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A ruthenium(II) complex containing a p-cresol group induces apoptosis in human cervical carcinoma cells through endoplasmic reticulum stress and reactive oxygen species production.
Article
Mining Unexplored Chemistries for Phosphors
for High-Color-Quality White-Light-Emitting
Diodes
Zhenbin Wang, Jungmin Ha,
Yoon Hwa Kim, Won Bin Im,
Joanna McKittrick, Shyue Ping
Ong
imwonbin@jnu.ac.kr (W.B.I.)
jmckittrick@eng.ucsd.edu (J.M.)
ongsp@eng.ucsd.edu (S.P.O.)
HIGHLIGHTS
Data-driven prediction and
screening identified Sr2LiAlO4 as
promising phosphor host
Sr2LiAlO4 is the first synthesized
quaternary oxide in the Sr-Li-Al-O
chemical space
Sr2LiAlO4:Eu2+/Ce3+ exhibits
green-yellow/blue emission with
good thermal stability
LEDs with Sr2LiAlO4:Eu2+/Ce3+
phosphors yield light with
excellent color quality
The discovery of new phosphors is key to the development of highly efficient and
environmentally friendly LED-based lighting. By applying data-driven structure
prediction and quantum mechanics-based screening on unexplored chemistries,
we identified a novel, earth-abundant phosphor host, Sr2LiAlO4, which was
successfully synthesized and integrated into prototype LEDs with high color
quality.
Wang et al., Joule 2, 1–13
May 16, 2018 ª 2018 Elsevier Inc.
https://doi.org/10.1016/j.joule.2018.01.015
Please cite this article in press as: Wang et al., Mining Unexplored Chemistries for Phosphors for High-Color-Quality White-Light-Emitting Diodes, Joule (2018), https://doi.org/10.1016/j.joule.2018.01.015
Article
Mining Unexplored Chemistries
for Phosphors for High-Color-Quality
White-Light-Emitting Diodes
Zhenbin Wang,1,4 Jungmin Ha,2,4 Yoon Hwa Kim,3,4 Won Bin Im,3,* Joanna McKittrick,2,*
and Shyue Ping Ong1,5,*
SUMMARY
Context & Scale
There is a critical need for new earth-abundant phosphors to enable next-generation, highly efficient solid-state lighting. Here we report the discovery of
Sr2LiAlO4, the first known Sr-Li-Al-O quaternary crystal, via a carefully targeted
data-driven structure prediction and screening effort using density functional
theory calculations. Sr2LiAlO4 is predicted and experimentally confirmed to
be a thermodynamically and thermally stable phosphor host that can be excited
with near-UV/blue sources. The Eu2+- and Ce3+-activated Sr2LiAlO4 phosphors
exhibit broad emissions at lmax 512 nm (green-yellow) and lmax 434 nm
(blue), respectively, with excellent thermal quenching resistance of >88% intensity at 150 C. A prototype phosphor-converted white LED utilizing Sr2LiAlO4based phosphors yields an excellent color-rendering index exceeding 90.
Sr2LiAlO4 therefore exhibits great potential for industrial applications in lowcost, high-color-quality WLEDs.
Solid-state lighting based on
phosphor-converted lightemitting diodes (pc-WLEDs) are
highly efficient, environmentally
friendly, and exhibit superior
durability and reliability. For
domestic lighting, a warm whitelight LED with good quantum
efficiency, resistance to thermal
quenching, high color-rendering
index (CRI), and low correlated
color temperature is desired.
Here, we report the discovery of
an earth-abundant Sr2LiAlO4
phosphor host using data-driven
structure prediction and
screening of unexplored
chemistries. The synthesized
Sr2LiAlO4:Eu2+ and
Sr2LiAlO4:Ce3+ phosphors exhibit
broad green-yellow and blue
emissions, respectively, with
excellent thermal quenching
resistance. A prototype pc-WLED
utilizing Sr2LiAlO4:Eu2+ yields an
excellent CRI > 90. This work
highlights the effectiveness of an
integrated in silico and
experimental approach in the
discovery of a technological
material in a novel chemistry.
INTRODUCTION
Phosphor-converted white-light-emitting diodes (pc-WLEDs) are among the most
promising solid-state lighting (SSL) technologies due to their high energy
efficiency and superior durability and reliability.1,2 For general illumination, we
need a warm white light with excellent quantum efficiency, resistance to thermal
quenching, and high color quality, i.e., a low color-correlated temperature (CCT)
of <3,000 K and a high color-rendering index (CRI) of >85.3 Typical commercial
WLEDs based on blue-emitting (450 nm) LED chips combined with a yellow-emitting phosphor (Y3Al5O12:Ce3+) have poor CRI <80 and high CCT >5,000 K.4
To improve the CRI and CCT, an alternative approach is to use near-UV
(380–420 nm) or blue LED chips with a mixture of red, green, and blue phosphors.
Lu3Al5O12:Ce3+ and (Ba,Sr)2SiO4:Eu2+ are two well-known green emitters
with good photoluminescence (PL) properties used for these phosphor mixtures.5
Unfortunately, the former requires the rare-earth Lu in large quantities, while the
latter suffers from severe thermal quenching. Meanwhile, most commercial
red-emitters are Eu2+-activated nitrides, such as CaAlSiN3:Eu2+ and Sr2Si5N8:Eu2+,
which are synthesized under harsh conditions (high temperature and high
pressure).6,7 Yet another way to generate white light with high color quality is
to leverage on a single-phase broad-band emitter that covers a wide range
of the visible spectrum (400–700 nm). Ba0.93Eu0.07Al2O4 is an example of a
recently discovered broad-band phosphor with good CRI >80 and CCT
<4,000 K,8 but its synthesis requires a high temperature of 1,450 C and a low pressure of 667 Pa.
Joule 2, 1–13, May 16, 2018 ª 2018 Elsevier Inc.
1
Please cite this article in press as: Wang et al., Mining Unexplored Chemistries for Phosphors for High-Color-Quality White-Light-Emitting Diodes, Joule (2018), https://doi.org/10.1016/j.joule.2018.01.015
There is therefore an urgent need to discover novel earth-abundant phosphors
with reasonably facile synthesis for pc-WLED applications. Hitherto, the
discovery of phosphor materials has largely taken place through painstaking
experiments, such as using exploratory crystal growth,1 combinatorial chemistry
screening,9 and single-particle diagnosis,10 in an Edisonian fashion. In recent years,
high-throughput density functional theory (DFT) calculations have emerged
as a powerful complementary tool to experiments to accelerate materials
discovery, with successes having been demonstrated in many application
areas.11–14 By enabling rapid evaluation across multiple application-specific
properties, DFT calculations can be used to effectively screen thousands of materials to identify a small subset of candidates for subsequent synthesis and experimental evaluation. Nevertheless, there have been no successful demonstrations
of in silico phosphor discovery to date, no doubt due in part to extensive experimental efforts in the field as well as the difficulty in predicting optical properties
with DFT.
In this work, we report the discovery of a novel, earth-abundant phosphor host,
Sr2LiAlO4, which to the authors’ knowledge is also the first known Sr-Li-Al-O quaternary compound. Sr2LiAlO4 was identified via a carefully targeted data-driven structure prediction and DFT screening effort guided by statistical analysis of known
phosphors in the Inorganic Crystal Structure Database (ICSD).15 We demonstrate
that Sr2LiAlO4 is predicted by DFT calculations to be thermodynamically and dynamically stable and to have the necessary bandgap, structural rigidity, and near-UV
excitation wavelength when activated with either Eu2+ or Ce3+. High-purity
Sr2LiAlO4 was synthesized via industrially scalable methods and characterized using
X-ray diffraction (XRD) and photoluminescence spectroscopy. The Eu2+- and Ce3+activated Sr2LiAlO4 phosphors exhibit broad emissions of lmax 512 nm (greenyellow) and lmax 434 nm (blue), respectively, with excellent thermal quenching
resistance of >88% intensity at 150 C. A prototype pc-WLED utilizing Sr2LiAlO4:Eu2+
yields an excellent CRI of >90.
RESULTS AND DISCUSSION
Data-Driven Discovery of New Phosphors
We began our search for novel phosphor hosts by constructing a ‘‘solid-state lighting’’ periodic table (Figure 1A) from a statistical analysis of all compounds in the
2017 version of the ICSD with the word ‘‘phosphor’’ in the publication title. The
high frequency of elements N, S, F, and Cl can be ascribed to the fact that (oxy)
nitrides, (oxy)halides, and sulfides are some of the most well-studied phosphor materials.16–18 Nevertheless, oxides are overwhelmingly preferred in practical SSL applications due to their typically more facile synthesis and better chemical stability
under ambient conditions; therefore, these other anion types will not be further
considered in this work. We may observe a preponderance of the alkalineearth metals (Mg/Ca/Sr/Ba), alkali metals (Li/Na/K), and main group elements
(Al/Si/P/B) among known phosphors. The presence of Ca and Sr is not surprising,
given that the ionic radii of Ca2+ and Sr2+ are similar to those of the common Ce3+
and Eu2+ activators.19 Na+ also presents a high frequency in phosphor hosts due to
its similar size to Eu2+. However, activation via aliovalent substitution of Na+ by
Eu2+ would require the identification of the most stable charge-compensating
defect and the use of large-supercell DFT calculations. Therefore, we have chosen
to focus on systems containing Sr2+/Ba2+/Ca2+ in this work as these are isovalent
with Eu2+. Given that phosphates, silicates, aluminates, and borates are among
the most commonly studied oxides in SSL,16–18 we then proceeded to identify
2
Joule 2, 1–13, May 16, 2018
1Department of Nanoengineering, University of
California, San Diego, 9500 Gilman Drive,
La Jolla, CA 92093, USA
2Materials Science and Engineering Program,
University of California, San Diego, 9500 Gilman
Drive, La Jolla, CA 92093, USA
3School of Materials Science and Engineering
and Optoelectronics Convergence Research
Center, Chonnam National University,
77 Yongbong-ro, Buk-gu, Gwangju 61186,
Republic of Korea
4These authors contributed equally
5Lead Contact
*Correspondence: imwonbin@jnu.ac.kr (W.B.I.),
jmckittrick@eng.ucsd.edu (J.M.),
ongsp@eng.ucsd.edu (S.P.O.)
https://doi.org/10.1016/j.joule.2018.01.015
Please cite this article in press as: Wang et al., Mining Unexplored Chemistries for Phosphors for High-Color-Quality White-Light-Emitting Diodes, Joule (2018), https://doi.org/10.1016/j.joule.2018.01.015
A
B
C
Figure 1. Data-Driven Discovery of Sr2LiAlO4
(A) Frequency at which each element appears in compounds having the word ‘‘phosphor’’ in the publication title in the 2017 version of ICSD. Only nonrare-earth elements are shown. Elements with frequency 0 are shaded in gray.
(B) Calculated 0 K SrO-Li 2 O-Al 2 O3 phase diagram. Blue circles, known stable phases in the Materials Project database; red square, new stable
quaternary phase, Sr 2 LiAlO 4 .
(C) Unit cell of the Sr 2 LiAlO 4 ðP21 =mÞ crystal and the two symmetrically distinct Sr sites. Numbers indicate Sr-O bond lengths in angstroms.
opportune chemistries for novel phosphor host discovery in the ternary M-X-O
(M = Ba/Sr/Ca, X = P/Si/Al/B) and quaternary M-L-X-O (M = Ba/Sr/Ca,
L = Li/Mg/Y, X = P/Si/Al/B) oxides.
We find that while ternary M-X-O oxides have been relatively well explored, significant opportunities exist in quaternary M-L-X-O oxides. In particular, there are no
reported compounds in the ICSD in seven chemistries: Ba/Sr/Ca-Li-Al-O, Sr-Li-P-O,
Ba/Sr-Y-P-O, and Ba-Y-Al-O (Figure S1). A further search of the larger Pauling File
database20 turned up one known compound each in the Sr-Li-P-O, Ba/Sr-Y-P-O,
and Ba-Y-Al-O chemistries, and still no compounds in the Ba/Sr/Ca-Li-Al-O chemistries. We generated 918 new crystal structure candidates in these seven unexplored chemical systems by applying a data-mined ionic substitution algorithm21
on the entire ICSD (see Experimental Procedures). These candidates were then systematically evaluated via an efficiently tiered series of DFT property calculations
(Figure S2). The first criterion that any technological material must satisfy is synthesizability and stability. Thermodynamic stability is estimated by calculating the energy above the linear combination of stable phases in the 0 K DFT phase diagram,
also known as Ehull. A typical threshold for synthesizability used in previous DFT
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screening works is an Ehull of <30–50 meV per atom.13,22 We find that the four
known phosphors in the Sr-Li-P-O, Ba/Sr-Y-P-O, and Ba-Y-Al-O chemistries reported in the Pauling database are indeed among the candidates generated
by the structure prediction algorithm, and all four compounds have a calculated
Ehull < 35 meV per atom (Table S1). The successful prediction of these ‘‘unseen’’
known phosphors from the ICSD gives us confidence that viable candidates are
identified via the combination of the data-mined ionic substitution algorithm
and DFT calculations. Among the remaining Ba/Sr/Ca-Li-Al-O chemical systems
with no known compounds, we will focus primarily on the Sr-Li-Al-O chemistry
due the fact that Sr2+ has an ionic radius (1.26 Å) that is closest to Eu2+ (1.25 Å),
compared with Ba2+(1.42 Å) or Ca2+(1.12 Å).19 Also, Li and Al are inexpensive,
earth-abundant elements that tend to form rigid bonds in crystals.
Figure 1B shows the calculated 0 K SrO-Li2O-Al2O3 phase diagram. We find that one
of the new candidates, Sr2LiAlO4, is predicted to be thermodynamically stable
(Ehull = 0). The computed phonon spectrum (Figure S3) confirms it to be also dynamically stable. The crystal structure of Sr2LiAlO4 (space group: P21 =m) is shown in Figure 1C, and the structural parameters are provided in Table S2. Sr2LiAlO4 is derived
from Ba2LiReN4 (ICSD no. 411453) via a multi-species substitution of Ba2+ with Sr2+,
Re7+ with Al3+, and N3 with O2. This is clearly a non-trivial crystal prediction that
cannot be easily replicated using traditional chemical intuition.
We evaluated the potential PL properties of the stable Sr2LiAlO4 host by calculating
its electronic structure and Debye temperature (QD). The calculated bandgap Eg of
Sr2LiAlO4 using the Perdew-Burke-Ernzerhof (PBE) functional13 is 4.19 eV, which suggests that it would yield a green-yellow emission with Eu2+ activation based on the
inverse relationship between experimental wavelength and the PBE Eg previously reported by the current authors.13 In general, host materials that have a large photoionization barrier, defined as the energy gap between conduction band minimum
and excited 5d level, are rigid and tend to exhibit excellent thermal quenching resistance.23,24 The calculated bandgaps of Sr2LiAlO4 using the more accurate HeydScuseria-Ernzerhof (HSE)25,26 functional and G0W027 are 5.91 eV and 6.00 eV,
respectively. This large bandgap suggests a strong likelihood of a large photoionization barrier. The calculated QD of Sr2LiAlO4 is 466 K, indicating that it has a rigid
crystal structure.28
There are two symmetrically distinct Sr crystallographic sites (labeled Sr1 and Sr2 in
Figure 1C) in Sr2LiAlO4, both of which are 8-fold coordinated with oxygen atoms. By
performing an isovalent substitution a single Eu2+ into a 2 3 2 3 2 supercell of
Sr2LiAlO4 (16 formula units), we determined using DFT calculations that the Eu2+
activator prefers the Sr1 site to the Sr2 site by about 35 meV. For Ce3+ activation,
we comprehensively evaluated various charge-neutral defect configurations, taking
into account typical experimental synthesis conditions such as excess Li from its volatility at elevated temperature. We find that the 2Ce$Sr + Li00Al defect combination
(Kröger-Vink notation) has the lowest defect formation energy compared with
2Ce$Sr + V00Sr (0.22 eV/Ce3+ higher) and Ce$Sr + Li0Sr (1.48 eV/Ce3+ higher). Hence, we
conclude that the substitution of Ce3+ on Sr2+ is likely to be charge compensated
by excess Li+ on the Al3+ tetrahedra, as opposed to vacancy formation or excess
Li+ on the Sr2+ site. This is consistent with the fact that the small Li+ ion (ionic radius =
0.9 Å) is likely to prefer the AlO4 tetrahedron rather than the much larger SrO8 site.
Henceforth, we will use the shorthand notation commonly used in the phosphor
community, Sr2LiAlO4:xEu2+ and Sr2LiAlO4:yCe3+, to denote the activated structures with compositions Sr2xEuxLiAlO4 and Sr2yCeyLi1+y/2Al1y/2O4, respectively.
4
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A
B
Figure 2. Computed Absorption Spectra for Sr2LiAlO4:Eu2+ and Sr2LiAlO4:Ce3+
(A and B) Absorption spectrum (characterized by absorption wavelength, labs ) for
Sr 2 LiAlO 4 :0.0625Eu 2+ (with Eu 2+ locating at the most stable site, Sr1) (A) and Sr 2 LiAlO 4 :0.125Ce 3+
(with the lowest energy 2Ce$Sr + Li00Al configuration) (B) calculated using the Bethe-Salpeter equation
method.
All subsequent DFT results on activated hosts, unless otherwise stated, were performed using the configuration with the lowest defect formation energy.
For efficient conversion, a phosphor should have absorption spectrum peaking
at the maximum emission of LED chips. The absorption spectra for
Sr2LiAlO4:0.0625Eu2+ and Sr2LiAlO4:0.125Ce3+ were calculated using the BetheSalpeter equation (BSE) method29 on top of G0W0, as shown in Figure 2. For both
activated phosphors, the main absorption peaks are observed at 380–420 nm, which
can be attributed to 4f7 / 4f65d1 transition in Eu2+ or 4f1 / 4f05d1 transition in
Ce3+. These findings suggest that both Eu2+- and Ce3+-activated Sr2LiAlO4 can be
efficiently excited by near-UV LED chips.
Synthesis and Photoluminescence Properties
The Sr2LiAlO4 host and the Eu2+- and Ce3+-activated phosphors were successfully
produced using solid-state reaction as well as combustion synthesis. Here, we will
discuss primarily the results from the samples synthesized via solid-state reaction,
given that this is the preferred approach in commercial applications due to its low
cost, availability of precursors, and potential for production on an industrial scale.
In general, other than a higher purity (94% versus 86% for solid-state reaction), the
measured structural parameters and PL properties of the combustion-synthesized
samples are very similar. Figure 3 shows the simulated and measured XRD profiles
of the host and activated phosphors, which are in excellent agreement and confirm
the successful synthesis of the predicted Sr2LiAlO4 compound. The residual factors
of Rietveld refinement analysis of the XRD profile (see Table S3) are Rwp = 9.11%,
Rp = 6.69%, and goodness of fit (GOF) = 2.79. The refined structure parameters
are also in excellent agreement with those from the DFT relaxed structure (see
Tables S2 and S3).
The measured PL spectrum of the Sr2LiAlO4:0.005Eu2+ phosphor excited at 394 nm
(Figure 4A) shows a green-yellow emission peaking at 512 nm with a shoulder peak
of 559 nm. The emission spectrum is broad (full width at half maximum [FWHM] =
73.6 nm) and asymmetric, indicating that Eu2+ ions occupy two distinct sites in the
Sr2LiAlO4 host. The excitation spectrum monitored at 512 nm shows a broad
band with two main peaks at 310 nm and 394 nm. The PL spectrum of the
Sr2LiAlO4:0.005Ce3+ phosphor measured at 384 nm excitation (Figure 4B) shows a
broad blue emission with a main peak at 434 nm and an FWHM of 70.3 nm. The
PL excitation recorded at 434 nm also presents two peaks: one major peak at
Joule 2, 1–13, May 16, 2018
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Figure 3. Simulated and Measured X-Ray Diffraction Patterns of Sr2LiAlO4, Sr2LiAlO4:0.005Eu2+,
and Sr2LiAlO4:0.005Ce3+
384 nm and one minor peak at 291 nm. The measured excitation spectra are consistent with the 4f-5d transitions of Eu2+/Ce3+ ions, and the BSE-computed excitation
wavelengths in Figure 2.
A careful optimization of the PL properties of Sr2LiAlO4:xEu2+ and Sr2LiAlO4:yCe3+
was carried out with respect to activator concentration (x, y). As shown in Figure 4C,
the emission intensity slightly increases and then decreases with increasing activator
concentration, with the maximum emission intensity occurring at x or y = 0.005
for both activators. The measured internal quantum efficiencies of the
Sr2LiAlO4:0.005Eu2+ and Sr2LiAlO4:0.005Ce3+ phosphors are 25% (lex = 394 nm)
and 32% (lex = 392 nm), respectively.
Figure S4 presents the unnormalized and normalized PL spectra of Sr2LiAlO4:xEu2+
with respect to a series of Eu2+ concentrations (0.0025 % x % 0.0500) measured at
room temperature. With increasing Eu2+ concentration, the relative intensity of short
excitation wavelength (at 320 nm) gradually decreases, while the relative intensity of
long wavelength (at 480 nm) slightly increases, as shown in Figure S4C. At the same
time, a corresponding increase of emission intensity at 559 nm is also observed with
increasing of Eu2+ concentration when normalized based on emission intensity at
512 nm, as shown in Figure S4D. We believe the lower energy emission (longer
wavelength) peaks are associated with more Eu2+ occupying the energetically
more favorable Sr1 site. The calculated average bond length (lav) of EuO8 polyhedron in the Sr1 site and Sr2 site are 0.269 nm and 0.272 nm, respectively, while
the distortion indices (D) (see Experimental Procedures for definitions) are 0.056
and 0.044, respectively. A shorter lav and larger D is associated with a larger crystal
field splitting (CFS).30,31 The larger CFS of Eu2+ in the Sr1 site leads to a red shift in
emission, as illustrated in Figure 4D. These conclusions are further supported by deconvolution of the PL emission spectra of Sr2LiAlO4:0.005Eu2+ at 10 K and 298 K
(Figures S5A and S5B), which shows a significant decrease in the long wavelength
emission at room temperature. The lower thermal stability of the Eu in the Sr1 site
6
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A
B
C
D
Figure 4. Photoluminescence Properties of the Sr2LiAlO4:xEu2+ and Sr2LiAlO4:yCe3+ Phosphors
(A–C) Measured excitation and emission spectra of (A) Sr 2 LiAlO4 :0.005Eu 2+ and (B)
Sr 2 LiAlO 4 :0.005Ce 3+ phosphor. Colors are indicated under the emission spectra for easy reference.
(C) Normalized emission intensity of Sr2 LiAlO4 :xEu 2+ (under 394 nm excitation, green curve) with
respect to the Eu 2+ (x) concentration and Sr 2 LiAlO4 :yCe 3+ (under 384 nm excitation, blue curve) with
respect to the Ce 3+ (y) concentration.
(D) Schematic energy level diagram for Eu 2+/Ce 3+ ions in the Sr 2 LiAlO 4 host crystal structure. DEne
and 3 cfs denote the centroid shift due to the nephelauxetic effect and the crystal field splitting of
Eu 2+/Ce 3+ ions in the Sr 2 LiAlO 4 host, respectively. DEA is the photoionization barrier.
is also a consequence of its larger CFS, which leads to a smaller barrier for photoionization ðDEA Þ.23 To obtain further support for these conclusions, we calculated the
HSE projected density of states (Figure S6) for a Sr2LiAlO4:Eu2+ structure where
Eu is introduced into both Sr1 and Sr2 simultaneously. We find that Eu in the Sr1
site has a smaller gap (longer excitation wavelength) between the 4f and 5d states
compared with Eu in the Sr2 site.
In contrast, there are no significant changes in the relative intensities of both
the excitation and emission peaks at different wavelengths with increasing of
Ce3+ concentration in Sr2LiAlO4:yCe3 (Figure S7). The emission spectra of
Sr2LiAlO4:0.005Ce3+ can be deconvoluted into four Gaussian peaks at 10 K with position of 433 (peak 1: 23,095 cm1), 468 (peak 2: 21,368 cm1), 497 (peak 3:
20,121 cm1), and 532 nm (peak 4: 18,797 cm1), as shown in Figure S5C. The energy difference between peak 1 and peak 3 is about 2,974 cm1 (0.37 eV), and between peak 2 and peak 4 is about 2,571 cm1 (0.32 eV). These values correspond to
the spin-orbit splitting energy of the lowest 4f level (2F5/2 and 2F7/2) in Ce3+.32 However, at 298 K (Figure S5D) only two Gaussian peaks are observed at 427 nm
(23,419 cm1) and 460 nm (21,739 cm1). These observations again suggest that
the Sr1 site, associated with the long wavelength emission, is thermally unstable
with no PL at room temperature.
Thermal Stability
In practical applications, WLEDs typically operate at elevated temperatures
(150 C), and a key metric of phosphor performance is its resistance to thermal
quenching. Figure 5 shows the measured temperature-dependent emission intensity for Sr2LiAlO4:0.005Eu2+. At 150 C, the emission intensity of the main peak
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A
B
Figure 5. Thermal Quenching and Site-Dependent Photoluminescence Properties of
Sr2LiAlO4:0.005Eu2+
(A) Temperature-dependent emission spectra under 394 nm excitation in the temperature range of
25 –200 C.
(B) Normalized temperature-dependent emission intensity under the peak emission wavelength
(l max ), 559 nm and integrated emission intensity (total area) with a temperature interval of 25 C.
(lem 512 nm) is about 88% of that at room temperature. The emission intensity of
the secondary peak (lem 559 nm), which is associated with Eu2+ in the thermally
less stable Sr1 site, on the other hand, reduces significantly with increasing temperature. Overall, Sr2LiAlO4:0.005Eu2+ exhibits excellent resistance to thermal quenching. The Sr2LiAlO4:0.005Ce3+ phosphor is found to have a slightly better resistance
to thermal quenching, retaining 91% of room temperature peak emission intensity at
150 C (Figure S8).
Performance of WLEDs
Finally, we constructed prototype WLED devices using Sr2LiAlO4:Eu2+ and
Sr2LiAlO4:Ce3+ as well as a mixture of Sr2LiAlO4:Eu2+/Ce3+. The electroluminescence (EL) spectra of these WLEDs are shown in Figures 6A–6C. The prototype
WLED using Sr2LiAlO4:Eu2+ exhibits CIE color coordinates of 0.301 and 0.323,
with high CRI of 93 and a CCT of 7,527 K at a forward bias current of 60 mA
(Table S4). The excellent CRI of the Sr2LiAlO4:Eu2+ phosphor can be attributed to
the feature of its broad-band emission by two Sr sites. The CCT of the WLED can
be further lowered by using a mixture of Sr2LiAlO4:Eu2+/Ce3+, as shown in
Table S5, as well as via the addition of a red component. For instance, the recently
reported SrLiAl3N4:Eu2+ narrow-band red phosphor1 would be a particularly interesting option, which would allow for the construction of an oxide + nitride device using the same earth-abundant elements. The measured luminous efficacy of WLEDs
with Sr2LiAlO4:Eu2+/Ce3+ phosphors are 19–36 lm W1, which is comparable with
that of WLEDs utilizing some recently reported novel phosphors, but still somewhat
lower than that of commercial WLEDs utilizing the Y3Al5O12:Ce3+ phosphor
(Table S6). To further improve the quantum efficiency of Sr2LiAlO4, crystal-site engineering techniques,17,33 e.g., co-doping with Ba or Ca, may be used to shift the energetic preference and/or tune the CFS of the two Sr sites.
To conclude, we have identified a novel earth-abundant Sr2LiAlO4 phosphor host—
the first known Sr-Li-Al-O quaternary compound—by composing a ‘‘solid-state lighting’’ periodic table based on statistical analysis of the ICSD to identify unexplored
yet promising chemical spaces with data-mined structure prediction and highthroughput DFT property computations. Sr2LiAlO4 comprises inexpensive, earthabundant elements (other than the rare-earth activator, which is required in small
quantities), and the DFT and PL results show Sr2LiAlO4:Eu2+/Ce3+ to have efficient
near-UV excitation, good thermal quenching resistance, and broad green-yellow/
blue emission. High-purity Sr2LiAlO4-based phosphors can be synthesized with
8
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Figure 6. Performance of Prototype pc-WLED Using the Single Sr2LiAlO4:Ce3+ and Sr2LiAlO4:Eu2+ Phosphor
(A) Electroluminescence (EL) spectra and photograph of the InGaN LED (l max = 400 nm) + Sr 2 LiAlO4 :0.005Ce 3+ phosphor.
(B) EL spectra and photograph of the InGaN LED (l max = 400 nm) + Sr 2 LiAlO 4 :0.005Ce 3+ + Sr 2 LiAlO 4 :0.005Eu 2+ phosphor.
(C) EL spectra and photograph of the InGaN LED (l max = 450 nm) + Sr 2 LiAlO 4 :0.005Eu 2+ phosphor.
(D) Photograph of WLEDs packaged with Sr 2 LiAlO 4 :0.005Eu 2+ phosphor.
(E) CIE chromaticity coordinates of the fabricated Sr 2 LiAlO 4 -based phosphors WLED under various forward bias currents.
scalable, industrially relevant methods. We therefore believe the novel
Sr2LiAlO4:Eu2+/Ce3+ phosphors to be highly promising candidates for low-cost,
high-CRI WLED applications.
EXPERIMENTAL PROCEDURES
Candidate Structure Generation
New crystal structure candidates for the target chemistries were generated by
applying a retrained version of the ionic substitution algorithm developed by Hautier
et al.21 on the ICSD.15 This algorithm codifies data-mined probabilities for substitution of one species by another. New candidates are generated by performing highprobability substitutions on a list of known crystal structures, in this case all ordered
crystal structures in the 2017 version of the ICSD.
Density Functional Theory Calculations
All DFT calculations were carried out using the Vienna ab initio simulation package,
VASP, within the projector-augmented wave method.34,35 The generalized gradient
approximation PBE functional36 was used for structural relaxations and energy calculations. The plane wave energy cutoff was 520 eV, and the Brillouin zone was integrated with a k-point grid at least of 100 per Å3 (reciprocal lattice volume). All
host crystal structures were relaxed with parameters in line with Materials Project.37
The Eu2+/Ce3+-activated phosphor structures were fully relaxed until the energies
and forces were converged to 105 eV and 0.01 eV Å1, respectively. All crystal
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structure manipulations and data analysis were carried out using the Python Materials Genomics package.38
The phase stabilities of the predicted Sr-Li-Al-O compounds were estimated by
calculating the energy above the linear combination of stable phases in the 0 K
DFT phase diagram,39 also known as the energy above hull, Ehull. Previous studies
have shown that 60% of ICSD oxides have Ehull less than 30 meV per atom;22 we
therefore use this threshold as a reasonable cutoff for synthesizability. For phase
diagram construction, the energies of all compounds other than those of direct interest in this work were obtained from the Materials Project37 via the Materials Application Programming Interface.40 The dynamical stability of the identified Sr2LiAlO4
host was evaluated by computing the phonon spectrum using density functional
perturbation theory as implemented in the Phonopy code41 in conjunction with
VASP34 as the force calculator. More stringent energy and force criteria of 108 eV
and 104 eV Å1, respectively, were used for these calculations.
Defect formation energies were calculated using the formalism presented by Wei
et al.42:
X
D
B
Ef = Etot
Etot
ni mi ;
i
D
B
and Etot
are the total energies of the structure with and without the dewhere Etot
fect(s), respectively; mi is the atomic chemical potential of species i; ni indicates
the number of atoms of species i being added (ni > 0) or removed (ni < 0) to form
the defect. mi is estimated based on the chemical pontentials for each specie based
on the relevant region of the 0 K DFT phase diagram.
The host bandgaps were calculated using PBE for the initial rapid screening, and
more detailed investigations of the electronic structure of Sr2LiAlO4 were carried
out using the more computationally expensive and accurate screened hybrid HSE
functional25,26 and single-shot GW method.27 The BSE29 simulation was performed
on top of G0W0 to calculate the absorption spectrum of Eu2+/Ce3+-activated
Sr2LiAlO4 phosphors. A large-supercell model (2 3 2 3 2, 128 atoms) was adopted
to mimic the relatively low Eu2+/Ce3+ concentrations in the experiment and to avoid
interactions between periodic images of activators.
The Debye temperature, QD, was evaluated using the quasi-harmonic model. The
elastic tensor was calculated with more stringent electronic convergence criterion
of 106 eV, and the elastic moduli were calculated using the Voigt-Reuss-Hill
approximation.13,43
Local Environment Analysis
The local environment of the Eu2+ activator was characterized using the average
Eu-O bond length ðlav Þ and the distortion index (D) of the EuO8 coordination polyP
av j
hedron. As per previous works,30,31 D is defined as D = 1n ni= 1 jli l
lav , where li is the distance from the center atom to the ith coordinating atom, and n is the total number of
Eu-O bonds (n = 8 in this case).
Synthesis
Solid-state reaction synthesis was performed using SrO (Kojundo, 99.9%), Li2CO3
(Kojundo, 99.9%), a-Al2O3 (Kojundo, 99.9%), and Eu2O3 (Kojundo, 99.9%) or
CeO2 (Kojundo, 99.99%). Stoichiometric amounts of the starting materials were
ground in agate mortar, placed in alumina crucibles, and fired at 900 C for 4 hr in
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a horizontal tube furnace using a 25% H2/N2 gas mixture to obtain Sr2LiAlO4:xEu2+
and Sr2LiAlO4:yCe3+ (0.0025 % x, y % 0.0500). After heat treatment, the samples
were cooled to room temperature and ground well with agate mortar into fine powders for further analysis. Excess Li source of up to 10 wt% was introduced to compensate for Li evaporation during synthesis.
Combustion reaction synthesis was performed using Sr2(NO3)2 (99.99%, Sigma
Aldrich), LiNO3 (ReagentPlus, Sigma Aldrich), Al(NO3)3,9H2O (ACS reagent, J.T.
Baker), Eu(NO3)3 from Eu2O3 (99.99%, Alfa Aesar) with nitric acid (69.3%, Fisher
Scientific), and Ce(NO3)3 (99.99%, Alfa Aesar) as precursors, assisted by the
exothermal reaction between urea (Certified ACS, Fisher Scientific) and ammonium
hydroxide (Certified ACS, Fisher Scientific) at 600 C. A post-annealing condition
was 800 C for 5 hr in a 5% H2/95% N2 atmosphere to transform Eu3+ to Eu2+ to
obtain Sr2LiAlO4:xEu2+ and Sr2LiAlO4:yCe3+ (0.001 % x, y % 0.040). Excess Li
source of up to 25 wt% was introduced to compensate for Li sublimation during
synthesis.
Crystal Structure Characterization
The powders by combustion reaction were analyzed by an X-ray diffractometer
(Bruker D2 Phaser, Karlsruhe, Germany) using CuKa radiation and a step size of
0.014 over a 2q range of 20 –80 . XRD data of powders by solid-state reaction
were obtained using CuKa radiation (Philips X’Pert). XRD data were collected
over angles of 10 % 2q % 120 with a step size of 0.026 . Structural information
of the synthesized samples was derived by refinement using the TOPAS software
suite from the XRD result. The VESTA program44 was used to draw the crystal
structure.
Optical Measurements
Photoluminescence of the solid-state-reaction synthesized samples was measured
using a Hitachi F-4500 fluorescence spectrophotometer over a wavelength range
of 200–700 nm. The quantum yield was measured with 394 nm and 450 nm excitation
using a xenon laser (Hamamatsu C9920-02) at the Korea Photonics Technology Institute (KOPTI), Gwangju, South Korea. The thermal quenching characteristics were
measured in the temperature range of 25 –200 C, connected to the Hitachi
F-4500 fluorescence spectrophotometer with integrated heater, temperature
controller, and thermal sensor. Low-temperature PL spectra were obtained under
excitation at 325 nm He-Cd laser connected to the cryostat system at the temperature of 10 K in the KOPTI.
Fabrication of pc-WLED Prototype
The white LED device prototype was fabricated by integrating the Sr2LiAlO4:0.005Ce3+
and mixed Sr2LiAlO4:0.005Ce3+/Eu2+ phosphors on an InGaN near-UV LED chip
(lmax = 400 nm) and the Sr2LiAlO4:0.005Eu2+ phosphor on an InGaN blue LED chip
(lmax = 450 nm). The device was then encapsulated in a phosphor/silicon resin mixture,
with the mixture placed directly on the headers of LED chip and cured at 150 C for
1 hr. After the packaging was completed, the WLED device was measured in an integrating sphere under direct current forward bias condition.
SUPPLEMENTAL INFORMATION
Supplemental Information includes eight figures and six tables and can be found
with this article online at https://doi.org/10.1016/j.joule.2018.01.015.
Joule 2, 1–13, May 16, 2018
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ACKNOWLEDGMENTS
This work was supported by the National Science Foundation, Ceramics Program,
under grant no. 1411192. The computational resources were provided by the Triton
Shared Computing Cluster (TSCC) at the University of California, San Diego, the National Energy Research Scientific Computing Center (NERSC), and the Extreme Science and Engineering Discovery Environment (XSEDE) supported by the National
Science Foundation under grant no. ACI-1053575. This work was also financially supported by the National Research Foundation of Korea (NRF) funded by the Ministry
of Science and ICT under grant no. NRF-2017R1A2B3011967.
AUTHOR CONTRIBUTIONS
Conceptualization, S.P.O. and Z.W.; Methodology, Z.W.; Investigation, Z.W., J.H.,
and Y.H.K.; Writing – Original Draft, Z.W., J.H., and Y.H.K.; Writing – Review and Editing, S.P.O., J.M., and W.B.I.; Supervision, S.P.O., J.M., and W.B.I.; Funding Acquisition, S.P.O., J.M., and W.B.I.
DECLARATION OF INTERESTS
A US provisional patent application Serial No. 62/572,084 has been filed on the
novel phosphors reported in this work.
Received: December 1, 2017
Revised: January 11, 2018
Accepted: January 26, 2018
Published: February 19, 2018
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