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Author’s Accepted Manuscript
Ca9Na1/3M2(1-x)/3(PO4)7:2x/3Eu3+ (M = Gd, Y): A
promising red-emitting phosphor without
concentration quenching for optical display
Yue Guo, Sung Heum Park, Byung Kee Moon,
Jung Hyun Jeong, Jung Hwan Kim
To appear in: Journal of Luminescence
Received date: 12 September 2017
Revised date: 16 October 2017
Accepted date: 17 October 2017
Cite this article as: Yue Guo, Sung Heum Park, Byung Kee Moon, Jung Hyun
Jeong and Jung Hwan Kim, Ca9Na1/3M2(1-x)/3(PO4)7:2x/3Eu3+ (M = Gd, Y): A
promising red-emitting phosphor without concentration quenching for optical
applications, Journal
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Ca9Na1/3M2(1-x)/3(PO4)7:2x/3Eu3+ (M = Gd, Y) : A promising
red-emitting phosphor without concentration quenching for
optical display applications
Yue Guo1, Sung Heum Park1, Byung Kee Moon1, Jung Hyun Jeong1,* and Jung Hwan Kim2
Department of Physics, Pukyong National University, Busan 608-737, South Korea
Department of Physics, Dongeui University, Busan 614-714, South Korea
Zero-quenching red-emitting phosphors Ca9Na1/3M2(1-x)/3(PO4)7:2x/3Eu3+ (M = Gd, Y)
were successfully synthesized by solid-state method. The analysis of XRD, SEM, UV-vis
DRS, luminescent spectra, and the emission dynamics curves were applied to investigate
the obtained powders. These results showed that Eu3+-activated Ca9Na1/3M2(1-x)/3(PO4)7
(M = Gd, Y) phosphors can absorb the wavelength from 220 to 480 nm, which is matched
well with commercial near-UV chips, and emit intense red light without concentration
quenching. Notably, the integral intensities calculated from Ca9Na1/3Gd2(1-x)/3(PO4)7:2x/3
Eu3+ and Ca9Na1/3Y2(1-x)/3(PO4)7:2x/3Eu3+ phosphors were about 3.05 and 2.64 times
higher than that of commercial phosphor Y2O3:Eu3+, respectively. In addition, the CIE
color coordinate was calculated to be (0.65, 0.34). It should be pointed out that the
FWHM of 5D0→7F2 transition band was greatly expanded due to the presence of multisites for Eu3+ ions in the hosts. These results suggest that the obtained phosphors have the
potential for lighting and optical display applications.
Keywords: Rare-earth; Zero-quenching; Luminescence; Phosphors; Eu3+
Currently, phosphor-converted white light-emitting diodes (pc-wLEDs) technology is
penetrating deeply into indoor and outdoor lighting, backlit display source, medical
applications, automotive lighting, lifestyle products, and other sub-areas, due to its low
electric consumption, high electro-optical conversion efficiency, environmentally
friendly nature, robustness, and long lifetime [1-5]. According to the United States
Department of Energy’s report, this technology is projected to reduce lighting energy
consumption by 15% in 2020 and by 40% in 2030—saving 3.0 quads in 2030 alone,
worth over $26 billion in saving at today’s energy prices and equivalent to the current
total energy consumption of about 24 million homes in the United States [6].
Until now, the most prevalent method of w-LEDs is produced by the blend of yellowemitting Y3Al5O12:Ce3+ phosphor excited by the blue InGaN chip with the reminding
blue light. Nevertheless, due to lack of a red emission in the entire spectrum, this method
suffers from poor white light quality with low color rendering index and high correlated
color temperature problems, which couldn’t long-term to meet the general illumination
requirements and also restrict its further application [7, 8]. In this case, some red
phosphors are added to this system to solve the above problems. Frequently, many
investigations indicate that sulfide and nitridosilicate phosphors are used as a redemitting component in this system. As we know, sulfide and nitridosilicate phosphors
suffer from poor chemical stability and efficiency, or require complicated synthesis
techniques [9-11]. As an alternative, another method to produce w-LEDs is to employ red,
green and blue phosphors that are stimulated by near-UV emitting LED chip. The
obvious advantage of this method is that the near-UV LED chip has much broader
phosphor options than the blue InGaN LED chip. Since some effective blue
(Ca2PO4Cl:Eu2+, BaMgAl10O17:Eu2+) and green ((Ba,Sr)2SiO4:Eu2+, SrSi2O2N2:Eu2+)
phosphors have been found, finding suitable and inexpensive red phosphors capable of
having high efficiency and excellent chemical stability under near-UV excitation is the
main problem.
As we know, trivalent europium ions (Eu3+) acts as one of the most popular activators
for red-emitting rare earth (RE) ions doped in various host lattices. Recently, many
investigations were devoted to growing the Eu3+-based red-emitting phosphors, such as
Eu3+-doped perovskite-type SrLaMgTaO6[10], an intense
D0→7F4 transition from
Ca2Ga2SiO7: Eu3+ [12], a bifunctional phosphor Sr3Sn2O7:Eu3+[13], etc. On the basis of the
characteristic emission features, Eu3+ ion possesses pure red emission at ~ 615 nm, which
comes from the 5D0 excited state relaxed to the lower 7FJ (J = 0-6) state. Another more
important economic advantage is that Eu3+ ion can be excited with low-cost near-UV or
blue LED chips [14]. In addition, Eu3+ ions have been extensively studied for phosphors,
electroluminescent devices, high-density optical storage, and optical amplifiers because
of the longer decay time of 5D0 state (ms). It should be pointed out that the concentration
quenching is a common phenomenon in the RE ion doped phosphors since energy can be
consumed in non-radiative and radiated manners, the latter manner usually occurs in high
doping concentration systems. In this context, we reported promising red-emitting
phosphors Ca9Na1/3Gd2(1-x)/3(PO4)7:2x/3Eu3+ and Ca9Na1/3Y2(1-x)/3(PO4)7:2x/3Eu3+, which
are abbreviated as CNGP: 2x/3Eu3+ and CNYP:2x/3Eu3+, without concentration
quenching for optical display applications. The concentration of Eu3+ ions can be doped
up to 100 % in CNGP and CNYP host, which is derived from β-Ca3(PO4)2 Whitlockite
structure (space group R3c, Z = 21) [15]. Besides, the phase purity, morphological, the
emission dynamics, and optical properties of the obtained phosphors were investigated by
X-ray diffraction, scanning electron microscope, decay time, and luminescence spectra,
2. Material and methods
2.1 Sample preparation
The compounds CNMP:2x/3Eu3+ (M = Gd, Y) (x = 0.05, 0.10, 0.15, 0.30, 0.50, 0.75,
and 1.00) were synthesized by the conventional solid-state reaction method. All starting
materials CaCO3 (99.99 %), (NH4)2HPO4 (99.95 %), Gd2O3 (99.99 %), Y2O3 (99.99 %)
and Eu2O3 (99.99 %) were weighed according to the stoichiometric ratio. In addition,
Li2CO3, Na2CO3, and K2CO3 were added as charge compensator. Typically, all the
materials were mixed and ground in an agate mortar for about 30 min. After that, the
mixed powders were subjected to three heat treatments in a muffle furnace, namely 350,
750, and 1200 °C for 10, 5, and 10 h, respectively, with intermediate grindings. Finally,
the obtained phosphors were reground into powder for further characterization. This work
was supplied by the Display and Lighting Phosphor Bank at Pukyong National University.
2.2 Characterization
The powder X-ray diffraction (XRD) analysis was collected on a Bruker D8 Advance
with a step size of 0.02°, with Cu Kα irradiation (λ = 1.5406 Å). The microstructural
morphology of the obtained phosphors were investigated using scanning electron
microscope (SEM) system (JSM-6490, JEOL Ltd., Tokyo, Japan). UV-vis diffuse
reflectance spectra (DRS) were recorded on a V-670 UV-vis spectrophotometer (JASCO
Corp., Japan). Photoluminescence (PL) and excitation spectra (PLE) were measured
using Photon Technology International (PTI, USA) fluorimeter, and a 60 W xenon lamp
was used as excitation source. The representative PL decay curves were measured with a
phosphorimeter attached to the fluorescence spectrophotometer equipped with a 25 W
xenon flash lamp. The PL quantum yield was carry out on a spectrofluorometer (Jasco,
FP-8500, Japan) with a fluorescence integrating sphere unit (Jasco ISF-834). All
measurements were performed at room temperature.
3. Results and Discussion
3.1 Phase characterization
With the contents of x(Eu3+) ions increased from 5% to 100%, the XRD patterns of
CNGP:2x/3Eu3+ and CNYP:2x/3Eu3+ phosphors are presented in Fig. 1(a) and Fig. 1(b),
respectively. By comparison, every diffraction peak in these obtained phosphors share the
same position and relative intensity with the standard pattern Ca9LiGd0.667(PO4)7
(PDF#48-1194), confirming that these obtained phosphors are a single-phase compound,
while no secondary phases and any impurities were found. In order to evaluate the
influence of Eu3+ substitution for Gd3+ on the lattice parameters, a simple structural
refinement was carried out by Jade 6. Fig. 1(c) and Fig. 1(d) show the variation in lattice
parameters a and V as a function of Eu3+ contents of CNGP:2x/3Eu3+ and
CNYP:2x/3Eu3+ samples, respectively. Since Eu3+ is slightly larger than Gd3+ and Y3+ in
the same coordination number, the increase of Eu3+ content induces a linear increase of
lattice parameters a and V [16]. The R2 values of the linear fitting of experimental data
are close to unity as shown in Fig. 1(c) and Fig. 1(d). It is apparent that Gd3+ and Y3+ are
replaced by Eu3+ ions in CNGP and CNYP lattice, respectively.
A high resolution SEM analysis was performed to investigate the particle size and
surface morphology of CNGP:2x/3Eu3+ and CNYP:2x/3Eu3+ phosphors with x = 0.05
under different magnifications as shown in Fig. 2. The obtained images clearly
demonstrate that the particle size and morphology of CNGP:2x/3Eu3+ and
CNYP:2x/3Eu3+ phosphors are similar with each other. The distribution of the particles
size is rather uniform, the average size is about 1.5 to 2.5 µm. In addition, there are very
few impurity particles on the surface and internal of the larger particles. It needs to point
out that there are many voids in these images. We think the inadequate sintering
temperature profiles should be responsible for this result. Moreover, the solid
microcrystalline structures have appeared clustering phenomenon among the crystalline
grains in both of the obtained phosphors because of the high-temperature solid-state
reaction. Therefore, the PL performance can be greatly improved by selecting the
appropriate sintering temperature scheme because of the large effect of the morphology,
particle size and voids on the luminescence properties.
3.2 Luminescent properties of Eu3+-doped CNGP and CNYP phosphors
For powder samples, the UV-vis DRS measurement is a convenient technique for
assessing the optical absorption properties [17]. Fig. 3 shows the UV-vis DRS of
CNGP:2x/3Eu3+ and CNYP:2x/3Eu3+ samples, each with x = 0.00 and x = 0.50. We can
see that all the samples have a similar absorption platform, which appears in the
wavelength range of 380-800 nm. Besides, a series of absorption lines from the intraconfigurational 4f-4f transitions of Eu3+ ions are obviously visible in CNGP:2x/3Eu3+ and
CNYP:2x/3Eu3+ samples with x = 0.50. We know that the bandgap is a basic material
property of a compound. The bandgap of pure CNGP and pure CNYP samples can be
obtained from respective reflection spectrum employing the following function [7, 18]:
in which hv is the energy per photon, Eg represents the value of the band gap, C is a
proportional constant, and the transition coefficient n = 1/2 means an indirect allowed
electronic transition, n = 2 stands for a direct allowed electronic transition, n = 3/2
indicates a direct forbidden transition, and n = 3 represents for an indirect forbidden
electronic transition). F(R∞) is the Kubelka-Munk equation (R∞ = Rsample/Rstandard), which
can be formulated from the Kubelka-Munk function [19]:
where S, R, and K are the scattering, reflectance, and absorption parameters, respectively.
As shown in the inset of Fig. 5(a) and Fig. 5(b), the band gap Eg of pure CNGP and pure
CNYP samples are estimated to be 5.85 and 5.76 eV, respectively, when [ (
Since both of CNGP:2x/3Eu3+ and CNYP:2x/3Eu3+ samples emit red emission under a
365 nm UV lamp and are of white color under daylight, suggesting that both samples
have similar spectroscopic properties, whereupon the following luminescent properties
study is focused on one of them, namely CNGP:2x/3Eu3+. With doping Eu3+ ions at the
level of 0.50 mol, the PL and PLE spectra of CNGP:2x/3Eu3+ sample excited at 397 nm
and monitored at 613.5 nm are displayed in Fig. 4(a) and Fig. 4(b). The PLE spectrum
contains a broad charge-transfer band (CTB) and a series of sharp peaks as shown in Fig.
4(a). The CTB locating in 220-300 nm with a maximum value at 270 nm is derived from
the completely filled 2p orbital of O2- ion and the partially filled 4f orbital of the Eu3+
ions (Eu3+-O2-). Besides, these sharp peaks are coming from Gd3+ and Eu3+ ions. Among
them, Gd3+ ions show two absorption peaks at 275 and 321 nm, which can be attributed
to the 8S7/2→6IJ and 8S7/2→6PJ, respectively. Other sharp peaks located at 350-500 nm
stem from the 4f-4f transitions of the Eu3+ ions, which are at 365, 384, 397, 418, and 468
nm belonging to 7F0→5D4, 7F0→5G2, 7F0→5L6, 7F0→5D3, and 7F0→5D2, respectively.
Obviously, the strongest peak at 397 nm indicates that the phosphor matches well with
commercial near-UV chips and is expected to be used for phosphor-converted white
Fig. 4(b) records the related PL spectrum of CNGP:2x/3Eu3+ sample with x = 0.50
upon 397 nm excitation. The PL spectrum consists of several peaks at 536, 581, 591,
613.5, 654, and 700 nm of Eu3+ ions originating from 5D1→7F1, 5D0→7F0, 5D0→7F1,
D0→7F2, 5D0→7F3, and 5D0→7F4, respectively. It can be seen that the PL intensity of
D0→7F0 transition is the weakest peak due to the forbidden transition (J = 0 ↔ J’ = 0)
[20]. Another point should be noted that the most intensive at 613.5 nm belongs to an
electric dipole transition (ED, 5D0→7F2) with ∆J = 2, whereas the peak at 591 nm belongs
to a magnetic dipole transition (MD, 5D0→7F1) with ∆J = 1. As we known, the MD
transition is parity-allowed and insensitive to the site symmetric, while the ED transition
is hypersensitive and its intensity is varied orders of magnitude attaching to the
surrounding environment. As shown in Fig. 4(b), the 5D0→7F2 emission peak is stronger
than that of 5D0→7F1, the ratio (5D0→7F2)/(5D0→7F1) was calculated to be 3.40. These
results suggests that Eu3+ ion occupy an asymmetric site in CNGP host matrix.
Fig. 4(c) gives the corresponding CIE chromaticity diagram with images under
daylight and a 365 nm UV lamp. Notably, the phosphor exhibits high purity and intense
red emission under a 365 nm UV lamp. By calculation, the CIE color coordinate was
calculated to be (x = 0.65, y = 0.34), which was particularly close to the National
Television Standards Committee red (x = 0.67, y = 0.33). In addition, quantum yield (QY)
is another important indicator parameter for phosphor in practical applications. QY can
be obtained based on the following equation [21]:
in which,
represents the spectrum of the excitation light from the empty integrated
means the spectrum of the light used for exciting the sample.
indicates the
emission spectrum of the obtained sample. The internal QY of CNGP:2x/3Eu3+ and
CNYP:2x/3Eu3+ phosphors with x = 0.50 were measured as 45.6% and 42.9%,
respectively, under 397 nm excitation.
In order to further study the energy transfer among Eu3+ ions with different doping
concentrations, a series of CNGP:2x/3Eu3+ and CNYP:2x/3Eu3+ (x = 0.05-1.00)
phosphors were synthesized and measured as shown in Fig. 5. One can see that the
integral PL intensity keeps on growing with Eu3+ concentration increased from 5% up to
100%, without concentration quenching. Typically, the quenching occurs at a lower
doping content in many Eu3+-doped materials. For example, the quenching concentration
for commercial red phosphor Y2O2S:Eu3+ is obtained to be 5 mol%, and that of
commercial red phosphor Y2O3:Eu3+ also be found to be 5 mol% [22, 23]. We know, the
concentration quenching means that the distance between the luminescent centers
becomes short as the activator ion concentration increases. Till the distance is shorter
than the critical distance, causing the cascade energy transfer, that is, the energy transfer
from a center to the next one, and then to the next one. Eventually, these energy-transfer
chains trigger energy migration to energy-sinks in the lattice, resulting in the
concentration quenching. Through detailed analysis of the abnormal phenomenon in our
study, the blocked cascade energy transfer processes can be ascribed to the following
reasons: (1) the exchange interaction among Eu3+ ions is low because the 4f electrons
dragged in the energy-transfer processes are shielded by the outer 5s and 5p electrons; (2)
the energy migration by multipole-multipole interactions is hampered since the weak
oscillator strength of the forbidden 4f-4f transitions; (3) the reabsorption hardly takes
place owing to few overlap between the PLE and PL spectra; (4) the resonance between
adjoining Eu3+ ions also hardly occurs because of the mismatch between the energy
levels; (5) In the Whitlockite structure, the shortest distance between two Gd(Eu3+) sites
is up to 7.53 Å [15]. This result implies that the distance is long enough to favor the Eu3+
emission while attenuating energy transfer between them.
The representative PL decay curves from the higher energy level 5L6 to the 7F2 level of
Eu3+ ions in CNGP:2x/3Eu3+ and CNYP:2x/3Eu3+ (x = 0.05-1.00) phosphors were
measured with 397 nm excitation by monitoring the emission at 613.5 nm as shown in
Fig. 6. After the curve fitting analysis, all decay curves were successfully fitted with a
single-exponential decay equation:
( )
where I(t) means the PL intensity at a given time t, I0 represents the initial intensity, and
is the decay time. By using equation 4, the decay times as a function of Eu3+
concentration were calculated and are put in Fig. 6. The values for CNGP:2x/3Eu3+ are
1.684, 1.733, 1.759, 1.776, 1.762, 1.754, and 1.740 ms for x = 0.05, 0.10, 0.15, 0.30, 0.50,
0.75, and 1.00 mol, respectively. And the values for CNYP:2x/3Eu3+ are 1.700, 1.738,
1.751, 1.756, 1.774, 1.772, and 1.758 ms for x = 0.05, 0.10, 0.15, 0.30, 0.50, 0.75, and
1.00 mol, respectively. It can be observed that the value
in both hosts gradually
increases with the increase of Eu3+ contents, and then decreases due to the distances
between the Eu3+ ions greatly shorter in higher doping concentrations.
3.3 Photoluminescence comparison
To investigate the influence of different charge compensators on the luminescent
performance and characteristics of Eu3+ ions in the obtained phosphors, three types of
charge compensators (Li, Na, and K) doped C□GP:2x/3Eu3+ and C□YP:2x/3Eu3+ (□=Li,
Na, K) phosphors under 397 nm excitation with x=0.15 mol were synthesized and are
compared in Fig. 7(a) and Fig. 7(b). We can see that the effect of charge compensators is
different in the two kinds of phosphors. Sodium ion (Na) doped C□GP:2x/3Eu3+ has the
strongest emission, while lithium ion (Li) doped C□YP:2x/3Eu3+ owns the most intense
emission. According to the previous report, Du et al reported the concentration quenching
in Eu3+ doped Ca9LiGd2/3(PO4)7 can reach to 90% [15]. Nevertheless, there was no
concentration quenching in our phosphors. Clearly, Sodium ion (Na) is a more suitable
charge compensator for C□GP:2x/3Eu3+ system.
Under 397 nm excitation, the PL spectra of CNGP:2x/3Eu3+ and CNYP:2x/3Eu3+
phosphors with commercial phosphor Y2O3:Eu3+ were measured and are compared in Fig.
7(c). Specifically, the intense red emission was observed from CNGP:2x/3Eu3+ and
CNYP:2x/3Eu3+ phosphors and their integral intensities were about 3.05 and 2.64 times
higher than that of commercial phosphor Y2O3: Eu3+. It is interesting to note that the fullwidth at half-maximum (FWHM) of 5D0→7F2 transition band in CNGP:2x/3Eu3+ and
CNYP:2x/3Eu3+ were much broader (~ 9.29 nm) than that of Y2O3: Eu3+ (~ 3.93 nm). The
D0→7F2 transition band is broadened because of the presence of multi-sites emissions.
The obtained phosphors are derived from β-Ca3(PO4)2 structure, which contains Ca(1)Ca(5) five types of Ca sites, Ca(1)-Ca(3) and Ca(5) sites coordinated with 7-, 7-, 8-, and
6- oxygens are fully occupied while Ca(4) site is half-occupied. In CNGP:2x/3Eu3+ and
CNYP:2x/3Eu3+, Ca(1)-Ca(3) and Ca(5) sites were confirmed to be substituted by Eu3+
ions [15, 24]. On the basis of the analysis, it is reasonable to understand that the 5D0→7F2
transition band was broadened.
4. Conclusions
In summary, we have synthesized a promising red-emitting phosphor CNMP:2x/3Eu3+
(M = Gd, Y) without concentration quenching by solid-state reaction. XRD analysis
confirmed that these obtained phosphors are in a single-phase and Eu3+ ions could
successfully occupy Gd3+ and Y3+ sites. High resolution SEM analysis showed a uniform
distribution of the particles size of about 1.5 to 2.5 µm. The band gap Eg of pure CNGP
and CNYP samples were estimated to be about 5.85 and 5.76 eV, respectively. Besides,
the Eu3+ ion occupied an asymmetric site in CNGP and CNYP hosts showing intense red
emissions, which were about 3.05 and 2.64 times higher than that of commercial
phosphor Y2O3:Eu3+. By studying the effects of different charge compensators (Li, Na,
and K), we can see that Na+ doped C□GP:2x/3Eu3+ has the strongest emission, while Li+
doped C□YP:2x/3Eu3+ owns the most intense emission. In addition, the internal QE of
CNGP:2x/3Eu3+ and CNYP:2x/3Eu3+ phosphors with x = 0.50 could reach 45.6% and
42.9% upon 397 nm excitation, respectively.
This research was supported by the Basic Science Research Program through the
National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT
and Future Planning (No. 2015060315). The Eu3+ ions activated Ca9Na1/3M2(1x)/3(PO4)7:2x/3Eu
(M = Gd, Y) phosphor was supplied by the Functional Phosphor Bank
at Pukyong National University.
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Figure captions
Figure 1 XRD patterns of (a) CNGP:2x/3Eu3+ and (b) CNYP:2x/3Eu3+ samples with the
standard pattern (PDF#48-1194). Variation in lattice parameters a and V as a function of
Eu3+ content of (a) CNGP:2x/3Eu3+ and (b) CNYP:2x/3Eu3+ samples.
Figure 2 SEM images of (a, b) CNGP:2x/3Eu3+ and (c, d) CNYP:2x/3Eu3+ samples with
x = 0.05 under different magnifications.
Figure 3 UV-vis DRS of (a) CNGP:2x/3Eu3+ and (b) CNYP:2x/3Eu3+ samples, each with
x = 0.00 and x = 0.50. Inset puts the determination of the band gap Eg of pure CNGP and
pure CNYP samples.
Figure 4 The (a) PLE and (b) PL spectra of CNGP:2x/3Eu3+ (x = 0.50 mol) sample. (c)
The corresponding CIE chromaticity diagram with images under daylight and a 365 nm
UV lamp.
Figure 5 Concentration dependence of PL spectra of (a) CNGP:2x/3Eu3+ and (b)
CNYP:2x/3Eu3+ phosphors under 397 nm excitation.
Figure 6 Decay curves from the higher energy level 5L6 to the 7F2 level of Eu3+ ions in (a)
CNGP:2x/3Eu3+ and (b) CNYP:2x/3Eu3+ (x = 0.05-1.00) phosphors excited at 397 nm
and monitored at 613.5 nm.
Figure 7 Comparison of the PL intensities in (a) C□GP:2x/3Eu3+ (□=Li, Na, K) and (b)
C□YP:2x/3Eu3+ (□=Li, Na, K) samples under 397 nm excitation with x=0.15. (c)
Comparing PL intensities of (M) CNGP:2x/3Eu3+, (N) CNYP:2x/3Eu3+ phosphors with that
of commercial phosphor (P) Y2O3: Eu3+ under 397 nm excitation.
Figure 1
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