Development of novel Na2Mg3Zn2Si12O30:Eu3+ red phosphor for white light emitting diodes

Optical Materials 96 (2019) 109350

Contents lists available at ScienceDirect

Optical Materials journal homepage: www.elsevier.com/locate/optmat

Development of novel Na2Mg3Zn2Si12O30:Eu3+ red phosphor for white light emitting diodes

T

G.V. Kanmania, V. Ponnusamya,∗, G. Rajkumara, M.T. Joseb a b

Department of Physics, MIT Campus, Anna University, Chennai, Tamil Nadu, 600044, India Radiological Safety Division, HSEG, IGCAR, Kalpakkam, 603102, Tamilnadu, India

ARTICLE INFO

ABSTRACT

Keywords: Milarite structure CIE chromaticity coordinates Color purity Thermal stability

A novel Eu3+ activated Na2Mg3Zn2Si12O30 red emitting phosphor was synthesized by solid state reaction at ambient conditions. The phase formation of prepared samples were confirmed by powder X-ray diffraction (XRD) and their luminescent properties were studied by photoluminescence(PL) spectroscopy. The surface morphology of samples were studied using SEM technique. The XRD profile of prepared samples are well matched with JCPDS card no(48–0418). The crystallite size is calculated using Scherrer's formula and found to be 0.5623 μm. The PL emission spectra shows intense peak around 614 nm arising as a result of electric dipole transition upon 394 nm excitation. The influence of increasing europium ion concentration on photoluminescence emission intensity were also studied. It is observed that PL intensity initially increases up to 11 mol % Eu3+ and then decreases. The concentration quenching is due to dipole-dipole interaction. Further, the commission International del’Eclairagethe (CIE) chromaticity coordinates, CCT and color purity for optimized Na2Mg3Zn2Si12O30:Eu3+(11 mol%) are calculated for 394 nm excitation and found to be (0.6391,0.3592), 1110K and 99.6% respectively. Moreover Eu3+activated Na2Mg3Zn2Si12O30 shows good thermal stability having activation energy 0.258eV. All the above results reveal that Na2Mg3Zn2Si12O30 doped Eu3+ can serve as potential candidates for red phosphor of WLED applications.

1. Introduction Nowadays white light emitting diodes (WLEDs) have spurred wide interest and become focus of research area because of its various advantages over conventional light sources such as incandescent lamps, fluorescent lamps and high pressure sodium vapour lamps. The advantages of LEDs are relatively high energy efficiency, cool operating temperature, brightness, compactness, long persistence, low power consumption and environmentally benign [1–5]. Currently white light emitting diodes(WLEDs) have been used in various fields such as automative headlights, indicators in traffic lighting, and flat panel backlighting in liquid crystal displays [6,7]. Commercially available WLEDs are based on combining blue emitting InGaN chip with yellow Y3Al5O12:Ce3+. Due to lack of red emission contribution, it exhibits low CRI (color rendering index) and high CCT (correlated color temperature) [8,9]. This drawback is overcome by combining blue, green and red phosphor with UV-LED [9]. Red phosphor Y2O2S is used for the construction of WLED and it is chemically unstable and has low absorption efficiency in the near uv-region [10]. Thus there is an urgent need to develop a red phosphor with high chemical stability and good

∗

spectral qualities, which can be activated by near UV and blue region. Luminescent materials based on silicate phosphors are of special interest due to their unique properties such as visible light transparency, high stability, high temperature strength, low thermal expansion, low cost, chemical resistance, moisture resistance and high conductivity. These silicate based materials were exhibiting high luminous efficacy, when doped with rare earth ions [1,11]. Among numerous rare earth ions, trivalent europium ion (Eu3+) has been considered as an outstanding activator that can produce intense red light. This is due to 5 D0 transition, the lowest excited level of the 4f6 configuration in Eu3+ is lying below the 4f5 5d configuration [12]. Among these silicate phosphors, the alkaline–alkaline earth-silicates are of special interest for host materials because of the following advantages, chemical stability, low sintering temperature, waterproofing, and it is easy to achieve electric charge balance [13]. However, most of the reported phosphors were required high temperature for synthesis. Thus we need to explore the novel host with low temperature synthesis and improved luminescence properties for tri-colour LED applications. In recent past researchers are interested in silicate based red phosphor such as Ca8ZrMg(PO4)6(SiO4):Eu3+ [14], Ca2YScMgSiO12:Eu3+

Corresponding author. E-mail address: [email protected] (V. Ponnusamy).

https://doi.org/10.1016/j.optmat.2019.109350 Received 13 July 2019; Received in revised form 24 August 2019; Accepted 25 August 2019 0925-3467/ © 2019 Published by Elsevier B.V.

Optical Materials 96 (2019) 109350

G.V. Kanmani, et al.

[15], Ca3Sc2Si3O12:Eu3+ [15], K3GdSi2O7:Eu3+ [16], Na2MgSiO4:Eu3+ [13], Ca2SiO4:Eu3+ [17], Y2MoSiO8:Eu3+ [18] etc. Among the various silicate materials, AxM3M′2Si12O30 milarite type structure exhibit favourable properties such as high thermal stability and chemical flexibility almost equivalent to those of spinel and pervoskites structure making it suitable candidate for luminescent host. Recently Qun et al. reported the luminescence property of Mn doped Na2Mg5Si12O30 [19]. Na2Mg3Zn2Si12O30 is a milarite type structure with general formula AxM3M′2Si12O30, where M occupies tetrahedral site and M′ occupies octahedral sites [20]. The structure presents two distinct cationic sites for element A, one sites is coordinated by 12 anions lying in the tunnel bounded by the Si12O30 rings and the other one site is coordinated by 9 anions situated between these rings. In present work, silicate based phosphor, Eu3+-doped Na2Mg3Zn2Si12O30 have been synthesized by solid-state reaction and the resulting phosphor materials exhibits high intense emission in the red region. The crystalline structure and morphology of host material was characterized by XRD and SEM technique. The luminescence properties and concentration effect of Eu3+on Na2Mg3Zn2Si12O30 were studied and reported for the first time. 2. Experimental studies 2.1. Synthesis of phosphor materials A series of Na2Mg3Zn2Si12O30 doped with various mol% of Eu3+ (1–15 mol%)with the interval of 2 mol% were synthesized by conventional solid state reaction. Stoichiometry ratio of Na2CO3, MgCO3, Zn2(CH3COO)2.2H2O and SiO2 were taken and well-grounded using agate mortar and pestle. The mixed powders were pre-heated for 550 °C for 2 h to eliminate CO2. The obtained samples were grounded again and sintered at 900 °C for 6 h in muffle furnace and cooled down to room temperature gradually. Thus, as prepared samples were analysed by various characterization techniques. 2.2. Characterization of phosphors

Fig. 1. XRD pattern of Na2Mg3Zn2Si12O30 and Na2Mg3Zn2Si12O30:Eu3+(11 mol %).

X-ray diffraction(XRD) patterns of synthesized Na2Mg3Zn2Si12O30 (NMZS) samples were determined using BRUKER D8 ADVANCE POWDER X-Ray diffractometer with CuKα radiation (k = 1.5406 Å) between the 2θ values 5°–90° with step size of 0.03° operating at 40 kV and 30 mA at room temperature. Surface morphology of the phosphor material was examined by VEGA3TESCAN. Photoluminescence excitation and emission spectra of the phosphors were recorded using Spex Spectrofluorometer (Jobin Yvon Fluorolog version-3; Model FL3-11) with a 150 W xenon lamp as the excitation source. All these measurements were carried out at room temperature. The thermal stability of the phosphor was studied using the same instrument equipped with the temperature controlling system.

tetrahedron through oxygen and edges of Zn tetrahedral. Each Zn and (Mg2) tetrahedra bonds four oxygen with adjacent Si tetrahedra by linking the Si12O30 clusters in the framework. The channel formed by Si12O30 clusters is occupied by Na1 atom and is surrounded by 12 anions(Fig. 2) [20,21]. The average crystallite size is calculated by Scherrer's equation: D = 0.89λ/β cosθ

(1)

where λ = wavelength of X-ray beam, β = full width half maxima, θ = diffraction angle and average crystallite size is found out to be 0.5623 μm. The lattice parameter is calculated using the following lattice geometry equation for hexagonal structure [22].

3. Result and discussion 3.1. Structural analysis

h2 + k2+hk l2 + 2 a2 c

1 4 = d2 3

Fig. 1 shows XRD pattern of Na2Mg3Zn2Si12O30 and Na2Mg3Zn2Si12O30: Eu3+(11 mol%) synthesized by conventional solid state reaction. The XRD profile of the as prepared sample are well matched and in consistent with the JCPDS card No(48–0418). Na2Mg3Zn2Si12O30 crystallizes in hexagonal structure with space group of P6/mcc(192) and z = 2. The additional weak peaks around 22.9°, 25.59° and 39.8° are due to residual SiO2 (JCPDS card no-89-7499). From the XRD pattern, it is confirmed that the doping of Eu3+ ion does not have any significant impact on the crystal structure of Na2Mg3Zn2Si12O30. In Milarite, Na2Mg3Zn2Si12O30 structure, Na2 atom is situated on the three fold axis, between (Si12O30) clusters and directly above and below through Mg1 octahedron, which is coordinated by nine oxygen atoms. The Mg1 octahedron shares corner with the Si

(2)

where h, k and l are the miller indices and d is the interplanar spacing in the atomic lattice is given by nλ = 2dsinθ V=

3 2

(3)

a2c where V=Volume of the hexagonal structure and is

found out to be a = 10.005 Å, c = 14.05 Å, and V = 1218.59 Å3.

By considering the effective ionic radii of cations corresponding to their coordination number as given by Shannon [23], the discrepency (Dr) in ionic radii between the activators (Eu3+ ions) and possible substituted cations [Na+(12), Na+(9), Mg2+(4), Mg2+(6), Zn2+(4), 2

Optical Materials 96 (2019) 109350

G.V. Kanmani, et al.

Fig. 2. Crystal structure of Na2Mg3Zn2Si12O30. Table 1 Discrepancy percentage(Dr) in ionic radii between the host cations and doped ions. Doped ion

Eu3+

Rd(CN) Å

Dr = 100 × (Rm(CN)-Rd(CN)/Rm(CN)

Eu3+(6) = 1.087 Eu3+(9) = 1.260

Na(9):(1.38)Å

Na(12):(1.53)Å

Mg(4):(0.71)Å

Mg(6):(0.860)Å

Si(4):(0.40)Å

Zn(4):(0.730)Å

– 8.69

– –

– –

26.3 –

– –

– –

Si4+(4)] in the host are calculated using equation (4) and summarized in Table 1 and acceptable percentage difference in ionic radii of dopant and possible substituted ions must not be greater than 30% [24].

Dr = 100 ×

Rm (CN ) Rd (CN ) Rm (CN )

SEM analysis. Fig. 3 displays the magnified SEM images of the Na2Mg3Zn2Si12O30:Eu3+(11 mol%) phosphor. The obtained micrograph shows that the synthesized samples are agglomerated with irregular morphology. The sizes of the particles are in μm range. This submicron sized phosphor is suitable for lighting, display and illuminating applications as at high sintering temperature, it can be attached to the substrate for solid state lighting (SSL) device fabrication [25].

(4) 3+

From the obtained Dr values, it is concluded that Eu can be preferably occupied at both Na+ and Mg2+ sites. Due to unavailability of ionic radii of Eu3+ for coordination number 4 and coordination number 12 thus the calculations for Mg2+(4), Zn2+(4) and Si4+(4) and Na+(12)are not performed.

3.3. Photoluminescence studies of the Na2Mg3Zn2Si12O30:Eu3+ phosphors The excitation and emission spectra of Na2Mg3Zn2Si12O30:Eu3+ (11 mol%) and their corresponding energy level diagram are displayed in Fig. 4a and b respectively. Fig. 5 represents the photoluminescence excitation spectra of Na2Mg3Zn2Si12O30 doped with various molar concentration of Eu3+ were recorded under the 614 nm emission

3.2. Morphology of Na2Mg3Zn2Si12O30: Eu3+ phosphor materials The surface morphology of the prepared samples were studied using

Fig. 3. SEM image of Na2Mg3Zn2Si12O30:Eu3+(11 mol%) phosphor with different magnification. 3

Optical Materials 96 (2019) 109350

G.V. Kanmani, et al.

Fig. 4. (a) Photoluminescence excitation and emission spectra of Na2Mg3Zn2Si12O30:Eu3+(11 mol%) phosphor. (b)Energy level diagram showing the excitation and emission mechanism of Eu3+ in Na2Mg3Zn2Si12O30:Eu3+(11 mol%).

Fig. 5. Photoluminescence excitation spectrum of Na2Mg3Zn2Si12O30 doped with various molar concentration of Eu3+ (λem = 614 nm).

Fig. 6. Photoluminescence emission spectrum of Na2Mg3Zn2Si12O30 doped with various molar concentration of Eu3+ (λem = 394 nm).

(5D0→7F2 transitions of Eu3+). The excitation spectra consist of various sharp peaks arising due to the excitation of ‘ground state Eu3+ ions’ to excited energy levels (4f-4f transition). These sharp peaks are found at 320 nm, 362 nm, 376 nm, 382 nm, 394 nm, 400 nm, 414 nm, 464 nm, 526 nm and 533 nm arising as a result of transitions, 7F0→5H3, 7 F0→5D4, 7F0→5G4, 7F0→5L7, 7F0→5L6, 7F1→5L6, 7F1→5D3, 7F0→5D2, 7 F0→5D1, and 7F1→5D1 respectively [26–29]. The broad band from 250 nm to 350 nm centred around 299 nm is attributed to charge transfer band arising as a result of charge transfer to the partially filled 4f orbital of Eu3+ ions from 2p orbital of O2−ions. The charge transfer band is weaker than the 4f-4f transition and the intense peaks occurring at 394 nm and 464 nm which overlap with the near-UV or blue LED chip emission, making it suitable for WLEDs applications.

Fig. 6 represents the photoluminescence spectrum of Na2Mg3Zn2Si12O30 doped with various molar concentration of Eu3+ were recorded in the spectral range from 500 nm to 750nm excited under 394 nm. The spectrum consist of a series of well resolved peaks at 579 nm, 586 nm, 592 nm, 597 nm, 614 nm, 627 nm, 654 nm, and 706 nm, which can be attributed to 5 the following D0→7F0(570 nm–585nm),5D0→7F1(585 nm–600nm), 5 5 D0→7F2 (610 nm–630nm), D0→7F3 (640 nm–660nm)and 5 7 D0→ F4(680 nm–710nm) transitions respectively [26,30]. From Fig. 6, it is clear that the emission arises only from low lying excited state, 5D0. The absence of transitions from the higher energy level 5DJ (J = 1, 2) may be due to depopulation of Eu3+ions from these level to lower 5D0 energy level via cross-relaxation process [31]. In addition, high energy lattice phonon can also quench 5DJ level via multiphonon relaxation process [31]. The 4

Optical Materials 96 (2019) 109350

G.V. Kanmani, et al.

Fig. 7. The dependence of the asymmetry ratio (R) on Eu3+ ion concentrations. Table 2 Comparison of CIE chromaticity coordinates of different Eu3+ doped red phosphors having R value greater and less than 1. S.No

Phosphor

R value

R CIE Values with color purity

Ref

1 2 3 4 5 6 7

CaZrO3:Eu3+(11 mol%) CaBi1.93B2O7:0.17Eu3+ Ba3Y1.3Eu0.7B6O15 Na2Y2B2O7:0.35Eu3+ NaSrB5O9:Eu3+ (9 mol%) Y2O2S:Eu3+ (commercial red phosphor) Na2Mg3Zn2Si12O30:Eu3+ (11 mol%)

<1 <1 <1 >1 >1 >1 >1

(0.5237,0.3090), 70% (0.601,0.392), 98% (0.601,0.396),not reported (0.652,0.349),98.2% (0.637,0.361),99.7% (0.622, 0.351), 91.6% (0.6391.0.3592),99.6%

[40] [41] [42] [39] [3] [43] present work

emission peak around 576 nm corresponding to 5D0-7F0 transition is strictly forbidden according to ΔJ (0-0 transition) selection rule of the judd-ofelt theory [26]. Fig. 6, it is observed that 5D0→7F0 transition is present and is almost comparable with the 5D0-7F1 transition indicating the presence of a linear crystal field strength namely Cnv, Cn or Cs symmetry in host material [32]. There are six different cationic sites in milarite structure namely Si tetrahedral sites, Mg1tetrahedral sites, Mg2 octahedral, NaO9 polyhedral, Zn tetrahedral, NaO12 polyhedral sites and corresponding symmetries are C1, D2, D3, S3, D2 and D6 [33]. It is believed that presence of 5D0-7F0 is due to Eu3+ occupying C1 linear symmetry. The orange emission peak around 592 nm corresponds to 5D0→7F1 magnetic dipole transition and the intensity of this transition is independent of the surrounding of the Eu3+ ion; while the red emission peak centred at 614 nm corresponds to electric dipole transition. This transition is also called hypersensitive transition as intensity depends on the local symmetry of Eu3+ and the nature of the ligand in the host matrix. The spectra shows the electric dipole-dipole transition is dominant than the magnetic dipole transition due to Eu3+ occupying low symmetric sites of octahedral Mg(2) and NaO9 polyhedral which are having site symmetry D3 and S3 respectively. The asymmetry ratio, I5D0→7F2/I5D0→7F1, is the ratio of the intensities of electric and magnetic dipole transition is calculated for all the concentrations of Eu3+ ions under excitation wavelength 394 nm and represented in Fig. 7. The asymmetry ratio provides information about the degree of distortion of the local structure symmetry around Eu3+ ions in host [34]. The slight increase in R with increasing Eu3+ concentration is due to deviation from the local

structure symmetry around Eu3+. When Eu3+ ions are introduced into the host, due to small ionic difference between the activators (Eu3+) and the possible substituted cations in the host(Na+ and Mg2+) the lattice parameter get altered and subsequently the interplanar spacing d changes, resulting in crystal distortion thus lowering the local structure symmetry around Eu3+ ions in the host [35,36]. The large R > 1 values indicates Eu3+ is occupying low symmetry with no inversion center, which is advantageous in achieving bright red phosphor with good color purity [36–39]. Table 2 compares the CIE chromaticity coordinates of different Eu3+ doped red phosphor having different R value (1 < R > 1). It is clear from Table 2 that phosphor having R value less than 1 shows orange emission while R value greater than 1 shows red emission. Thus Na2Mg3Zn2Si12O30 doped Eu3+ can be used as a potential red phosphor excited by near uv/blue LED for white LED applications. 3.4. Critical transfer distance (Rc) and concentration quenching Fig. 8 displays the variation of emission intensity at 614 nm (5D0→ F2) as a function of different Eu3+ molar concentration under 394 nm excitation. Initially, the photoluminescence intensity increases up to11 mol% and further increasing the concentration of Eu3+ result in decrease of luminescence intensity. The decrease in intensity is due to non-radiative energy transfer mechanism between neighbouring Eu3+ ions. As the concentration of Eu3+ increases the average distance between the adjacent Eu3+ decreases resulting in cross relaxation process. 7

5

Optical Materials 96 (2019) 109350

G.V. Kanmani, et al.

Fig. 8. Variation of emission intensity at 614 nm (5D0→ 7F2) as a function of different Eu3+ molar concentration under 394 nm excitation.

Fig. 9. Linear relationship between log(I/C)and log(C) for the 5D0→7F2 transition of Eu3+ in Na2Mg3Zn2Si12O30:Eu3+ phosphors.

Non-radiative energy transfer arises mainly due to exchange interaction, radiation reabsorption and multipole-multipole interaction [44]. According to Blasse [45], the average smallest distance between the adjacent activators (Eu3+ions), i.e critical energy transfer distance (Rc) can be calculated using equation (5).

Rc = 2

3V 4 xc N

where N is the number of Z cations (Z) in the unit cell, xc is the optimal concentration and V is the volume of the unit cell. By taking, N = 2, xc = 0.11 and V = 1218.59 Å3; the critical distance is found out to be 27.66 Å, which is much greater than 0.5 nm. Thus the possibility of exchange interaction among the Eu3+ ions is ruled out. Since the energy transfer among Eu3+ via exchange interaction their critical distance should be less than 5 Å [46]. Since, there is no overlap of the excitation and emission spectrum of Na2Mg3ZnSi12O30:Eu3+ the

1/3

(5) 6

Optical Materials 96 (2019) 109350

G.V. Kanmani, et al.

Therefore, Eq. (6) could be simply written as:

Table 3 CIE coordinates of Na2Mg3Zn2Si12O30 doped with different mol% of Eu3+monitored at 394 nm and 464 nm excitation wavelength. Mole concentration (%)

X

Y

Color purity

1 3 5 7 9 11 13 15

0.5917 0.6234 0.6354 0.6357 0.6372 0.6391 0.6373 0.6374

0.4005 0.3229 0.3625 0.3622 0.3609 0.3592 0.3608 0.3607

97.8 99 99.5 99.5 99.5 99.6 99.5 99.5

log(I/C) = A-s/3 × log(C)

where A is a constant, s is equal to 3, 6, 8, and 10 for exchange, dipoledipole, dipole-quadrupole and quadrupole-quadrupole interactions respectively [46]. The linear relationship between log(I/C)and log(C) for the electric dipole transition 5D0→7F2 transition of Eu3+ in Na2Mg3Zn2Si12O30:Eu3+ phosphors was displayed in Fig. 9 and the value of slope (-s/3) is −1.6915 and thus the value of s is found to be 5.07 which is closer to 6. This indicates the dipole-dipole interaction is responsible for the energy transfer among Eu3+ ions in Na2Mg3Zn2Si12O30 phosphors.

possibility of radiation reabsorption is also ruled out [47]. Thus, the concentration quenching arises due to the multipolar interaction of Eu3+ ions in the host. The electric multipolar interaction comprises of three types of interactions, viz., quadrupole–quadrupole(q–q), dipole–quadrupole(d–q) and dipole–dipole (d–d) interactions. Furthermore, the Dexter's theory [48] gives relation between luminescence intensity (Iem) and activator concentration and can be expressed as

I/C = k[1 +

× C S/3]

1

( > > 1)

(7)

3.5. Commission internationaldel’ Eclairage(CIE) chromaticity coordinates The CIE 1931 chromaticity coordinates are the important parameter for finding the luminescent color of the phosphor materials. CIE chromaticity coordinates and color purity are calculated for Na2Mg3Zn2Si12O30:Eu3+ phosphors doped with different molar concentration of Eu3+ and are summarized in Table 3. Color purity is calculated by the following formula [49].

(6)

where, C is the doping concentration greater than critical concentration; k and β are constants. The product β × Cs/3 is much larger than 1.

Color purity =

(x

x i ) 2 (y

(x d

xi )2 (yd

yi ) 2 yi )2

Fig. 10. Chromaticity diagram of Na2Mg3Zn2Si12O30:Eu3+ (11 mol%) phosphors. 7

(8)

Optical Materials 96 (2019) 109350

G.V. Kanmani, et al.

Fig. 11. (a) The emission spectrum of Na2Mg3Zn2Si12O30: Eu3+(11 mol%) phosphors (a) at different temperatures (303 K–453K). (b) Normalized PL intensities of Na2Mg3Zn2Si12O30:Eu3+(11 mol%) phosphors at different temperatures (303 K–453K). Table 4 The relative PL intensity at 423K with respect to room temperature of different Eu3+activated phosphors. S.No

Phosphor

Relative PL intensity at 423K

Ref

1 2 3 4 5. 6. 7. 8

Gd1.86ZnTiO6:0.14Eu3+ NaY0.7Eu0.3TiO4 NaGd0.7Eu0.3TiO4 Na3Sc2(PO4)3:0.35Eu3+ BaLa(1-y)AlO4:Euy3+ (6 mol%) Y2O2S:Eu3+ (commercial red phosphor) Y2O3:Eu3+ (commercial red phosphor) Na2Mg3Zn2Si12O30:Eu3+

51.7% 72.0% 59.0% 73.4% 55.0% 75.0% 90.0% 75.0%

8 10 10 24 50 54 54 Present work

where (x, y) is the CIE chromaticity coordinate of the phosphors, (xi,yi) is the CIE chromaticity coordinate of the white illumination, and (xd, yd) presents the CIE chromaticity coordinate of the dominant wavelength. From Table 3, it is observed that CIE chromaticity coordinates are shifting towards red with increasing concentration of Eu3+ and color purity also increases with increasing Eu3+ concentration and attain optimized value 99.6% for Na2Mg3Zn2Si12O30:Eu3+(11 mol%). The Chromaticity coordinates for Na2Mg3Zn2Si12O30: Eu3+(11 mol%) is found to be x = 0.6391, y = 0.3592, which is close to National Television System Committee (NTSC) standard red color coordinate values (0.67,0.33) and is marked in chromaticity diagram as shown in Fig. 10. The CCT value is calculated by using McCamy empirical formula [50]. CCT = 449n3+3525n2–6823.3n+5520.33 Where, n =

x y

xe ye

3.6. Thermal stability Thermal stability of a phosphor is one of the key parameter to consider its practical application in the high temperature WLEDs which operate at temperature greater than 100 °C [52]. Fig. 11a represents the PL emission spectra of Na2Mg3Zn2Si12O30:Eu3+ (11 mol%) measured at different temperature ranges from 303K–453K upon excitation at 394 nm. It is evident from Fig. 11a that PL emission intensity decreases with increasing temperature due to thermal quenching effect. Fig. 11b shows the normalized PL intensity values of Na2Mg3Zn2Si12O30:Eu3+(11 mol%) at 614 nm at various temperature ranging from 303K–453K under 394 nm excitation and it is shown that intensity at 423K is 75% of its initial emission intensity at 303K. The relative PL intensity at 423K with respect to room temperature of different phosphors are shown in Table 4. To comprehend the effect of temperature on PL intensity of phosphor, an important parameter, activation energy (Ea) is calculated using Arrhenius equation as illustrated below [52–54].

(9)

is the inverse slope of the line, (x, y) are the CIE color

coordinates and chromaticity epicentre is at xe = 0.332 and ye = 0.186. For commercial lighting purpose, the cold white light of CCT values higher than 5000K is favourable and for residence appliances, warm white light (CCT value < 5000K) is used [51]. The CCT value for Na2Mg3Zn2Si12O30:Eu3+ under 394 nm is found to be 1110K and serve as potential candidate for warm WLED applications.

I (T) =

I0 1 + exp

(

Ea kT

)

Taking ln on both sides and rearranging equation (10) 8

(10)

Optical Materials 96 (2019) 109350

G.V. Kanmani, et al.

Fig. 12. The relation of ln

( ) I0 I

1 vs 1/kT of the Na2Mg3Zn2Si12O30:Eu3+(11 mol%) phosphors.

behaviour were studied and exhibit maximum luminescence intensity at 11 mol % of Eu3+ ion. The optimized Na2Mg3Zn2Si12O30 exhibits significant red emission with color coordinates (color 0.6391, 0.3592) which is close to National Television System Committee (NTSC) standard red color coordinate values (0.67,0.33). Furthermore Na2Mg3Zn2Si12O30:Eu3+(11 mol %) shows good thermal stability and maintains 75% of initial intensity (303K) at 423K. All these results demonstrate that Na2Mg3Zn2Si12O30 doped Eu3+ phosphor can find applications in red phosphor applicable to white LED applications.

Table 5 Activation energy of different Eu3+ activated phosphors. S.No

Phosphors

Activation energy (eV)

Ref

1 2 3 4 5. 6. 7.

LiGd3(MoO4)5:Eu3+ BaZrGe3O9:Eu3+ CaLa4(SiO4)3O:Eu3+ Bi4Si3O12:Eu3+ NaCaGaSi2O7: Eu3+, Li+ Y2O3:Eu3+ (commercial phosphor) Na2Mg3Zn2Si12O30:Eu3+

0.240 0.175 0.270 0.240 0.230 0.170 0.258

[55] [56] [57] [58] [59] [57] Present study

ln

I0 I

1 =

Ea + lnC kT

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

(11)

where, I0 and I (T ) represent initial emission intensity and at various temperature respectively, ΔEa is the activation energy, k is Boltzmann constant 8.629 × 10−5 eV K−1 and C is constant. The plot between 1/

kT and ln

( ) I0 I

Acknowledgements One of the author GVK gratefully acknowledges Council of Scientific & Industrial Research, New Delhi, India for CSIR-JRF Fellowship (09/ 468(0506)/2017-EMR-1) and also thanks Anna University for Anna Centenary Research Fellowship(CFR/ACRF/2017/37).

1 yields linear fit as shown in Fig. 12. The obtained

value of slope is around −0.258, thus the activation energy is estimated to be 0.258eV which is compared with the other Eu3+ activated phosphor as shown in Table 5. The relative high activation energy suggests Eu3+ activated Na2Mg3Zn2Si12O30 have high thermal stability and can serve as potential red phosphor for its application in WLEDs.

References [1] Y. Li, M. Geceviciusa, J. Qiu, Long persistent phosphors—from fundamentals to applications, Chem. Soc. Rev. 45 (2016) 2090–2136, https://doi.org/10.1039/ c5cs00582e. [2] Z. Xia, Q. Liu, Progress in discovery and structural design of color conversion phosphors for LEDs, Prog. Mater. Sci. 84 (2016) 59–117 https://doi.org/10.1016/j. pmatsci.2016.09.007. [3] G.R. Dillip, K. Mallikarjuna, S.J. Dhoble, B. Devaprasadraju, The luminescence and structural characteristics of Eu3+ doped NaSrB5O9 phosphor, J. Phys. Chem. Solids 75 (2014) 8–14 https://doi.org/10.1016/j.jpcs.2013.07.008. [4] V.R. Bandi, B.K. Grandhe, H.J. Woo, K. Jang, D.S. Shin, S.S. Yi, J.H. Jeong, Luminescence and energy transfer of Eu3+ or/and Dy3+ co-doped in Sr3AlO4F phosphors with NUV excitation for WLEDs, J.Alloys.Comp. 538 (2012) 85–90 https://doi.org/10.1016/j.jallcom.2012.05.057. [5] Y.N. Xue, F. Xiao, Q.Y. Zhang, A red-emitting Ca8MgLa(PO4)7:Ce3+,Mn+ phosphor for UV-based white LEDs application, Spectrochim. Acta 78 (2011) 1445–1448,

4. Conclusion A novel silicate based Na2Mg3Zn2Si12O30:Eu3+ phosphor doped with different mole % of Eu3+ were synthesized by solid state reaction. The XRD analysis reveals that phosphor crystallizes in hexagonal structure with a space group p6/mcc(192) and there is no significant impact on the structure of the host materials by doping of Eu3+ ions. From SEM analysis, it is found that average particle size of the red phosphor host materials is in micrometer dimensions. The photoluminescence emission spectra of red phosphor exhibit strong electric dipole transition at 614 nm, which is due to Eu3+occupying non-centrosymmetric site and it is important factor for color purity and exhibits high purity around 99.6%. The concentration quenching 9

Optical Materials 96 (2019) 109350

G.V. Kanmani, et al. https://doi.org/10.1016/j.saa.2011.01.025. [6] Y. Wang, G. Zhu, S. Xin, Q. Wang, Y. Li, Q. Wu, C. Wang, X. Wang, X. Ding, W. Geng, Recent development in rare earth doped phosphors for white light emitting diodes, J. Rare Earths 33 (2015) 1–12, https://doi.org/10.1016/S10020721(14)60375-6. [7] G. Annadurai, S.M.M. Kennedy, V. Sivakumar, Luminescence properties of a novel green emitting Ba2CaZn2Si6O17:Eu2+ phosphor for white light Emitting diodes applications, Superlattice Microstruct. 93 (2016) 57–66 https://doi.org/10.1016/j. spmi.2016.02.045. [8] S.H. Lee, Y. Cha, H. Kim, S. Lee, J.S. Yu, Luminescent properties of Eu3+-activated Gd2ZnTiO6 double perovskite red-emitting phosphors for white light-emitting diodes and field emission displays, RSC Adv. 8 (2018) 11207–11215, https://doi. org/10.1039/c8ra00700d. [9] C. He, H. Ji, Z. Huang, T. Wang, X. Zhang, Y. Liu, M. Fang, X. Wu, J. Zhang, X. Min, Red-shifted emission in Y3MgSiAl3O12:Ce3+ garnet phosphor for blue light-pumped white light-emitting diodes, J. Phys. Chem. C 122 (27) (2018) 15659–15665 https://doi.org/10.1021/acs.jpcc.8b03940. [10] N. Zhang, C. Guo, H. Jing, Photoluminescence and cathode-luminescence of Eu3+doped NaLnTiO4 (ln = Gd and Y) phosphors, RSC Adv. 3 (2013) 7495–7502, https://doi.org/10.1039/c3ra40375k. [11] O.S. Rajamohan, V. Ponnusamy, Synthesis and emission properties of Y2Si2O7:Eu3+ Phosphor, Mater. Res. Innov. (2018), https://doi.org/10.1080/14328917.2018. 1503799. [12] Z. Liang, F. Mo, X. Zhang, L. Zhou, Luminescence of the LiMgBO3:Eu3+, Bi3+ phosphor, J. Lumin. 151 (2014) 47–51 https://doi.org/10.1016/j.jlumin.2014.02. 001. [13] H. Tang, R. Yang, Y. Huang, A novel red-emitting Eu3+-doped Na2MgSiO4 phosphor with high intensity of 5D0 →7F4 transition, J. Alloy. Comp. 714 (2017) 263–269, https://doi.org/10.1016/j.jallcom.2017.04.243. [14] Fengping Ruan, Degang Deng, Ming Wu, Bowen Chen, Chengxiao Wu, Shiqing Xu Eu3+ doped self-activated Ca8ZrMg(PO4)6(SiO4) phosphor with tunableluminescence properties, Opt. Mater. 79 (2018) 247–254 https://doi.org/10.1016/j. optmat.2018.03.055. [15] Gorbenko, T. Zorenko, A.M. Kaczmarek, R. Van Deun, A. Fedorov, Yu Zorenko, Eu3+ multicenter formation and luminescent properties of Ca3Sc2Si3O12:Eu and Ca2YScMgSiO12:Eu single crystalline films, Opt. Mater. 90 (2019) 70–75 https:// doi.org/10.1016/j.optmat.2019.02.030. [16] Naoyoshi Nunotani, Yoshiki Asakawa, Mizuki watanabe & nobuhito imanaka crystal structure and photoluminescent property of Eu3+-doped K3GdSi2O7, J. Asian Ceram. Soc. 5 (3) (2017) 377–380 https://doi.org/10.1016/j.jascer.2017.07.002. [17] L.Lakshmi DeviC.K. Jayasankar, Spectroscopic investigations on high efficiency deep red-emitting Ca2SiO4:Eu3+ phosphors synthesized from agricultural waste, Ceram. Int. 44 (12) (2018) 14063–14069 https://doi.org/10.1016/j.ceramint.2018. 05.003. [18] Guoyi Dong, Jinxing Zhao, Mengdie Li, Li Guan, Li Xu, A novel red Y2MoSiO8:Eu3+ phosphor with high thermal stability for white, LEDs Ceram. Int. 45 (2019) 2653–2656 https://doi.org/10.1016/j.ceramint.2018.10.050. [19] Qina Lin, Cuili Chen, Jing Wang, Shala Bi, Yanlin Huang, Hyo Jin Seo, Luminescence properties of red-emitting Mn2+-Activated Na2Mg5Si12O30 phosphors Materials Research, Bulletin 118 (2019) 110494 https://doi.org/10.1016/j. materresbull.2019.110494. [20] N. Nguyen, J. Choisnet, Et B. Raveau, Silicates synthetiques a structure milarite, J. Solid State Chem. 34 (1980) 1–9 https://doi.org/10.1016/0022-4596(80)90395-3. [21] O.C. Gagnte, F.C. Hawthrone, Chemographic exploration of the milarite-type structure, Can. Mineral. 54 (2016) 1229–1247, https://doi.org/10.3749/canmin. 1500088. [22] J. Sivasankari, S. Sankar, S. Selvakumar, K. Sivaji, Room-temperature ferromagnetism induced by Na co-doping and K co-doping in the rare earth Eu doped ZnO nanocrystallites, J. Alloy. Comp. 615 (2014) 4–11 https://doi.org/10.1016/j. jallcom.2014.06.146. [23] R.D. Shannon, Revised effective ionic radii and systematic studies of interatomie distances in halides and chaleogenides, Acta Crystallogr. A32 (1976) 751 https:// doi.org/10.1107/S0567739476001551. [24] H. Guo, X. Huang, Y. Zeng, Synthesis and photoluminescence properties of novel highly thermal-stable red-emitting Na3Sc2(PO4)3:Eu3+ phosphors for UV-excited white-light-emitting diodes, J. Alloy. Comp. 741 (2018) 300–306 https://doi.org/ 10.1016/j.jallcom.2017.12.316. [25] V.R. Bandi, M. Jayasimhadri, J. Jeong, K. Jang, H.S. Lee, S.S. Yi, J.H. Jeong, Host sensitized novel red phosphor CaZrSi2O7 : Eu3+ for near UV and blue LED-based white LEDs, J. Phys. D Appl. Phys. 43 (2010) 395103–395110 https://doi.org/10. 1088/0022-3727/43/39/395103. [26] K. Binnemans, Interpretation of europium(III) spectra, Coord. Chem. Rev. 295 (2015) 1–45 https://doi.org/10.1016/j.ccr.2015.02.015. [27] K. KaviRasu, D. Balaji, S. Moorthy Babu, Photoluminescence properties of Eu3+:RbGd(WO4)2 red phosphors prepared by sol–gel method, J. Lumin. 170 (2016) 825–834 https://doi.org/10.1016/j.jlumin.2015.09.030. [28] V. Dubey, R. Tiwari, R.K. Tamrakar, J. Kaur, S. Dutta, S. Das, H.G. Visser, S. Som, Estimation of spectroscopic parameters and colour purity of the red-light-emitting YBa3B9O18 phosphor: judd–Ofelt approach, J. Lumin. 180 (2016) 169–176 https:// doi.org/10.1016/j.jlumin.2016.08.029. [29] K. Maheshvaran, K. Marimuthu, Concentration dependent Eu3+ doped boro-tellurite glasses—structural and optical investigations, J. Lumin. 132 (9) (2012) 2259–2267 https://doi.org/10.1016/j.jlumin.2012.04.022. [30] R. Cao, H. Xiao, F. Zhang, Z. Luo, T. Chen, W. Li, P. Liu, G. Zheng, Red-emitting phosphor Na2Ca2Nb4O13:Eu3+ for LEDs: synthesis and luminescence properties, J. Lumin. 208 (2019) 350–355 https://doi.org/10.1016/j.jlumin.2018.12.064.

[31] X. Liu, X. Wang, Preparation and luminescence properties of BaZrO3:Eu phosphor powders, Opt. Mater. 30 (4) (2007) 626–629, https://doi.org/10.1016/j.optmat. 2007.02.032. [32] J. Shena, Z. Wanga, J. Zhoua, X. Liu, W. Chen, Photoluminescence properties of NUV light excited Ba(Mg1/3Nb2/3)O3:Eu3+ red phosphor with high color purity, Ceram. Int. 45 (2019) 11844–11849, https://doi.org/10.1016/j.ceramint.2019.03. 065. [33] Warren C. Forbes, Werner H. Baur, Aijaz A. Khan, Crystal chemistry of Milarite-type minerals, Am. Mineral. 57 (1972) 463–472. [34] T.S. Sreena, P. Prabhakar Rao, K.N. Ajmal, Athira K.V. Raj, Influence of morphology on luminescence properties of xenotime-type phisphors NaYP2O7:Eu3+ synthesied via solid state and citrate gel routes, J. Mater. Sci. Mater. Electron. 29 (2018) 7458 https://doi.org/10.1007/s10854-018-8737-5. [35] U. Ramababu, Sang-Do Han, Luminescence optimization with superior asymmetric ratio(red/orange) and color purity of MBO3:Eu3+@SiO2 (M = Y, Gd and Al) nano down-conversion phosphors, RSC Adv. 3 (2013) 1368 https://doi.org/10.1039/ c2ra21304d. [36] B.-S. Tsai, Y.-H. Chang, Y.-C. Chen, Synthesis and luminescent properties of MgIn2xGaxO4:Eu3+ phosphors, Electrochem. Solid State Lett. 8 (2005) H55–H57, https:// doi.org/10.1149/1.1921128. [37] M. Janulevicius, P. Marmokas, M. Misevicius, J. Grigorjevaite, L. Mikoliunaite, S. Sakirzanovas, A. Katelnikovas, Luminescence and luminescence quenching of highly efficient Y2Mo4O15:Eu3+ phosphors and ceramics, Sci. Rep. 6 (2016) 26098 https://doi.org/10.1038/srep26098. [38] T.-C. Chien, J.-C. Yang, C.-S. Hwang, M. Yoshimura, Synthesis and photoluminescence properties of red-emitting Y6WO12:Eu3+ phosphors, J. Alloy. Comp. 676 (2016) 286–291 https://doi.org/10.1016/j.jallcom.2016.03.083. [39] Thangavel Sakthivel, G. Annadurai, R. Vijayakumar, Xiaoyong Huang, Synthesis, luminescence properties and thermal stability of Eu3+-activated Na2Y2B2O7 red phosphors excited by near-UV light for pc-WLEDs, J. Lumin. 205 (2019) 129–135 https://doi.org/10.1016/j.jlumin.2018.09.008. [40] D. Navamia, R.B. Basavaraja, S.C. Sharma, B. Daruka Prasad, H. Nagabhushana, Rapid identification of latent fingerprints, security ink and WLED applications of CaZrO3:Eu3+ fluorescent labelling agent fabricated via bio-template assisted combustion route, J. Alloy. Comp. 762 (2018) 763–779, https://doi.org/10.1016/j. jallcom.2018.05.016. [41] Jiangong Li, Huifang Yan, Fengmei Yan, Luminescence properties of a novel orange–red CaBi2B2O7:Eu3+ phosphor for near-UV pumped, W-LEDs Optik 127 (2016) 4541–4544, https://doi.org/10.1016/j.ijleo.2016.01.155. [42] G. Annadurai, Bin Li, Balaji Devakumar, Heng Guo, Liangling Sun, Xiaoyong Huang Synthesis, structural and photoluminescence properties of novel orange-red emitting Ba3Y2B6O15:Eu3+ phosphors, J. Lumin. 208 (2019) 75–81 https://doi.org/10. 1016/j.jlumin.2018.12.028. [43] Amit K. Vishwakarma, Kaushal Jha, M. Jayasimhadri, A.S. Rao, Kiwan Jang, B. Sivaiah, D. Haranath, Red light emitting BaNb2O6:Eu3+ phosphor for solid state lighting applications, J. Alloy. Comp. 622 (2015) 97–101 https://doi.org/10.1016/ j.jallcom.2014.10.016. [44] Xinguo Zhang, Fangxiang Zhou, Jinxin Shi, Menglian Gong, Sr3.5Mg0.5Si3O8Cl4: Eu2+ bluish–green-emitting phosphor for NUV-based LED, Mater. Lett. 63 (2009) 852–854, https://doi.org/10.1016/j.matlet.2009.01.024. [45] G. Blasse, Energy transfer in oxidic phosphors, Phy.Lett. A 28 (6) (1968) 444–445 https://doi.org/10.1016/0375-9601(68)90486-6. [46] G. Li, Y. Wei, Z. Li, G. Xu, Synthesis and photoluminescence of Eu3+ doped CaGd2(WO4)4 novel red phosphors for white LEDs applications, Opt. Mater. 66 (2017) 253–260, https://doi.org/10.1016/j.optmat.2017.02.018. [47] M. Dalal, V.B. Taxak, S. Chahar, J. Dalal, A. Khatkar, S.P. Khatkar, Judd-Ofelt and structural analysis of colour tunable BaY2ZnO5:Eu3+ nanocrystals for single-phased white, LEDs J.Alloy. Compd. 686 (2016) 366–374 https://doi.org/10.1016/j. jallcom.2016.06.040. [48] D.L. Dexter, J.H. Schulman, Theory of concentration quenching in inorganic phosphors, J. Chem. Phys. 22 (1954) 1063–1070 https://doi.org/10.1063/1. 1740265. [49] A. Azhagiri, V. Ponnusamy, R. Satheeshkumar, A development of new red phosphor based on europium doped as well as substituted Barium Lanthanum Aluminate (BaLaAlO4: Eu3+), Opt. Mater. 90 (2019) 127–138, https://doi.org/10.1016/j. optmat.2019.02.024. [50] C.S. McCamy, Correlated color temperature as an explicit function of chromaticity coordinates, Color Res. Appl. 17 (1992) 142–144 https://doi.org/10.1002/col. 5080170211. [51] A.K. Ambast, J. Goutam, S. Som, S.K. Sharma, Ca1-x-yDyxKyWO4: A novel near UV converting phosphor for white light emitting diode, Spectrochim. Acta A Mol. Biomol. Spectrosc. 122 (2014) 93–99 https://doi.org/10.1016/j.saa.2013.11.032. [52] B. Han, B. Liu, Y. Dai, J. Zhang, H. Shi, Alkali metal ion substitution induced luminescence enhancement of NaLaMgWO6:Eu3+ red phosphor for white lightemitting diodes, Ceram. Int. 45 (2019) 3419–3424, https://doi.org/10.1016/j. ceramint.2018.10.256. [53] S.A. Arrhenius, Über die Dissociationswärme und den Einfluß der Temperatur auf den Dissociations grad der Elektrolyte, Z. Phys. Chem. 4 (1889) 96–116 https://doi. org/10.1515/zpch-1889-0408. [54] Chien-Hao Huang, Te-Wen Kuo, Teng-Ming Chen, Thermally stable green Ba3Y (PO4)3:Ce3+,Tb3+ and red Ca3Y(AlO)3(BO3)4:Eu3+ phosphors for white-light fluorescent lamps, Opt. Express 19 (S1) (2011) A1–A6 https://doi.org/10.1364/OE. 19.0000A1. [55] D.S. Lu, X.H. Gong, Y.J. Chen, J.H. Huang, Y.F. Lin, Z.D. Luo, Y.D. Huang, Synthesis and photoluminescence characteristics of the LiGd3(MoO4)5:Eu3+ red phosphor with high color purity and brightness, Opt. Mater. Express 8 (2018) 259–269

10

Optical Materials 96 (2019) 109350

G.V. Kanmani, et al. https://doi.org/10.1364/OME.8.000259. [56] Q. Zhang, X. Wang, X. Ding, Y. Wang, A potential red-emitting phosphor BaZrGe3O9:Eu3+ for WLED and FED applications: synthesis, structure, and luminescence properties, Inorg. Chem. 56 (12) (2017) 6990–6998 https://doi.org/10. 1021/acs.inorgchem.7b00591. [57] J. Zhong, D. Chen, H. Xu, W. Zhao, J. Sun, Z. Ji, Red-emitting CaLa4(SiO4)3O:Eu3+ phosphor with superior thermal stability and high quantum efficiency for warm wLEDs, J. Alloy. Comp. 695 (2017) 311–318, https://doi.org/10.1016/j.jallcom.

2016.10.211. [58] Y. Zhang, Jiayue Xu, Qingzhi Cui, Bobo Yang, Eu3+ -doped Bi4Si3O12 red phosphor for solid state lighting: microwave synthesis, characterization, photoluminescence properties and thermal quenching mechanisms, Sci. Rep. 7 (2017) 42464, https:// doi.org/10.1038/srep42464 doi: 10.1038/srep42464 (2017). [59] K.Y. Yeh, W.R. Liu, Luminescence properties of NaCaGaSi2O7: RE, Li+ (RE = Ce3+, Eu3+ or Tb3+) phosphors for UV excitablewhite light emitting diodes, Mater. Res. Bull. 80 (2016) 127–134, https://doi.org/10.1016/j.materresbull.2016.02.022.

11