Eu3+ co-doped ZPBT glasses for epoxy free w-LEDs application

Journal of Non-Crystalline Solids xxx (xxxx) xxx

Contents lists available at ScienceDirect

Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol

Multicolor emission and energy transfer dynamics in thermally stable Dy3+/Eu3+ co-doped ZPBT glasses for epoxy free w-LEDs application Kaushal Jha a, d, Amit K Vishwakarma a, Mula Jayasimhadri a, *, Divi Haranath b, Kiwan Jang c, * a

Luminescent Materials Research Lab (LMRL), Department of Applied Physics, Delhi Technological University, Delhi 110 042, India Department of Physics, National Institute of Technology, Warangal 506004, India c Department of Physics, Changwon National University, Changwon 641 773, Republic of Korea d Department of Physics, Bhagalpur College of Engineering, Sabour, Bhagalpur-813210, India b

A R T I C L E I N F O

A B S T R A C T

Keywords: White light emitting diodes Energy transfer Thermal stability Luminescent glass

Dy3+ doped and Dy3+/Eu3+ co-doped zinc phosphate barium titanate (ZPBT) glasses were successfully synthe­ sized by melt quenching procedure. The co-doped glasses revealed emission peaks in the blue, yellow, orange, and red spectral regions. The combinations of these emission yield white light (cool, neutral, and warm) and orangish-red light. The tunable emission is attained by appropriately regulating Eu3+ concentrations and exci­ tation wavelengths. The emission and decay analysis revealed the energy transfer arising due to dipolar-dipolar interaction from Dy3+ to Eu3+. The emission intensity and the chromaticity shift observed at 423 K were 72.54 % and 7.74 ×10− 3, respectively signifying the strong thermal stability of the co-doped ZPBT glass. An epoxy-resin free device was demonstrated using 1.0 mol% Dy3+/Eu3+ co-doped glass and 385 nm n-UV LED chip. The results mentioned-above indicate that Dy3+/Eu3+ co-doped synthesized glasses are promising candidates for epoxy-free high powered w-LED applications under n-UV excitation.

1. Introduction In recent times, solid-state lighting (SSL) became an essential and feasible alternative to conventional appliances in the lighting industry. This SSL technology can save an enormous amount of electrical energy and decrease the carbon emission globally by several million metric tons annually. White-Light Emitting Diodes (w-LEDs) based on the SSL is gaining importance as futuristic lighting source as these offer unique characteristics such as high energy efficiency, longer persistence and lifetime, environmental friendliness, and low power consumption over conventional traditional lamps. Presently, w-LEDs are realized by combining single/multiple phosphors with n-UV/blue InGaN LED chips, and these are termed as phosphor-converted (pc)-w-LEDs [1–4]. The pc-w-LEDs have three critical components: the excitation source (InGaN LED chip), phosphor as the luminescent converter, and organic-resin as the encapsulant material. The highly powered InGaN LED chips are the optical excitation source that has input current in the range of 350-1000 mA. This high current creates local heat flux in the LED chips, and the junction temperature can reach 150 ºC. This high temperature leads to thermal quenching (reduction in the emission intensity of the phosphor) cracking, delamination, yellowing, and carbonization of the

organic-resin. Hence, the optical performance of pc-w-LEDs deteriorates due to the deprivation in luminous efficiency and shift in the chroma­ ticity coordinates. Therefore, thermal management is an important aspect of the manufacturing of high-power w-LEDs. Luminescent glass has excellent optical characteristics, lower production cost, a more straightforward manufacturing method, and high thermal resistance as compared with phosphors. Moreover, glass serves as both, the solution for wavelength converter and encapsulant as it provides organic-resin free assembly. Further, the scattering loss arising due to the mismatch in refractive indices of the phosphor and the organic-resin is eliminated [5–8]. Therefore, luminescent inorganic glass is the potential choice for w-LED for the current and futuristic applications. Phosphate glasses offer several exceptional properties like low melting and softening temperatures, excellent solubility for rare earth dopants, and higher transparency over the full spectral region [9, 10]. Phosphate glasses doped with rare-earth ions are used for an extensive variety of applications such as optical amplifiers, displays, greenhouses, solid-state lighting, and lasers [11–15]. The addition of zinc oxide (ZnO) to phosphate glasses offers better chemical durability, thermal stability, and makes the glass moisture resistant [16–18]. The incorporation of alkaline earth metal oxide helps in lowering the melting temperature

* Corresponding authors. E-mail addresses: [email protected] (M. Jayasimhadri), [email protected] (K. Jang). https://doi.org/10.1016/j.jnoncrysol.2020.120516 Received 14 May 2020; Received in revised form 17 October 2020; Accepted 29 October 2020 0022-3093/© 2020 Published by Elsevier B.V.

Please cite this article as: Kaushal Jha, Journal of Non-Crystalline Solids, https://doi.org/10.1016/j.jnoncrysol.2020.120516

K. Jha et al.

Journal of Non-Crystalline Solids xxx (xxxx) xxx

and enhances the mechanical strength of the glass network [19]. Further, due to the incorporation of TiO2 in the zinc phosphate glasses, chemical durability is improved [20, 21]. The rare earth oxides Dy3+ ions having 4f9 configuration give rise to strong emission bands in the yellow (4F9/2→6H15/2) and blue (4F9/2→6H13/2) regions. Therefore, by altering the intensity of yellow to blue (Y/B) emission peaks appropri­ ately, emission in the white light region can be achieved [18, 22]. Yet, Dy3+ doped material suffers from a deficiency of red emission. The lack of a red component can be compensated by doping another suitable rare-earth ion. Eu3+ is the most appropriate activator for this purpose [23]. In the present manuscript, the Dy3+ doped and Dy3+/Eu3+ co-doped quaternary zinc phosphate barium titanate (ZPBT) glasses were syn­ thesized through the melt quenching method. The photoluminescence (PL), energy transfer process, and decay studies have been discussed in detail. Besides, thermal quenching of the co-doped glass was also studied for developing these glasses as a probable candidate for the production of white LEDs under n-UV excitation.

recorded with the Edinburgh FLS920 with xenon lamp as the excitation source. The Home-made assembly, along with the ocean optics spec­ trophotometer (HR4000), was used to record the emission profile with temperature variation. For LED device preparation, 385 nm n-UV LED was purchased from FUTURELED, Berlin. The DC voltage was supplied by the Agilent E3633A DC power supply. 3. Results and discussion 3.1. Luminescence properties of Dy3+ doped and Dy3+/Eu3+ co-doped ZPBT glasses Fig. 1 (a) denotes the Photoluminescence Excitation (PLE) and emission spectra for DY04 (1 mol% Dy3+) glass. PLE (spectrum (i)) was obtained by setting emission wavelength at 575 nm. The excitation spectrum consists of numerous bands centered at 350, 362, 384, 425, 452 and 473 nm initiating from the 6H15/2 ground level to 6P7/2, (4I11/2, 6 P5/2), (4F7/2,4I13/2), 4G11/2, 4I15/2, and 4F9/2 electronic transitions of Dy3+, respectively [24, 25]. The intensity of excitation peaks at 362 and 384 nm are the most intense and almost comparable. Therefore, the emission spectra were recorded for the optimized glass (DY04) at both these excitations. The emission spectra at λex=384 nm (spectrum (ii)) and λex=362 nm (spectrum (iii)) represents two intense emission bands centered at 482 (blue), 575 nm (yellow) and a minute band at 660 nm (red) ascribed to transitions 4F9/2→6H15/2, 4F9/2→6H13/2, and 4 F9/2→6H11/2 of Dy3+, respectively. The 4F9/2→6H15/2 transition is attributed due to magnetic dipole (MD), while 4F9/2→6H13/2 is ascribed to the forced electric dipole (ED) transition. The 4F9/2→6H13/2 transition intensity was slightly more than 4F9/2→6H15/2, which indicates that the Dy3+ ions were present at the low-symmetry sites without inversion center [18]. The emission intensity observed at 575 nm (4F9/2→6H13/2) enhanced with an increment in the Dy3+ ions doping concentration up to 1 mol% and decreased after that as it is evident from Fig. 1 (b). This decrease in the emission (concentration quenching phenomenon) comes into the picture as a result of resonant energy transfer taking among Dy3+-Dy3+ ions [26, 27]. The ratio of yellow to blue (Y/B) emission intensity with Dy3+ con­ centration for both 362 and 384 nm excitations are represented in Fig. 1 (b). The Y/B emission intensity ratio for both the excitation wavelengths lies in the vicinity of unity. This Y/B ratio decreases slightly on increasing the concentration of Dy3+ ions, as the environment of Dy3+ in the ZPBT glass host differs with variation in the Dy3+ amount [28]. The CIE chromaticity coordinates calculated from emission data for DY04

2. Experimental section The glass samples with molar chemical composition (40-x)ZnO35P2O5-20BaO-5TiO2-xDy2O3 (x = 0.25, 0.50, 0.75, 1.0, and 2.0 mol%) were termed DY01, DY02, DY03, DY04, and DY05, respectively. Another glass series with molar composition (39-y)ZnO-35P2O5-20BaO5TiO2-1Dy2O3-yEu2O3 (where y=0.25, 0.50, 1.0 and 2.0 mol%) and the samples were named as DE01, DE02, DE03, and DE04, respectively. The glass with molar composition 39ZnO-35P2O5-20BaO-5TiO2-1Eu2O3 was named EU01. The precursors utilized were analytical reagent (A.R) grade NH4H2PO4, zinc oxide (ZnO), titanium dioxide (TiO2), barium carbonate (BaCO3) and highly pure (99.99 %) Dy2O3 and Eu2O3. The stoichiometry amount of chemicals were weighed and ground using agate mortar pestle for 1 hour to get a uniform mixture. The uncertainty in the weight measurement of the precursors were ±0.1 mg. The uni­ formly mixed chemical was kept in an alumina crucible and the placed inside the programmable muffle furnace at 1150◦ C for 1 hour to obtain the desired melt of the mixture. The molten liquid was transferred on the brass plate and was preseed with an additional brass plate and kept at 400◦ C for 3 hours to eliminate the stress from the glass. The excitation and emission properties were performed with Shi­ madzu RF-5301PC spectrofluorophotometer having xenon flash lamp as the source of excitation. The uncertainty in the measurement was ±0.2 nm and the results of this applies from Fig. 1–5. The decay profiles were

Fig. 1. (a) The excitation spectrum under 575 nm emission and emission spectra under 362 and 384 nm excitations and (b) Yellow to blue intensity ratio and integrated emission intensity variation of 575 nm under 362 and 384 nm excitations for DY04 glass. 2

K. Jha et al.

Journal of Non-Crystalline Solids xxx (xxxx) xxx

Fig. 2. The excitation spectra for DE03 glasses under 575 and 613 nm emissions (Inset represents excitation spectra in the 350-400 nm and the excitation wave­ lengths are marked for emission analysis).

Fig. 3. The spectral overlap of donor (Dy3+) emission with acceptor (Eu3+) absorption.

particular intense emissions of Dy3+ (λem= 575 nm) and Eu3+ (λem= 613 nm) which are represented in Fig. 2. In the case of 1.0 mol% Eu3+/Dy3+ (λem= 613 nm), two peaks were observed due to Dy3+ at 350 and 452 nm corresponding 6H15/2 → 6P7/2 and 6H15/2 → 4I15/2 transitions, along with characteristic peaks of Eu3+. The excitation peaks observed at 350 nm and 452 nm may have arisen due to the transfer of energy from Dy3+ to Eu3+ ions [29]. The characteristic excitation peaks of Eu3+ were observed at 360, 380, 392, and 462 nm corresponding to the 7F0→5D4, 7 F0→5L7, 7F0→5L6, and 7F0→5D2 transitions, respectively [30]. The PLE spectra of DE03 glasses in the n-UV range (350- 400 nm) is represented in the inset of Fig. 2. One of the essential conditions for energy transfer

glass at 362 and 384 nm excitations were found to be (0.298, 0.387) and (0.296, 0.386), respectively. The corresponding values of Correlated Color Temperature (CCT) were 6788, and 6877 K. The obtained values of CCY suggest emission of white light. However, the CCT value should be below 5000 K for white-light emitting applications. Furthermore, a feeble red band was observed in Dy3+ doped glasses. Therefore, opti­ mized Dy3+ doped ZPBT glasses were co-doped with different concen­ trations of Eu3+to enhance the red component, which is a pre-requisite for w-LEDs applications. The PLE spectra of DE03 (1.0 mol% Dy3+ and 1 mol% Eu3+) glass were recorded by monitoring wavelengths corresponding to the 3

K. Jha et al.

Journal of Non-Crystalline Solids xxx (xxxx) xxx

Fig. 4. The emission spectra for (a) DY04, DE01, DE02, DE03, and DE04 glasses under 384 nm excitation and (b) DE01, DE02, DE03, and DE04 glasses 462 nm excitation.

emission peaks observed at 536, 590, 613, 655, and 700 nm are ascribed to the 5D1→7F1, 5D0→7F1, 5D0→7F2, 5D0→7F3, and 5D0→7F4, respectively are distinctive transitions of the Eu3+. The forced electric dipole (ED) transition 5D0→7F2 is hypersensitiveand is strongly influenced due to the crystal field of the ligand atoms. However, the magnetic dipole (MD) transition 5D0→7F1 is insensitive and is not influenced by the crystal environment. The red to orange emission intensity ratio (5D0→7F2/5D0→7F1) is termed as asymmetric ratio and is a measure to quantify the site symmetry of rare-earth ions [31, 32]. In the prepared glasses, the asymmetric ratio was found to be greater than unity, which suggests that Eu3+ are at low symmetry sites with greater Eu3+ - O2− covalency in the glass system. It is apparent from Fig. 4 (a) that the emission intensity of Dy3+ falls, while that of the Eu3+ increases with enhancement in the concentration of Eu3+. The decrement in the emission intensities of Dy3+ with an increment in Eu3+ concentration arises owing to the non-radiative energy transfer from Dy3+ to Eu3+. A similar trend in the emission spectra was observed under 380 nm, 389 nm, and 392 nm excitations, although not shown in the manuscript. Fig. 4 (b) represents the emission spectra of DE01, DE02, DE03, and DE04 glasses under 462 nm. The emission spectra indicate an additional transition at 578 nm corresponding to the 5D0→7F0 transition, which was not observed in Fig. 4 (a) as the 4F9/2→6H13/2 transition of Dy3+ overlapped with the 5D0→7F0 transition of Eu3+. As per the Judd-Ofelt theory, the 5D0→7F0 transition at 578 nm is strictly forbidden. The ex­ istence of this band comes into picture due to parity mixing (d-f) or J-J mixing at the excited (5DJ=0, 1, and 3) or ground state (7FJ=0, 1, 2, 3, and 4). The parity mixing and J-J mixing at the excited state is ruled out as there is a substantial energy gap between them. Therefore, the possibility of J-J mixing at the ground state only exists. Moreover, the magnetic dipole transitions (5D0→7F1 and 7F3) are independent of the local crystal field strength and are out of the picture. The only possible reason is the mixing of J=2, and 4 with J=0, but the higher energy level of J=4 and 6 from J=0 and their emission intensities are weak as compared with J=2. So, the existence of the 5D0→7F0 transition is due to the J-J mixing at the ground state between J=2 and J=0 [31, 33]. From Fig. 4 (b), it is noted, emission due to Dy3+ is not detected due to the lack of energy transfer from Eu3+ to Dy3+. This is because 462 nm excitation is specific for the peak belonging to Eu3+. Another important consideration is that the 5 D1→7F1 transition at 536 nm is not observed when Dy3+/Eu3+co-doped glasses were excited at 462 nm, suggesting that energy is transferred from Dy3+ to Eu3+. To understand the variation in the emission property

Fig. 5. The emission spectra for DE03 glasses under 380, 384, 389, and 392 nm excitations (Inset represents the emission spectra in the 520-550 nm spec­ tral region).

to arise is the overlap of the donor/sensitizer (Dy3+) emission and activator/acceptor (Eu3+) absorption spectra. The emission band, 4 F9/2→6H15/2 of Dy3+, overlaps the absorption band 7F0→5D2 of Eu3+, as represented in Fig. 3. As presented in Fig. 2 inset, the emission spectra for Dy3+ doped and Dy3+/Eu3+ co-doped ZPBT glasses should be observed under various wavelengths (380 nm, 384 nm, 389 nm, and 392 nm). The emission spectra were recorded at different wavelengths, but in the manuscript were shown at only two wavelengths (384 and 462 nm), as 384, 389, and 392 nm falls in the n-UV region. Fig. 4 (a) represents the emission spectra under 384 nm excitation for fixed Dy3+ (1 mol%) and for various Eu3+ (0.0, 0.25, 0.5, 1.0, and 2.0 mol%) concentrations i.e. for DY04, DE01, DE02, DE03, and DE04 glasses. For DY04 glass, two strong emission bands centered at 482 nm and 575 nm plus a weak band centered at 660 nm corresponding to 4F9/ 6 4 6 4 6 3+ were 2→ H15/2, F9/2→ H13/2 and F9/2→ H11/2 transitions of Dy observed, respectively as discussed earlier. While, for DE01, DE02, DE03, and DE04 glasses, five other emissions were also observed. The 4

K. Jha et al.

Journal of Non-Crystalline Solids xxx (xxxx) xxx

with variable excitation wavelengths, the emission spectra for the DE03 glass sample were detected at different wavelengths in the n-UV region, as depicted in Fig. 5. It is easily noted that there is a significant change in the emission bands intensity with the variation in excitation wave­ length. The emission bands of Dy3+ has minimum intensity at 392 nm excitation, while that of Eu3+ is the most intense. Also, it is depicted in Fig. 5 inset that the emission centered at 536 nm is most prominent at 392 nm excitation. The variation in the intensity of the emission bands at different excitation wavelengths may be due to competition between Dy3+ and Eu3+ to accept the excitation energy for a particular wave­ length [34]. Thus, from emission spectra analysis, it may be stated that white light for lighting and display can be obtained by combining the blue (482 nm), yellow (575 nm) emission of the Dy3+ and the orange (590 nm) and red emission (613 nm) of the Eu3+ in Eu3+/Dy3+ co-doped ZPBT glasses by varying excitation wavelengths and Eu3+concentration. However, the colorimetric study needs to be performed to confirm the warm white light emission, which will be discussed in the succeeding sections. The energy transfer process captivating in Dy3+/Eu3+ co-doped ZPBT glasses is explained in Fig. 6. The energy level diagram em­ bodies the different energy levels of Dy3+ and Eu3+ in the present glass system and reveals the channel for the energy transfer process in Dy3+/ Eu3+ co-doped glass system. The energy corresponding to 4F9/2 state/ level (~21056 cm− 1) of Dy3+ is marginally more significant than that of 5 D1 (19028 cm− 1) and 5D0 levels (~17277 cm− 1) of Eu3+. The variation in the energy between 4F9/2 and 5D1 is 2028 cm− 1 and that between 4F9/ 5 − 1 2 and D0 is 3779 cm . However, the phonon energy of the barium phosphate glass is near to 1100 cm− 1, which makes the phonon aided the non-radiative transition from 4F9/2 level to the 5D1 and 5D0 level. This result recommends that Dy3+ acts as an effective sensitizer for the Eu3+ [29, 34–36]. The energy transfer mechanism arises either via exchange or the multipolar interaction from Sensitizer (Dy3+) to the activator (Eu3+). The multipolar interaction is further classified into dipolar-dipolar, dipolar-quadrupolar, and quadrupolar-quadrupolar interaction. The nature of the interaction is identified through Dexter’s Energy Transfer (ET) formula along with the Reisfeld’s approximation as per the relation

given below [37]:

η0 n ∝ C3 η

(1)

where η0 and η are the luminescent quantum efficiencies of the sensitizer (Dy3+) without and with (Eu3+) respectively, C is a summation of the mole percent of sensitizer (Dy3+) and activator (Eu3+), and the nature of the multipole interaction is referred by n, which take the value of 6, 8, and 10 for dipolar-dipolar, dipolar-quadrupolar, and quadrupolarquadrupolar interactions, respectively. The ratio of η0/η is approxi­ mately estimated as per the relation given below: Iso ∝ Cn/3 Is

(2)

where Iso and Is are the emission intensities of Dy3+ without and along with Eu3+. Fig. 7 denotes the plot for the Iso/Is versus Cn/3 for the value of n=6, 8, and 10. The most accurate fitting achieved for n=6 establishes the nature of interaction from Dy3+ to Eu3+ because of non-radiative dipolar-dipolar interaction. From the emission data, CIE chromaticity coordinates were evalu­ ated for DE03 glass under different excitations of 380, 384, 389, and 392 nm as signified in Fig. 8 (a). The chromaticity coordinates were (0.392, 0.378), (0.365, 0.381), (0.423, 0.377), and (0.528, 0.366) and the cor­ responding values of the CCT were 3720, 4465, 3027, and 1759 K. The values of CCT were found to be lower than 5000 K for every excitation wavelength. The CIE chromaticity values for DE01, DE02, DE03, and DE04 glasses were also evaluated under 384 nm, and all the values are in the white light region, as represented in Fig. 8 (b). It can be concluded that the color tone for the Dy3+/Eu3+ co-doped glasses is easily modu­ lated in the white light region (cool, neutral, and warm white light) and orangish-red region with the variation in Eu3+ concentrations and excitation wavelengths. The value of chromaticity coordinates for DE03 glass under 384 nm is very close to the typical white light point (0.333, 0.333), with emission falling in the warm white light region. Hence, white light with different CCTs can be achieved in Dy3+/Eu3+ co-doped glasses, which makes it an appropriate candidate for w-LEDs.

Fig. 6. The Energy level diagram showing energy transfer involved in Dy3+/Eu3+ co-doped ZPBT glasses. 5

K. Jha et al.

Journal of Non-Crystalline Solids xxx (xxxx) xxx

activator is estimated as per the equation given below [39]: ( ) τd ηT(Dy→Eu) = 1 −

τd0

(5)

where τdo and τd represent the decay time of donor in acceptor absence and presence, respectively. The probability rate of the energy transfer in lifetime aspect is estimated by the equation given below [34]: PT(Dy→Eu) =

1

τd

−

1

(6)

τd0

4 5 Fig. 7. The dependence Iso/Is Dy of versus of3+C6/3 x10 C8/3 ,x10 and , Dy+Eu Dy+Eu 6 3+ 3+ co-doped glasses under 384 nm excitation. C10/3 Dy+Eu x10 for the Dy /Eu

The values for the energy transfer efficiencies (ηT) and probabilities (PT) are given in Table 1. The values of both ηT and PT increase with the increase in acceptor concentration. Fig. 9 (c) represents the effect on the average lifetime for the 4F9/2 level and energy transfer efficiency with variation in the Eu3+concentration for Dy3+/Eu3+ co-doped ZPBT glass system. The Inokuti- Hirayama (I-H) model is very useful in determining the nature of multipolar interaction. The I-H model was applied to the decay profile for DE03 glass under 384 nm excitation The luminescence decay after pulsed excitation between donor (Dy3+) - acceptor (Eu3+) is given by the equation shown below [40, 41]:

3.2. Decay measurements of Dy3+/Eu3+ co-doped ZPBT glasses

{ ( )3s } − t t It = I0 exp − Q

τo

Fig. 9 (a) represents decay profiles for DY04, DE01, DE02, DE03, and DE04 glasses by fixing emission wavelength at 575 nm (4F9/2 level) under 384 nm excitation wavelength. The decay profiles were best tailored with the bi-exponential equation as [38] ( ( ) ) t t I = Io + A1 exp − + A2 exp − (3)

τ1

τ2

where I and Io represent the emission intensities at a certain time t and 0,

are the constant related to fitting. The average lifetimes for the different ZPBT glasses were evaluated by the formula [38] and presented in Table 1.

τavg

(7)

where It is the emission intensity at a particular time t, t represents the time after excitation, τo signifies the decay time of the donors (Dy3+) without acceptors (Eu3+), Q denotes energy transfer parameter, and S is the parameter of multipolar interaction having the value of 6, 8 and 10 for dipolar-dipolar, dipolar-quadrupolar, and quadrupolar-quadrupolar interactions, respectively. From Fig. 9 (b), it is clear that S=5.78 fits the decay curve for DE03 glass, which is close to 6, indicating dipolardipolar interaction. The results attained from both the I-H model and Dexter ET formula and Reisfeld’s approximation confirms the dipoledipole interaction exists between Dy3+ and Eu3+.

τ1 and τ2 represent exponential components of the lifetimes, A1 and A2

A1 τ21 + A2 τ22 = A1 τ 1 + A2 τ 2

τo

3.3. Thermal quenching and organic-resin free device based on Dy3+/ Eu3+ co-doped ZPBT glass

(4)

It is evident from Fig. 9 (c) that the values of the average lifetime of the donor (Dy3+) for 4F9/2 level reduces with increment in the activator (Eu3+) concentration confirming energy transfer takes place from Dy3+ to Eu3+. The value of energy transfer efficiency (ηT) from the donor to

Thermal quenching is a significant characteristic of luminescent materials that have practical application in w-LEDs. Fig. 10 (a) repre­ sents the variation on the emission spectra with temperature for DE03 glass under the excitation of 384 nm. The emission intensity reduces

Fig. 8. The CIE diagram representing chromaticity coordinates for (a) DE03 glass under 380, 384, 389, and 392 nm excitations and (b) DE01, DE02, DE03, DE04 glasses under 384 nm excitation. The error estimate in the CIE calculation is based on the wavelength shift of ±0.2 nm. 6

K. Jha et al.

Journal of Non-Crystalline Solids xxx (xxxx) xxx

Fig. 9. (a) Decay profiles of DY04, DE01, DE02, DE03, and DE04 fitted with bi-exponential equations for 4F9/2 level, (b) DE03 glass fitted with I-H model under 384 nm excitation, and (c) average lifetime and energy transfer efficiency variation with Eu3+ concentration in Dy3+/Eu3+ co-doped ZPBT glasses. The measurement of the lifetime is result from both the instrumental error and fitting estimation, the error estimate is not more than ±10 %.

with increment in the temperature from 298 to 498K. The relative in­ tegrated emission intensity variation in percentage with temperature is represented in Fig. 10 (b). The emission intensity of the DE03 glass re­ mains around 85.83 % and 72.54 % at 373 K and 423 K, respectively. The activation energy (Ea) was calculated for the thermal quenching using the below expression:

Table 1 Average lifetime (τavg), Energy transfer efficiency (ηT) and Energy transfer probability rate (PT) for DY04, DE01, DE02, DE03, and DE04 ZPBT glasses under 384 nm excitation. ZPBT glass samples (Dy3+, Eu3+)

Sample name

(1.0, 0.0) (1.0, 0.25) (1.0, 0.50) (1.0, 1.0) (1.0, 2.0)

DY04 DE01 DE02 DE03 DE04

Average lifetime (τavg) (ms) For 4F9/2 level

Energy transfer efficiency % (ηT)

Energy transfer probability rate (x 103 s− 1) (PT)

0.620 0.582 0.525 0.447 0.372

0.000 6.43 15.59 28.13 40.19

0.000 0.110 0.297 0.629 1.080

IT =

Io ( 1 + Aexp −

) Ea KB T

(8)

where Io and IT are the emissions at initial and emission at a particular temperature, Ea is the activation energy of thermal quenching, KB (8.67 × 10-5 eV/K) is the Boltzmann constant and A is constant. The graph between ln(Io/IT-1) and 1/KBT is plotted in Fig. 11 (a), the slope of this graph gives the value of activation energy (Ea). The value of the acti­ vation energy for thermal quenching was found to be 0.254 eV, which is higher than that of the SLSAKP: Ce3+, Tb3+, Mn2+and SLSAKP:

The error estimate is ±10 % on the lifetime measurement, so the same per­ centage of errors apply for estimation of energy transfer efficiencies and prob­ ability rate.

Fig. 10. (a) Emission spectra of DE03 glass with various temperature from 298-498 K and (b) Integrated emission intensity variation in the temperature range 298498 K. The magnitude of error estimate due to experimental uncertainty is ±5.0 %. 7

K. Jha et al.

Journal of Non-Crystalline Solids xxx (xxxx) xxx

Fig. 11. (a) Plot of ln(Io/IT-1) and 1/KBT for the DE03 glass (b) variation in CIE chromaticity coordinates at different temperatures (298, 398, and 498 K).

Eu2+glasses [5, 42]. The effect of temperature on the emission profiles leads to a shift in the chromaticity coordinates. The chromaticity shift (∆E) was evaluated using the equation given below: ( ′ ( ′ ( ′ ′ )2 ′ )2 ′ )2 ΔE = √ ut − u0 + vt − v0 + wt − w0

(9)

where u , v and w are the chromaticity coordinates in the u v uniform ′ ′ color space. The value of u = 4x/(3-2x+12y),v = 4x/(3-2x+12y) and ′ ′ ′ w = 1-u -v , xand y are the chromaticity coordinates of 1931 color space. The evaluated values of the chromaticity coordinates at temperatures 298 K, 398 K, and 498 K are represented in Fig. 11 (b). The values of chromaticity shift for DE03 glass at 373 K, 423 K, and 498 K were 3.11 X 10-3, 7.74 X 10-3, and 13.8 X 10-3. A very minute chromaticity shift was observed, which is due to the similar effect of the temperatures on each emission peak. The chromaticity shift values are smaller than that re­ ported for SLSAKP: Ce3+, Tb3+, Mn2+ glass, and is comparable to SLSAKP:Eu2+ [5, 42]. A device was prepared to demonstrate the prac­ tical applicability of Dy3+/Eu3+ co-doped ZPBT glass by appending the glass on a 385 nm n-UV LED chip. The emission profile for the DE03 glass sample at V=3.5 V and I= 120 mA along with the EL (electrolu­ minescent) spectrum of n-UV LED chip is represented in Fig. 12. The emission spectrum revealed peaks due to both the Eu3+ and Dy3+, as discussed previously. The inset of Fig. 12 represents the photograph of the device under V=0 and 3.5 V, which clearly indicates the white light emission under 3.5 V. Hence, the above result suggests its strong po­ tential applicability for the w-LED applications.

encapsulating material for the preparation of epoxy-free w-LEDs under n-UV excitation.

4. Conclusions

Authorship contributions

In conclusion, the emission properties of Dy3+ doped and Dy3+/Eu3+ co-doped ZPBT transparent glasses synthesized by the melt quenching method were investigated for the first time. The emission band falls in the yellow, blue, orange, and red spectral region for the co-doped glasses under n-UV excitation, and the combination of these bands finally emit warm white light. The value of CIE chromaticity coordinates (0.365, 0.381) was close to the standard white light point with 4465 K as the CCT value. The energy transfer was due to electric dipolar-dipolar interaction from Dy3+ to Eu3+. The Dy3+/Eu3+ co-doped glasses show strong thermal stability and inconspicuous chromaticity shift. A prac­ tical epoxy-free device was demonstrated by appending the luminescent ZPBT glass on a 385 nm n-UV LED chip. These results properties enable the ZPBT glasses to serve as both the luminescent converter and

Conception and design of study: Dr. Kaushal Jha, Dr. Amit K Vish­ wakarma, Dr. M. Jayasimhadri, Dr. Divi Haranath Acquisition of data: analysis and/or interpretation of data: Kaushal Jha, Amit K Vishwakarma, Dr. Divi Haranath Drafting the manuscript: revising the manuscript critically for important intellectual content: Dr. Kaushal Jha, Dr. M. Jayasimahdri, Prof. Kiwan Jang

′

′

′

′

′

Fig. 12. Emission spectrum of 1 mol% Dy3+ and 1 mol% Eu3+ co-doped ZPBT glasses under 385 nm n-UV LED chip driven at 3.5 volt (Inset represents the photographs at V=0 and V= 3.5 volt). The error estimate in the wavelength measurement is ±0.2 nm.

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. 8

K. Jha et al.

Journal of Non-Crystalline Solids xxx (xxxx) xxx

References [1] M. Jiao, N. Guo, W. Lü, Y. Jia, W. Lv, Q. Zhao, B. Shao, H. You, Tunable blue-greenemitting Ba3LaNa(PO4)3F:Eu2+,Tb3+ phosphor with energy transfer for near-UV White LEDs, Inorg. Chem. 52 (2013) 10340–10346. [2] X. Wang, Z. Zhao, Q. Wu, Y. Li, Y. Wang, A garnet-based Ca2YZr2Al3O12 :Eu3+ red-emitting phosphor for n-uv light emitting diodes and field emission displays: electronic structure and luminescence properties, Inorg. Chem. 55 (2016) 11072–11077. [3] K. Jha, M. Jayasimhadri, Spectroscopic investigation on thermally stable Dy3+ doped zinc phosphate glasses for white light emitting diodes, J. Alloys Compd. 688 (2016) 833–840. [4] K. Jha, M. Jayasimhadri, Structural and emission properties of Eu3+ -doped alkaline earth zinc-phosphate glasses for white LED applications, J. Am. Ceram. Soc. 100 (2017) 1402–1411. [5] X. Zhang, L. Huang, F. Pan, M. Wu, J. Wang, Y. Chen, Q. Su, Highly thermally stable single-component white-emitting silicate glass for organic-resin-free whitelight-emitting diodes, ACS Appl. Mater. Interfaces. 6 (2014) 2709–2717. [6] H. Masai, Y. Yamada, Y. Suzuki, K. Teramura, Y. Kanemitsu, T. Yoko, Narrow energy gap between triplet and singlet excited states of Sn2+ in borate glass, Sci. Rep. 3 (2013) 3541. [7] X.Y. Liu, H. Guo, Y. Liu, S. Ye, M.Y. Peng, Q.Y. Zhang, M. Peng, M. Wu, J. Ballato, R.S. Liu, X. Chen, Thermal quenching and energy transfer in novel Bi3+/Mn2+ codoped white-emitting borosilicate glasses for UV LEDs, J. Mater. Chem. C. 4 (2016) 2506–2512. [8] K. Jha, A.K. Vishwakarma, M. Jayasimhadri, D. Haranath, Multicolor and white light emitting Tb3+/Sm3+ co-doped zinc phosphate barium titanate glasses via energy transfer for optoelectronic device applications, J. Alloys Compd. 719 (2017) 116–124. [9] C. Ming, F. Song, L. An, X. Ren, Turning ultraviolet-green into red light in transparent phosphate glasses for greenhouses, Appl. Phys. Lett. 102 (2013) 20–23. [10] R.O. Omrani, S. Krimi, J.J. Videau, I. Khattech, A.El Jazouli, M. Jemal, Structural and thermochemical study of Na2O–ZnO–P2O5 glasses, J. Non. Cryst. Solids. 390 (2014) 5–12. [11] S. Jiang, T. Luo, B.C. Hwang, F. Smekatala, K. Seneschal, J. Lucas, N. Peyghambarian, Er3+-doped phosphate glasses for fiber amplifiers with high gain per unit length, J. Non. Cryst. Solids. 263 (2000) 364–368. [12] Y.C. Yan, A.J. Faber, H. De Waal, P.G. Kik, A. Polman, Erbium-doped phosphate glass waveguide on silicon with 4.1 dB/cm gain at 1.535 μm, Appl. Phys. Lett. 71 (1997) 2922. [13] H. Segawa, N. Hirosaki, S. Ohki, K. Deguchi, T. Shimizu, Exploration of metaphosphate glasses dispersed with Eu-doped SiAlON for white LED applications, Opt. Mater. (Amst). 42 (2015) 399–405. [14] J.H. Campbell, T.I. Suratwala, Nd-doped phosphate glasses for high-energy / highpeak-power lasers, J. Non. Cryst. Solids. 264 (2000) 318–341. [15] C. Ming, F. Song, L. An, X. Ren, Research phosphate glass on turning sunlight into red light for glass greenhouse, Mater. Lett. 137 (2014) 117–119. [16] K. Jha, M. Jayasimhadri, Effective sensitization of Eu3+ and energy transfer in Sm3+/Eu3+ co-doped ZPBT glasses for CuPc based solar cell and w-LED applications, J. Lumin. 194 (2018) 102–107. [17] N. Deopa, A.S. Rao, S. Mahamuda, M. Gupta, M. Jayasimhadri, D. Haranath, G. V. Prakash, Spectroscopic studies of Pr3+ doped lithium lead alumino borate glasses for visible reddish orange luminescent device applications, J. Alloys Compd. 708 (2017) 911–921. [18] M. Jayasimhadri, K. Jang, H.S. Lee, B.J. Chen, S.S. Yi, J.H. Jeong, White light generation from Dy3+-doped ZnO-B2O3-P2O5 glasses, J. Appl. Phys. 106 (2009) 1–4. [19] D.V.R. Murthy, A.M. Babu, B.C. Jamalaiah, L.R. Moorthy, M. Jayasimhadri, K. Jang, H.S. Lee, S.S. Yi, J.H. Jeong, Photoluminescence properties of Er3+-doped alkaline earth titanium phosphate glasses, J. Alloys Compd. 491 (2010) 349–353. [20] D.V.R. Murthy, A.M. Babu, B.C. Jamalaiah, L.R. Moorthy, M. Jayasimhadri, K. Jang, H.S. Lee, S.S. Yi, J.H. Jeong, Photoluminescence properties of Er3+-doped alkaline earth titanium phosphate glasses, J. Alloys Compd. 491 (2010) 349–353. [21] A. Kiani, JV Hanna, S.P. King, G.J. Rees, M.E. Smith, N. Roohpour, V. Salih, J. C. Knowles, Structural characterization and physical properties of P2O5-CaO-

[22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]

[33] [34] [35] [36] [37]

[38] [39] [40] [41] [42]

9

Na2O-TiO2 glasses by Fourier transform infrared, Raman and solid-state magic angle spinning nuclear magnetic resonance spectroscopies, Acta Biomater 8 (2012) 333–340. J. Kuang, Y. Liu, J. Zhang, White-light-emitting long-lasting phosphorescence in Dy3+-doped SrSiO3, J. Solid State Chem. 179 (2006) 266–269. G. Gao, L. Wondraczek, Heavily Eu3+-doped boroaluminosilicate glasses for UV/ blue-to-red photoconversion with high quantum yield, J. Mater. Chem. C. 2 (2014) 691. ˙ W.A. Pisarski, L. Zur, T. Goryczka, M. Sołtys, J. Pisarska, Structure and spectroscopy of rare earth – doped lead phosphate glasses, J. Alloys Compd. 587 (2014) 90–98. X.-Y. Sun, S. Wu, X. Liu, P. Gao, S.-M. Huang, Intensive white light emission from Dy3+-doped Li2B4O7 glasses, J. Non. Cryst. Solids. 368 (2013) 51–54. J. Pisarska, L. Zur, W.A. Pisarski, Structural and optical characterization of Dydoped heavy-metal oxide and oxyhalide borate glasses, Phys. Status Solidi Appl. Mater. Sci. 209 (2012) 1134–1140. K. Linganna, C.S. Rao, C.K. Jayasankar, Optical properties and generation of white light in Dy3+-doped lead phosphate glasses, J. Quant. Spectrosc. Radiat. Transf. 118 (2013) 40–48. L. Mishra, A. Sharma, A.K. Vishwakarma, K. Jha, M. Jayasimhadri, B.V. Ratnam, K. Jang, A.S. Rao, R.K. Sinha, White light emission and color tunability of dysprosium doped barium silicate glasses, J. Lumin. 169 (2016) 121–127. Y. Deng, S. Yi, J. Huang, J. Xian, W. Zhao, White light emission and energy transfer in Dy3+/Eu3+ co-doped BaLa2WO7 phosphors, Mater. Res. Bull. 57 (2014) 85–90. G. Gao, S. Reibstein, M. Peng, L. Wondraczek, Tunable dual-mode photoluminescence from nanocrystalline Eu-doped Li2ZnSiO4 glass ceramic phosphors, J. Mater. Chem. 21 (2011) 3156. H. Wen, G. Jia, C.-K. Duan, P.A. Tanner, Understanding Eu3+ emission spectra in glass, Phys. Chem. Chem. Phys. 12 (2010) 9933–9937. B.V. Ratnam, M.K. Sahu, A.K. Vishwakarma, K. Jha, H-J. Woo, K. Jang, M. Jayasimhadri, Optimization of synthesis technique and luminescent properties in Eu3+-activated NaCaPO4 phosphor for solid state lighting applications, J. Lumin. 185 (2017) 99–105. A.K. Parchur, R.S. Ningthoujam, Behaviour of electric and magnetic dipole transitions of Eu3+, 5D0→7F0 and Eu–O charge transfer band in Li+ co-doped YPO4:Eu3+, RSC Adv 2 (2012) 10859. D. Rajesh, K. Brahmachary, Y.C. Ratnakaram, N. Kiran, A.P. Baker, G.-G. Wang, Energy transfer based emission analysis of Dy3+/Eu3+ co-doped ZANP glasses for white LED applications, J. Alloys Compd. 646 (2015) 1096–1103. W. Xu, Q. Peng, J. Dong, Y. Gao, X. Liang, S. Wang, G. Chen, Photoluminescence properties and energy transfer of Dy3+-Eu3+ codoped phosphate glasses, J. Am. Ceram. Soc. 93 (2010) 3064–3067. J.-L. Cia, R.-Y. Yi, C.-J. Zhao, S.-T. Tie, Ju.-Y. Shen, White light emission and energy transfer in Dy3+/Eu3+ co-doped aluminoborate glass, Mater. Res. Bull. 57 (2014) 85–90. G. Li, D. Geng, M. Shang, C. Peng, Z. Cheng, J. Lin, Tunable luminescence of Ce3+/ Mn2+-coactivated Ca2Gd8(SiO4)6O2 through energy transfer and modulation of excitation: potential single-phase white/yellow-emitting phosphors, J. Mater. Chem. 21 (2011) 13334. K. Li, X. Liu, Y. Zhang, X. Li, H. Lian, J. Lin, Host-sensitized luminescence properties in CaNb2O6 :Ln3+ (Ln3+ = Eu3+/Tb3+/Dy3+/Sm3+) phosphors with abundant colors, Inorg. Chem. 54 (2015) 323–333. W.J. Yang, L. Luo, T.M. Chen, N.S. Wang, Luminescence and energy transfer of Euand Mn-coactivated CaAl2Si2O8 as a potential phosphor for white-light UVLED, Chem. Mater. 17 (2005) 3883–3888. S.N Rasool, L.R Moorthy, C.K. Jayasankar, Spectroscopic Investigation of Sm3+ doped phosphate based glasses for reddish-orange emission, Opt. Commun. 311 (2013) 156–162. M. Inokuti, F. Hirayama, Influence of energy transfer by the exchange mechanism on donor luminescence, J. Chem. Phys. 43 (1965) 1978. X. Zhang, J. Wang, L. Huang, F. Pan, Y. Chen, B. Lei, M. Peng, M. Wu, Tunable luminescent properties and concentration-dependent, site-preferable distribution of Eu2+ ions in silicate glass for white LEDs applications, ACS Appl. Mater. Interfaces. 7 (2015) 10044–10054.