Concentration dependent Dy3+ activated LiPbB5O9 phosphor: Structure and luminescence studies for white LED applications

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Concentration dependent Dy3þ activated LiPbB5O9 phosphor: Structure and luminescence studies for white LED applications T. Raghu Raman 1, Y.C. Ratnakaram * Sri Venkateswara University, Tirupati, India

A R T I C L E I N F O

A B S T R A C T

Keywords: William-Hall analysis Photoluminescence Lifetime White light Energy transfer

In this study, distinct concentrations of Dy3þ ions (0.04, 0.07, 0.1, 0.4 and 0.7 mol%) doped LiPbB5O9 phosphors were fabricated via solid state reaction technique. X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), thermo-gravimetric studies (TG-DSC) and photoluminescence characterizations were performed to LiPbB5O9:Dy3þ phosphors. Photoluminescence excitation and emission spectra were recorded at 576 nm and 349 nm, respectively. The stress, the critical distance (Rc) and energy transfer parameters (Q) between the Dy3þ ions were evaluated for all concentrations. To interpret the white light emission ability of the LiPbB5O9:Dy3þ phosphor, yellow to blue (Y/B) intensity ratios have been calculated. The decay curves of 4F9/2 level of Dy3þions were studied. The CIE color co-ordinates and coordinate color temper­ atures (CCT) were also calculated. From these studies, it is mentioned that LiPbB5O9:0.4Dy3þ phosphor is a suitable material for fabrication of WLEDs and laser applications.

1. Introduction

single-phase w-LED phosphors that are excited by near UV. The ad­ vantages of this method are chemical stability, lattice stiffness, ease of fabrication and low cost [5]. Among various rare earth elements, Dy3þ is teeming element and it is used in a wide spread way in fiber amplifiers, infrared lasers, solar cells, mid infrared wavebands, telecommunications and lighting. Only Dy3þ doped phosphors can bring about white light. Due to peculiar 4f9 elec­ tronic configuration of Dy3þ ions it can absorb UV light effectively [6]. Besides these applications, Dy3þ ions show prominent laser transitions in mid-infrared and near-infrared regions, e.g. 6H11/2 → 6H13/2 (4.3 μm), 6 H13/2 → 6H15/2 (3.0 μm) and 6H9/2 þ6F11/2 → 6H15/2 (1.3 μm) transi­ tions. There are two main emission lines in Dy3þ doped phosphor, namely blue and yellow component transitions. The (Y/B) intensity ratio of phosphor is directly related to white light emission. As yellow color arises due to hypersensitive transition, it is sensitive to crystal field environment and it can be easily varied. It also represents the covalence degree between O2 and Dy3þ ions. By modifying the (Y/B) ratio, white light emission can be accomplished, further, (Y/B) ratio can be varied by varying the rare earth (RE) ion concentration, phosphor chemical composition and wavelength of excitation. In getting the white light emission, host composition play the vital role [7,8]. The luminescence properties of Dy3þ- ions can be altered by varying the Dy3þ ions

For the last few decades, consciousness about global warming, climate change, energy crisis, eco-friendly environment and harmful impact from fossil fuels have been increased and people are in quest to create alternative to fossil fuels [1]. Today in society major fraction of energy is utilized for lighting purpose. So scientists/engineers are concentrating on producing light with minimum energy consumption with minimum environmental damage/pollution. Solid state lighting (SSL) technology is emerging as option for engineers/scientists to fulfill all the afore mentioned demands [2]. Lot of research work is going on about white light emitting diodes (w-LEDs) in SSL technology. W-LEDs use 20–30% of all electrical energy consumption. White light can be produced by coating a tri-color phosphor on ultraviolet (UV) chip. Phosphors which converted into white light emitting diodes (pc-wLEDs) are replacing conventional light sources. Fluorescence conversion method is widely used for the production of W-LEDs. In this procedure phosphors are mixed with blue or near ultraviolet (n-UV) chip and it is efficiently excited by this chip [3]. Even though this method produces high output power and high color rendering index, it suffers from the poor quantum efficiency [4]. The reason is blue light re-absorption in the tri-color phosphors. Alternative approach is production of

* Corresponding author. E-mail address: [email protected] (Y.C. Ratnakaram). 1 Lecturer in Physics, P.V.K.N. Govt. Degree College, Chittoor https://doi.org/10.1016/j.optmat.2019.109515 Received 25 September 2019; Received in revised form 27 October 2019; Accepted 5 November 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.

Please cite this article as: T. Raghu Raman, Y.C. Ratnakaram, Optical Materials, https://doi.org/10.1016/j.optmat.2019.109515

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of LiPbB5O9:Dy3þ phosphors is as follows,

concentration, excitation wavelength and chemical composition [9,10]. Emission properties of phosphors not only depend on chemical compo­ sition, but also depend upon the lattice defects (intrinsic defects). The lattice defects are introduced in the crystal during the synthesis of the phosphor. To compensate the charge and elastic stress, intrinsic defects are formed in the vicinity of the dopants. To explain the concentration quenching, the excitation energy is assumed to be transferred to the lattice defects or impurity ions and they acts as the energy acceptors. Borate network former has many advantages like good physicochemical stability and homogeneous optical characteristics. B2O3 is the best one among the available formers, because it has high material strength, high thermal stability, low melting point and high RE ions solubility. Because of stretching vibrations, B2O3 has high phonon energy (1300 cm 1). High phonon energy reduces the luminescence efficiency, because it encourages the non-radiative emission. Heavy metal oxides (HMO) are used to reduce phonon energy in B2O3 system. The lead borate crystals give vital properties in visible region, they are used in electro-opto modulator, solid state laser materials, electro-optic switches and non-linear parametric converters. By adding the PbO to borate host, it modifies the boroxyl rings of borate network and produces the complex crystals with one, two or four-fold coordinated boron atoms. Pb2þ ions can form both the ionic and covalent nature of bonding with phosphor. This kind of dual nature of PbO produces asymmetrical crystal field for RE3þ ions and this favors the electric-dipole (E-D) transitions of RE3þ ions. PbO content leads to enhance the thermal stability, mechanical strength and chemical durability [11]. In the present work, heavy metal oxide, PbO is added to get higher quantum efficiency and lower multi-phonon decay rates. The quantum efficiency can be altered by increasing RE ion concentration [12]. Li3þ ions in phosphor improve the crystallinity and the charge compensation of the phosphors. Doping concentration of Li3þ ions also adjust color co-ordinates of phosphor in white LEDs application. Further Li3þ, 157Gd and 155Gd have strong ability to capture neutrons in nuclear reactions [13]. Rajesh et al. [14] reported the photoluminescence properties of NaPbB5O9:Dy3þ, as white light emitting phosphor. The present phosphor, LiPbB5O9:Dy3þ is also one of the novel white light emitting phosphors. In the present work, luminescence properties of LiPbB5O9:Dy3þ phosphors for different concentrations were reported. This work reports on structural, thermal and luminescence properties of LiPbB5O9 phosphor with varying Dy2O3 concentration, in particular white light emission point of view. XRD, SEM, FTIR, TG-DSC and PL characterizations were carried out for all the prepared samples. Based on the data obtained from the measurements, the effect of Dy3þ ion con­ centration on concentration quenching, (Y/B) ratios and color co-ordinates were studied. From the decay curves, lifetime measure­ ments of 4F9/2 excited state of Dy3þ in all the phosphors were also done. The energy transfer process was explained using the energy level diagram.

10H3BO3 þ 2PbO þ Li2CO3 → 2LiPbB5O9 þ 15H2O ↑ þ CO2↑

2.2. Characterization For the prepared phosphors, X-ray diffraction studies were executed by Bruker, D8 advance diffractometer provided with Cu Kα radiation with wavelength, 0.154 nm. FTIR characterization was performed with Perkin Elmer 100 IR spectrometer. SEM analysis was performed with Evo MA15, Carl Zeiss model scanning electron microscope. TG-DSC studies were performed up to 800 � C at the heating rate 10 � C/min in nitrogen atmosphere. Photoluminescence and decay curve character­ izations were performed by FL-21TCSPC model fluorescence spectrom­ eter equipped with 150 W xenon lamp. 3. Results and discussion 3.1. X-ray diffraction Fig. 1 indicates the XRD patterns of LiPbB5O9:xDy3þ (x ¼ 0.04, 0.07, 0.1, 0.4 and 0.7 mol %) phosphors prepared in the present work. The intensities and positions of different peaks coincide with the JCPDS No. 78–0294. Hence it can be concluded that LiPbB5O9:Dy3þ phosphor has monoclinic structure with space group P21/C and lattice parameters: a ¼ 6.463 A0, b ¼ 13.932 A0, c ¼ 7.858 A0, β ¼ 109.5500 and Z ¼ 4. The mean crystallite size can be calculated using [15]:

2. Experimental 2.1. Sample synthesis The LiPbB5O9:xDy3þ(x ¼ 0.04, 0.07, 0.1, 0.4 and 0.7 mol %) phos­ phors were prepared through solid state reaction procedure. The analytical grade (A.R) powders, boric acid (H3BO3), lead oxide (PbO), lithium carbonate (Li2CO3) and dysprosium oxide (Dy2O3) are utilized in the preparation of the phosphors. All these chemical have purity of 99.9%. The reactants were homogeneously mixed according to their stoichiometric proportion and milled evenly by agate mortar. Acetone of required amount was added for homogeneous mixing of the chemicals. The mixture was transferred to silica crucible and it was preheated for 2 h at 500 � C. After this, the products were grounded to fine powders. Finally, the mixture was re-sintered for 10 h at 630 � C then cooled to the room temperature. After this, samples were grounded well to fine powders for characterization. The chemical equation in the preparation

Fig. 1. X-ray diffraction profiles of LiPbB5O9:Dy3þ phosphor for different concentrations. 2

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Dh

k l

¼

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0:89 λ β2θ cos θ

Table 1 Crystallite sizes and microstrains for LiPbB5O9:Dy3þ phosphor for different concentrations.

(1)

where β2θ is fullwidth at half maximum, λ is the X-ray wavelength, θ is angle of diffraction. 3.1.1. Williamson-Hall analysis The merit of Williamson-Hall analysis compared with the DebyeScherrer model is that in addition to crystallite size (D), the micro­ strain (ԑ) can also be calculated. The equation for the W–H analysis is [16]: βhkl Cos ​ θ ​ ​ ¼ ​ ε ​ Sin ​ θ ​ ​ þ

Kλ D

S. No

Formula

Crystallite size (μm)

Strain (ԑ %)

1 2 3 4 5

LiPb0.96B5O9:0.04Dy3þ LiPb0.93B5O9:0.07Dy3þ LiPb0.90B5O9:0.10Dy3þ LiPb0.60B5O9:0.40Dy3þ LiPb0.30B5O9:0.70Dy3þ

0.073 0.068 0.098 0.060 0.069

0.01 0.05 0.10 0.10 0.05

[18].

(2)

3.3. FTIR analysis

where D is the crystallite size, β is full-width at half maxima in radians and it gives the broadening of XRD peaks, θ is the Bragg’s angle in ra­ dians, ԑ is the microstrain of the lattice and K is the constant of the crystallite shape. Fig. 2 shows the linear fitted graph between sin θ and βhkl cos θ. Crystallite sizes and microstrains are obtained from the in­ tercepts and slopes of the graphs respectively [17]. These values for different concentrations of Dy3þ doped phosphors are presented in Table 1. From the table it is noticed that there is not much variation in microstrain values and the crystallite sizes with the variation of Dy3þ concentration in the phosphor. These observations indicated that the effect of Dy3þ concentration on crystal structure is small.

The FTIR spectrum of LiPbB5O9:0.4Dy3þ phosphor recorded by employing the KBr pellet method is shown in Fig. 4. FTIR spectroscopy is the powerful technique and it gives information about the structural groups and vibrations. There are three characteristic vibrational modes in LiPbB5O9:Dy3þ phosphor in the regions 600-800 cm 1, 8001200 cm 1 and 1200-1600 cm 1. The first one stands for B–O–B bending vibrations, second one stands for BO4 units stretching vibrations and third one stands for BO3 units asymmetric stretching vibrations [19]. Fig. 4 shows 10 well defined bands at 423, 462, 505, 542, 652, 680, 773, 876, 1141 and 1268 cm 1. The band about 423 cm 1is ascribed to the metal–oxide bond and the band at 462 cm 1 corresponds to oxygen-deficiency related defect complex. The band about 542 cm 1 may be due to Pb–O vibrations. The bands below 700 cm 1 are assigned to the asymmetric and symmetric bending of BO4 and BO3 groups respectively. The vibrations around 773 cm 1are attributed to BO3 groups asymmetric bending. The band around 876 cm 1 is due to stretching vibrations of di-borate groups in BO4 units [19]. In pure borate phosphors (B2O3), addition of Li2CO3 and Na2O3 provides more non-bridging oxygen atoms and leads to BO4 units. The intensity of BO4 units is more compared with the BO3 units. The peak about 1141 cm 1 is ascribed to BO3 asymmetric stretching vibrations. The maximum phonon energy of LiPbB5O9:0.4Dy3þ phosphor is 1268 cm 1 and it represents the asymmetric stretching vibrations of B–O bonds from ortho-borate groups [19].

3.2. SEM analysis The surface morphology of LiPbB5O9:Dy3þ phosphor precursor is examined by the scanning electron microscope and the SEM images are depicted in Fig. 3 for 4 mol% Dy3þ doped phosphor. Irregular particle shapes were noticed from the SEM images. SEM images of phosphors prepared through the solid state reaction method are found to be condensed and agglomerated because of high temperature. If the par­ ticle size varies in a large quantity then the emission intensity varies. If the reaction temperature is decreased, extra impurity phases may occurred with less emission intensity. If the temperature is increased below the melting point, the emission intensity will increase. At present, commercially available phosphor particle sizes are in the range 2–10 μm

Fig. 2. W–H plot between Sinө and β Cosө for 0.4 mol % of Dy3þ doped LiPbB5O9 phosphor. 3

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Fig. 3. SEM images of LiPbB5O9:0.4Dy3þ phosphor precursor.

Fig. 4. FTIR spectra of LiPbB5O9:0.4Dy3þ phosphor.

3.4. TG – DSC analysis

3.5. Luminescence properties

The TG-DSC curves obtained for LiPb0.6B5O9:0.4Dy3þphosphor are shown in Fig. 5. In TG curve, the first weight loss of sample was observed between room temperature and 100 � C, which is due to evaporation of water molecules and the weight loss between 100 � C and 350 � C is attributed to conversion of boric acid (H3BO3) into metaboric acid (HBO2). Correspondingly, in a DSC curve, a tiny exothermic peak is observed at around 110 � C and another exothermic peak is observed at 240.9 � C. Next weight loss of the sample is observed between 330 � C and 630 � C due to accompanied by release of CO2. The endothermic peak at 588 � C is ascribed to the melting temperature of LiPbB5O9 phosphor. The exothermic peak at 683 � C may be related to the formation of LiPbB5O9 powder. The glass transition temperature for LiPbB5O9:0.4Dy3þ phos­ phor is observed 450 � C and the crystallization temperature is observed at 630 � C [14].

The excitation spectrum of LiPbB5O:0.4Dy3þ phosphor monitored at 576 nm is recorded in the range 300–550 nm and is submitted in Fig. 6. Fig. shows seven peaks indicating the excitations in the UV region and they are situated nearly at 323, 349, 364, 385, 421, 451 and 479 nm for the transitions, 6H15/2 → 6P3/2, 6H15/2 → 6P7/2, 6H15/2 → 6P5/2, 6H15/ 4 6 4 6 4 6 4 2 → I13/2, H15/2 → G11/2, H15/2 → I15/2 and H15/2 → F9/2 respec­ tively [20]. Most of the excitation bands are in the range 323–480 nm indicating that Dy3þ ions may be used as efficient activators for white LEDs. Among these transitions the most intense transition (6H15/2 → 6P7/2) is observed at 349 nm. Hence it is chosen as excitation wavelength for LiPbB5O9:Dy3þ phosphor. The emission spectra recorded in the 400–650 nm range using exci­ tation wavelength 349 nm are shown in Fig. 7 for all the concentrations of Dy3þ doped phosphors. The emission spectra shows two prominent peaks, one is at 479 nm (blue region) and the another is at 570 nm (yellow region) which represents the transitions, 4F9/2 → 6H15/2 and 4F9/ 4

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Fig. 5. Combined profiles of TGA and DSC for LiPbB5O9:0.4Dy3þ phosphor.

Fig. 7. Emission spectra of LiPbB5O9:Dy3þ phosphor under 349 nm excitation for different concentrations.

Fig. 6. Photoluminescence excitation spectrum of LiPbB5O9:0.4Dy3þ phosphor under 576 nm emission.

The phase deformation around Dy3þ ions in the LiPbB5O9 phosphor can be find out by the relative (Y/B) intensity ratio. In the present study, the obtained (Y/B) values are 1.01, 1.07, 1.10, 1.15 and 1.11 for 0.04, 0.07, 0.1, 0.4 and 0.7 mol% Dy3þ doped phosphors respectively as shown Table 3. Variation of Y/B values with the variation of Dy3þ concentration is shown in Fig. 8. From the results it is observed that by increasing the Dy3þ ions concentration, the (Y/B) values increased up to

H13/2 serially. Between the two transitions, 4F9/2 → 6H13/2 is elec­ tric dipole (ED) allowed transition and it is hypersensitive. As this transition is hypersensitive, it’s intensity is strongly depends upon the environment and crystal field of host matrix. The selection rules for this electric dipole allowed transition are ΔJ ¼ 0 or �2 and ΔL ¼ �2 [21]. 4 F9/2 → 6H15/2 transition is the magnetic dipole (MD) allowed transition and the selection rules for this transition are ΔJ ¼ 0, �1 (but 0↔0 transition is forbidden). As 4F9/2 → 6H15/2 is magnetic dipole allowed transition it’s intensity is invariant to the host lattice. Normally, the ratio of ED and MD transition is used to recognize the position and co-ordination rate of Dy3þ ions in the host crystal [22]. If blue band (B) is more intense than yellow band (Y) then Dy3þ ions occupy the sym­ metry site with inversion center in the lattice. If the yellow transition is salient than blue band then Dy3þ ion occupy low symmetry site with the lack of inversion center. In this study, it is identified that, yellow tran­ sition is more intense than blue transition, so Dy3þ ions are accommo­ dated in the asymmetric site with non inversion of host matrix [23]. 2→

6

Table 2 The critical distance (Rc) and energy transfer parameters (Q) of LiPbB5O9:Dy3þ phosphor for different concentrations.

5

Concentration (mol %)

Rc (A0)

Q (%)

0.04 0.07 0.10 0.40 0.70

20.37 16.90 15.00 9.45 7.84

25.11 25.04 25.04 25.04 25.04

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5 A0 multi-polar interaction is the cause for non-radiative transfer [28]. The critical distance is estimated by Blasse’s equation [29]:

0.4 mol% and then attenuated at 0.7 mol%. The maximum (Y/B) value is obtained for 0.4 mol% of Dy3þ doped phosphor. Also, the covalence degree between RE ion and oxide ion is estimated by (Y/B) intensity ratio and its value is higher for higher covalence degree [24]. The maximum intensity is obtained for 0.4 mol % of Dy2O3 concentration; hence this concentration of Dy3þ doped phosphor may be used for WLEDs fabrication.

� �13 3V RC ¼ 2 4π Xc N =

The average critical distance is calculated as 13.91 A0. The result indicates that transfer of energy between Dy3þ ions is due to multi-polar interaction. The type of multi-polar interaction between Dy3þ ions can be identified by Dexter’s theory [30]:

Between the two emission transitions, i.e. 4F9/2 → 6H15/2 (magnetic dipole transition, blue region) and 4F9/2 → 6H13/2 (electric dipole tran­ sition, yellow region), longer lifetime was found to yellow transition when excited with 349 nm [25]. Therefore, in the present work, 4 F9/2 → 6H13/2 transition decay curves under the excitation 349 nm have been obtained and are depicted in Fig. 9. It is observed that all the decay curves are matched to bi-exponential behavior. The bi-exponential behavior is described by the following equation [26]:

1

χ

(3)

A1 τ 1 2 þ A2 τ 2 2 A1 τ1þ A2 τ2

(4)

The reason for bi-exponential behavior of the decay curves is, transfer of energy between excited and ground state ions, i.e. donor and acceptor ions. Lifetime of 4F9/2 energy level is given by expression R IðtÞ⋅t⋅dt (5) τexp ¼ R IðtÞ⋅dt

� where Γ 1

3.7. Concentration quenching From the emission spectra, it was found that by raising the Dy3þ concentration, emission intensity also raised up to 0.4 mol% and then decreased at 0.7 mol% because of concentration quenching. Variation of emission intensity with the variation of Dy3þ concentration is shown in Fig. 10. Non-radiative energy transfer is the major cause of concentra­ tion quenching. Intensity of emission was increased without any change in the position of the peaks. There are two kinds of energy transfer phenomenon between one ion and other ion; exchange interaction and radiation re-absorption or multi-polar interaction. By using the critical distance (Rc) between the dopant ions, the type of interaction can be estimated. If the critical distance (Rc) is less than 5 A0, then exchange interaction is the cause for forbidden energy transfer, if it is greater than

Formula

(Y/B) ratio

FormulaLiPb0.96B5O9:0.04Dy3þ FormulaLiPb0.93B5O9:0.07Dy3þ LiPb0.90B5O9:0.10Dy3þ LiPb0.60B5O9:0.40Dy3þ LiPb0.30B5O9:0.70Dy3þ

1.0168 1.0756 1.1069 1.1543 1.1199

� 3 s

is gamma function, s ¼ 6 for dipole-dipole interaction.

4

F9/2 → 6H5/2 þ 6F3/2 ↔ 6H15/2 → 6H7/2 þ 6F9/2

4

F9/2 → 6F11/2 þ 6H9/2 ↔ 6H15/2 → 6F5/2 þ 6F3/2

The excited Dy3þ ions in 4F9/2 level will come to low-lying energy state 6F5/2þ6F3/2 by emitting the photon of suitable energy. The Dy3þ ions in ground state 6H15/2 will absorb this photon and jumps to the higher energy state 6H7/2 þ 6F9/2. Similarly, CR-2 process happens in the same manner. Due to above processes the Dy3þ ions will come to the ground state and causes the 4F9/2 energy level luminescence quenching [33].

Table 3 The (Y/B) ratios of LiPbB5O9:Dy3þ phosphor for different concentrations. 1 2 3 4 5

(7)

Energy transfer parameters are calculated for all the concentrations and they are shown in Table 2. It is noticed from the table that with increasing Dy3þ ion concentration, critical distance decreases while energy transfer parameter almost remains constant for all the concen­ trations due to luminescence decay. The different mechanisms engaged in concentration quenching of LiPbB5O9:Dy3þ phosphor can be understood well using the partial en­ ergy level diagram and it is shown in Fig. 12. The two vital mechanisms responsible for the concentration quenching are cross-relaxation chan­ nels (CRC) and resonance energy transfer (RET). With the excitation of 349 nm, the electrons in the ground state, 6H15/2 will absorb the photons and goes to the meta-stable state, 6P7/2. Due to non-radiative multiphonon assisted transition a part of electrons in 6P7/2 jumps to the 4F9/2 level. This non radiative transition involves multistage process, because of many levels between 6P7/2 and 4F9/2, any level can be occupied by the electron during this process. From Fig. 12 it can be noticed that reso­ nance energy transfer occurred between 4F9/2 and 6H15/2 levels. Crossrelaxation occurs between nearest adjacent Dy3þ ions followed by en­ ergy matching rule [32]. The probable cross relaxation mechanisms are

In the present study, the lifetime values of 4F9/2 level of Dy3þ ions are evaluated using eqns. (4) and (5). These values are obtained as 34.4, 34.8, 34.8, 36.0 and 37.6 μs using eqn. (4) and from eqn. (5), the lifetime values are 39, 40, 42, 45 and 39 μs for 0.04, 0.07, 0.1, 0.4 and 0.7 mol% concentrations of Dy3þ doped LiPbB5O9 phosphor respectively. It is observed that in both the cases the lifetime values are slightly changed with the variation of concentration. The non-exponential behavior is due to energy transfer among the Dy3þ ions from cross-relaxation channels. The bending of the non-exponential decay curves is attributed to the dipole-dipole interaction [27].

S. No.

1

where’s’ is the constant of multi-polar interaction and it is 6, 8 and 10 for dipole-dipole (D-D), dipole quadrupole (D-Q) and quadrupolequadrupole (Q-Q) interactions respectively. Fig. 11 shows the varia­ tion of log (I/X) with log X from which the slope of the graph observed is 1.6 and the observed ‘s’ value is 4.8. Hence the cause for concentration quenching beyond the 0.4 mol% in LiPbB5O9:Dy3þ phosphor is dipoledipole (D-D) interaction. The radiation re-absorption will be effective only when there is an intersection of excitation and emission spectra of activator and sensitizer. For dipole-dipole mechanism between acceptor and donor, the energy transfer parameter is given by Ref. [31]: � � 4π 3 XC R3c Q¼ (8) Γ 1 s 3

where τ1 and τ2 are fluorescence lifetimes, A1 and A2 are the amplitudes of decay. The mean lifetime in bi-exponential case is

τmean

3 � � �s3 5 ¼ ​ Κ 1þβ x =

3.6. Decay curve analysis

I (t) ¼ A1 exp (-t/τ1) þ A2 exp (-t/τ2)

(6)

3.8. Photometric characterization Determination of emission color characteristics is important for whiter LED applications. Color co-ordinates, co-ordinate color temper­ atures (CCT) and color purity are certain parameters to know about the 6

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Fig. 8. Variation of yellow to blue (Y/B) ratio with Dy3þconcentration in LiPbB5O9 phosphor.

Fig. 9. Lifetime decay curves of 4F9/2 level of Dy3þ in LiPbB5O9 phosphor for different concentrations.

characteristics of emitted light which were defined by CIE. The color coordinates x and y are evaluated using equations given in Ref. [34]. Color coordinates of LiPbB5O9:xDy3þ phosphors were evaluated for distinct concentrations of Dy3þ ions and they are tabulated in Table 4. These values are matched nearly with white light co-ordinates, x ¼ 0.333 and y ¼ 0.333, which are nearer to the commercially avail­ able white LEDs [35]. The commercial white LED phosphors are formed by combining the blue LED chip with Y3Al5O12:Ce3þ (YAG: Ce3þ). The main drawback of this phosphor is low color rendering index (CRI) and high coordinate color temperature (CCT). These drawbacks are due to

lack of red emission. So this phosphor does not meet the requirements of medical and domestic users. Y2O5S:Eu3þ and (Ca, Sr)S:Eu3þ sulfide based phosphors are used to overcome the defects, but these sulfide based phosphors have poor chemical stability. Zhang Zw et al. [36] reported that LiSr4(BO3)3:Dy3þ phosphor has CCT of 10,049 K and CIE color coordinates are (0.266, 0.291), Dillip G.R. et al. [37] reported that NaSrB5O9:Dy3þ phosphor has CCT of 7,570 K and (0.299, 0.298) are color coordinates, Meng Lu et al. [38] reported that Ca3(P1-xBxO4)2: Dy3þ, Ce3þ phosphor has (0.302, 0.325) as CIE color coordinates and Rajesh et al. [14] reported that NaPbB5O9:Dy3þ phosphor has color 7

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Fig. 10. Dependence of emission intensity on Dy3þconcentration in LiPbB5O9 phosphor.

Fig. 11. Variation of log (I/x) with log x in LiPbB5O9:Dy3þ phosphor.

coordinates (0.32, 0.34). Now the present LiPbB5O9: 0.4Dy3þ phosphor shows CCT as 5826 K and CIE color coordinates are (0.325, 0.345). From the above results, it can be mentioned that LiPbB5O9:Dy3þ phosphor is most preferable phosphor for production of white light by near UV excitation. Mc Camy empirical formula for CCT of phosphor is [39]:

source and is measured in Kelvin. CCT values of neutral white light can be separated into three regions like warm-white (�3700 K), neutral white (3700–5000 K) and cool white light (�5000 K) [40]. Generally cool CCT is favorable for active atmosphere and warm CCT is for mellow atmosphere. Cool white light is not preferable for indoor applications because it tends to have harsh feeling while the safe and pleasant feeling is created by the warm white light. Low CCT value is appreciable for human eye. Warm white light luminescent lamps are highly suggestible for domestic lighting applications [41]. By observing Table 4, it is concluded that CCT values of LiPbB5O9 phosphor fall in the cool white light region.

(9)

CCT ¼ 449 n3 þ 3525 n2 -6823.3 n þ 5520.33 3þ

The CCT values of distinct concentrations of Dy ions for LiPbB5O9 phosphor are calculated and they are presented in Table 4. CCT is the crucial parameter to recognize the luminescence characteristics of the 8

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Fig. 12. Energy level diagram of Dy3þshowing excitation, photoluminescence, resonant energy transfer and cross relaxation channel for LiPbB5O9 phosphor. Table 4 Colour co-ordinates and CCT values of LiPbB5O9:Dy3þ phosphor for different concentrations. Formula LiPb0.96B5O9:0.04Dy3þ LiPb0.93B5O9:0.07Dy3þ LiPb0.90B5O9:0.10Dy3þ LiPb0.60B5O9:0.40Dy3þ LiPb0.30B5O9:0.70Dy3þ

colour coordinates

CCT (K)

x

y

0.312 0.316 0.324 0.325 0.322

0.336 0.334 0.342 0.345 0.337

6495 6300 5878 5826 5987

recorded at 576 nm and eight transitions were noticed, among them the 6 H15/2 → 6P7/2 transition located at 349 nm is the most profound tran­ sition. Emission spectrum showed two prominent peaks when recorded using excitation at 349 nm and these transitions are 4F9/2 → 6H13/2 (yellow) and 4F9/2 → 6H15/2 (blue). Between these two transitions, 4F9/ 6 2 → H13/2 is hypersensitive and it is electric dipole allowed transition. With increasing the Dy3þ ion concentration, the (Y/B) ratio slightly increased and then decreased. The optimum concentration was identi­ fied as 0.4 mol%. In the present work, the average critical distance be­ tween Dy3þ ions is about 13.91 A0, hence the energy transfer among Dy3þ ions takes place due to multi-polar interaction. The constant of multi-polar interaction is closer to 6, so energy transfer takes place due to dipole-dipole (D-D) interaction. In the present work, the lifetime decay curves were bi-exponential behavior. Also, the maximum phonon energy of the borate phosphors is 1300 cm 1 but in the present LiPbB5O9: 0.4 Dy3þ phosphor, it is 1268 cm 1. The decrease in phonon energy leads to higher quantum efficiency. Hence the present phosphors may be used in the production of opto-electronic devices. As the CIE color coordinates of LiPbB5O9:Dy3þ phosphors are very close to ideal white light region, LiPbB5O9:Dy3þ phosphor is the potential material in the production of white LEDs. The CIE color co-ordinates of LiPbB5O9: Dy3þ phosphor for distinct concentrations of Dy3þ ions were calculated, all the values fall in the white region. These values are very closer to the commercial WLED phosphor CIE values (0.32, 0.33). By investigating CCT values for different concentrations, it is concluded that LiPbB5O9: Dy3þ phosphor can be utilized in cool white light generation.

Fig. 13. The CIE 1931 chromaticity co-ordinate diagram for LiPbB5O9:xDy3þ phosphor under 350 nm excitation.

4. Conclusions LiPbB5O9 phosphor doped with distinct concentrations of Dy3þ ions (0.04, 0.07, 0.1, 0.4 and 0.7 mol %) were synthesized by solid state re­ action method. From powder XRD patterns, it is concluded that LiPbB5O9:Dy3þ phosphor has monoclinic structure with space group P21/C. From the SEM investigation, the size of the particle in LiPbB5O9: Dy3þ phosphor is in micrometer range. The excitation spectrum was 9

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T.R. Raman and Y.C. Ratnakaram

Declaration of competing interest

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