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Supplementary information

A zero thermal quenching phosphor

Yoon Hwa Kim1, Paulraj Arunkumar1, Bo Young Kim1, Sanjith Unithrattil1, Eden Kim1,

Su-Hyun Moon1, Jae Young Hyun1,2, Ki Hyun Kim2,3, Donghwa Lee1, Jong-Sook Lee1, and

Won Bin Im1,*

1School of Materials Science and Engineering and Optoelectronics Convergence Research

Center, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju, 61186, Republic of

Korea.

2Lighting Sources and Materials Team, Korea Photonics Technology Institute (KOPTI), 9

Cheomdan venture-ro 108beon-gil, Buk-gu, Gwangju, 61007, Republic of Korea.

3Present address: Medical Photonics Research Center, Korea Photonics Technology Institute

(KOPTI), 9 Cheomdan venture-ro 108beon-gil, Buk-gu, Gwangju, 61007, Republic of Korea.

A zero-thermal-quenching phosphor

© 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.

SUPPLEMENTARY INFORMATIONDOI: 10.1038/NMAT4843

NATURE MATERIALS | www.nature.com/naturematerials 1

http://dx.doi.org/10.1038/nmat4843

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Table S1. Rietveld refinement parameters of combined X-ray and neutron diffraction profiles of Na2.94Eu0.03Sc2(PO4)3 (NSPO:0.03Eu2+). The numbers in parentheses are the estimated standard deviations of the last significant figure.

Combined X-ray and neutron diffraction

T/˚C 25

symmetry monoclinic

space group C2/c (#15)

a/Å 16.0937(6)

b/Å 8.9275(3)

c/Å 9.1070(3)

β/º 127.021(2)

volume/Å3 1044.70(4)

Z 4

Rwp (%) 6.83

Rp (%) 5.34

χ2 2.756





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Table S2. Structural parameters of NSPO:0.03Eu2+ as obtained from the combined Rietveld refinement of X-ray and neutron diffraction at room temperature. The numbers in parentheses are the estimated standard deviations of the last significant figure.

atom Wyckoff position

x y z 100 × Uiso

(Å2) g

Na1 4d 1/4 1/4 1/2 0.7800 (4) 0.87

Na2 8f 0.1783 (11) 0.0948 (16) 0.1913 (18) 0.0740 (5) 0.70

Na3 4e 1/2 0.1055 (15) 3/4 0.0048 (1) 0.74

Sc1 8f 0.4014 (2) 0.2544 (5) 0.3500 (4) 0.0222 (6) 1.00

P1 8f 0.1413 (5) 0.3898 (7) 0.0480 (7) 0.0133 (14) 1.00

P2 4e 1/2 0.4507 (10) 3/4 0.0380 (3) 1.00

O1 8f 0.2416 (4) 0.3094 (7) 0.1968 (8) 0.0020 (13) 1.00

O2 8f 0.0524 (6) 0.3210 (9) 0.0396 (11) 0.0580 (2) 1.00

O3 8f 0.1182 (4) 0.3689 (7) −0.1412 (8) 0.0168 (16) 1.00

O4 8f 0.1434 (6) 0.5534 (8) 0.0687 (10) 0.0340 (2) 1.00

O5 8f 0.5773 (5) 0.3423 (7) −0.0873 (9) 0.0178 (17) 1.00

O6 8f 0.5627 (5) 0.5481 (9) −0.2891 (8) 0.0226 (19) 1.00

The Eu2+ ions could occupy the available cationic sites in the NSPO host, based on the ionic radii

(rM) of the cations. Due to the smaller size of Sc (rSc = 0.74 Å) and P (rP = 0.38 Å) at the six-

coordinated site, the Eu2+ ions would occupy only Na sites with permissible coordination numbers

(CN) (rNa = 1.02 Å, rEu = 1.17 Å, CN = 6; rNa = 1.12 Å, rEu = 1.20 Å, CN = 7; rNa = 1.18 Å, rEu =

1.25 Å, CN = 8)1. Therefore, Eu2+ could occupy any of these three crystallographic Na sites.





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Table S3. DFT calculated relative energies of NSPO polymorphs with different Na coordinations.

coordination monoclinic (α-phase)

hexagonal (-phase)

6 0.99 0.27

7 0.81 -

8 0.00 0.00





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Table S4. Decay time of NSPO:xEu2+ (x = 0.01, 0.03, and 0.07) monitored at 355 nm excitation and 453 nm emission in the temperature range of 25 – 200ºC.

Temp. (oC)

decay time (μs)

NSPO:0.01Eu2+ NSPO:0.03Eu2+ NSPO:0.07Eu2+

25 0.41 0.39 0.28

50 0.42 0.40 0.29

100 0.44 0.43 0.30

150 0.43 0.45 0.33

200 0.42 0.42 0.32





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Table S5. Optical properties of white LEDs fabricated using NSPO:0.03Eu2+ phosphor as blue component with yellow-emitting La3Si6N11:Ce3+ (LSN:Ce3+) and red-emitting (Sr,Ca)AlSiN3:Eu2+ (SCASN:Eu2+ ) phosphor with UV LED (λmax = 365 nm) excitation in the high flux operating current of 100 - 1000 mA.

current (mA)

CIE x CIE y CRI luminous flux (lm)

luminous efficacy (lm/W)

100 0.2989 0.3502 93 4.5 13.01

200 0.2968 0.3531 90 8.0 11.11

300 0.2958 0.3574 89 10.7 9.58

400 0.2952 0.3602 88 13.3 8.69

500 0.2953 0.3617 88 15.8 8.00

600 0.2957 0.3625 88 17.9 7.38

700 0.2963 0.3632 88 19.7 6.78

800 0.2970 0.3633 88 20.8 6.14

900 0.2975 0.3639 88 22.7 5.86

1000 0.2982 0.3639 88 23.6 5.43





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Table S6. Temperature-dependent QE of NSPO:0.03Eu2+ phosphor with flux under 370 nm excitation in comparison with commercial phosphors used namely yellow-emitting LSN:Ce3+

and red-emitting SCASN:Eu2+.

Temp. (oC)

QE

NSPO:0.03Eu2+ LSN:Ce3+ SCASN:Eu2+

25 74.2 % 58.0 % 62.5 %

50 74.3 % 57.0 % 61.0 %

75 81.8 % 58.0 % 60.2 %

100 85.0 % 57.0 % 60.5 %

125 86.9 % 55.0 % 59.9 %

150 88.3 % 55.0 % 59.0 %

175 92.8 % 54.0 % 58.4 %





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Table S7. Temperature-dependent QE and absorption fraction of NSPO:xEu2+ (x = 0.01, 0.03, and 0.07) without flux under 370 nm excitation in the temperature range of 25 – 175ºC with temperature interval of 25ºC.

The emission intensity of NSPO:0.01Eu2+ at RT (α-phase), is too low compared to the

NSPO:xEu2+ (x = 0.03 and 0.07) phosphors (Table S7). Considering the infinitesimal amount of

defects at RT, and multiple absorption events arising from Eu2+ and defect centers, may not result

in the energy transfer and also accompanied by a non-radiative decay. In the NSPO:xEu2+ (x =

0.03 and 0.07), the direct absorption of Eu2+ competes more efficiently in absorption with the

defect, leading to higher emission in the α-phase. The NSPO:0.03Eu2+ exhibited high QE at RT

(64 %) and high-temperature (175˚C; 78 %), and superior TQ property, hence was chosen for

demonstrating the WLED properties.

Temp. (oC)

NSPO:0.01Eu2+ NSPO:0.03Eu2+ NSPO:0.07Eu2+

QE absorption QE absorption QE absorption

25 31.5 % 0.211 64.1 % 0.596 50.0 % 0.432

50 33.0 % 0.209 66.2 % 0.599 50.7 % 0.432

75 47.7 % 0.209 69.8 % 0.598 50.7 % 0.432

100 59.7 % 0.208 67.5 % 0.598 52.6 % 0.431

125 65.4 % 0.208 70.0 % 0.599 53.7 % 0.436

150 69.6 % 0.208 72.4 % 0.598 54.7 % 0.432

175 76.2 % 0.207 78.1 % 0.593 54.8 % 0.433





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Fig S1. Differential thermal analysis curves of NSPO:Eu2+ upon (a, b) heating and (c, d) cooling

up to 180˚C.

The differential thermal analysis curve of NSPO upon heating and cooling up to 250˚C are

illustrated in Fig. S1. Under heating, two exothermic reactions were observed at ~66˚C and ~164˚C,

corresponding to the structural transformation of the monoclinic α-phase to the hexagonal -phase

and to the -phase. Upon cooling, the reversible transformation from hexagonal to monoclinic

phase is evidenced by the endothermic reactions at the slightly lower temperature of ~62 ˚C and

~161˚C. These results are in accordance with the high temperature XRD results (Figs. 3a and 3b).





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Fig S2. (a) 23Na and (c) 31P MAS NMR spectra of NSPO:Eu2+ and normalized (b) 23Na and (d) 31P

MAS NMR spectra of NSPO:Eu2+ in the temperature range 25 – 180˚C.

The local structure of NSPO:Eu2+, namely Na symmetry and number of non-equivalent Na

sites were investigated using 23Na solid-state magic angle spinning (MAS) NMR spectroscopy.

Since 23Na is a quadrupolar nucleus, the NMR spectra may provide useful information on the static

and dynamic nature (at elevated temperatures) of the material. The large quadrupole interaction of 23Na nuclei increases the difficulty in deriving information due to the broad spectrum, and becomes

complicated when the sample contains two or more non-equivalent Na sites, which are partially

reduced by high speed spinning. The 23Na (spin I = 3/2) and 31P MAS (spin I = 1/2) NMR of the

NSPO:Eu2+ from room temperature (25˚C) to 180˚C are illustrated in the Fig. S2. A single





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asymmetric broad band was observed from 23Na NMR for the sample at all temperatures. The

central transition line maxima vary from ~ -20.3 (at 25˚C) to -2.8 (180˚C) ppm with the increase

in temperature, which is associated with the structural transitions from low symmetry monoclinic

(25˚C) to high symmetry hexagonal (>150˚C) with systematic peak shift. The stronger central

transition line positioned at ~ -20 ppm for monoclinic phase corresponds to the eight-coordinated

(NaO8) Na site as evidenced for the similar monoclinic NASICON structures.2 From the diffraction

results, three nonequivalent Na sites were observed for monoclinic phase (≤65˚C) and two Na sites

for hexagonal (- and -) phase (>50˚C). However, only single peak is observed, due to the line

broadening associated with the anisotropic effect. In general, the solid-state NMR signal is broader

due to the anisotropic nature in the spin-lattice and spin-spin relaxation processes.3 It is evident

that a straightforward assignment of different crystallographic sites (NaO6, NaO7 and NaO8) with 23Na NMR spectra is impossible due to the large broadening. Hence, 31P MAS NMR spectra were

measured to understand the nature of Na sites in the close proximity of 31P nuclei in the NSPO:Eu2+.

The 31P MAS NMR spectra of the NSPO:Eu2+ in the temperature range of 25 – 180˚C (Fig.

S2c) show a single component ~ 12.0 (±0.6) ppm which corresponds to the PO4 tetrahedra. In the

NSPO structure, each PO4 tetrahedron shares oxygen with four polyhedra, three with the Sc2(PO4)3

and one with the adjacent Na units. The position of 31P MAS NMR peak depends on the polarizing

strength of cations that share oxygen with PO4 tetrahedra.4 The peak move towards less negative

chemical shift with the structural transformation and the number of neighboring Na sites in the

monoclinic (three Na) and hexagonal (two Na) phases.

The non-equivalent crystallographic Na sites decrease from three (six-, seven-, and eight-

coordination) to two (six- and eight-coordination) during polymorphic modification from

monoclinic to hexagonal phase with increasing temperature.





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Fig S3. Temperature dependent excitation spectra of NSPO:xEu2+ with (a) x = 0.01, (b) x = 0.03,

and (c) x = 0.07 under 370 nm excitation in the temperature range of 25 – 200ºC.





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Fig S4. Diffused reflectance spectra (DRS) of NSPO host and NSPO:0.03Eu2+ in the room

temperature (α-phase).





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Fig S5. Temperature-dependent emission spectra of NSPO:xEu2+ (x = 0.01, 0.03, and 0.07) under

370 nm excitation, upon heating from room temperature to 250˚C and subsequent cooling to room

temperature with a temperature interval of 50˚C.

1) The emission spectra of NSPO:xEu2+ (x = 0.01, 0.03, and 0.07) in all the three composition

during cooling (from 250C) showed that the phosphor possess a closely similar emission

intensities to that of the emission during heating at the corresponding temperature (Fig. S5).

This indicates the presence of contributing trapped energy even during cooling process from

250C, and sustains until room temperature. This result substantiates that the contributing traps

influencing an increase in emission intensity (x = 0.01 and 0.03) or zero TQ (x = 0.07) property

of phosphor during heating, shows its existence even under decreasing temperature till room

temperature. This demonstrates that the contributing traps persists in a wide temperature

window of 25 – 250C under both heating as well as cooling process. The above result

reaffirms that the contributing traps are not short-lived and are very stable, which could be

utilized as the zero TQ phosphor in a wide temperature range of 25 – 250˚C and would certainly

be of practical use.

2) The source of these contributing traps in the NSPO:Eu2+ phosphor basically arises from the

generation of highly conducting Na+ ions with increasing temperature and its associated phase





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transformation to ionic conducting β- and γ-phases at 65˚C and 165˚C, respectively. The

formation and filling of contributing traps are driven by the reversible phase transformation

from conducting α-phase to highly ionic conducting β- and γ-phase in the temperature range

of 25 - 250˚C upon heating and cooling and are temperature-dependent. Therefore, whenever

the temperature of the sample rises, the formation of defects are initiated through the thermal

activation (supply of heat) leading to an assistance in the increase in emission intensity or zero

TQ property.

3) The beauty of the NSPO:Eu2+ phosphor is that the contributing traps favoring zero TQ property,

is highly reversibly formed during heating and cooling process with the reversible formation

of high conducting phases (β- and γ-phase) at high temperature to conducting phase (α-phase)

at room temperature. The efficiency of phosphor with increasing temperature is determined by

the ratio of radiative to non-radiative transitions.

4) In addition, the magnitude of two-fold emission of NSPO:0.01Eu2+ at 175ºC is also determined

by the amount of electron-hole pairs generated during UV excitation. In the low activator

sample (NSPO:0.01Eu2+), under UV excitation the large number of defect centers that were

formed due to the high content of trap forming Na+ ions (Eu2+ substitutes Na+ in the NSPO

host), serve as trapping centers, leading to a large formation of electron-hole pairs.





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Fig S6. The emission spectra and relative PL intensity of NSPO:0.03Eu2+ phosphor with respect

to time, monitored under 370 nm excitation continuously for 90 min at (a, d) 100˚C, (b, e) 150˚C,

and (c, f) 200˚C, for 30 min at each temperature with a time interval of 5 min.

The lifetime of the contributing traps at elevated temperature were investigated, from the

emission spectra of phosphors measured continuously over 90 min by holding the sample

temperature at 100˚C, 150˚C, and 200˚C for 30 min at each temperature, with a time interval of 5

min (Fig. S6). The lifetime of contributing traps which assist in the emission of phosphor with

increasing temperature, can be identified by maintaining the phosphor at a specific temperature

and monitoring the emission profile of phosphor as a function of time.

1) The emission profile of phosphor maintained at 100˚C, 150˚C, and 200˚C continuously for 30

min at each temperature showed that the emission intensity and emission area are very stable

and remain unchanged over 90 min, as well as without changing the emission wavelength.

2) This observation further confirms that the contributing traps that assist in the emission with

increasing temperature are very stable and these traps are not emptied, rather they exist during

continuous operation and contribute to the emission of the phosphor.

3) Moreover, the contributing traps are very active at temperature above room temperature

especially above 65˚C, which confirms that these traps are active in the complete temperature

range of 25 – 200˚C. The contributing traps that assist in the emission has high stability and

longer lifetime during the operation in the fabricated device. These results further substantiates





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that the contributing traps causing zero TQ property of phosphors are long-lived and would

surely be of practical use in the high-power white LED lighting.

4) The long-term stability of the defects displays a strong note for its suitability as a sustainable

blue component for the high-power white LED lighting.





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Fig S7. Decay time measurement of NSPO:xEu2+ (a) x = 0.01, (b) 0.03, and (c) 0.07, monitored at

355 nm excitation and 453 nm emission in the temperature range of 25 – 200ºC. The inset graph

clearly depicts the difference in decay curves.

The lifetime of the NSPO:xEu2+ (x = 0.01, 0.03, and 0.07) phosphors monitored at 453 nm were

measured as a function of temperature and presented in the Fig. S7. With an increase in the Eu2+

concentration, the decay time of NSPO:xEu2+ decreases in the order (x) 0.01 < 0.03 < 0.07 at room

temperature, which are in agreement with the reported literature.5,6,7 The decrease in the decay

time with increasing Eu2+ could be attributed to the increase in energy migration between

neighboring Eu2+ ions at higher Eu2+ concentration in the NSPO:xEu2+ phosphor.7 Moreover,

generally phosphors exhibit a decrease in the decay time with increasing temperature when TQ

occurs. In the NSPO:Eu2+, an increase in the decay time was observed which confirms the zero

TQ property of NSPO:Eu2+ phosphor. The TL result (Fig. 3e) also confirms the trend in the

concentration of defects among these three polymorphs. At RT, energy transfer process is

negligible due to an infinitesimal amount of traps in the α-phase.





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Fig S8. Density of states of the NSPO:Eu2+ phosphor in (a) monoclinic (α-phase) and (b)

hexagonal (β- and γ-phase) phases from DFT calculations.

The structural information of NSPO:Eu2+ phosphor could be correlated to the optical properties by

calculating the density of states through DFT calculations (Fig. S8). The structure of monoclinic

(α-phase) and hexagonal (β- and γ-phase) phases shows an interesting result of identical electronic

band structure, which substantiates the similar Eu2+ emission profile (453 nm) in both the

polymorphs. The band gap of monoclinic and hexagonal phases was estimated to be 4.65 eV and

4.54 eV, respectively. Moreover, Eu2+ was identified to occupy the most favorable eight-

coordinated NaO8 site in both the monoclinic and hexagonal phases which had identical electronic

d-band structure. This further corroborates the explanation for an unaltered emission position of

NSPO:Eu2+ in both monoclinic and hexagonal phases.





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Fig S9. Temperature-dependent normalized emission spectra of NSPO:0.03Eu2+ (in terms of

emission area) upon heating from 25 – 200ºC under different excitation wavelength ranging 350 –

420 nm.





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Fig S10. The excitation spectrum of commercial yellow-emitting La3Si6N11:Ce3+ and red-emitting

(Sr,Ca)AlSiN3:Eu2+ phosphors monitored at λem = 533 and 630 nm, respectively, overlapped with

the emission spectrum of NSPO:0.03Eu2+ phosphor under 370 nm excitation.

1) There is a significant absorption of the blue component (420 – 490 nm) of NSPO:Eu2+

phosphor by the yellow-emitting LSN:Ce3+ phosphor, which would reduce the blue emission

intensity of NSPO:Eu2+ from the EL spectra of WLED (Fig. S10).

2) A similar blue absorption of NSPO:Eu2+ by the red-emitting SCASN:Eu2+ phosphor had

resulted in the decrease of blue component during white-LED fabrication. Hence, the

decrement in the NSPO:Eu2+ emission may be attributed to the reabsorption process of the

other additive phosphor components and not the worse thermal quenching property. Therefore,

NSPO:Eu2+ is anticipated to show the zero thermal quenching property with the increase in

applied current in the EL spectra.

3) Moreover, this significant absorption of NSPO:Eu2+ emission by other phosphor component

namely green-emitting (LSN:Ce3+) and red-emitting (SCASN:Eu2+) phosphors can be an

advantage for the NSPO:Eu2+ based WLED fabrication due to the fact that the blue component

of NSPO:Eu2+ could overcompensate the drop in the efficiency of other phosphor components

at higher temperature. This added advantage would significantly improve the optical properties

of WLED with high color purity, color stability, and CRI for high-power LED lighting.





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References:

1. Shannon R. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr, Sect A: Found Crystallogr 32, 751-767 (1976).

2. Jian Z, et al. Atomic Structure and Kinetics of NASICON NaxV2(PO4)3 Cathode for Sodium‐Ion Batteries. Adv Funct Mater 24, 4265-4272 (2014).

3. Macomber RS. A complete introduction to modern NMR spectroscopy. Wiley New York, 1998.

4. Losilla ER, et al. Sodium Mobility in the NASICON Series Na1+xZr2-xInx(PO4)3. Chem Mater 12, 2134-2142 (2000).

5. Wang L, et al. Europium(ii)-activated oxonitridosilicate yellow phosphor with excellent quantum efficiency and thermal stability - a robust spectral conversion material for highly efficient and reliable white LEDs. Phys Chem Chem Phys 17, 15797-15804 (2015).

6. Xia Z, Liu R-S, Huang K-W, Drozd V. Ca2Al3O6F:Eu2+: a green-emitting oxyfluoride phosphor for white light-emitting diodes. J Mater Chem 22, 15183-15189 (2012).

7. Zhou J, Xia Z, Yang M, Shen K. High efficiency blue-emitting phosphor: Ce3+-doped Ca5.45Li3.55(SiO4)3O0.45F1.55 for near UV-pumped light-emitting diodes. J Mater Chem 22, 21935-21941 (2012).





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