Optimizing white light luminescence in Dy -doped

JOURNAL OF APPLIED PHYSICS 116, 174308 (2014)

Optimizing white light luminescence in Dy31-doped Lu3Ga5O12 nano-garnets n-Luis,2 P. Haritha,1 I. R. Martın,2,3 K. Linganna,1 V. Monteseguro,2,3 P. Babu,4 S. F. Leo C. K. Jayasankar,5 U. R. Rodrıguez-Mendoza,2,3 V. Lavın,2,6 and V. Venkatramu1,a) 1

Department of Physics, Yogi Vemana University, Kadapa - 516 003, India Department of Physics, and MALTA Consolider Team, Universidad de La Laguna, 38200 San Crist obal de La Laguna, Santa Cruz de Tenerife, Spain 3 Instituto Universitario de Materiales y Nanotecnologıa, Universidad de La Laguna, 38200 San Crist obal de La Laguna, Santa Cruz de Tenerife, Spain 4 Department of Physics, Government Degree College, Satyavedu - 517 588, India 5 Department of Physics, Sri Venkateswara University, Tirupati - 517 502, India 6 Instituto Universitario de Estudios Avanzados en At omica, Molecular y Fot onica, Universidad de La Laguna, 38200 San Crist obal de La Laguna, Santa Cruz de Tenerife, Spain 2

(Received 11 July 2014; accepted 22 October 2014; published online 6 November 2014) Trivalent dysprosium-doped Lu3Ga5O12 nano-garnets have been prepared by sol-gel method and characterized by X-ray powder diffraction, high-resolution transmission electron microscopy, dynamic light scattering, and laser excited spectroscopy. Under a cw 457 nm laser excitation, the white luminescence properties of Lu3Ga5O12 nano-garnets have been studied as a function of the optically active Dy3þ ion concentration and at low temperature. Decay curves for the 4F9/2 level of Dy3þ ion exhibit non-exponential nature for all the Dy3þ concentrations, which have been wellfitted to a generalized energy transfer model for a quadrupole-quadrupole interaction between Dy3þ ions without diffusion. From these data, a simple rate-equations model can be applied to predict that intense white luminescence could be obtained from 1.8 mol% Dy3þ ions-doped nano-garnets, which is in good agreement with experimental results. Chromaticity color coordinates and correlated color temperatures have been determined as a function of temperature and are found to be within the white light region for all Dy3þ concentrations. These results indicate that 2.0 mol% C 2014 Dy3þ ions doped nano-garnet could be useful for white light emitting device applications. V AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4900989] I. INTRODUCTION

Nanomaterials have attracted much attention in display and lighting technologies due to their excellent properties, which are often uniquely different from those exhibited by their macroscopic (bulk) counterparts. The development of these nanocrystalline materials has become pivotal in miniaturization of today’s display device technology. Rare-earth (RE3þ)-doped nanocrystalline materials find a broad range of applications such as lighting, cathode ray tubes (CRTs), displays, biological labeling, and detector systems.1–4 Over the past few years, white light-emitting-devices (WLEDs) have been widely studied as potential lighting sources to replace the conventional incandescent and fluorescent lamps as they exhibit higher brightness, lower power consumption, longer lifetime, higher efficiency, better reliability, environment-friendly as well as excellent low temperature performance.5–7 Typically, white light can be generated from cerium(III)doped yttrium aluminum garnet (YAG:Ce3þ) yellow phosphor excited by GaN-based blue LED. However, they suffer from chromatic aberration and poor white light performance after long period of working due to their individual degradation rates.8 Hence, it is interesting to develop a single-phase white light emitting phosphor excited by GaN LED that may be an excellent option to replace YAG:Ce3þ yellow phosphor. In a)

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this direction, several efforts are being made for the development of white light sources. For this, garnets are good candidates as host structures as they exhibit interesting properties such as high chemical stability, high thermal conductivity, hardness, and show intense luminescence useful for white LEDs application when activated with RE3þ ions.9,10 Among all the garnets, Lu3Ga5O12 (LuGG) is a good garnet host material since luminescence intensities of many Lu3þ-based compounds are significantly higher than corresponding Yttrium (Y3þ) or Gadolinium (Gd3þ) based compounds.11,12 Trivalent dysprosium ion (Dy3þ) is a promising activator to have white light emission because of its characteristic blue and yellow emission bands. The blue band centered at around 493 nm corresponds to the 4F9/2!6H15/2 transition and the yellow band located at around 581 nm corresponds to the hypersensitive 4F9/2!6H13/2 transition. Thus, the crystal-field environment of Dy3þ has a significant effect on the intensity of yellow emission, but has only marginal effect on the blue emission. Therefore, it is possible to obtain white light emission from Dy3þ-activated luminescent materials by tuning the intensity ratio of yellow to blue (Y/B) emissions by changing the host matrix, the excitation wavelength or the laser pump power.13 The main aim of the present investigation is to optimize the single dopant Dy3þ ion concentration in LuGG nano-garnet synthesized by low cost, environmentally benign, and non-toxic technique to get white light emission under low cost single excitation light source, instead of using multiple excitation sources and multiple RE3þ dopant activators.8

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C 2014 AIP Publishing LLC V

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In the present work, Dy3þ-doped LuGG nano-garnets have been prepared by sol-gel method and investigated their optical properties through luminescence and decay curve analysis. The characteristic blue and yellow emissions of Dy3þ in LuGG nano-garnets have been observed under 457 nm laser excitation. The luminescence decay curves have been measured as a function of Dy3þ ion concentration and analyzed using suitable and simple theoretical models in order to obtain information on the excited state dynamics of Dy3þ ions in LuGG nano-garnet. Finally, the Y/B luminescence intensity ratios, the color coordinates, and correlated color temperatures (CCT) of these nano-garnets have been calculated as a function of temperature to know their utility for WLED applications. II. EXPERIMENTAL

Garnets of composition (Lu(1-x)Dyx)3Ga5O12, where x ¼ 0.01, 0.02, 0.03, 0.04, and 0.05 labeled as LuGG1Dy, LuGG2Dy, LuGG3Dy, LuGG4Dy, and LuGG5Dy, respectively, were prepared by sol-gel method.14 The X-ray diffraction pattern of the LuGG1Dy nano-garnet was measured on ˚) the powder X-ray diffractometer using the CuKa1 (1.5406 A radiation in the range of 2h ¼ 15–80 , with a step size of 0.02 (PANalytical X’Pert Pro). The high-resolution transmission electron microscopy (HRTEM) micrographs (FEI-TECNAI G2 microscope at 200 kV) were used to study the morphology and the nano-structure of the LuGG powder. The particle size distribution was characterized using a Zetasizer Nano-S90 (Malvern Instruments, USA) by dynamic light scattering (DLS) technique. The nano-garnet powder of 1 mg was added into 15 ml of ethanol solution, bath sonication was carried out for 3 h. The diffuse reflectance in UV-Vis-NIR region was measured with a spectrophotometer (Agilent Technologies Cary 5000). The luminescence spectra in the range of 460–1000 nm were measured by exciting at 457 nm using diode pumped solid state laser. These emissions were focused with a convergent lens onto a fiber coupled 0.303 m single grating spectrograph (Andor Shamrock SR-303i-B) and then detected with a cooled CCD detector (Newton DU920N). The luminescence decay curves for the 4F9/2 level of Dy3þ ions were measured under 462 nm laser excitation using a 10 ns optical parametric oscillator (EKSPLA/NT342/3/UVE) and a digital storage oscilloscope (Lecroy WS424) coupled to a PMT (Hamamatsu R928) used in the detection system. For the low temperature measurements from 12 to 300 K a closedcycled cryostat (APD Cryogenics DE204) was used. All the spectra were corrected for instrumental response. III. RESULTS AND DISCUSSION A. Structure

The garnet structure can be viewed as a network of GaO6 octahedra and GaO4 tetrahedra linked by shared oxygen ions at the corners of the polyhedral, and arranged in chains along the three crystallographic directions forming dodecahedral cavities which are occupied by the Lu3þ ions.14 The structure of Dy3þ-doped LuGG nano-garnet has been determined using powder X-ray diffraction (see Fig. 1).

FIG. 1. XRD pattern of 1.0 mol% Dy3þ ions-doped LuGG nano-garnets. The vertical marks (blue) are the allowed reflections for this material in the Ia-3d (No. 230) space group.

The well-defined Bragg reflections in measured XRD pattern indicate that the nanomaterial under study was well crystallized in a single phase of cubic structure. All the peaks in XRD spectrum could be well-indexed to a space group Ia-3d (No. 230), and no second phase was detected. This confirms the formation of single phase cubic Dy3þ-doped LuGG nano-garnets, in which the Dy3þ ions are assumed to occupy the Lu3þ dodecahedral sites. The average crystallite size has been calculated using the Scherrer equation15 and is around 55 nm. Morphology of LuGGDy nano-garnets has been studied by HRTEM micrographs and is shown in Fig. 2(a) for LuGG1Dy nano-garnets. The HRTEM micrographs of the LuGG1Dy nano-garnets reveal that the nano-garnets have been agglomerated in different shapes and sizes ranging from 40 to 60 nm. These results are in good agreement with scanning electron micrograph of the LuGG nano-garnets codoped with 1.0 mol% Ho3þ ions and 10.0 mol% Yb3þ ions.14 The size distribution of the LuGG1Dy nano-garnets was measured by DLS technique and is shown in Fig. 2(b). The average diameter of the particle (dp) is calculated from the diffusion coefficient D using the Stokes–Einstein equation:16 dp ¼ kT/3plD, where k is the Boltzmann constant (in J K–1), T is the absolute temperature (in K), and l is the viscosity of the medium (in kg m1 s1). The distribution of nanogarnets in different sizes ranging from 15 to 80 nm with a maximum at 37 nm is observed and is in good agreement with HRTEM results. As can be seen from Fig. 2(b), maximum number of nano-garnet crystals are grown to about 37 nm in size. B. Absorption and luminescence

For optical applications special interest is devoted to the analysis of the local structure around the RE3þ ions in the matrix, since it rules the fine structure splitting of the freeion multiplets and the forced intra-configurational 4f-4f electric-dipole transition probabilities in the visible range.


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FIG. 3. Diffuse reflectance spectra of 5.0 mol% Dy3þ-doped LuGG nanogarnet. The inset (a) shows absorption band in UV region and inset (b) shows partial energy level structure and main emission lines and crossrelaxation (CR) channels of Dy3þ ions in LuGG nano-garnet.

FIG. 2. (a) HRTEM micrograph (b) Particle size distribution histogram of 1.0 mol% Dy3þ ions–doped LuGG nano-garnets.

As already mentioned, the Dy3þ ions will predominantly enter the distorted dodecahedral sites by replacing the Lu3þ ions. Thus, the eight oxygen ligands surrounding the optically active Dy3þ ion create a local environment with orthorhombic D2 point symmetry. As a consequence, the D2 crystal-field interaction felt by the optically active ion will completely remove the degeneracy of the 2Sþ1LJ multiplets of the free-Dy3þ ion giving rise to (2J þ 1)/2 Stark or crystal-field levels labelled according to the irreducible representations obtained from the group theory. The room temperature (RT) diffuse reflectance spectrum of the LuGG5Dy nano-garnet in the UV-Vis-NIR range is given in Fig. 3. The peaks observed correspond to intraconfigurational 4f9-4f9 electronic transitions starting from the 6H15/2 ground state to the different excited levels of the Dy3þ ion. All the transitions are assumed to be electric dipole in nature, except 6H13/2 level that shows magnetic dipole contribution. The labels of the different transitions of the Dy3þ ion in the LuGG nano-garnet have been assigned according to the well-known Dieke’s diagram for this ion in

LaCl3 crystal.17 The sharp peak profiles found for all the electronic transitions confirm that the Dy3þ ions are incorporated in the nanocrystalline structure of the garnet. A partial energy level diagram of the Dy3þ ion in the LuGG nanogarnet is given in inset (b) of Fig. 3, which illustrates the excitation, de-excitation and cross-relaxation processes of Dy3þ ions in LuGG nano-garnet. If any level above the 4F9/2 level is excited (blue region), a fast non-radiative decay from these levels to the 4F9/2 level takes place resulting in a radiative emission from this level. Luminescence spectra of LuGG:Dy3þ nano-garnets with varying Dy3þ concentrations were measured under a cw 457 nm laser excitation in resonance with the 6H15/2!4I15/2 transition. As commented before, the departure emitting level corresponds to the 4F9/2 one, which spontaneously emits photons (see Fig. 4(a)). The luminescence spectra show characteristic emission bands of the Dy3þ ions in the blue (460–510 nm), yellow (570–610 nm), red (660–690 nm), and NIR (750–1000 nm) regions, which are ascribed to the 4 F9/2!6HJ(J ¼ 15/2, 13/2, 11/2, 9/2, and 7/2) transitions, respectively. It is worth noting that, just as it has happened with the absorption measurements, the sharp structure of all these transitions confirms the crystalline nature of the dodecahedral sites occupied by the Dy3þ ions, with D2 orthorhombic local point symmetry. The luminescence spectra are comparable to other Dy3þ-doped Y2CaZnO5,18 Gd3Ga5O12,19 Li2Gd4(MoO4)7,20 Y2(MoO4)3,21 NaGdTiO4,22 Y3Ga5O12,23 Y4Al2O9,24 and Gd2Mo3O925 phosphors. On the other hand, the variations of the excitation wavelength and/or the Dy3þ ion concentration have no significant effect on the emission profile and hence all the properties were characterized by exciting the samples with cw 457 nm laser. The strong yellow emission band centered at 581 nm corresponds to the hypersensitive 4F9/2!6H13/2 transition (DL ¼ 2; DJ ¼ 2) and its intensity is strongly influenced by the surrounding environment around the Dy3þ ion. Blue emission band at 493 nm corresponds to the 4F9/2!6H15/2 transition. When Dy3þ is located in sites with inversion


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FIG. 5. Variation of blue and yellow emissions and Y/B ratios as a function of Dy3þ ion concentration in LuGG nano-garnets.

and yellow bands, besides the Y/B ratio, as a function of temperature, in the range of 12–300 K, are shown in Fig. 6 for the LuGG2Dy sample. It is noticed that, with increase of temperature, the intensity of the yellow emission increases whereas there is no considerable change in intensity of the blue emission although, as it will be shown later, this change does not affect much the color of the white emission. C. Luminescence decay curves

FIG. 4. (a) Luminescence spectra of LuGG:Dy nanogarnets obtained for different concentrations under 457 nm excitation. (b) Commission International d’Eclairage (CIE) color diagram. The color coordinates obtained for all LuGG:Dy nano-garnets are inside of the circle near to the white region.

The luminescence decay curves for the 4F9/2 level of Dy ion for different concentrations obtained under 462 nm pulsed laser excitation, in resonance with the 6H15/2!4F9/2 absorption transition, and monitoring the 4F9/2!6H13/2 emission at 581 nm are shown in Fig. 7. The decay curves exhibit non-exponential nature for all the Dy3þ concentrations in this nano-garnet. The enhancement of non-exponential nature and shortening of the decay curves with increase in the Dy3þ ion concentration is attributed to energy transfer from the Dy3þ ion in an excited 4F9/2 state to a nearby Dy3þ ion in the ground 6H15/2 state (see Fig. 7). Resonant or quasi

symmetry, the yellow emission is zero since electric-dipole transitions are forbidden in those sites.26 In the present system, the yellow 4F9/2!6H13/2 emission at 581 nm is slightly more intense than the blue 4F9/2!6H15/2 emission at 493 nm in Dy3þ-doped LuGG nano-garnets. This can be understood as distortions of the dodecahedral sites of the Dy3þ ions. It can also be found from Fig. 5 that with the increase of the Dy3þ ions concentration, there is no considerable variation in the intensities of both blue (B) and yellow (Y) emissions. The Y/B ratios as a function of active ion concentration have been calculated to be 2.57, 2.57, 2.64, 2.48, and 2.59 for 1.0, 2.0, 3.0, 4.0, and 5.0 mol% of Dy3þ, respectively. As can be seen, Y/B ratio is practically independent of the Dy3þ ion concentration. Similar observations were reported in Ln2Zr2O7 (Ln ¼ La, Gd and Y) crystals,13 Y2CaZnO5 nanophosphors,18 and NaGdTiO4 phosphors.22 Moreover, the integrated emission intensities of both blue

FIG. 6. Variation of blue and yellow emissions and Y/B ratios as a function of the temperature for 2.0 mol% Dy3þ ions-doped LuGG nano-garnets.

3þ


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J. Appl. Phys. 116, 174308 (2014) TABLE I. The values for Pade approximant coefficients in Eq. (2) for different multipolar interactions.

FIG. 7. Luminescence decay curves of the 4F9/2 level as a function of the Dy3þ ion concentration in LuGG nano-garnets. The solid lines (in red colour) correspond to the Martin’s model fits for S ¼ 10.

resonant cross-relaxation channels responsible for this energy transfer process are (see inset (b) of Fig. 3), 4

F9=2 þ 6 H15=2 ! 6 H5=2 þ 6 H7=2

4

F9=2 þ 6 H15=2 ! ð6 F3=2 ;6 F1=2 Þ þ ð6 H9=2 ;6 F11=2 Þ:

(1)

When energy transfer processes between excited donor ions are dominant, the generalized Yokota-Tanimoto energy transfer model given by Martin et al.27 and hereafter named as Martin0 s model, can be used to study the non-exponential nature of the decay curves. In this model, the intensity of the luminescence is given by "

S3 3=S t t 1 þ a1 X þ a2 X2 S2 IðtÞ ¼ Ið0Þexp Q 1 þ b1 X s0 s0

#

(2) with 4p 3 ðCDA s0 Þ3=S Q ¼ CA C 1 3 S 2=S

X ¼ D CDA t12=S ;

(3) (4)

where I(t) is the luminescence intensity at time t; I(0) is the luminescence intensity at t ¼ 0 s; s0 is the intrinsic lifetime in the absence of acceptors; Q is the energy transfer parameter and is dimensionless in Eq. (3); CA is the acceptor concentration; C is the gamma function; CDA is the donor-acceptor interaction parameter; and D is the diffusion parameter. The value for the parameter S ¼ 6, 8, and 10 corresponds to an electric dipole-dipole, dipole-quadrupole and quadrupolequadrupole interaction, respectively. The values of ai and bi coefficients are given in Table I. In the case of dipole-dipole interaction (S ¼ 6) the Yokota-Tanimoto expression is obtained,28 whereas when the migration processes in Eq. (2) are negligible, i.e., D ¼ 0, then the Inokuti-Hirayama expression is obtained.29 Therefore, the Eq. (2) can be considered as a generalization of these models.

S

a1

a2

b1

6 8 10

10.866 17.072 24.524

15.500 35.860 67.909

8.734 13.882 20.290

The expression (2) gives the emission decay after pulsed excitation. Similar to Yokota-Tanimoto model, Martin’s model is also restricted to strong donor-acceptor interaction with weak diffusion. The best fitting of the experimental decay curves to S ¼ 6, 8, or 10 determines the character of the donoracceptor interaction and the energy transfer and diffusion parameters, CDA and D, respectively. The decay curves have been fitted to Eq. (2) by considering Q, so, and D as adjustable parameters. The three possible interaction mechanisms have been taken into consideration and observed that the fit for S ¼ 10 is significantly better than those with S ¼ 6 or 8, with negligible diffusion (D ¼ 0) among Dy3þ ions and with an intrinsic lifetime of 1.45 ms. This result indicates that the interaction between Dy3þ ions is quadrupole-quadrupole in nature, which is consistent with the cross-relaxation processes that also follows quadrupolar selection rule (see Eq. (1)). The quadrupole-quadrupole type of interaction could be due to energy transfer between Dy3þ ions and/or defect sites located on the surface of the nano-garnets. The defects are randomly created within the nano-garnets as they grow and are located randomly from the core to the surface of the nano-garnets. When nano-garnets are calcined at high-temperature, the defects within the core start to diffuse towards the surface of nano-garmet, thereby increasing the defect densities near the surface of the nano-garnet. The non-radiative energy transfer is dominated by the surface defect-sites due to quadrupolequadrupole type of interaction between Dy3þ ions and/or surface defect sites. Moreover, the energy transfer parameter Q is found to be 1.02, 1.2, 2.0, 2.63, and 3.76 for 1.0, 2.0, 3.0, 4.0, and 5.0 mol. % Dy3þ-doped nano-garnets, respectively. This clearly indicates that Q values increase linearly with increasing concentration (see Fig. 8), and is according to Eq. (3). The similar quadrupole-quadrupole type of interaction has been observed by Cavalli et al.30 in Dy3þ: CaMoO4 single crystals. It is known that the luminescence efficiency depends on active ion concentration. In order to know the dependence of the luminescence efficiency with the Dy3þ concentration in the LuGG nano-garnets, the rate equations for the population of the 4 F9/2 emitting level of Dy3þ ion can be written as follows:31 dn 1 ¼ r/CA n WT n ; dt s0

(5)

where n is the population of the 4F9/2 level of Dy3þ ion; r is the absorption cross-section; U is the incident pumping flux; CA is the concentration; s0 is the intrinsic lifetime of 4F9/2 level; and WT is the energy transfer probability. Under stationary condition, dndt ¼ 0, therefore, the popu4 lation of the excited F9/2 level of Dy3þ ion can be evaluated from the following:


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FIG. 8. Variation of the energy transfer parameter as a function of Dy3þ ion concentration in LuGG nano-garnets. The solid line (in red) is a linear fit between Q parameter and the Dy3þ ion concentration.

r/CA n ¼ 1 : s0 þ WT

(6)

The transfer probability WT in Eq. (6) can be calculated from the following:31 WT ¼

1 gg

0

s0 gg

;

(7)

0

where gg is the donor quantum yield. According to the previ0 ous fits, the dominant interaction is quadrupole-quadrupole between the donor and acceptor ions with negligible diffusion, therefore the quantum yield is given by29 1 3=S ! ð g 1 t t ¼ exp Q dt: (8) g0 s0 s0 s0 0

The intensity of the emission is proportional to the number of ions in the excited state, i.e., n*. The results obtained using Eq. (6) have been plotted in Fig. 9. It can be easily observed from Fig. 9 that the LuGG nano-garnet doped with 2.0 mol% of Dy3þ ions exhibits high luminescence efficiency and this concentration is quite close to the theoritically predicted concentration of about 1.8 mol% Dy3þ ions obtained using Eqs. (6)–(8) by assuming quadrupole-quadrupole interactions. D. Colour coordinates

It is well-known that a white luminescence can be obtained not only by mixing the appropriate ratio of the three primary colors (Red, Green, and Blue) but also with the appropriate mixture of yellow (Y) and blue (B) emissions. However, it is difficult to get white light with the former method whereas the later one is relatively simple, i.e., by changing the Y/B ratio.32,33 In general, color is represented by CIE chromaticity coordinates and color ratios. In our present work, in order to evaluate the colorimetric performance of the nano-garnets, the color coordinates for the LuGG:Dy3þ (2.0 mol%) sample are

FIG. 9. Variation of relative emission intensity as a function of the Dy3þ ion concentration in LuGG nano-garnets. The solid line corresponds to the theoretical fit to the experimental data using Eq. (6).

calculated as a function of temperature using the intensitycorrected luminescence spectra obtained by 457 nm laser excitation (see Table II), for which the detailed calculation procedure is described elsewhere.34 They are almost invariant with change of temperature and are shown in the CIE chromaticity coordinate diagram (see Fig. 4(b)). These values correspond to the white region in the coordinate diagram and similar results have also been obtained for the other concentration of Dy3þ ions in LuGG nano-garnets and for Y2O3:Dy3þ nanophosphors.35 The calculated color coordinates of the present materials are only sligthly far from the equal energy point (x ¼ 0.333, y ¼ 0.333). An advantage of this material is that it can be used for the UV excited warm white LEDs. The luminescence quality of LuGG nano-garnet is also evaluated in terms of CCT, which illustrates the temperature of a closest Plankian black-body radiator to the operating point on the chromaticity diagram.36 McCamy37 has proposed the empirical formula to evaluate CCT using the color co-ordinates given by CCT ¼ 449n3 þ 3525n2 6823n þ 5520:33;

(9)

TABLE II. Colour coordinates and colour temperatures of 2 mol. % Dy3þ ion-doped LuGG nano-garnet as a function of temperature. Color coordinates Temperature (K) 12 25 50 75 100 125 150 175 200 225 250 275 300

x 0.386 0.385 0.390 0.388 0.387 0.387 0.388 0.386 0.383 0.391 0.388 0.389 0.386

y

Color temperatures (K)

0.387 0.387 0.391 0.390 0.390 0.391 0.392 0.390 0.386 0.394 0.389 0.391 0.391

3933 3958 3862 3904 3928 3935 3917 3953 4002 3858 3897 3886 3959


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where n ¼ (x-xe)/(y-ye) is the inverse slope line and (xe ¼ 0.332, ye ¼ 0.186) is the epicenter. The CCT values obtained for the present LuGG:Dy3þ nano-garnet are found to be in the range of 3858–4002 K for the temperature range from 12 to 300 K, which are close to the warm CCT (i.e., CCT < 4000 K)35 and are presented in Table II. The CCT values of the LuGG nano-garnet are in between to those of fluorescent tube (3935 K) and day light (5500 K),38 and gives relatively warm white light compared to the commercial YAG:Ce3þ,39 Y2CaZnO5,18 and Y3Ga5O1223 phosphors whose CCT values are higher than 4000 K. This indicates that LuGG:Dy3þ garnet might be a single-component UV-converting candidate for white LEDs. IV. CONCLUSIONS

Lu3Ga5O12 nano-garnets doped with different Dy3þ ion concentrations have been prepared by sol–gel method. All the nano-garnets of the present study emit white light through the characteristic blue and yellow emissions under 457 nm laser excitation. The interaction for energy transfer between Dy3þ ions is found to be of quadrupole-quadrupole type from the analysis of fluorescence decay curves using the Martin’s model. The variation of the emission intensity with the Dy3þ concentration has been measured and is found that 2.0 mol% Dy3þ-doped Lu3Ga5O12 nano-garnet produces the more intense emission, which is in good agreement with the theory based on a simple rate equations considering the energy transfer processes. The determination of CIE chromaticity coordinates, Y/B ratios and CCT values as a function of temperature reveals that the Dy3þ-doped Lu3Ga5O12 nano-garnet could be a suitable candidate for the generation of warm white-light. The main goal of these matrices is the ability to address white light as a sum of blue and yellow emissions under laser excitation with commercially available diode lasers or an Argon laser, at a unique excitation wavelength and using the predicted concentration of single Dy3þ ion in Lu3Ga5O12 nano-garnets. ACKNOWLEDGMENTS

V. Venkatramu is grateful to Council of Scientific and Industrial Research (CSIR), New Delhi for the sanction of major research project (No. 03(1229)/12/EMR-II, dated: 16th April, 2012). The authors are grateful to Department of Science and Technology, Government of India and Ministerio de Economıa y Competitividad (MINECO) from Spain for financial support within the Indo-Spanish Joint Programme of Cooperation in Science and Technology (DST-INT-Spain-P-38-11/PRI-PIBIN-2011-1153). This work has been also supported by Ministerio de Ciencia e Innovaci on of Spain (MICINN) under the National Program of Materials (MAT2010-21270-C04-024 and MAT201346649-C4-4-P) and the Consolider-Ingenio 2010 Program (MALTA CSD2007-00045), and by the EU-FEDER funds. V. Monteseguro wishes to thank MICINN for the FPI Grant (BES-2011-044596).

J. Appl. Phys. 116, 174308 (2014) 1

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