Structural and photoluminescence studies of Eu3+ doped cubic Y2O3 nanophosphors

Structural and photoluminescence studies of Eu3+ doped cubic Y2O3 nanophosphors

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Journal of Luminescence ∎ (∎∎∎∎) ∎∎∎–∎∎∎

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Structural and photoluminescence studies of Eu3 þ doped cubic Y2O3 nanophosphors P. Packiyaraj a,b, P. Thangadurai a,n a b

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

art ic l e i nf o

a b s t r a c t

Article history: Received 16 May 2013 Received in revised form 18 July 2013 Accepted 31 July 2013

Structural and photoluminescence properties of undoped and Eu3 þ doped yttrium oxide (Y2O3:Eu3 þ ) nanoparticles heat-treated at 600 and 900 1C were reported. Three concentrations of Eu3 þ (1, 3 and 5 mol%) were doped in Y2O3. The heat-treated Y2O3:Eu3 þ nanoparticles were cubic in structure without any impurity phase as studied by X-ray diffraction and transmission electron microscopy. The samples showed high crystallinity and average particle size was in the range of 10–16 nm and 20–25 nm when annealed at 600 and 900 1C respectively. The 900 1C annealed Y2O3:Eu3 þ exhibited a strong red photoluminescence due to homogeneously occupied Eu3 þ 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 ms in the case of 600 and 900 1C annealing respectively. Emission becomes faster with higher loading of Eu3 þ . Strong photoluminescence characteristics at most commonly available UV-blue excitation wavelengths make these phosphors suitable for LEDs and other display applications. & 2013 Elsevier B.V. All rights reserved.

Keywords: Eu3 þ doped Y2O3 Nanophosphors Photoluminescence PL lifetime CIE diagram

1. Introduction Luminescent nanomaterial have potential applications in the field of photonics, nanotechnology, electronics, display, lasing, detection, optical amplification, fluorescent sensing to biomedical engineering, and environmental control [1–3]. And also it provides challenges to both fundamental research and breakthrough development of technologies. Yttrium oxide (Y2O3) is an excellent host material 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. It has high thermal stability because of high melting point (2450 1C) [2]. The small phonon energy (380 cm  1) is enough for effective radiative transitions between electronic energy levels of the rare earth ions in Y2O3 host and it is a good choice of host material [4]. Nanophosphors should have a spherical shape and high luminescence efficiency for the lighting applications. The efficiency of light emission and the brightness of a phosphor screen are improved because of their minimized light scattering on their surfaces [6–8]. However, the shape and size of luminescent nanoparticles depend on their synthesis method. Nanoparticles synthesized by different methods show variation in their size, shape, and optical properties. The Eu3 þ ions doped Y2O3 (Y2O3:Eu3 þ ) is one of the main oxide based red-emitting phosphors and is widely used in the lighting industry and in solid-state-laser-based devices [5], was studied for a long time

because of its efficient luminescence under ultraviolet (UV) and cathode-ray excitation. Reduction of sizes to the nanoscale levels, improve the brightness and resolution of displays in devices of present technology. Y2O3:Re3 þ nanostructures of different morphologies have been synthesized by using different methods such as gas phase condensation technique [9], sol–gel route [10,11], spray pyrolysis [12], chemical vapor technique [13], combustion synthesis [14], and hydrothermal method [15]. Specially, the hydrolysis assisted co-precipitation method has advantages due to its simplicity in nature over other methods. Co-precipitation synthesis involves dissolution of compound salt precursor in aqueous media and subsequent precipitation from the solution by pH adjustment. Apart from its simplicity, atomic mixing of the constituents by chemical co-precipitation yields a final product of near-perfect stoichiometry without high temperature treatment [16]. However, it is very important to develop phosphors with controlled morphology ultra-small sizes [17,18] and in particular, the site-selectivity and relative behavior of the strongest 5D0  7F2 (red) transition from Eu3 þ needs to be further investigated for redphosphor applications[19,20]. In this work, we used hydrolysis assisted co-precipitation method to prepare pure and Eu3 þ -doped yttrium oxide nanoparticles and annealing them at different temperatures. Their structural and emission characteristics were studied in detail and reported.

2. Experimental n

Corresponding author. Tel.: þ 91 413 265 4974; fax: þ 91 413 265 6758. E-mail addresses: [email protected], [email protected] (P. Thangadurai).

Pure and Eu3 þ doped yttrium oxide nanoparticles were prepared by hydrolysis assisted co-precipitation method for three

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

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dopant concentrations such as 1, 3 and 5 mol% of Eu3 þ . Yttrium (III) nitrate hexahydrate (Y(NO)3  6H2O, 99.8% pure, Sigma Aldrich), 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 purification for the preparation. In the case of preparation of pure (or undoped) Y2O3, yttrium nitrate hexahydrate was used as precursor and it was dissolved in ultrapure water to form 0.1 N homogeneous solution. The precursor solution was placed in a 500 ml flask fitted with a reflux condenser and was hydrolyzed for 20 h. After hydrolysis, the reaction mixture was cooled down to room temperature. Then ammonium hydroxide [NH4OH] solution was added to the hydrolyzed solution to control the pH of the solution and a precipitate was obtained. Using centrifuge, the precipitate was separated and washed several times with distilled water and dried at room temperature. The as-prepared samples were annealed at 600 and 900 1C for 2 h in air to obtain different grain sizes and good crystallinity. On the other hand, Y2O3:Eu3 þ nanoparticles were prepared by the same method using Eu(NO)3d6H2O as Eu3 þ source in Y(NO)3d6H2O solution. Europium ions were added to a doping level of 1, 3, 5 mol % relative to yttrium ions. The precursor solutions have undergone same 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 by using the Cu-Kα1 radiation (λ ¼1.5406 Å) in a Rigaku powder Xray diffractometer at a scanning rate of 0.021 per second in the 2θ range of 151 r 2θ r 601. The X-ray diffractometer was operated at 40 kV and 30 mA. Crystallite size was estimated from XRD peak broadening using the Scherrer calculation [21]. The morphology of the samples were recorded from Carl Zeiss Scanning Electron Microscope (SEM) (Model EVO 18) working at the accelerating voltage of 15 kV. Carbon coating was given on the Y2O3:Eu3 þ powder to avoid charging under electron beam for SEM measurements. Microstructures were studied by transmission electron microscopy (TEM) and TEM studies were conducted in a FEI Tecnai G2 TEM microscope operated at a voltage of 200 kV. The specimen for TEM studies were made by dispersing the sample powder in ethanol by ultrasonication for about 30 min and a drop from the dispersed solution was put on the standard carbon film coated Cu TEM grids with 300 mesh. The grid was dried under light for the solvent to evaporate and mounted on the TEM sample holder for measurements. The steady-state and timeresolved photoluminescence (PL) measurements were carried out using a home-built set-ups with 410 nm (CW laser) and 532 nm (CW laser) lasers as the excitation source. The emission from the samples was coupled into a monochromator (Andor SP2300) that is coupled to CCD through the appropriate lenses and filters for recording the emission spectra. For time-resolved photoluminescence measurements (PL lifetime studies), a frequency generator (5 Hz), lock-in amplifier, digital storage oscilloscope and a monochromator (Acton SP2300) coupled to a photo multiplier tube (PMT) were employed. The PL excitation spectra were acquired in a RF-5301 PC Spectrofluorophotometer. For PL and time resolved PL measurements, the samples were made as thin layer on the ultrasonically cleaned (in acetone) glass slide and loaded in the sample holder. The PL imaging were done by a modified laser scanning confocal microscope (Olympus, BX51) equipped with XY-piezo stage and excited with a 410 nm CW laser. Both PL white light and conventional confocal bright and dark field images were recorded using an ALP (All long pass) 4 410 nm filter. For PL imaging, Y2O3:Eu3 þ powder was kept on the sample stage which was mounted on the microscope, then laser was allowed to fall on the sample and images were recorded by using a CCD detector.

3. Results and discussion 3.1. X-ray diffraction analysis The phase and crystal structure of the as-prepared and annealed Y2O3:Eu3 þ at various temperatures above 600 1C of the pure and doped were analyzed by XRD. Fig. 1 shows the powder XRD patterns of 0, 1, 3 and 5 mol% Y2O3:Eu3 þ nanoparticles, as prepared and annealed at 900 1C. All the as-prepared samples showed an amorphous nature, with no well-defined crystalline peaks (except a broad and weak diffraction at 291) and the phase purity of crystalline nature has been started evolving from the annealing temperatures above 600 1C (supplementary information). Such a low degree of crystallinity in as-prepared samples is expected because the precipitate is in the form of unreacted yttrium hydroxide [Y(OH)3] that should be either amorphous or very small crystallites of [Y(OH)3] has a hexagonal crystal structure, close to amorphous [22–24]. This would only transform to Y2O3 until they were subjected to appropriate annealing temperatures. When the samples were annealed at and above 600 1C, well defined diffraction peaks have appeared with a good intensity because of its improved crystallinity (Fig. 1b). All the XRD peak positions have been compared with reported values (JCPDS card No. 88-2162) and found that the structural phase of Y2O3 is a cubic phase. The corresponding peaks are marked in the Fig. 1. XRD data further suggest no traces of any unreacted constituents and/or other crystalline phases such as monoclinic, hexagonal [25]. Similar observations have also been made for the samples annealed at 600 1C and shown in the supplementary information.

Fig. 1. Powder X-ray diffraction patterns of Eu3 þ doped Y2O3 nanoparticles (a) assynthesized and the [2 2 2] reflection is from hexagonal [Y(OH)3] (JCPDS card no.83-2042) and (b) annealed at 900 1C with the Eu3 þ concentrations of 0, 1, 3 and 5 mol%. The diffraction peaks were indexed by comparing with the standard data for cubic Y2O3 (JCPDS card no. 88-2162).

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Since, Eu3 þ has a close ionic radius (of 0.109 nm) to that of Y3 þ (0.104 nm) ions, the minimum deviation in the estimated lattice parameters of cubic phase of Y2O3 suggest that Eu3 þ ions are incorporated into the Y3 þ sites even up to 5 mol% [26]. The FWHM of the peaks is found to decrease in the 900 1C sample compared to the 600 1C annealed samples. It would be expected that the width should get narrower when annealed at higher temperatures. This is obviously due to the increase in crystallinity of the Y2O3:Eu3 þ nanopowders. In addition, heat treatment has also led to an increase in crystallite size. The average crystallite size (d) of the Y2O3:Eu3 þ phosphors obtained from XRD data using Scherrer's equation are tabulated in Table 1. The average crystallite sizes are 13 and 23 nm for the annealing temperatures of 600 and 900 1C respectively. One another interesting observation has been made in the trend of the particle size increase with doping concentration at a given annealing temperature. Since Eu3 þ is little heavier atom compared to Y3 þ and this mass difference might have played a role in crystal growth process by diffusion at the time of synthesis reactions. Microstructure analysis was conducted by TEM and SEM. The bright field TEM image of as-prepared and 900 1C annealed 5 mol% Y2O3:Eu3 þ nanoparticles is shown in Fig. 2(a,c). The particles are found to be agglomerates containing many crystallites having more or less spherical shape. The sizes determined from TEM (for one particle it is marked as 30 nm in Fig. 2c) matches close to the calculated size from XRD data. Fig. 2(d) shows the SAED (selected area electron diffraction) pattern of the 900 1C annealed sample. The diffraction rings corresponding to the (222), (440), (622) planes are indicated in the SAED pattern confirming the crystal structure of Y2O3. The diffused SAED ring pattern (Fig. 2(b)) is observed for as-prepared sample indicating the less crystalline/

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amorphous nature. The SAED for 900 1C annealed sample (Fig. 2(d)) showed a clear diffraction spots indicating good crystallinity of the samples. This results corroborates with the XRD results. It is confirmed that the as-prepared precursor sample is amorphous in nature. The morphology of the all Eu3 þ doped Y2O3 nanoparticles was observed in SEM. The SEM images of the 3% and 5% Eu3 þ doped Y2O3 annealed at 600 and 900 1C are shown in Fig. 3. (a,b) and (c,d) respectively. The SEM micrographs show that these particles are agglomerated in nature. 3.2. Photoluminescence studies The room-temperature Photoluminescence (PL) spectra of Y2O3:Eu3 þ nanoparticles annealed at different temperatures and with various Eu3 þ concentrations were acquired by using various commercially excitation sources 337, 410 and 532 nm. Fig. 4(a, b) present the PL spectra of 900 1C annealed Y2O3:Eu3 þ excited at 410 and 532 nm respectively. The emission spectra have a series of emission peaks between 570 and 650 nm, well agree with the reported values of Eu3 þ emission transitions [22,27]. The emission spectral lines of Eu3 þ ion are sharp which is due to the screening of 4f orbital by 5s and 5p orbitals from crystal field of the host lattice. PL intensity increases with annealing temperature because of better crystallinity of the sample with increasing grain size and/ or the removal of impurities. Overall, the emission intensities from Eu3 þ rare earth ions were found to increase with the increase of Eu3 þ doping. The emission spectral lines observed at 581, 588, 594, 600, 612, and 632 nm are attributed to the 5D0-7FJ (J ¼0, 1, 2, 3, 4) transitions of Eu3 þ ions in the Y2O3 host matrix. The PL intensity of 5D0-7FJ transition and the Stark-crystal field splitting is

Table 1 Crystallite size and optical parameters of the Y2O3:Eu3 þ nanoparticles. Y2O3:Eu3 þ (x mol%)

Average crystalline size (nm)

600 1C

0 mol% 1 mol% 3 mol% 5 mol%

10 11 14 16

900 1C

21 22 24 25

R/O Ratio

CIE co-ordinate

410 nm laser

532 nm laser

600 1C

600 1C

– 8.25 9.46 9.66

900 1C

– 10.44 10.51 10.91

– 10.21 10.83 11.29

900 1C

– 10.34 10.87 11.54

Life-time (ms)

600 1C

900 1C

600 1C

900 1C

X

Y

X

Y

– 0.579 0.618 0.622

– 0.339 0.335 0.340

– 0.594 0.607 0.628

– 0.337 0.337 0.342

600 1C

900 1C

– 2.26 2.02 1.77

– 2.35 2.05 1.81

Fig. 2. TEM images with SAED pattern as inset of 5 mol% Y2O3:Eu3 þ nanoparticles (a, b) as prepared and (c, d) annealed at 900 1C.

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Fig. 3. Scanning electron microscopy images of 3 and 5 mol% Y2O3:Eu3 þ 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:Eu3 þ nanoparticles acquired with excitations (a) λex ¼ 410 nm and (b) λex ¼ 532 nm.

strongly dependent on the local symmetry of the crystal field of the Eu3 þ ion [26]. Among all emission spectral lines, 5D0–7F0, 5 D0–7F2 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 corresponding splitting of 5D0–7F1 electronic transition (at 588, 594, and 600 nm) emission is from the C2 and S6 sites [22–24]. Y2O3 presents a cubic structure with lattice constant a¼ 1.0604 Å [19–24]. The primitive unit cell contains 80 atoms (48 O and 32 Y), Y atoms occupy two sites with the C2 and S6 (C3i) symmetry site. TheY2O3:Eu3 þ nanoparticles that is reported in this work present a clearly dominant typical C2 symmetry site, where almost 75% of Eu3 þ ions occupy the C2 sites and the rest occupy the S6 sites [5]. The intense peak at 612 nm can be assigned to the 5D0–7F2, hypersensitive forced electric dipole transitions from the C2 site which is found to be the strongest among all other emission lines. It has been established that there is a strong energy transfer from the Eu3 þ ions occupying S6 site to C2 site resulting in the increase in the emission efficiency at 612 nm. The second strongest is the orange emission line at 588 nm, corresponding to the magnetic

dipole transition 5D0-7F1. The intensity of energy transfer increases with the increase in the Eu3 þ concentration which is shown in the Fig. 4(a) and (b). Hence the presence of PL emission peaks confirms that the Eu3 þ ions occupy the C2 and S6 yttria sites. In general, the transition probability of the magnetic-dipole transition 5D0-7F1 is nearly independent of the host matrix and other electric-dipoles allowed for 5D0-7FJ (J¼ 2, 4 and 6) transitions are strongly influenced by the local structure and site asymmetry around the Eu3 þ ion [31]. Therefore, the photoluminescence intensity ratio of 5D0-7F2 to 5D0-7F1 (Red to Orange, R/ O) provides valuable information about the symmetry at the site occupied by Eu3 þ ions and covalent nature of the host matrix [32,33]. Comparatively higher values of R/O suggest the strong covalent nature of the Eu3 þ bonding with the surroundings. Overall, the dominating 5D0-7F2 transition provides the redcolor richness, confirmed from Commission International de I'Eclairage (CIE) calculations, which can be very helpful in developing red luminescent optical systems [32,33]. The emission intensity ratio between red and orange (R/O) color transitions corresponding to 5 D0–7F2 and 5D0–7F1 for different Eu3 þ doping concentration, are

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Fig. 5. Photoluminescence excitation spectra of 900 1C annealed 1, 3 and 5 mol% Y2O3:Eu3 þ nanoparticles acquired with a emission wavelength of λem ¼ 612 nm.

Fig. 6. Variation of spectral intensity of the PL spectra with the power of the excitation source for 5 mol% Y2O3:Eu3 þ nanoparticles annealed at 900 1C (λex ¼ 410 nm).

calculated from the emission spectra and the values are listed in Table 1. The relative R/O ratio of present sample clearly shows (8.25–11.54) that red emission is high intense compared to orange emission and it's dominate throughout the experiments. Fig. 5 illustrates the excitation spectrum of 900 1C annealed Y 2 O 3 :Eu 3 þ nanoparticles recorded at 612 nm emissions of Eu 3 þ ion. The excitation spectra show several excitation bands between 350 and 550 nm spectral region, suggesting possibility of using many commercial and commonly available UV-green LEDs. These characteristic transitions are from the ground states 7F0 and 7 F1 (thermally populated) to various excited states of Eu3 þ , as indicated in Fig. 5. Both the photoluminescence and excitation spectra show strong intensity, further confirming that the photoluminescence centers of Eu3 þ ions increase greatly with the annealing temperature reaching at 900 1C. The spectra show characteristic features of Eu3 þ excitation without any spectral shifts in all the samples under 612 nm emission. Among all PL spectra, 900 1C annealed Y2O3:Eu3 þ nanoparticles, show relatively high intense higher order emission peaks at 364, 383, 395, 406, 415, 467, 528 and 532 nm, corresponding to 5D4,5G2,5L6, 5L6,5D3, 5D2, 5D1 and 5D1 transitions of Eu3 þ ion, respectively. Generally, multiphonon relaxation, concentration quenching and the cross relaxation between

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neighboring Eu3 þ ions, substantially quench the higher-order photoluminescence [26]. The excitation laser power dependent PL studies were carried out at the excitation wavelength of 410 nm with the power levels from 5 mW to 100 mW. Fig. 6 illustrates the PL spectral intensities for various emission lines for various powers of excitation laser. The interesting thing is that a proportional variation of the spectral intensity against laser power was observed up to 65 mW of power. Above this power it loses this nature. It confirms that the emission from Eu3 þ ion is linearly depending on the energy of the lasers excitation from 10 mW to 65 mW. The photoluminescence lifetimes (τ) for the most intense emission line at 612 nm of Eu3 þ ( 5 D 0 – 7 F2 ) doped Y 2 O 3 nanophosphors of all samples were recorded using 410 nm as excitation source. The highest lifetime was obtained from the PL decay curves by fitting them to an exponential decay function and they mainly follow a single exponential decay with the expression for intensity, I ¼ I 0 exp(  t/τ), where I 0 is the initial emission intensity and τ is the PL life time [34]. The PL decay time values were obtained from the single exponential curve fits (solid lines in the Fig. 7). The obtained PL lifetimes shows a systematic decrease from 2.26 to 1.77 ms in the case of 600 1C annealed samples and from 2.35 ms to 1.81 ms for the 900 1C annealed samples with increasing dopant concentrations. The corresponding lifetimes are presented in Table 1. Single exponential behavior shows the homogeneous distribution of Eu 3 þ ions in the Y2 O 3 matrix and no influence of inter-ion energy transfer between the rareearth ions [35]. Upon increasing the doping concentration, the decay becomes faster. It was reported already [36] in Eu 3 þ doped Y2 O 3 , the decay rate becomes faster in both the transitions 5 D 0 – 7 F2 and 5 D 1 – 7 F2 . In the case of 5 D 1 – 7 F2 transition, the decay time decreases from a few tens of microsecond for 0.005% and 0.1% sample to 10 μs for the 1 and 5% Eu 3 þ doped Y 2 O 3 samples. The decay time for the 5 D 0 – 7 F 2 transition in their case lies in the range from 1.6 to 1.9 ms [36]. In our case the maximum decay time is 2.35 ms for the 900 1C annealed samples with 1 mol% doping concentration. Increasing doping content may increase the number of ions occupying the surface states in nanoscale dimensional materials and these states may be leading to concentration - quenching behavior [37]. Thus in our samples, decay time decreases with increasing doping content. This result supports our emission and excitation spectral analysis. Specifically, the highest emission lifetimes

Fig. 7. Photoluminescence lifetime decay curves of 1, 3 and 5 mol% of Y2O3:Eu3 þ nanoparticles annealed at 900 1C recorded using λex ¼410 nm. Dotted lines are the experimental data and the solid lines show the single exponential curve fitting.

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

Fig. 8. The emission CIE coordinate diagram of 5 mol% Y2O3:Eu3 þ nanophosphors annealed at 900 1C.

In order to indicate the color purity, the PL spectra were converted into the Commission International de I'Eclairage (CIE) 1931 chromaticity values. Typical emission CIE coordinate diagram for the 900 1C annealed Y2O3:Eu3 þ is presented in Fig. 8. The chromaticity coordinates of the Y2O3:Eu3 þ annealed 900 1C phosphors excited at 410 nm were calculated and listed in Table 1. It can be seen that the typical red emission is achieved in the present phosphors. The 5 mol% doped and 900 1C annealed Y2O3:Eu3 þ exhibits a reddish orange color upon the excitation of 410 nm, whereas the 600 1C annealed samples showed relatively less emission color intensities. To demonstrate the PL characteristics of these nanocrystalline phosphors over a broad area, we have studied the modified confocal microscope images (white light illumination, bright field and dark field mode), as well as the PL (excited with 410 nm highpower laser) images. The PL images (Fig. 9), as visible to the naked eye, show dominant red color photoluminescence, even to the naked eye. From these PL images, it has been observed that (Fig. 9) the photoluminescence (612 nm) related to Eu3 þ is high for 5 mol % Eu3 þ doping and more intense in the 900 1C annealed Y2O3: Eu3 þ . While Eu3 þ emission is uniform throughout the sample annealed at 600 1C, comparatively less intense Eu3 þ emission (supplementary information) could be due to OH bonds associated with matrix at such relatively low annealing temperatures [27].

Fig. 9. Confocal photoluminescence image of 1, 3 and 5 mol% Eu3 þ doped Y2O3 nanoparticles annealed at 900 1C (λex ¼ 410 nm).

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4. Conclusions Highly luminescent Eu3 þ (0%, 1%, 2%, 3%) doped Y2O3 nanoparticles was synthesized by a modified hydrolysis assisted coprecipitation method in the size range of 10–25 nm. The cubic nature of the Y2O3 is confirmed by XRD results. The optical properties, especially in Y2O3 host matrix, have been investigated and elucidated for improving the luminescence. Luminescence investigation confirmed that majority of the Eu3 þ ions occupy the Y2O3 site and luminescence intensity increases with particle size/ crystallinity. The highly crystalline nanoparticles of uniform size distribution favors the high PL emission intensities. It was confirmed that the red emitting Y2O3:Eu3 þ nanoparticles of size in the 10–25 nm range with high PL emission can be prepared through the co-precipitation. The luminescence observed was strong in Y2O3:Eu3 þ with slow decay (maximum decay time of 2.35 ms) and it is a good candidate for interesting applications such as biosensors and red components for while light LEDs. It could be a promising approach for fabricating optoelectronic thin films with high optical quality. Acknowledgments The financial supports from the start-up grant (PU/PC/Start-up Grant/2011-12/311) of the Pondicherry University and the UGCDAE-CSR (CSR-KN/CRS-14/2011-12/586) are gratefully acknowledged. The author PP acknowledges the National Photonics fellowship, DeitY, Govt. of India, New Delhi for project fellowship. This work is part of High-Impact Research scheme of IIT Delhi, Nano Research Facility (DeitY, Govt. of India), and UK-India Education and Research Initiative (UKIERI) programmes of Dr. G. Vijaya Prakash, Nanophotonics Laboratory, Department of Physics, IIT Delhi. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2013.07.074. References

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Please cite this article as: P. Packiyaraj, P. Thangadurai, J. Lumin. (2013), http://dx.doi.org/10.1016/j.jlumin.2013.07.074i