ar ticle

Copyright © 2013 by American Scientific Publishers All rights reserved. Printed in the United States of America

Science of Advanced Materials Vol. 5, pp. 1–7, 2013 (www.aspbs.com/sam)

Synthesis, Structural and Luminescent Properties of Novel Eu3+:Y2CaZnO5 Nanophosphor for White Light-Emitting Diodes R. Rajeswari1 , L. Jyothi1 , C. K. Jayasankar1 ∗ , S. Surendra Babu2 , N. Vijayan3 , and D. Haranath3 1

Department of Physics, Sri Venkateswara University, Tirupati 517502, India Directorate of Laser Systems, Research Centre Imarat, Vignyana Kanchi, Hyderabad 500069, India 3 National Physical Laboratory, Council of Scientific and Industrial Research (CSIR), Dr. K. S. Krishnan Road, New Delhi 110012, India 2

ABSTRACT

KEYWORDS:

1. INTRODUCTION With the recent advances in efficiency, output power and white light quality, solid state lighting (SSL) technology in the form of white light-emitting diodes (w-LEDs) is poised to grab major share of lighting market by replacing the incandescent lamps since they are highly energy efficient with low power consumption and environmentally friendly.1–3 SSL systems can be categorized into dichromatic and trichromatic, and independently, into systems that are based on active (i.e., current-injected) emitters and non-current-injected phosphor wavelength-converters. Out of these technical strategies, the best way of achieving white light is phosphor down-converted (DC) LEDs where the luminescent materials used in LEDs convert the emission from the near ultraviolet (n-UV) or blue chip to longer wavelength light.4 The phosphor material for w-LED must possess several important characteristics such as strong ∗

Author to whom correspondence should be addressed. Email: [email protected] Received: xx Xxxx xxxx Accepted: xx Xxxx xxxx

Sci. Adv. Mater. 2013, Vol. 5, No. 11

absorption, namely broad excitation band in the blue or n-UV region (380–480 nm), good physical and chemical stability, high thermal stability and luminescence quenching temperature (> 150 C), since the radiation density of LED is about 200 W/cm2 , which is about three orders of magnitude higher than that of traditional fluorescent lamp. The popular w-LED in the lighting market is achieved via pumping a yellow phosphor (em = 520–850 nm), typically a Y3 Al5 O12 :Ce (YAG:Ce) phosphor, using a GaInN blue LED (em = 450–480 nm).5 The drawbacks of such a combination are low color rendering index (Ra < 80), lack of sufficient red emission, and different degradation rates of chip and phosphor, resulting in chromatic aberration and poor color stability for longer operation time. Another way of generating white light with high Ra and color reproducibility is to combine an UV chip with red, green and blue (RGB) phosphor or blue chip with red and green phosphors. Hence, it is timely to develop a stable, efficient red phosphor with high color purity that can be well coupled with blue and n-UV LEDs using inexpensive and efficient methods. As an active luminescence centre, europium (Eu) exhibits attractive red fluorescence due to its 5 D0 → 7 FJ (J = 1 2 3 and 4) transitions, which

1947-2935/2013/5/001/007

doi:10.1166/sam.2013.1671

1

ARTICLE

Novel red-emitting phosphors, Eu3+ -doped Y2 CaZnO5 (YCZ), were synthesized by modified urea assisted sol–gel combustion method. The X-ray diffraction analysis indicates the uniform formation of single-phase triclinic YCZ nanoparticles even when the concentration of Eu3+ ions is as high as 50 mol%. TEM measurements revealed that the particles have uniform size distributions in the range of 10–30 nm. Luminescence properties have been characterized using photoluminescence excitation, emission spectra and decay time measurements. A strong charge transfer band (CTB) at around 250 nm is observed which is due to the Eu O interaction in the host along with the 4f –4f excitation bands due to Eu3+ ions in UV and blue regions. The excitation bands at 395 nm 7 F0 → 5 L6 and 465 nm 7 F0 → 5 D2 are found to be stronger than the CTB for Eu3+ :YCZ nanophosphor, which match very well with the emission wavelength of commercial InGaN blue LED chip. Upon excitation to these wavelengths, a bright red emission from the 5 D0 level of Eu3+ ion is observed with CIE chromaticity coordinates (0.65, 0.34), which are found to be very close to the National Television System Committee (NTSC) for red phosphor (0.67, 0.33).

ARTICLE

Synthesis, Structural and Luminescent Properties of Novel Eu3+ :Y2 CaZnO5 Nanophosphor for White Light-Emitting Diodes

provides excellent red contribution to raise the colour rendering index (CRI) in novel w-LEDs.5–9 Host matrix in the form of small and spherical particles is desirable in phosphors due to their ease of processing into devices with intense emission as well as longer life.10 The conventional solid-state reaction synthesis normally results in irregular particle shape and size, irrespective of the high temperature and cumbersome grinding and firing processes. Moreover, it has been observed that in some cases this process can lead to the reduction of Eu3+ to Eu2+ valence state.11 Therefore, over the last few years various solution phase routes have been tried to reduce the reaction temperature and to obtain high-quality nanoparticles as phosphors.10 However, the simple and mass fabrication of nanocrystals with narrow grain size distribution and uniform morphology still remains a challenge. The sol–gel process is superior to the other preparation methods since the intimate mixing of components in the solution form ensures homogeneity of the final product, on the other hand combustion of these solutions using suitable fuel or complexing agent will result in a dry, crystalline and fine oxide powder. Among the red emitting oxide phosphors, Y2 O3 :Eu3+ has been studied extensively, which suffers significantly from the lack of the strong absorption/excitation band at n-UV and blue wavelength regions.12–16 In the present work, synthesis, structural and luminescent properties of a new inorganic phosphor, Y2 CaZnO5 (YCZ), doped with Eu3+ ions have been studied. The proposed synthesis method can be used to produce the nanoparticles in mass quantity. The YCZ:Eu3+ phosphor can be effectively excited by n-UV and blue light and the current study reveals the potentiality of the phosphor to be used for w-LEDs. To the best of our knowledge, this is the first report on the preparation of these nanoparticles by combustion method using metal nitrate precursors and urea as fuel. The synthesis method is technically simple, affordable, and versatile compared to the other existing chemical routes.

2. EXPERIMENTAL DETAILS 2.1. Synthesis of Y2 CaZnO5 :Eu3+ Nanophosphor Y2 CaZnO5 nano-phosphors doped with various Eu3+ ion concentrations (1, 5, 10, 25 and 50 mol%) were prepared by modified urea assisted sol–gel combustion method. High-purity Y2 O3 , ZnO, CaCO3 , Eu2 O3 and urea (H2 NCONH2 from Sigma-Aldrich were taken as starting materials. In a typical experiment, stoichiometric amount of all the oxides and carbonates were dissolved in concentrated nitric acid to obtain Y(NO3 3 , Zn(NO3 2 , Ca(NO3 2 and Eu2 (NO3 3 . All these nitrate precursor solutions were taken together and were rigorously stirred. Urea was used as a fuel to initiate the ignition and its amount was calculated using total oxidizing and reducing valencies.17 Urea acts as the monomer to form transparent complex gel 2

Rajeswari et al.

from the initial solution upon drying it for 10–12 hours in an oven at 75 C. Further, the gel was taken in a home made quartz boat and fired in air in a tubular furnace which was pre maintained at 800 C. Initially in the furnace the urea complex gel transforms to a black mass nearly ten times the gel volume and has undergone rapid dehydration and foaming followed by decomposition, generating combustible gases such as CO2 , N2 , NO2 and H2 O. These volatile combustion gases ignite and generate large amount of heat for a short period of time before the process terminates with white mass. The combustion process utilizes the enthalpy of combustion for the formation and crystallization of the nanocrystals at low ignition temperature. The white mass that was formed was again fired at 800 C for 1–2 hours to yield a white fluffy mass of YCZ:Eu3+ nanophosphor which could easily be crushed to ultra-fine powder for further characterization. The over all pH of the solution was maintained around 1–2 in order to obtain size controlled nanophosphor particles. Synthesis of size-controlled nanoparticles with high yield (∼ 90%) is one of the highlights of this method. Also, un-doped YCZ and 10 mol% Eu3+ ion doped YCZ samples were synthesized at 1300 C for 6 hours by the usual solid-state reaction technique for comparative study. 2.2. Characterization Structural characterization was carried out using a Bruker D-8 advanced powder X-ray diffractometer (XRD) with Cu K radiation operated at 35 kV and 30 mA. For transmission electron microscopic (TEM) observations, the samples were re-dispersed in methanol by ultrasonic treatment and dropped on carbon-copper grids. TEM images were collected using a Tecnai G2 F30 S-Twin (FEI; Super Twin lens with Cs = 12 mm) instrument operating at an accelerating voltage of 300 kV, having a point resolution of 0.2 nm and lattice resolution of 0.14 nm. The room temperature photoluminescence (PL) spectra were recorded using an Edinburgh Luminescence Spectrometer (Model FLSP900) equipped with a xenon lamp. The excitation and emission spectra were recorded in the fluorescence mode over the range of 200–700 nm. The PL lifetime measurements were performed with the same instrument but with a microsecond xenon flash lamp, as the source of excitation.

3. RESULTS AND DISCUSSION 3.1. Structure X-ray diffraction (XRD) was taken to examine the crystal structure and phase purity of the as prepared samples. To obtain the structure of pure Y2 CaZnO5 (YCZ) powders, typical XRD patterns of un-doped and 10 mol% Eu2 O3 doped YCZ phosphors sintered at 1300 C for 6 hours along with the samples prepared by the sol–gel combustion method were investigated and the corresponding patterns Sci. Adv. Mater., 5, 1–7, 2013


Y2 CaZnO5 Eu3+ :Y2 CaZnO5 (Bulk un doped) (Bulk)

(–2 0 3)

(0 3 6)

(2 3 7)

(2 2 2)

(2 0 1)

Table I. Unit cell parameters of doped and un-doped Y2 CaZnO5 phosphors.

(0 2 0)

(c)

(–1 0 1) (–1 1 0) (–1 1 1) (0 2 1) (1 2 4)

(0 1 0)

Intensity (arb. units)

Rajeswari et al.

(b)

Eu3+ :Y2 CaZnO5 (Nano)

Crystal structure

Triclinic

Triclinic

Triclinic

a (Å) b (Å) c (Å) Volume, V (Å3 ( ) ( ) ( )

74544 125761 553445 50357 973101 945774 100275

751151 920094 6531 451348 903394 903279 895591

47106 553814 127888 222329 549866 548356 676204

(a)

20

30

40

50

60

70

2θ (degree) Fig. 1. XRD patterns of (a) un-doped (bulk), (b) 10 mol% Eu2 O3 doped Y2 CaZnO5 (bulk) and (c) 10 mol% Eu2 O3 doped Y2 CaZnO5 nanophosphor powders.

there is always a chance of substitution of Eu3+ ions for combined sites of Ca and Zn. Further, it is clear from the Figure 1, that the pattern shows a line broadening, indicating the reduction in crystalline size. The crystallite size of the material was calculated using the Scherrer’s formula; D=

Sci. Adv. Mater., 5, 1–7, 2013

(1)

where D is the crystallite size, K is the dimension-less shape factor (0.9), is the X-ray wavelength, is the FWHM and is the Bragg’s angle. The crystallite size of the samples prepared by the combustion method is found to be in the range of 20–25 nm. As nanoparticles are known to have a number of surface atoms which have unsaturated in co-ordination, significant extent of strains are associated with them. The strain in the lattice of the sol–gel combustion grown nanocrystals has been evaluated using the Hall-Williamson relation;20 k Cos = + Sin (2) D A plot is drawn between cos versus sin , which is shown in Figure 2. Since Eq. (2) resembles to that of a 0.0110 0.0105 0.0100

β*Cos(θ) = (k*λ/τ) + η*Sin(θ)

βcos(θ) (radians)

0.0095 0.0090 0.0085 0.0080 0.0075 0.0070 0.0065 0.0060 0.0055 0.0050 0.225 0.250 0.275 0.300 0.325 0.350 0.375 0.400 0.425

Sin(θ) Fig. 2. Hull-Williamson plot for 10 mol% Eu2 O3 doped Y2 CaZnO5 nanophosphor powders.

3

ARTICLE

are shown in Figure 1. The XRD measurements were carried out in an identical way using pellets of the equal mass and same dimensions. As can be seen from the Figure 1, a considerable variation is observed in the crystallinity of the same as a function of the way they were prepared. Since the material of the present investigation, YCZ is an entirely new lattice and the respective JCPDS data is not available, WIN-INDEX (ver. 3.08) software has been used for the structure refinement studies and determination of (h k l) values corresponding to the crystalline planes. It is clear from the Figure 1 that the XRD patterns of undoped, bulk and nano crystalline samples are identical with respect to the line positions and relative intensities. Also there is no considerable change in the XRD pattern with increase in Eu3+ ion concentration, even at 50 mol%. Such a high rare earth solubility has also been observed in MaMoO4 :Eu3+ 9 and YVO4 :Eu3+ 18 phosphors. This indicates that the incorporation of Eu3+ ions into the YCZ host is complete and both doped and un-doped samples have the same crystal structure. The structural refinement studies reveal that the YCZ phosphor has a triclinic structure and the unit cell parameters of the three compounds under study are collected in Table I. From the table it is clear that the sample prepared by the sol–gel combustion method has a considerable lesser unit cell volume without change in the crystal system. Such a change in the cell parameters and volume due to the synthesis process has also been observed in the Er3+ -doped ZnO.19 The variation in the unit cell volume may be due to the variation in the oxygen vacancies during the synthesis process. Considering the ionic radii and valence states of Y3+ and Eu3+ ions, it could be presumed that Y3+ ions are being substituted by Eu3+ ions in the YCZ lattice giving strong red emission at ∼ 610 nm, which will be discussed in the forthcoming sections. However,

K cos

Rajeswari et al.

ARTICLE


Fig. 3. TEM micrographs of images of 10 mol% Eu2 O3 doped Y2 CaZnO5 nanophosphors with 100 nm scale (a) and (b), 50 nm scale (c), 5 nm scale (d), histograms of particle size in the prepared sample along with average fit (e) and (f).

straight line, the slope of the linear fit of the data gives the value of . The strain value is found to be 0.0028 for nano Eu3+ :YCZ particles, which is lower than the reported value for ZnO–Eu–Y2 O3 system (0.007) prepared by the urea hydrolysis synthesis.21 The as-synthesized nanophosphor samples were subjected to transmission electron microscopy (TEM) observations at different magnifications. Typical TEM and high-resolution TEM (HRTEM) micrographs of the nanophosphor samples are shown in Figure 3. Fluffy, voluminous and delicate porous nanophosphor samples are formed due to evolution of large amount of gases during 4

the self-proliferating sol–gel combustion reaction. As can be seen from the images, the grain size of the samples prepared by modified urea assisted sol–gel method is in the nano scale range. Loosely bound agglomerates of nanophosphor particles are formed with uniform size distributions. At low magnification, long chain-like morphology with polygon shaped nanostructures and sharp boundaries are evidently seen in Figures 3(a) and (b). At higher magnification (Figs. 3(c) and (d)), the fringes are clearly apparent that reflect the strong crystalline nature of the nanophosphor particles. From TEM observations, the average particle size has been attributed to be in Sci. Adv. Mater., 5, 1–7, 2013

Rajeswari et al.


the range of 10–30 nm, which is also marked on the images. Figures 3(e) and (f) shows the histogram of the particle size in the prepared samples. It is clear from the histograms that the particle size has a distribution of 10–30 nm region with average sizes of 15 and 20 nm. 3.2. Photoluminescence

Fig. 4. Excitation spectra of 10 mol% Eu2 O3 doped Y2 CaZnO5 nanophosphors by monitoring the red emission at 610 nm, along with the partial energy level diagram of Eu3+ ions. Sci. Adv. Mater., 5, 1–7, 2013

Fig. 5. Red emission intensity dependence on excitation wavelength (250,395 and 465 nm) for 10 mol% Eu2 O3 doped Y2 CaZnO5 phosphor powders. The inset shows the emission spectra for bulk and nanophosphors.

The inset of Figure 5 shows the emission spectra of bulk and nano YCZ:Eu3+ samples at RT excited to various excitation bands/peaks available in the excitation spectrum. The PL spectrum exhibits typically Eu3+ emission lines at 578 nm (5 D0 → 7 F0 ; 585, 590 and 600 nm (5 D0 → 7 F1 ; 610 and 621 nm (5 D0 → 7 F2 ; 651 nm (5 D0 → 7 F3 ; 689 nm (5 D0 → 7 F4 and 708 nm (5 D0 → 7 F5 . As can be seen from the figure, that the red emission peaked at 610 nm (5 D0 → 7 F2 is the strongest among the other transitions, which is electric-dipole allowed and depends strongly on the local symmetry around the Eu3+ ion. Whereas, the 5 D0 → 7 F1 transition is magneticdipole allowed and is independent of the local symmetry. The fluorescence intensity ratio (R) of 5 D0 → 7 F2 to 5 D0 → 7 F1 transition is used to establish the degree of asymmetry in the vicinity of Eu3+ ions and Eu O covalence for various Eu3+ -doped systems.22 Moreover, R value also depends on the Judd-Ofelt parameter 2 , which is used to explain the short range effects. Therefore, the variation of R and in turn 2 gives the information about the short range effect on local structure around Eu3+ ions and Eu O covalence. The higher the value of R, lower the symmetry around the Eu3+ ions and the higher the Eu O covalence, and vice-versa. The R value for both nano and bulk YCZ:Eu3+ samples is found to be 6 suggesting that the Eu3+ ions are located in an asymmetric environment. The R value of YCZ:Eu3+ is considerably lower than Y2 O3 :Eu3+ and higher than YAG:Eu3+ .12 13 22 The appearance of the non-degenerate 5 D0 → 7 F0 transition indicates that the Eu3+ ion is in an environment of low symmetry22 in the YCZ system. Further, the magneticdipole 5 D0 → 7 F1 transition splits into three components indicating that the crystallographic sites of the Eu3+ ions in the present system is as low as orthorhombic, monoclinic 5

ARTICLE

The photoluminescence (PL) excitation spectra of 10 mol% Eu2 O3 doped YCZ powder phosphor were recorded by monitoring a red emission at 610 nm and is shown in Figure 4. A typical excitation and emission process is shown in the partial energy level diagram (inset of Fig. 4). A broad excitation band observed in the lower wavelength region, 250∼280 nm, can be attributed to the O2− Eu3+ charge transfer (CT) transitions. In addition to the CT band, sharp excitation peaks were observed in the 380–550 nm region due to the typical f –f transition of Eu3+ ions. The strong absorption lines in the excitation spectrum is located at 395 nm and 465 nm, which corresponds to the 7 F0 → 5 L6 and 7 F0 → 5 D2 transitions of Eu3+ ions, respectively. It is interesting to note that the 4f –4f excitation bands in the wavelength region of 380–480 nm, are much stronger for Eu3+ :YCZ nano phosphor than the bulk counter partner, which matches very well with the emission wavelength of commercial InGaN LED chips. This characteristic feature of YCZ phosphor make it suitable candidate for application in w-LEDs combined with both blue and n-UV light. Upon varying the excitation wavelength (ex = 250, 395 and 465 nm) a sharp increase in the integrated emission intensity is observed as shown in Figure 5, without any significant change in the emission profile. It is worth noting that the YCZ:Eu3+ phosphor has shown higher PL efficiency when excitation wavelength is 395 or 465 nm.


ARTICLE

or triclinic in a crystalline lattice,23 which is in good agreement with the XRD data discussed above. It is also evident from the appearance of the electric-dipole transition at 610 nm (5 D0 → 5 F2 that the Eu3+ ions occupied the site of non-inversion symmetry.23 The chromaticity coordinates for the emission at different excitation wavelengths have been computed based on the procedure made available by the Commission of International de L’Eclairage (CIE), France. The CIE chromaticity coordinate of both the bulk and nano YCZ:Eu3+ phosphor are found to be (0.65, 0.35) which are very close to the commercial Y2 O3 :Eu3+ phosphors (0.65, 0.35), Y2 O2 S:Eu3+ phosphors (0.645, 0.345) and National Television System Committee (NTSC) for red phosphor (0.67, 0.33),6 indicating that the YCZ:Eu3+ phosphor has a good color purity. All these results supplement that YCZ:Eu3+ is a potential candidate for w-LED applications as there are strong excitation bands in the 380–480 nm (blue/n-UV region) which can efficiently down convert the light from InGaN LED chip. 3.3. Concentration Dependence of Eu3+ Ion Emission The emission intensity of Eu3+ ions always depends strongly on Eu3+ ion doping concentration and hence, the effect of Eu3+ ion concentration on the emission intensity, particularly red emission, has been investigated. Figure 6 shows the emission spectra of YCZ:Eu3+ samples with various concentrations of Eu3+ (x = 1–50 mol%) under the 395 nm excitation. As can be seen from Figure 6, the emission intensity increases with increasing Eu3+ content and reaches to the maximum value at about 10 mol% of Eu3+ ions and then descends at higher contents. In general, the concentration quenching of emission intensity in the lanthanide (Ln3+ ion doped systems can be ascribed to the migration of excitation energy to the quenching centers (traps) or to the cross-relaxation

Fig. 6. Concentration dependent emission spectra of Eu2 O3 doped Y2 CaZnO5 nanophosphors. Inset shows the variation of red emission intensity with Eu2 O3 concentration.

6

Rajeswari et al.

(exchange interaction) between neighboring Ln3+ ions.24 In case of Eu3+ ions, cross relaxation is possible from the 5 D1 level with the following channel;18 5

D1 + 7 F0 → 5 D0 + 7 F2

In the present study, red emission is observed from the 5 D0 level of Eu3+ ions in YCZ samples. The cross-relaxation of excitation energy from the 5 D0 level of Eu3+ ions may not be a dominant quenching mechanism as the energy level matching is not effective from this level and hence, the emission quenching can be attributed to the migration of excitation energy to the quenching centers. The average distance between neighboring Eu3+ ions is longer at lower doping concentrations, and thus, the energy migration is hindered. As the doping concentration increases, the average distance between the activators will decrease. At shorter distances, the possibility of energy transfer to the quenching centers is dominant and hence the excitation energy is lost through non-radiative transitions. In other words, at higher concentration the energy transfer exceeds the radiative emission, which can lead to the quenching of the luminescent intensity. In addition, the occupation of Ca2+ sites by smaller Eu3+ ions resulting in creation of oxygen vacancy to compensate the negative charge of Eu → Ca in the lattice, such as 3Ca2+ → 2Eu3+ + vacancy (Ca2+ , leading to lowering the symmetry of the surroundings of Eu3+ ions, and thus enhance the 5 D0 → 7 F2 red emission transition.25 However, excessive calcium vacancies will be created when the Eu3+ concentration exceeds certain degree and thus will destroy the crystallinity and lead to the luminescence quenching. Similar kind of concentration quenching is also observed due to excess oxygen vacancies.9 12 18 3.4. Decay Time Measurements The decay profile of the 5 D0 level of Eu3+ ions was recorded by exciting the samples with 395 nm radiation. Figure 7 shows the decay curves for 5 D0 level of Eu3+ ions in YCZ nanoparticles obtained by monitoring the 5 D0 → 7 F2 transition at 610 nm. The decay is found to be single exponential for lower Eu3+ ion concentrations with almost constant lifetime and tends to be non-exponential at high concentrations with shortening of lifetime. The average decay time ( ) was evaluated using the expression; tIt dt = (3) It dt The value for Eu3+ :YCZ ranges from 1.3 ms to 0.4 ms which is comparable to the Eu3+ -doped Y2 O3 (1.1 ms),15 higher than MgMoO4 (0.44 ms),9 and YVO4 :Eu3+ (0.7– 0.2 ms),18 and lower than KCaBO3 :Eu3+ (2.5–3 ms).11 The non-exponential nature of decay at higher concentrations of Eu3+ ions also indicates the existence of energy transfer from the 5 D0 level at these concentrations. The emission Sci. Adv. Mater., 5, 1–7, 2013

Rajeswari et al.


Acknowledgments: One of the authors (C. K. Jayasankar) is grateful to the Governments of India and Spain for the award of a project within the Indo-Spanish Joint Programme of Cooperation in Science and Technology (DST-INT-Spain-P-38-11 and PRI-PIBIN-2011-1153).

References and Notes

Fig. 7. Decay curves for the 5 D0 → 7 F2 transition of Eu3+ -doped Y2 CaZnO5 nanophosphor with various Eu2 O3 concentrations. Inset shows the dependence of lifetime with concentration.

4. CONCLUSION To conclude, new and novel red emitting phosphors, Y2 CaZnO5 doped with Eu3+ ions, were prepared by the simplified sol–gel combustion method using urea as a fuel. The modified method resulted in good quality singlephase triclinic nanoparticles with a particle size distribution of 10–30 nm. Intense and broad excitation peak in the near UV and blue regions were observed for nano Y2 CaZnO5 :Eu3+ phosphor which can efficiently down convert the radiation from commercial InGaN LED chip. Upon excitation to these levels, highly color pure red emission from the 5 D0 level of Eu3+ ion with CIE chromaticity coordinates (0.65, 0.34) were observed, which is comparable or higher than other reported red phosphor. Lifetime and concentration dependent luminescent study reveals that the optimum concentration of Eu3+ ions in Y2 CaZnO5 is found to be 10 mol% for the lightening applications.

Sci. Adv. Mater., 5, 1–7, 2013

7

ARTICLE

intensity and decay time of the 5 D0 → 7 F2 transition with different Eu3+ concentrations follow the same trend as shown in the inset of Figures 6 and 7. From the analysis of PL emission, excitation and decay time measurements, the optimum concentration of Eu3+ ions in YCZ is found to be 10 mol% for the application in white light emitting diodes.

1. L. Chen, C.-C. Lin, C.-W. Yeh, and R.-S. Liu, Materials 3, 2172 (2010). 2. S. Nukamura, T. Mukai, and M. Senoh, Appl. Phys. Lett. 64, 1687 (1994). 3. A. A. Setlur, J. J. Shiang, and U. Happek, Appl. Phys. Lett. 92, 081104 (2008). 4. L. S. Rohwer and A. M. Srivastava, Electrochem. Soc. Interface 12, 36 (2003). 5. (a) Z. Xia, J. Zhuang, and L. Liao, Inorg. Chem. 51, 7202 (2012); (b) S. Nakamura and G. Fasol, The Blue Laser Diode: GaN Based Light Emitters and Lasers, Springer, Berin, Germany (1997), p. 216. 6. H. Guo, H. Zhang, R. Wei, M. Zheng, and L. Zhang, Opt. Express. 19, A201 (2011). 7. D. Wang, P. Yang, Z. Cheng, W. Wang, P. Ma, X. Zhai, and J. Lin, J. Nanopart. Res. 14, 707 (2012). 8. C. E. Rodríguez-García, N. Perea-López, G. A. Hirata, and S. P. DenBaars, J. Phys. D: Appl. Phys. 41, 092005 (2008). 9. V. H. Romero, E. De La Rosa, and P. Salas, Sci. Adv. Mat. 4, 551 (2012). 10. S. Ye, F. Xiao, Y. X. Pan, Y. Y. Ma, and Q. Y. Zhang, Mater. Sci. Eng. 71, 1 (2010). 11. A. Amarnath Reddy, Subrata Das, Shahab Ahmad, S. Surendra Babu, Jose M. F. Ferreira, and G. Vijaya Prakash, RSC Adv. 2, 8768 (2012). 12. A. Paulraj, P. Natarajan, K. Munnisamy, M. K. Nagoor, K. P. Nattar, B. Abdulrazak, and J. Duraisamy, J. Am. Ceramic. Soc. 94, 1627 (2011). 13. J. Dhanaraj, R. Jagannathan, T. R. N. Kutty, and C.-H. Lu, J. Phys. Chem. B 105, 11098 (2001). 14. P. A. Tanner and K. L. Wong, J. Phys. Chem. B 108, 136 (2004). 15. J. Lian, L. Yang, X. Y. Chen, G. K. Liu, L. M. Wang, R. C. Ewing, and D. Shi, Nanotechnology 17, 1351 (2006). 16. C. A. Traina and J. Schwartz, Langmuir 23, 9158 (2007). 17. S. Ekambaram and K. C. Patil, J. Allos Comp. 248, 7 (1997). 18. Y.-S. Chang, F.-M. Huang, Y.-Y. Tsai, and L.-G. Teoh, J. Lumin. 129, 1181 (2009). 19. Rita John and Rajaram Rajakumari, Nano-Micro. Lett. 4, 65 (2012). 20. G. K. Williamson and W. H. Hall, Acta Metall. 1, 22 (1953). 21. L. Robindro Singh, R. S. Ningthoujam, V. Sudarsan, S. Dorendrajit Singh, and S. K. Kulshreshth, J. Lumin. 128, 1544 (2008). 22. R. Reisfeld, E. Zigansky, and M. Gaft, Molec. Phys. 102, 139 (2004). 23. C. Gorller-Walrand and K. Binnemans, Rationalization of crystal field parameterization, Handbook on the Physics and Chemistry of Rare-Earths, edited by K. G. Gschneidner, Jr, and L. Eyring, North-Holand, Amsterdam (1996). 24. F. Auzel, Chem. Rev. 104, 139 (2004). 25. M. Kang, J. Lin, G. F. Yin, and R. Sun, Rare Met. 28, 439 (2009).