Photoluminescence Studies of ZnO , ZnO : Eu and ZnO : Eu Nanoparticles Covered with Y 2 O 3 Matrix

Development of advanced display and lighting technology such as field emission displays and plasma display panels requires phosphor which has a high efficiency and low degradation. Particle sizes and the locations of dopants in the hosts take an important role in the luminescence emissions of phosphors. ZnO nanoparticles are widely employed in plasma field emission display devices and well investigated; however, lanthanide (Ln3+) doped ZnO needs more investigations. In ZnO:Eu the lanthanide ions (Eu3+) may occupy either Zn2+ lattice site or on surface of ZnO crystal. The emissions of Eu3+ ion on the surface are the characteristic of Eu2O3, which leads to weak luminescence emission. To observe such phenomena, nanoparticles of ZnO, 2 at.% Eu3+ doped ZnO (ZnO:Eu) and ZnO:Eu covered with yattria matrix were prepared by wet chemical method at low temperature. The prepared nanoparticles were characterized by XRD and TEM. XRD data reveal the significant phase segregation of the annealed nanoparticles compared with lower heated samples. This phase segregation of Eu3+ ion establishes responsible for the decrease luminescence intensity of annealed ZnO:Eu nanoparticles compared with the as-prepared ZnO:Eu nanoparticles. Improvement on luminescence emissions could be achieved only for the as-prepared ZnO:Eu nanoparticles while covered with yattria matrix.


Introduction
ZnO is a direct band gap semiconductor of band gap 3.2 eV at room temperature and one of the potential mate-rials in various display devices [1] [2].Luminescent properties of ZnO can be improved by doping ZnO with transition metals or lanthanide ions.Extensive luminescence studies have been carried out some of the systems like ZnO:Eu, ZnO:Mn, ZnO:V, ZnO:W etc. [3]- [7], since the last three decades.As the luminescent properties of Eu 3+ ions in different crystallographic environments are well understood, it can be used as a probe to monitor the structural changes taking place around it, when doped in different inorganic materials.Among them, luminescent properties of ZnO:Eu system have been investigated by a number of authors [3]- [7] keeping in mind its potential applications as materials in field emission display devices.Our previous luminescence studies of ZnO:Eu nanoparticles showed that there were no energy transfer between the host ZnO and Eu 3+ ions [8].As the Eu 3+ ions have very poor solubility in ZnO, most of the Eu 3+ ions are lying on the surface of the ZnO particles, only few ions may be in the lattice site while annealing at high temperature results migration of Eu 3+ ions towards the grain boundaries (distorted environment) and segregation of Eu 3+ into separate Eu 2 O 3 phase.Both the phenomena produce significant reduction of their luminescent properties.Hence low temperature synthesis will be suitable Eu 3+ doped ZnO nanoparticles.Low temperature synthesis normally leads to smaller particle size.One way to circumvent this problem is to cover the surface of the nanoparticles with suitable inorganic materials or dispersion in organic/inorganic materials wherein the Eu 3+ ions can migrate without phase separation.Y 2 O 3 turned out to be a suitable choice as it was a suitable host for the lanthanide ions.Shell formation with Y 2 O 3 , it is expected that there will not have any nanoparticle aggregation and phase separation of Eu 2 O 3 .
In this article, ZnO:Eu and ZnO:Eu nanocroystals covered with yatria matrix (ZnO:Eu/Y 2 O 3 ) prepared at low temperature (140˚C) and critical luminescence studies were also carried out for ZnO, ZnO:Eu and ZnO:Eu/ Y 2 O 3 nanoparticles of different particle sizes.

Preparation
Nanoparticle of ZnO, 2 at.%Eu 3+ doped ZnO (ZnO:Eu), ZnO:Eu covered with in Y 2 O 3 (ZnO:Eu/Y 2 O 3 ) were prepared by wet chemical in polyethylene glycol medium at 140˚C.The preparation procedures were similar to our earlier reports [8]- [10].To prepare ZnO:Eu covered with in Y 2 O 3 , 98 at.%Zn 2+ , 2 at.%Eu 3+ and 100 at.%Y 3+ were used, the precursors used for production of Zn 2+ , Eu 3+ and Y 3+ are Zn(CH 3 COO) 2 •2H 2 O (99.05%, E-Merck), Eu 2 O 3 (99.99%Aldrich) and Y 2 (CO 3 ) 3 •3H 2 O (99.99%, Alfa Aesar) respectively.Polyethylene glycol acts as reaction medium as well as the cappant during the preparation process.To prepare ZnO doped with 2 at.%Eu 3+ , 0.50 g of Zn(CH 3 COO) 2 •2H 2 O was dissolved in 50 ml of polyethylene glycol mixture (form by 80:20 ration of polyethylene and ethylene glycol), warm the mixture at 60˚C within few minutes a clear solution could be observed.Then, 0.00734 g of Eu 2 O 3 dissolved in dil•HCl, warmed the solution several times in double distill water to get rid out excess acids from the solution.This solution containing Eu 3+ ions were simply mixed to the above solution.Another solution containing 0.2 M of NaOH (Merk-GR) was made and simply added to the above mixtures then heated linearly up to 140˚C.The reaction temperature was maintained for one hour.A cloudy white precipitate could be observed and the reaction continued another two hours.The precipitate so obtain was extracted by centrifugation.The powder so obtained were the ZnO:Eu nanoparticles.In the second step, to cover ZnO:Eu nanoparticles with Y 2 O 3 matrix, the former solution containing ZnO:Eu refluxed again simply by addition of another solution containing Y 3+ (to get Y 3+ , 0.4785 g of Y 2 (CO 3 ) 3 •3H 2 O dissolve in dil•HCl, warm the solution several times in excess double distilled water).Further 20 ml solution of 0.2 M strength of NaOH added to the above mixture then heated slowly at the same reaction temperature.A thick milky white precipitate could be obtained, the reaction continues another three hours, the precipitate so obtained was extracted by centrifugation in excess ethanol.This powder sample was the ZnO:Eu/Y 2 O 3 nanoparties.The powder samples so obtained were dried for further characterization and luminescence measurements.

Characterization
X-ray diffraction studies were carried out using a Philips powder X-ray diffractometer (model PW 1071) with Ni filtered Cu-K α radiation.The lattice parameters were calculated from the least square fitting of the diffraction peaks.The average crystallite size was calculated from the diffraction line width based on Scherrer relation: d = 0.9λ/Bcosθ, where λ is the wavelength of X-rays and B is the half maximum line width.
All luminescence measurements were carried out at room temperature with a resolution of 3 nm, using a Hi-tachi Instrument (F-4500) having a 150 W Xe lamp as the excitation source.Powder samples were mixed with methanol, spread over a quartz plate, dried at 100˚C and mounted inside the sample chamber.All Transmission Electron Microscopy (TEM) images are done using CM 200 Fillips.The crystallite size of as-prepared sample is found to be 12 nm.With Eu 3+ incorporation, there is slight shift in the diffraction peak maxima towards lower 2θ values compared to as-prepared ZnO (Figure 1(b)), indicating the lattice expansion of the host lattice with Eu 3+ incorporation.The lattice parameter values are found to be a = 3.254(1), c = 5.208(1) Å.It is understandable as the ionic radius of Eu 3+ (0.95 Å) is higher than that of Zn 2+ (0.70 Å) and hence the Eu 3+ incorporation in the ZnO lattice is associated with lattice expansion.It may be quite possible that many of the Eu 3+ ions must be occupying the Zn 2+ lattice site of ZnO nanoparticles.Annealing the samples at 500˚C and 900˚C result in the slight shifting of the diffraction peaks to higher 2θ values, revealing that the lattice parameter decrease with increase in heat treatment temperatures.The lattice parameters of 500˚C heated sample are a = 3.252(1) Å and c = 5.208(1) Å, and for 900˚C heated sample the corresponding values are a = 3.249(1) Å and c = 5.207(1) Å respectively.From these results it can be confirmed that there are partial removal of Eu 3+ ions from the ZnO lattice with heat treatment.The crystallite sizes are calculated from the peak position of ZnO:(101) plane by Gaussian fitting.The crystallite sizes of 2 at.%Eu 3+ doped ZnO (ZnO:Eu) nanoparticles:as-prepared, 500˚C and 900˚C heated samples are 8, 16 and 36 nm respectively suggesting the agglomeration of particles on higher heat-treatment.

Luminescence Study
Figure 4 shows the emission spectra of pure (a) as-prepared, (b) 500˚C and (b) 900˚C heat-treated ZnO nanoparticles excited at 320 nm.In as-prepared sample the pattern consists of a sharp peak around 390 nm superimposed over a broad peak centered around 500 nm.Weak luminescence emission at 465 nm could be detected in this sample.Based on the previous photoluminescence studies of ZnO nanoparticles [12]-[14], the peak around   390 nm has been attributed to the near band edge emission due to the exciton emission [12], that of 465 nm emission is due to excitons on the surface of ZnO nanoparticles and that around 500 nm has been attributed to the defects present in the ZnO lattice or the deep level electrons [11]- [18].In 500˚C heat-treated sample the emission at 466 nm is dominant over the 390 and 500 nm emissions, this emission is due to the native defect of ZnO nanoparticle or the emission due to surface trap excitons.For the sample heat-treated at 900˚C, the green emission around 500 nm is dominated over the band edge emission 390 nm and that of surface trap emission 466 nm.The green emission of ZnO is still on debate [17]- [23], but in our observation this emission is due to oxygen vacancy [10].In order to confirm the exact peak positions of luminescence emissions, the photoluminescence emission spectrum of 900˚C heat-treated ZnO nanoparticle is deconvoluted with Gaussian fitting (chi square = 0.9988) as shown in Figure 5.It is observed two peaks centered at 390 and 504 nm.The emission of native defect state could not be detected.It is observed that the UV-emission and the visible light emissions of ZnO nanoparticles depend on the particle size.With incorporation of Eu 3+ in the ZnO lattice, no defect or exciton emissions could be observed.However strong Eu 3+ emissions due to the transition 5 D 0 → 7 F 1 (orange emission at 590 nm) and 5 D 0 → 7 F 2 (yellow emission at 615 nm) could be observed on direct excitation of Eu 3+ ion at 394 nm ( 7 F 0 → 5 L 6 ) as shown in Figure 6(a).Heating the sample at 500˚C there is significant reduction in the Eu 3+ luminescent intensity as seen from Figure 6(b) and on further heating at 900˚C no emission could be observed from the sample (Figure 6(c)).Partial phase segregation of Eu 3+ ions as well as Eu 3+ incorporation in the grain boundaries (distorted regions) are responsible for the significant reduction in the luminescence intensity of heated samples.These results are supported by XRD data, decreased in the lattice parameter values of the ZnO:Eu nanoparticles for higher heat-treatments at 500˚C and 900˚C.It is worth to mention, even though the luminescence emission intensities decrease with the raise heat-treatment temperatures, the peak positions of the emission lines  do not change i.e. the emission lines are still at 615 nm.
Figure 7 shows the emission spectra corresponding to as-prepared ZnO:Eu/Y 2 O 3 nanoparticles along with their heat-treatments at 500˚C and 900˚C.There are two strong emission lines due to f-f transitions of Eu 3+ ions at 5 D 0 → 7 F 1 and 5 D 0 → 7 F 2 .Here opposite phenomena could be observed compared to heat-treated ZnO:Eu nanoparticles, strong emission peaks of Eu 3+ ions could be detected for the heat-treated ZnO:Eu/Y 2 O 3 samples and luminescence intensity also increased with the increase heat-treatment temperature.It also observes in the emission lines that the peak positions of the heat-treated samples are shifting towards lower wavelength side (at 610 nm) compared to the as-prepared sample (remain at 615 nm).Why such phenomena could be observed in the emission spectra of ZnO:Eu/Y 2 O 3 samples?It is clearly observed the luminescence emission intensity of asprepared ZnO:Eu/Y 2 O 3 is at 615 nm and strong.In the as-prepared ZnO:Eu/Y 2 O 3 sample, Y 2 O 3 exists as amorphous as revealed by the XRD data in  ions i.e.Eu 3+ ions by forming weakly bonded Eu 2 O 3 and will be leading weak luminescence emission from such particles [9].To confirm these emissions are either from ZnO:Eu or Y(OH) 3 particles, 2 at.% of Eu 3+ ion doped Y 2 O 3 was prepared at the same reaction temperature (the preparation procedure was similar to LR Singh et al.Crystallization of Y 2 O 3 starts only when annealed at higher temperatures and substitution of Y 3+ by Eu 3+ starts at this condition only.When Eu 3+ ions occupy Y 3+ site strong emissions could be obtained in such systems that is why the emission of heat-treated ZnO:Eu/Y 2 O 3 samples are intense.These results are similar to earlier reports [9] [24]- [26].It is worth to mention that the peak positions of the heat-treated samples are found at 610 nm, these spectra are identical with heat-treated Y 2 O 3 :Eu [9].The substitution of Y 3+ by Eu 3+ are supported by XRD data that the unit cell volume of Y 2 O 3 increased with the increase of heat-treatment temperatures.As the heat treatment temperatures increased more Eu 3+ ions are leaving ZnO particles then migrate towards the Y 2 O 3 lattice, where it feels an environment different from that of ZnO.That is why narrow and sharp Eu 3+ emission lines at 610 nm could be observed in the luminescence emission spectra of annealed ZnO:Eu/Y 2 O 3 samples.Figure 8 shows the comparisons of luminescence emission spectra of as-prepared ZnO:Eu and the ZnO:Eu nanoparticles covered with in yattria matrix (ZnO:Eu/Y 2 O 3 ), it is obvious that the luminescence emission of ZnO:Eu nanoparticles covered with in yattra matrix can be significantly improved its luminescence intensity compared to ZnO:Eu particles.The luminescence emission intensity of ZnO:Eu nanoparticles in yattra matrix is10 times than the ZnO:Eu nanoparticles.This result will be useful for the further understanding of these systems of particles.It is expected the solubility of Eu 3+ ions in ZnO are poor and most of the lanthanide ions may be on the surface of the ZnO particles or on the grain boundaries.These lanthanide ions migrates towards the amorphous environment of   Eu 3+ ions are adapted in the amorphous environment of Y 2 O 3 , not forming the bond with Y 3+ site.The above observations have revealed that there are significant numbers of surface Eu 3+ ions on the ZnO nanoparticles and phase segregation of Eu 3+ ions can be significantly reduced by incorporation by the Y 2 O 3 matrix.The ratio of the integrated intensities of magnetic dipole transition 5 D 0 → 7 F 2 and electric dipole transition 5 D 0 → 7 F 1 transitions can be considered indicative of the asymmetry of coordination of Eu 3+ ions [9].
Asymmetric ratio A 21 for as-prepared ZnO:Eu/Y 2 O 3 nanoparicle is found to be 3.4 that of heat-treated at 500˚C and 900˚C are found to be 4.8 and 5.3 respectively.This simply indicting the that co-ordination of Eu-O could be improved for the annealed samples, the Eu 3+ ions are leaving ZnO and making co-ordination with Y 3+ .The co-ordinations of Eu-O are also increase with the increase of annealing temperature.But the result is contrary with ZnO:Eu systems, in these system the asymmetric ratio decreases with the increase of annealing tem-peratures.From the above observations in the luminescence emissions of both the systems (ZnO:Eu and ZnO:Eu/Y 2 O 3 ) it is evident that covering of ZnO:Eu nanoparticles in Y 2 O 3 can significantly reduced phase segregation of Eu 2 O 3 in these systems.

Conclusion
Nanocrystals of ZnO, Eu 3+ doped ZnO (ZnO:Eu) and ZnO:Eu covered with in Y 2 O 3 could be successfully prepared at low temperature by wet chemical method in polyethylene glycol medium.Luminescence emission of pure ZnO nanoparticles depends on their particle sizes.However, there is a significant reduction in the luminescence intensity of annealed ZnO:Eu compared with the as-prepared ZnO:Eu which can be attributed to the partial phase segregation of Eu 3+ ions as well as the incorporation of Eu 3+ ions in the grain boundaries leading to distorted environment.The phase segregation and the incorporation of Eu 3+ ions at the grain boundaries can be significantly reduced by incorporating the ZnO:Eu nanoparticles in Y 2 O 3 .

Figure 1 (
Figure 1(a) shows the XRD patterns of as-prepared ZnO and Figures 1(b)-(d) gives the XRD pattern of asprepared 2 at.%Eu 3+ doped ZnO (ZnO:Eu) nanoparticles along with their 500˚C and 900˚C heated samples.The particles are crystalline with hexagonal structure with S. G. P6 3mc , all the diffraction peaks corresponds to the JCPDS 36-1451.No other characteristic peaks of europium impurity such as Eu 2 O 3 could be detected at least within the resolution limit of the diffractometer.The broadening of the diffraction peaks are the characteristic of nanosized ZnO materials.The lattice parameters of as-prepared sample are found to be a = 3.249(1) Å and c = 5.205(1) Å. Bulk value of ZnO has lattice parameters a = 3.249 Å, c = 5.206 Å (JCPDS 36-1451).Lattice parameters are close to bulk value.The crystallite sizes are calculated from the peak position of ZnO:(101) plane.The crystallite size of as-prepared sample is found to be 12 nm.With Eu 3+ incorporation, there is slight shift in the diffraction peak maxima towards lower 2θ values compared to as-prepared ZnO (Figure1(b)), indicating the lattice expansion of the host lattice with Eu 3+ incorporation.The lattice parameter values are found to be a = 3.254(1), c = 5.208(1) Å.It is understandable as the ionic radius of Eu 3+ (0.95 Å) is higher than that of Zn 2+ (0.70 Å) and hence the Eu 3+ incorporation in the ZnO lattice is associated with lattice expansion.It may be quite possible that many of the Eu 3+ ions must be occupying the Zn 2+ lattice site of ZnO nanoparticles.Annealing the samples at 500˚C and 900˚C result in the slight shifting of the diffraction peaks to higher 2θ values, revealing that the lattice parameter decrease with increase in heat treatment temperatures.The lattice parameters of 500˚C heated sample are a = 3.252(1) Å and c = 5.208(1) Å, and for 900˚C heated sample the corresponding values are a = 3.249(1) Å and c = 5.207(1) Å respectively.From these results it can be confirmed that there are partial removal of Eu 3+ ions from the ZnO lattice with heat treatment.The crystallite sizes are calculated from the peak position of ZnO:(101) plane by Gaussian fitting.The crystallite sizes of 2 at.%Eu 3+ doped ZnO (ZnO:Eu) nanoparticles:as-prepared, 500˚C and 900˚C heated samples are 8, 16 and 36 nm respectively suggesting the agglomeration of particles on higher heat-treatment.

Figures 2 (
Figures 2(a)-(c) show XRD patterns of as-prepared 2 at.%Eu 3+ doped ZnO (ZnO:Eu) nanoparticles covered with in Y 2 O 3 (ZnO:Eu/Y 2 O 3 ) along with their 500˚C and 900˚C heated samples.In as-prepared sample only the characteristic peaks of ZnO could be observed indicating that Y 2 O 3 phases are amorphous.However on heating the samples at 500˚C and 900˚C, along with diffraction peaks characteristic of ZnO the characteristic peaks of cubic Y 2 O 3 are also appeared.The * indicates the characteristics peaks of Y 2 O 3 phases.The crystallite sizes are calculated from the peak position of ZnO:(101) plane.The crystallite sizes of 2 at.%Eu 3+ doped ZnO (ZnO:Eu) nanoparticles covered with Y 2 O 3 are 8, 13 and 32 nm respectively for the as-prepared, 500˚C and 900˚C heated samples, indicating the increase of particle size with heat-treatment.The lattice parameters of ZnO present in ZnO:Eu/Y 2 O 3 are a = 3.256(1) Å, c = 5.208(1) Å for as-prepared sample and for the 500˚C and 900˚C heattreated samples are a = 3.251(1) Å, c = 5.208(1) Å and a = 3.249(1) Å, c = 5.208(1) Å for 900˚C respectively.Bulk value of ZnO has lattice parameters a = 3.249 Å, c = 5.206 Å, and volume, V = 47.62 Å 3 .It suggests that lattice parameters decreases with the heat-treatment indicating Eu 3+ coming out from ZnO lattice with heattreatment.The lattice parameter and unit cell volume of Y 2 O 3 present in ZnO:Eu/Y 2 O 3 are a = 10.621(1)Å for 500˚C heat-treated sample; and a = 10.649(1)Å for 900˚C heat-treated sample.The lattice parameters for the bulk value of Y 2 O 3 is a = 10.604Å, and volume, V = 1192.40Å 3 [11].Unit cell parameter of Y 2 O 3 in ZnO:Eu/ Y 2 O 3 samples increase with heat-treatment.It indicates that Eu 3+ ions which are coming out from the ZnO lattice diffuse into Y 2 O 3 lattices occupying Y 3+ sites.TEM picture shows the as-prepared ZnO:Eu nanoparticles are in spherical shape covered with Y 2 O 3 particles (ZnO:Eu/Y 2 O 3 ), the particle size is found to be 13 nm as shown in Figure 3(a).The bright portion shows the ZnO:Eu nanoparticles surrounded by dark Y 2 O 3 particles.The inset figure shows the expansion of it, showing ZnO particles are covered with Y 2 O 3 particles.Figure 3(b) shows the TEM picture of 900˚C treated ZnO:Eu/ Y 2 O 3 nanoparticles.It shows that both the ZnO and the Y 2 O 3 particles are crystalline, though Y 2 O 3 fairy covered to ZnO:Eu particles.The particle size is 46 nm.The particle sizes determined by XRD and TEM are in agreement.

Figure 3 (
Figures 2(a)-(c) show XRD patterns of as-prepared 2 at.%Eu 3+ doped ZnO (ZnO:Eu) nanoparticles covered with in Y 2 O 3 (ZnO:Eu/Y 2 O 3 ) along with their 500˚C and 900˚C heated samples.In as-prepared sample only the characteristic peaks of ZnO could be observed indicating that Y 2 O 3 phases are amorphous.However on heating the samples at 500˚C and 900˚C, along with diffraction peaks characteristic of ZnO the characteristic peaks of cubic Y 2 O 3 are also appeared.The * indicates the characteristics peaks of Y 2 O 3 phases.The crystallite sizes are calculated from the peak position of ZnO:(101) plane.The crystallite sizes of 2 at.%Eu 3+ doped ZnO (ZnO:Eu) nanoparticles covered with Y 2 O 3 are 8, 13 and 32 nm respectively for the as-prepared, 500˚C and 900˚C heated samples, indicating the increase of particle size with heat-treatment.The lattice parameters of ZnO present in ZnO:Eu/Y 2 O 3 are a = 3.256(1) Å, c = 5.208(1) Å for as-prepared sample and for the 500˚C and 900˚C heattreated samples are a = 3.251(1) Å, c = 5.208(1) Å and a = 3.249(1) Å, c = 5.208(1) Å for 900˚C respectively.Bulk value of ZnO has lattice parameters a = 3.249 Å, c = 5.206 Å, and volume, V = 47.62 Å 3 .It suggests that lattice parameters decreases with the heat-treatment indicating Eu 3+ coming out from ZnO lattice with heattreatment.The lattice parameter and unit cell volume of Y 2 O 3 present in ZnO:Eu/Y 2 O 3 are a = 10.621(1)Å for 500˚C heat-treated sample; and a = 10.649(1)Å for 900˚C heat-treated sample.The lattice parameters for the bulk value of Y 2 O 3 is a = 10.604Å, and volume, V = 1192.40Å 3 [11].Unit cell parameter of Y 2 O 3 in ZnO:Eu/ Y 2 O 3 samples increase with heat-treatment.It indicates that Eu 3+ ions which are coming out from the ZnO lattice diffuse into Y 2 O 3 lattices occupying Y 3+ sites.TEM picture shows the as-prepared ZnO:Eu nanoparticles are in spherical shape covered with Y 2 O 3 particles (ZnO:Eu/Y 2 O 3 ), the particle size is found to be 13 nm as shown in Figure 3(a).The bright portion shows the ZnO:Eu nanoparticles surrounded by dark Y 2 O 3 particles.The inset figure shows the expansion of it, showing ZnO particles are covered with Y 2 O 3 particles.Figure 3(b) shows the TEM picture of 900˚C treated ZnO:Eu/ Y 2 O 3 nanoparticles.It shows that both the ZnO and the Y 2 O 3 particles are crystalline, though Y 2 O 3 fairy covered to ZnO:Eu particles.The particle size is 46 nm.The particle sizes determined by XRD and TEM are in agreement.

Figure 2 .
Figure 2. XRD pattern of 2 at.%Eu doped ZnO nanoparticles dispersed in Y 2 O 3 (a) as-prepared, (b) 500˚C and (c) 900˚C heat-treated nanoparticles, the * shows the Y 2 O 3 phase in the pattern.

Figure 3 .
Figure 3. TEM images of (a) as-prepared 2 at.%Eu doped ZnO nanoparticles covered with Y 2 O 3 along with its expansion in the inset and (b) shows the 900˚C heattreated ZnO:Eu/Y 2 O 3 nanoparticles.

Figure 4 .
Figure 4. Photoluminescence emission spectra of (a) as-prepared and heat-treated at (b) 500˚C and (c) 900˚C ZnO nanoparticles excited at 320 nm.

Figure 6 .
Figure 6.Photoluminescence emission spectra of ZnO:Eu nanoparticles (a) as prepared and its (b) 500˚C and (c) 900˚C heat treated samples excited at f-f transition of Eu 3+ ion (394 nm).

Figure 2 .
The emission lines of ZnO:Eu/Y 2 O 3 nanoparticles are from the ZnO:Eu particles, not from Y 2 O 3 .In such samples the Y 2 O 3 is amorphous that is Y(OH) 3 , the Eu 3+ ions occupy the Zn 2+ site in ZnO and results the emission lines.If the emissions are from Y 2 O 3 , it will be from surface
[9] except using NaOH in place of urea).Comparison of luminescence emissions of the as-prepared ZnO:Eu and Y(OH) 3 :Eu are shown in the supplementary Figure 1.It is observed the luminescence emission of ZnO:Eu is strong whereas that of Y(OH) 3 :Eu is weak as Y 2 O 3 is amorphous at this temperature and the emissions are from the surface ions i.e. from weakly bonded Eu 2 O 3 .
Y 2 O 3 consequently reduces surface ions in ZnO.The surface lanthanide ions enhanced non-radiative transition leading weak luminescence emission, due to this ZnO:Eu covered with in Y 2 O 3 have more intense compared to ZnO:Eu nanoparticles.For further confirmation of location of Eu 3+ ions in ZnO:Eu/Y 2 O 3 nanoparticles the excitation spectra are taken by monitoring at 615 nm for as-prepared and heat-treated samples as shown in Figure 9.The pattern consists of a broad peak around 250 nm for the heat-treated samples which are the characteristics of Eu-O charge transfer band in Y 2 O 3 lattice [1] [27].Such Eu-O charge transfer band could not be observed in as-prepared ZnO:Eu/Y 2 O 3 nanocrystals but only the f-f absorption band at longer wavelength could be observed.This simply confirms that in as-prepared sample the