Letter
www.acsami.org
Enhance Color Rendering Index via Full Spectrum Employing the
Important Key of Cyan Phosphor
Mu-Huai Fang,† Chenchen Ni,† Xuejie Zhang,† Yi-Ting Tsai,† Sebastian Mahlik,‡ Agata Lazarowska,‡
Marek Grinberg,‡ Hwo-Shuenn Sheu,§ Jyh-Fu Lee,§ Bing-Ming Cheng,§ and Ru-Shi Liu*,†,⊥
†
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan
Institute of Experimental Physics, Faculty of Mathematics, Physics and Informatics, University of Gdańsk, 80-308 Gdańsk, Poland
§
National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan
⊥
Department of Mechanical Engineering and Graduate Institute of Manufacturing Technology, National Taipei University of
Technology, Taipei 106, Taiwan
‡
S Supporting Information
*
ABSTRACT: A new concept called “full-spectrum lighting” has
attracted considerable attention in recent years. Traditional devices are
usually combined with ultraviolet−light-emitting diode (LED), red,
green, and blue phosphors. However, a cyan cavity exists in the 480−520
nm region. Hence, cyan phosphors are needed to compensate for the
cavity. (Sr,Ba)5(PO4)3Cl:Eu2+ phosphors feature an extremely unique
and tunable photoluminescence spectrum. Nevertheless, the tuning
mechanisms of these phosphors remain unclear. In this study, we
elucidate the mechanism of the cation size-controlled activator unevenoccupation and reoxidation in (Sr,Ba)5(PO4)3Cl:Eu2+ phosphors. This
mechanism could help tune the optical properties of related apatite
families and structures with multiple cation sites and strongly uneven
occupation of activators and cations. Finally, the package of the LED
device is constructed to show that both color rendering index Ra and R9 are higher than 95. Thus, the device could be a potential
candidate for full-spectrum lighting.
KEYWORDS: cyan phosphor, full spectrum, M5(PO4)3Cl, LED, activator redistribution
W
The emission wavelength of Sr5(PO4)3Cl:Eu2+ is approximately
450 nm. Ba2+ doping broadens the full width at half-maximum
of Sr5(PO4)3Cl:Eu2+. However, the luminescent intensity and
spectrum of this phosphor remain to be analyzed in detail. The
luminescent properties of Sr4.7−xBax (PO4)3Cl:Eu2+ with a high
Ba2+ concentration are unique and tunable. Previous studies
analyzed activator redistribution in some phosphor systems.14−18 To the best of our knowledge, a system featuring
activator-preferred occupation, activator-uneven distribution,
change of activator oxidation state, and squeezed-out behavior
remains to be observed and analyzed. In the present study, we
analyze the structure and luminescence properties of
Sr4.7−xBaxEu0.3(PO4)3Cl in detail. We also propose a mechanism for the unique behavior of the system. Finally, LED
devices are constructed to show that both color rendering index
Ra and R9 are higher than 95.
The X-ray diffraction (XRD) patterns of
Sr4.7−xBaxEu0.3(PO4)3Cl (0 ≤ x ≤ 2.5) are shown in Figure
1a. All patterns are pure phase except for some Eu2O3
hite light-emitting diodes (LEDs) have been employed
in different illumination systems worldwide. An
increasing number of traditional illumination devices are
being replaced by LEDs because of their high brightness, low
energy consumption, long lifetime, and eco-friendliness.1−6
LEDs are applied not only in lighting but also in extensive
fields, such as agriculture lighting, automobile, and backlighting.7−9 UV−LED chips with red, green, and blue
phosphors have attracted considerable attention as nextgeneration LED devices because of their high color rendering
index.10 A new concept called “full-spectrum lighting” (FSL)
has attracted much attention in recent years. However, the cyan
cavity in the 480−520 nm region (Figure S1) decreases color
vividness. Hence, a cyan phosphor is important to obtain real
full-spectrum lighting. The M5(PO4)3Cl:Eu2+ (M = Ca, Sr, Ba)
system has been selected as the candidate phosphor because of
its high water resistance and high quantum efficiency. Norzol et
al.11 first proposed that two Sr sites are available and that Ca
prefers to occupy one of the Sr sites in (Ca,Sr)5(PO4)3Cl:Eu2+.
In 2013, Shang et al.12 reported that Ca2Ba3(PO4)3Cl:Eu2+
phosphors could emit light with a peak maximum at 410 nm
but are hardly excited by light over 400 nm. In 2015, Deressa et
al.13 first reported Ba2+-doped Sr5(PO4)3Cl:Eu2+ phosphors.
© 2016 American Chemical Society
Received: August 15, 2016
Accepted: November 1, 2016
Published: November 1, 2016
30677
DOI: 10.1021/acsami.6b10233
ACS Appl. Mater. Interfaces 2016, 8, 30677−30682
Letter
ACS Applied Materials & Interfaces
Figure 1. (a) XRD patterns with different x values in Sr4.7−xBaxEu0.3(PO4)3Cl (0 ≤ x ≤ 2.5). (b) Sr coordination environment of Sr(I) and Sr(II).
(c) Ba occupation value at Sr(I) and Sr(II).
Figure 2. (a) Normalized room-temperature PLE and deconvolution PL spectra, (b) room-temperature PL spectra, (c) low-temperature PL and
PLE spectra and, (d) decay curve of Sr4.7−xBaxEu0.3(PO4)3Cl with different x values.
impurities when x is higher than 2. Both Sr5(PO4)3Cl and
Ba5(PO4)3Cl possess a hexagonal P63/m structure, which allows
them to form a solid solution. The XRD peaks slightly shift
toward low scattering angles as x increases because the ionic
size of Ba2+ is larger than that of Sr2+. Two crystallographic
coordination environments of Sr2+ are present and denoted as
Sr(I) and Sr(II). Sr(I) is coordinated by nine oxygen ions,
whereas Sr(II) is coordinated by six oxygen and two chloride
ions (Figure 1b). To further investigate the structure, we
employed synchrotron XRD and Rietveld refinement (Figure
S2). Lattice parameters a, c, and V linearly increase because of
the increased ionic size of Ba2+. Interestingly, Ba2+ is not equally
distributed within the Sr2+ site. The occupation value of Ba2+ at
the Sr(I) site is close to zero and slightly increases when x is
higher than 1.5 (Figure 1c). The result reveals that Ba2+ has a
strongly preferred occupation in different Sr2+ sites.
To investigate the effect of Ba2+-preferred occupation in the
system, the room-temperature photoluminescence (PL) spectra
30678
DOI: 10.1021/acsami.6b10233
ACS Appl. Mater. Interfaces 2016, 8, 30677−30682
Letter
ACS Applied Materials & Interfaces
determine the charge variation of Eu (Figure 3). The L3-edge
of Eu shows two peaks at 6974 and 6984 eV, suggesting the
and photoluminescence excitation (PLE) spectra are determined (Figure 2a). The excitation spectra range from 300 to
430 nm. The excitation spectra consist of several overlapping
broad bands that are independent of the Ba2+ concentration in
all considered samples. When excited by 330 nm UV light, the
emission peak maximum shifts from 446 to 472 nm as x
increases (Figure 2b). The XRD patterns indicate that two
crystallographic Sr2+ sites exist. Therefore, the PL spectra of
Sr4.7−xBaxEu0.3(PO4)3Cl are decomposed into two Gaussian
bands. The higher energy emission band with peak maximum at
448 nm corresponds to Eu2+ located at the Sr(II) site, whereas
the lower energy emission band with peak maximum at 488 nm
corresponds to Eu2+ at the Sr(I) site, which are denoted as
Eu(SrII) and Eu(SrI), respectively. Although the excitation
spectra appear similar in Figure 2a, we expect to observe
different excitation spectra for the two well-defined Eu(SrII)
and Eu(SrI) emission bands. As a result, the low-temperature
excitation spectra at 450 and 500 nm are determined. Although
the excitation spectra overlap largely with each other, the 420−
440 nm region is different. Under an excitation wavelength of
420 nm, both Eu(SrII) and Eu(SrI) are simultaneously excited,
but the excitation is dominant for Eu(SrII). The emission
spectrum shows the maximum peak at approximately 450 nm
and a tail at approximately 500 nm. By contrast, Eu(SrI) could
be independently excited under 440 nm, and the emission
maximum wavelength is approximately 500 nm.
Interestingly, the emission spectrum of Sr4.7Eu0.3(PO4)3Cl
seems to be a symmetry band, although both Sr(I) and Sr(II)
sites exist in the structure. This phenomenon conflicts with our
expectation that the emission spectrum of Sr4.7Eu0.3(PO4)3Cl is
an asymmetry band consisting of Eu(SrII) and Eu(SrI).
However, the emission spectra gradually change from a
symmetry to an asymmetry band after Ba2+ doping. In
consequence, we boldly assume that Eu2+ ions only occupy
the Sr(II) site at the beginning. After Ba2+ doping, Eu2+ ions are
partly squeezed from the Sr(II) to the Sr(I) site. To verify our
assumption, the decay curve is used to investigate the behavior
of the two Sr2+ sites (Figure 2d). First, λobs = 440−460 nm is
used to detect the luminescence overlap region of Eu(SrII) and
Eu(SrI). The luminescence decay of Sr4.7Eu0.3(PO4)3Cl is single
exponential, which indicates that Eu2+ occupies only one site.
By contrast, the PL decays of Sr4.7−xBaxEu0.3(PO4)3Cl (0.5 ≤ x
≤ 1.5) are nonexponential and considered as a superposition of
two exponents. This finding indicates that Eu2+ ions occupy
both Sr(II) and Sr(I) sites. In addition, the luminescence
decays of λobs = 480−520 nm are single exponential for
Sr4.7−xBaxEu0.3(PO4)3Cl (0.5 ≤ x ≤ 1.5) because of the Eu(SrI)
single site luminescence. The lifetime calculated from the decay
curve is shown in Figure S3b. The lifetime observed from 440
to 460 nm is short and gradually decreases, whereas the lifetime
observed from 480 nm to 520 increases after Ba2+ doping. This
phenomenon may be attributed to the nonradiative energy
transfer from Eu(SrII) to Eu(SrI) sites.
Some Eu2O3 impurities exist in the samples with high Ba
concentrations (Figure S4c). This phenomenon is uncommon
in solid state reaction. To investigate the underlying reasons,
the oxidation state of Eu ions with high Ba2+ concentrations is
analyzed by Eu3+ characteristic PL spectroscopy (Figure S4).
Three characteristic narrow peaks appear at approximately 625
nm, and the intensity slightly increases with increasing Ba2+
concentration. This condition means that Eu2+ gradually
oxidizes to Eu3+ after Ba2+ doping. Moreover, X-ray absorption
near-edge structure (XANES) spectroscopy is applied to
Figure 3. XANES spectrum of Sr4.7−xBaxEu0.3(PO4)3Cl (0 ≤ x ≤ 2.5).
2p3/2 → 5d transition of Eu2+ and Eu3+, respectively. Ba2+
doping slightly increases the signal of Eu3+ and decreases the
signal of Eu2+. However, the Eu3+ signal in the sample with a
high Ba2+ concentration considerably increases, and most of the
Eu2+ ions are changed to Eu3+ as x = 2.5. This phenomenon
may be attributed to the size difference between Ba2+ and Sr2+.
When x = 1.5, the Eu2+ is mainly at the Sr(I) site. However,
when large Ba2+ is doped into Sr(I), Eu2+ ions are forced to
shrink the size and hence be oxidized to small Eu3+ ions. The
direct effect of Eu oxidization is the extremely low PL intensity
when x = 2.5, and the results agree with the photoluminescence
spectrum shown in Figure 2b.
Integrating the results above, we propose a mechanism of
cation size control activator redistribution (Figure 4). Initially,
Figure 4. Mechanism of cation size-control activator redistribution.
Eu2+ ions only occupy the Sr(II) site; hence, the decay curve is
single exponential, and the PL spectrum is symmetrical (x = 0).
After a small amount of Ba ions is doped into the crystal (0.5 ≤
x ≤ 1.5), Ba prefers to occupy the Sr(II) site because of the
larger space of the Sr(II) site compared with that of the Sr(I)
site. Eu2+ ions partially transfer from the Sr(II) to the Sr(I) site
because of the limited space in the Sr(II) site, causing the
change in the decay curve and the PL spectrum. After further
Ba doping (1.5 ≤ x ≤ 2.5), the space for Ba at the Sr(II) site
reaches the limit, and Ba starts to dope in Sr(I), which is in
accordance with the results shown in Figure 1c. In addition, the
space of Eu2+ in the Sr(I) site is decreased by the large Ba2+
ions. This process forces Eu ions to change from large Eu2+ ions
30679
DOI: 10.1021/acsami.6b10233
ACS Appl. Mater. Interfaces 2016, 8, 30677−30682
Letter
ACS Applied Materials & Interfaces
Figure 5. Temperature-dependent (a) PL intensity and (b) decay time of Sr4.7−xBaxEu0.3(PO4)3Cl (0 ≤ x ≤ 1.5). (c) Vacuum UV absorption
spectrum of Sr4.7(PO4)3Cl:Eu3+. (d) Energy level diagram of Sr4.7Eu0.3(PO4)3Cl.
Figure 6. (a) CIE diagram of three different package devices. (b) Luminescence diagram combined with UV−LED chip, (Ca,Sr)AlSiN3 red
phosphor, (Ba,Sr)2SiO4:Eu2+ green phosphor, and different blue phosphors.
to small Eu3+ ions or exist as Eu2O3 to save space, which could
be observed in the XRD pattern and the PL spectrum.
Thermal stability is also an important property of phosphor
materials. In this study, we use temperature-dependent
photoluminescence (TDPL) spectroscopy to analyze the
thermal stability of Sr4.7−xBaxEu0.3(PO4)3Cl (Figure 5a). The
thermal stability of the phosphor is obviously enhanced by the
doping of Ba ions. For Sr3.2Ba1.5Eu0.3(PO4)3Cl, the PL intensity
at 150 °C retains 86% of the intensity at 25 °C. Normally, when
heavy atoms are doped, the lattice vibration decreases and
hence increases the thermal activation energy, which
consequently increases the thermal stability. To study the
thermal properties in depth, the decay times of different
detected wavelengths and temperatures are measured as shown
in Figure S5. To analyze the temperature shortening of the
emission decay time τ, we should consider the following
formula
τ=
30680
1
τr
Ea
kT
Ea
kT
( )
+ P exp( )
1 + exp
(1)
DOI: 10.1021/acsami.6b10233
ACS Appl. Mater. Interfaces 2016, 8, 30677−30682
Letter
ACS Applied Materials & Interfaces
where τr is the PL radiative decay time, Ea is the activation
energy, and P is the probability of the nonradiative
depopulation. Fitting relation 1 to the experimental data, we
estimate a single nonradiative pathway for Eu at the Sr(II) site
center in Sr4.7Eu0.3(PO4)3Cl, characterized by activation energy
Ea = 609 cm−1 and probability P = 14 × 106 s−1 (Figure 5b).
For Eu at the Sr(II) centers in Sr3.2Ba1.5Eu0.3(PO4)3Cl, we
obtain similar results of activation energy Ea = 776 cm−1 and
probability P = 98 × 106 s−1, indicating that we are dealing with
the same channel deactivation for Eu at the Sr(II) site. In
Sr3.2Ba1.5Eu0.3(PO4)3Cl, the Eu at Sr(I) site luminescence
quenching is characterized by activation energy Ea = 3273 cm−1
and probability P = 5 × 109 s−1. This phenomenon can be
related to the autoionization of Eu2+, considered as the
transition from the excited state 4f65d1 to the conduction
band. In addition, the charge transfer band and band absorption
are indispensable information to construct the band structure
and energy level diagram of Sr4.7Eu0.3(PO4)3Cl. In this study,
Sr4.7(PO4)3Cl:Eu3+ is synthesized to obtain the more accurate
charge transfer band compared with Sr4.7(PO4)3Cl:Eu2+.
Consequently, the vacuum UV excitation spectrum is
determined (Figure 5c). The energies of the charge transfer
band and optical band gap (Eex) are 5.12 and 7.42 eV,
respectively. The electrical band gap (EVC) is approximately
equal to 1.08Eex, that is, 8.02 eV for Sr4.7Eu0.3(PO4)3Cl.
Furthermore, the emission energies of Eu from Sr(II) and Sr(I)
are 2.78 and 2.45 eV, respectively. To verify the obtained
values, we combined the charge transfer energy, PL energy, and
Ea calculated from the decay curve as shown in Table S2. The
band gaps calculated from Eu(SrII) and Eu(SrI) are 7.98 and
8.07 eV, respectively, which agree with the spectral value of
8.02 eV as shown in Figure 5d.
To demonstrate the potential of (Sr,Ba)4.7Eu0.3(PO4)3Cl in
actual applications, LED packages are constructed. The devices
are combined with UV−LED, red phosphor (Ca,Sr)AlSiN3:Eu2+, green phosphor (Ba,Sr,Ca)2SiO4:Eu2+, and different blue phosphors. As the most well-known blue phosphor,
BaMgAl10O17:Eu2+ is chosen to be the comparison group. To
compare the three devices, the point in the CIE diagram should
be the same and on the blackbody radiation line; a correlated
color temperature of 3000 K is selected as shown in Figure 6a.
For the device combined with BAM phosphor, a cavity exists in
the blue to cyan region, causing the limited enhancement of the
color rendering index (Table S4). When Sr4.7Eu0.3(PO4)3Cl is
used as the blue phosphor, a huge cyan cavity appears in the
480−520 nm region, causing the color rendering index to
become even lower than that of the device with BAM
phosphor. Nevertheless, if the Ba/Sr ratio is tuned, the
luminescence spectra gradually show a red-shift to compensate
for the cyan cavity. Sr3.4Ba1.3Eu0.3(PO4)3Cl is chosen to be the
best candidate to compensate for the cyan cavity and to
enhance the color rendering index. As shown in Figure 6b, the
device using Sr3.4Ba1.3Eu0.3(PO4)3Cl shows a smooth spectrum
without a cyan cavity in the blue−cyan region. The measured
Ra and R9 can reach 95.7 and 98, respectively, which shows the
great application potential of the device in full-spectrum
lighting.
In summary, this study has comprehensively analyzed and
resolved a system featuring activator-preferred occupation,
activator-uneven distribution, change of activator oxidation
state, charge compensation via cation vacancies and squeezedout behavior. The preferred occupation of the Ba2+ ions at the
Sr(II) site compared with the Sr(I) site is analyzed by
synchrotron XRD and Rietveld refinement. In addition, the
phenomenon above triggers the compression of Eu(SrII) space
and its transfer to the Sr(I) site. The transfer of Eu broadens
the PL spectra, which could be assigned as two peaks by
deconvolution. In addition, the decay curve spectrum of
Sr4.7Eu0.3(PO4)3Cl is single exponential but becomes nonexponential after Ba2+ doping, which also proves the transfer of
Eu from the Sr(II) to the Sr(I) site. As further compressed by
Ba2+, Eu(SrI) is squeezed out as Eu2O3 because of the limited
Sr(II) space. Moreover, Eu2+ oxidization is examined by
XANES and Eu3+ characteristic PL spectroscopy. The
mechanism scheme is then provided. In addition, the thermal
stability and energy level diagram are examined by TDPL and
decay curve.
The mechanism of the size-controlled behavior with activator
uneven-occupation and reoxidation could help tune the PL
spectrum in related apatite families, such as
Ca5(PO4)3(F,Cl,OH), Ca5(PO4)3F, and their solid-solution
compounds. The mechanism is also expected to be applied in
structures with more than two cation sites and strong activatorpreferred occupation. The tuning method and provided
mechanism could help understand and explain the results.
Real full-spectrum lighting can be achieved by
Sr3.4Ba1.3Eu0.3(PO4)3Cl. The LED package device not only
possesses high Ra and R9 values (>95) but also compensates
for the 480−520 nm cyan gap, which widens the research on
full-spectrum lighting.
■
ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsami.6b10233.
Experimental methods, X-ray Rietveld refinement,
Eu3+characteristic emission spectra, temperature-dependent decay, energy level, quantum efficiency, package
result (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work was supported by the Ministry of Science and
Technology of Taiwan (Contracts MOST 104-2113-M-002012-MY3, MOST 104-2119-M-002-027-MY3, and MOST 1042923-M-002-007-MY3) and National Center for Research and
Development Poland Grant (PL-TW2/8/2015).
■
REFERENCES
(1) Wang, L.; Wang, X.; Kohsei, T.; Yoshimura, K. i.; Izumi, M.;
Hirosaki, N.; Xie, R. J. Highly Efficient Narrow-Band Green and Red
Phosphors Enabling Wider Color-Gamut Led Backlight for More
Brilliant Displays. Opt. Express 2015, 23, 28707−28717.
(2) Yeh, C. W.; Chen, W. T.; Liu, R. S.; Hu, S. F.; Sheu, H. S.; Chen,
J. M.; Hintzen, H. T. Origin of Thermal Degradation of Sr2−xSi5N8:Eux
Phosphors in Air for Light-Emitting Diodes. J. Am. Chem. Soc. 2012,
134, 14108−14117.
(3) Chen, W. T.; Sheu, H. S.; Liu, R. S.; Attfield, J. P. Cation-SizeMismatch Tuning of Photoluminescence in Oxynitride Phosphors. J.
Am. Chem. Soc. 2012, 134, 8022−8025.
30681
DOI: 10.1021/acsami.6b10233
ACS Appl. Mater. Interfaces 2016, 8, 30677−30682
Letter
ACS Applied Materials & Interfaces
(4) Daicho, H.; Iwasaki, T.; Enomoto, K.; Sasaki, Y.; Maeno, Y.;
Shinomiya, Y.; Aoyagi, S.; Nishibori, E.; Sakata, M.; Sawa, H.;
Matsuishi, S.; Hosono, H. A Novel Phosphor for Glareless White
Light-Emitting Diodes. Nat. Commun. 2012, 3, 1132.
(5) Xie, R. J.; Hirosaki, N. Silicon-Based Oxynitride and Nitride
Phosphors for White Ledsa Review. Sci. Technol. Adv. Mater. 2007,
8, 588−600.
(6) Höppe, H. A. Recent Developments in the Field of Inorganic
Phosphors. Angew. Chem., Int. Ed. 2009, 48, 3572−3582.
(7) Vogel, S. J. Structural Changes in Agriculture: Production
Linkages and Agricultural Demand-Led Industrialization. Oxford. Econ
Pap. 1994, 136−156.
(8) Su, S. J.; Chen, X. X.; Shi, W. B.; Zhu, W. T. Design of
Automobile Ellipsoid Headlight Based on High Power Led [J].
Semicond. Technol. 2007, 8, 007.
(9) Wei, L. L.; Lin, C. C.; Fang, M. H.; Brik, M. G.; Hu, S. F.; Jiao,
H.; Liu, R. S. A Low-Temperature Co-Precipitation Approach to
Synthesize Fluoride Phosphors K2MF6:Mn4+(M= Ge, Si) for White
Led Applications. J. Mater. Chem. C 2015, 3, 1655−1660.
(10) Sheu, J. K.; Chang, S. J.; Kuo, C.; Su, Y. K.; Wu, L.; Lin, Y.; Lai,
W.; Tsai, J.; Chi, G. C.; Wu, R. White-Light Emission from near Uv
Ingan-Gan Led Chip Precoated with Blue/Green/Red Phosphors.
IEEE Photonics Technol. Lett. 2003, 15, 18−20.
(11) Noetzold, D.; Wulff, H.; Herzog, G. Structural and Optical
Properties of the System (Ca, Sr, Eu)5(PO4)3Cl. Phys. Status Solidi B
1995, 191, 21−30.
(12) Shang, M.; Geng, D.; Yang, D.; Kang, X.; Zhang, Y.; Lin, J.
Luminescence and Energy Transfer Properties of Ca2Ba3(PO4)3Cl and
Ca2Ba3(PO4)3Cl:A (A= Eu2+/Ce3+/Dy3+/Tb3+) under Uv and LowVoltage Electron Beam Excitation. Inorg. Chem. 2013, 52, 3102−3112.
(13) Deressa, G.; Park, K.; Jeong, H.; Lim, S.; Kim, H.; Jeong, Y.;
Kim, J. Spectral Broadening of Blue (Sr,Ba)5(PO4)3Cl:Eu2+ Phosphors
by Changing Ba2+/Sr2+ Composition Ratio for High Color Rendering
Index. J. Lumin. 2015, 161, 347−351.
(14) Han, J.; Zhou, W.; Qiu, Z.; Yu, L.; Zhang, J.; Xie, Q.; Wang, J.;
Lian, S. Redistribution of Activator Tuning of Photoluminescence by
Isovalent and Aliovalent Cation Substitutions in Whitlockite
Phosphors. J. Phys. Chem. C 2015, 119, 16853−16859.
(15) Huang, K. W.; Chen, W. T.; Chu, C. I.; Hu, S. F.; Sheu, H. S.;
Cheng, B. M.; Chen, J. M.; Liu, R. S. Controlling the Activator Site to
Tune Europium Valence in Oxyfluoride Phosphors. Chem. Mater.
2012, 24, 2220−2227.
(16) Zhang, Y.; Li, X.; Li, K.; Lian, H.; Shang, M.; Lin, J. Crystal-Site
Engineering Control for the Reduction of Eu3+ to Eu2+ in CaYAlO4:
Structure Refinement and Tunable Emission Properties. ACS Appl.
Mater. Interfaces 2015, 7, 2715−2725.
(17) Shang, M.; Li, C.; Lin, J. How to Produce White Light in a
Single-Phase Host? Chem. Soc. Rev. 2014, 43, 1372−1386.
(18) Li, G.; Tian, Y.; Zhao, Y.; Lin, J. Recent Progress in
Luminescence Tuning of Ce3+ and Eu2+-Activated Phosphors for PcWLEDs. Chem. Soc. Rev. 2015, 44, 8688−8713.
30682
DOI: 10.1021/acsami.6b10233
ACS Appl. Mater. Interfaces 2016, 8, 30677−30682