corresponding to5D0–

7F2 and 5D0–7F1 for different Eu3þdoping concentration, are

Fig. 3. Scanning electron microscopy images of 3 and 5 mol% Y2O3:Eu3þ nanoparticles annealed at 600 1C (a, b) and 900 1C (c, d).

Fig. 4. Photoluminescence spectra of 900 1C annealed 1, 3 and 5 mol% Y2O3:Eu3þ nanoparticles acquired with excitations (a) λex¼410 nm and (b) λex¼532 nm.

P. Packiyaraj, P. Thangadurai / Journal of Luminescence 145 (2014) 997–10031000

calculated from the emission spectra and the values are listed inTable 1. The relative R/O ratio of present sample clearly shows(8.25–11.54) that red emission is high intense compared to orangeemission and it's dominate throughout the experiments.

Fig. 5 illustrates the excitation spectrum of 900 1C annealedY2O3:Eu3þ nanoparticles recorded at 612 nm emissions ofEu3þ ion. The excitation spectra show several excitation bandsbetween 350 and 550 nm spectral region, suggesting possibility ofusing many commercial and commonly available UV-green LEDs.These characteristic transitions are from the ground states 7F0 and7F1 (thermally populated) to various excited states of Eu3þ , asindicated in Fig. 5. Both the photoluminescence and excitationspectra show strong intensity, further confirming that the photo-luminescence centers of Eu3þ ions increase greatly with theannealing temperature reaching at 900 1C. The spectra show char-acteristic features of Eu3þ excitationwithout any spectral shifts in allthe samples under 612 nm emission. Among all PL spectra, 900 1Cannealed Y2O3:Eu3þ nanoparticles, show relatively high intensehigher order emission peaks at 364, 383, 395, 406, 415, 467, 528and 532 nm, corresponding to 5D4,5G2,5L6, 5L6,5D3, 5D2, 5D1 and 5D1

transitions of Eu3þ ion, respectively. Generally, multiphonon relaxa-tion, concentration quenching and the cross relaxation between

neighboring Eu3þ ions, substantially quench the higher-orderphotoluminescence [26].

The excitation laser power dependent PL studies were carriedout at the excitation wavelength of 410 nm with the power levelsfrom 5 mW to 100 mW. Fig. 6 illustrates the PL spectral intensitiesfor various emission lines for various powers of excitation laser.The interesting thing is that a proportional variation of the spectralintensity against laser power was observed up to 65 mW of power.Above this power it loses this nature. It confirms that the emissionfrom Eu3þ ion is linearly depending on the energy of the lasersexcitation from 10 mW to 65 mW.

The photoluminescence lifetimes (τ) for the most intenseemission line at 612 nm of Eu3þ (5D0–

7F2) doped Y2O3 nano-phosphors of all samples were recorded using 410 nm asexcitation source. The highest lifetime was obtained from thePL decay curves by fitting them to an exponential decayfunction and they mainly follow a single exponential decaywith the expression for intensity, I¼ I0 exp(�t/τ), where I0 isthe initial emission intensity and τ is the PL life time [34]. ThePL decay time values were obtained from the single exponen-tial curve fits (solid lines in the Fig. 7). The obtained PLlifetimes shows a systematic decrease from 2.26 to 1.77 ms inthe case of 600 1C annealed samples and from 2.35 ms to1.81 ms for the 900 1C annealed samples with increasingdopant concentrations. The corresponding lifetimes are pre-sented in Table 1. Single exponential behavior shows thehomogeneous distribution of Eu3þ ions in the Y2O3 matrixand no influence of inter-ion energy transfer between the rare-earth ions [35]. Upon increasing the doping concentration, thedecay becomes faster. It was reported already [36] in Eu3þ

doped Y2O3, the decay rate becomes faster in both the transi-tions 5D0–

7F2 and 5D1–7F2. In the case of 5D1–

7F2 transition, thedecay time decreases from a few tens of microsecond for0.005% and 0.1% sample to 10 μs for the 1 and 5% Eu3þ dopedY2O3 samples. The decay time for the 5D0–

7F2 transition intheir case lies in the range from 1.6 to 1.9 ms [36]. In our casethe maximum decay time is 2.35 ms for the 900 1C annealedsamples with 1 mol% doping concentration. Increasing dopingcontent may increase the number of ions occupying the surfacestates in nanoscale dimensional materials and these states maybe leading to concentration - quenching behavior [37]. Thus inour samples, decay time decreases with increasing dopingcontent. This result supports our emission and excitationspectral analysis. Specifically, the highest emission lifetimes

Fig. 5. Photoluminescence excitation spectra of 900 1C annealed 1, 3 and 5 mol%Y2O3:Eu3þ nanoparticles acquired with a emission wavelength of λem¼612 nm.

Fig. 6. Variation of spectral intensity of the PL spectra with the power ofthe excitation source for 5 mol% Y2O3:Eu3þ nanoparticles annealed at 900 1C(λex¼410 nm).

Fig. 7. Photoluminescence lifetime decay curves of 1, 3 and 5 mol% of Y2O3:Eu3þ

nanoparticles annealed at 900 1C recorded using λex¼410 nm. Dotted lines are theexperimental data and the solid lines show the single exponential curve fitting.

P. Packiyaraj, P. Thangadurai / Journal of Luminescence 145 (2014) 997–1003 1001

are useful for the optoelectronic applications such as displayand solid state lighting [38].

In order to indicate the color purity, the PL spectra wereconverted into the Commission International de I'Eclairage (CIE)1931 chromaticity values. Typical emission CIE coordinate diagramfor the 900 1C annealed Y2O3:Eu3þ is presented in Fig. 8. Thechromaticity coordinates of the Y2O3:Eu3þ annealed 900 1C phos-phors excited at 410 nm were calculated and listed in Table 1. Itcan be seen that the typical red emission is achieved in the presentphosphors. The 5 mol% doped and 900 1C annealed Y2O3:Eu3þ

exhibits a reddish orange color upon the excitation of 410 nm,whereas the 600 1C annealed samples showed relatively lessemission color intensities.

To demonstrate the PL characteristics of these nanocrystallinephosphors over a broad area, we have studied the modifiedconfocal microscope images (white light illumination, bright fieldand dark field mode), as well as the PL (excited with 410 nm high-power laser) images. The PL images (Fig. 9), as visible to the nakedeye, show dominant red color photoluminescence, even to thenaked eye. From these PL images, it has been observed that (Fig. 9)the photoluminescence (612 nm) related to Eu3þ is high for 5 mol% Eu3þ doping and more intense in the 900 1C annealed Y2O3:Eu3þ . While Eu3þ emission is uniform throughout the sampleannealed at 600 1C, comparatively less intense Eu3þ emission(supplementary information) could be due to OH bonds associatedwith matrix at such relatively low annealing temperatures [27].Fig. 8. The emission CIE coordinate diagram of 5 mol% Y2O3:Eu3þ nanophosphors

annealed at 900 1C.

Fig. 9. Confocal photoluminescence image of 1, 3 and 5 mol% Eu3þ doped Y2O3 nanoparticles annealed at 900 1C (λex¼410 nm).

P. Packiyaraj, P. Thangadurai / Journal of Luminescence 145 (2014) 997–10031002

4. Conclusions

Highly luminescent Eu3þ (0%, 1%, 2%, 3%) doped Y2O3 nano-particles was synthesized by a modified hydrolysis assisted co-precipitation method in the size range of 10–25 nm. The cubicnature of the Y2O3 is confirmed by XRD results. The opticalproperties, especially in Y2O3 host matrix, have been investigatedand elucidated for improving the luminescence. Luminescenceinvestigation confirmed that majority of the Eu3þ ions occupy theY2O3 site and luminescence intensity increases with particle size/crystallinity. The highly crystalline nanoparticles of uniform sizedistribution favors the high PL emission intensities. It was con-firmed that the red emitting Y2O3:Eu3þ nanoparticles of size in the10–25 nm range with high PL emission can be prepared throughthe co-precipitation. The luminescence observed was strong inY2O3:Eu3þ with slow decay (maximum decay time of 2.35 ms) andit is a good candidate for interesting applications such as biosen-sors and red components for while light LEDs. It could be apromising approach for fabricating optoelectronic thin films withhigh optical quality.

Acknowledgments

The financial supports from the start-up grant (PU/PC/Start-upGrant/2011-12/311) of the Pondicherry University and the UGC-DAE-CSR (CSR-KN/CRS-14/2011-12/586) are gratefully acknowl-edged. The author PP acknowledges the National Photonics fellow-ship, DeitY, Govt. of India, New Delhi for project fellowship. Thiswork is part of High-Impact Research scheme of IIT Delhi,Nano Research Facility (DeitY, Govt. of India), and UK-IndiaEducation and Research Initiative (UKIERI) programmes ofDr. G. Vijaya Prakash, Nanophotonics Laboratory, Department ofPhysics, IIT Delhi.

Appendix A. Supporting information

Supplementary data associated with this article can be found in theonline version at http://dx.doi.org/10.1016/j.jlumin.2013.07.074.

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