Blue-Emitting Sr3Si8–xAlxO7+xN8–x:Eu2+ Discovered by a Single

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Blue-emitting Sr3Si8-xAlxO7+xN8-x:Eu2+ Discovered by a Singleparticle-diagnosis Approach: Crystal Structure, Luminescence, Scaleup Synthesis, and and its Abnormal Thermal Quenching Behavior Xiao-Jun Wang, Le Wang, Takashi Takeda, Shiro Funahashi, Takayuki Suehiro, Naoto Hirosaki, and Rong-Jun Xie Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03252 • Publication Date (Web): 19 Oct 2015 Downloaded from http://pubs.acs.org on October 24, 2015

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Blue-emitting Sr3Si8-xAlxO7+xN8-x:Eu2+ Discovered by a Singleparticle-diagnosis Approach: Crystal Structure, Luminescence, Scale-up Synthesis, and its Abnormal Thermal Quenching Behavior Xiao-Jun Wang,1 Le Wang,2 Takashi Takeda,1 Shiro Funahashi,1 Takayuki Suehiro,1 Naoto Hirosaki,1 Rong-Jun Xie 1* 1

Sialon Group, Environment and Energy Materials Division, National Institute for Materials Science (NIMS), Namiki, Tsukuba, Ibaraki, 305-0044, Japan 2

College of Optical and Electronic Technology, China Jiliang University, Hangzhou, Zhejiang 310018, China

ABSTRACT: The single-particle-diagnosis approach allows for the fast discovery of novel luminescent materials using powdered samples. This paper reports a new blue-emitting Sr3Si8-xAlxO7+xN8-x:Eu2+ phosphor for solid state lighting and its scale-up synthesis. The structure-, composition- and temperature-dependent luminescence were investigated and discussed by means of various analytic techniques including single-crystal XRD diffractometer, single-particle fluorescence spectroscopy, FTIR spectra, decay time, low-temperature luminescence, and computed energy level scheme. Sr3Si8xAlxO7+xN8-x crystallizes in the monoclinic system (space group C2/c, no. 15) with a = 18.1828 (13) Å, b = 4.9721 (4) Å, c = 15.9557 (12) Å, β = 115.994 (10)ο, and Z = 2. The Sr atoms are coordinated to 8 and 6 O/N atoms and located in the voids along [010] formed by vertex-sharing (Si,Al)-(O,N)4 tetrahedra. Phase-pure powder samples of Sr3Si8-xAlxO7+xN8-x:Eu2+ were synthesized from the chemical composition of the single particle by controlling the x value. Luminescence of both a single particle and powders show a broad Eu2+ emission band centered at ~ 465 nm and a fwhm of ~ 70 nm, under the UV light irradiation. The title phosphor has a band gap of 5.39 eV determined from the UV-VIS spectrum, absorption efficiency of 83%, internal quantum efficiency of 44.9% and external quantum efficiency of 37.4% under the 355 nm excitation. An abnormal thermal quenching behavior is observed in Sr3Si8-xAlxO7+xN8-x:Eu2+ that has a high activation energy for thermal quenching (0.294 eV) but a low thermal quenching temperature (~ 370 K), which is ascribed to the partial overlap between the Eu2+ excited energy level and the conduction band of the host.

INTRODUCTION Nowadays, solid state lightings have become a crucial technology to the world’s energy saving.1 Nobel Prize in 2014 Physics was awarded jointly to Akasaki, Amano, and Nakamura “for the invention of efficient blue light emitting diodes which has enabled bright and energy-saving white light sources”,2 affirming the important contribution of white light emitting diodes (white-LEDs). Currently, white-LEDs are accepted as a superior white-light source over their incandescent and fluorescent counterparts, due to their high energy efficiency, long lifetime, robustness, and environmental compatibility.3-12 To date, phosphor-converted white-LEDs are considered as the simplest, cost-effective and more reliable solid state lighting, which are assembled by combining a blue, or ultraviolet (UV) LED chip with phosphors.7,12 Apparently, phosphor is a critical component in white-LEDs, playing a key role in determining the overall luminous efficacy, color rendering, color temperature, and lifetime of lighting devices. The ever-changing solid state lighting technology triggers an increasing demand of new phosphors with enhanced or specific properties. As a new comer in phosphor family, nitride phosphors (e.g., (oxo)nitridosilicate and (oxo)nitridoaluminosilicate) possess unique and promising luminescence, enabling them to be excellent candidates of down-conversion materials in solid state lighting devices.7,13-15 So far, a number of interesting nitride phosphors, such as AlN:Eu2+, β-sialon:Eu2+, α-sialon:Eu2+, CaAlSiN3:Eu2+, La3Si6N11:Ce3+, and Sr2Si5N8:Eu2+ have already been commercially available.16-21 Great efforts are continuously made to hunt for

nitride phosphors with new crystal structures or compositions. By using the single crystal X-ray diffraction method combined with high-resolution transmission electron microscope, Schnick et al. discovered a variety of new nitride luminescent materials, including SrAlSi4N7:Eu2+, Ba[Mg3SiN4]:Eu2+.22-24 Besides these, the material systems investigated are also extensively extended from nitridosilicate or nitridoaluminosilicate to nitridogallate (Mg3GaN3), nitridoaluminate (Sr[LiAl3]N4).25-27 Sohn et al. applied a heuristics optimization-assisted combinatorial chemistry method together with powder X-ray diffraction28,29, and reported several new nitride phosphors for solid state lighting, such as Ba(Si,Al)5(O,N)8:Eu2+, Ca1.5Ba0.5Si5N6O3:Eu2+, Ca15Si20O10N30: Eu2+, and Ce4-xCaxSi12O3+xN18-x:Eu2+.29-32 Some other research groups have also investigated the nitride material systems but not systematically, and found a few new phosphors including Sr3Si13Al3O2N21:Eu2+, and CaMg2AlN3:Eu2+.33,34 Recently, we have proposed a facile and high-speed method, i.e., the single-particle-diagnosis approach, to look for totally new nitride phosphors.35 Without the necessity of the growth of largesize single crystals (usually > 100 µm) or the preparation of phase-pure powder samples, this approach enables to clarify the crystal structure, composition and luminescence of a tiny single crystal with a diameter as small as 10 µm, by using a singlecrystal X-ray diffractometer and a home-built single-particle fluorescence spectrometer. Differing from those luminescent materials discovered through powder X-ray diffraction, nitride phosphors obtained by the single-particle-diagnosis approach face the prob-

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lem of the scale-up of powder synthesis that is required by industries. In some cases, the powder synthesis starting from the stoichiometric composition of single crystal usually fails to achieve phase-pure powder.35-37 A typical example for this case is SrAlSi4N7:Eu2+, its pure powder is only achieved by adding double amount of AlN.36 Another example is Ba2LiSi7AlN12:Eu2+, the phase purity of powder is only 50% starting from the chemical composition of the single particle.37 In this contribution, we report on the crystal structure and luminescence of Sr2.86Eu0.14Si8-xAlxO7+xN8-x (x = 2.5) discovered in the SrO-Si3N4-AlN-SiO2 system by using the single-particle-diagnosis approach. Furthermore, we explore the scale-up of powder synthesis of the Sr3Si8-xAlxO7+xN8-x:Eu2+ single crystal, and discuss the structure-, composition-, and temperature-dependent luminescence of the phosphor powder.

EXPERIMENTAL SECTION Synthesis. Powder phosphor mixtures in SrO-Si3N4-AlN-SiO2 system, with random Sr:Si:Al:O:N ratios, were prepared by mixing appropriate amounts of SrCO3 (99.9% purity, AR grade, Sigma-Aldrich), Si3N4 (SN-E10, Ube Industries, Japan), SiO2 (Kojundo Chemical Laboratory Co. Ltd., Tokyo, Japan), AlN (Tokuyama Corp., Type F, Tokyo, Japan), and Eu2O3 (Shin-Etsu Chemical, Tokyo, Japan). The Eu2+ concentration was fixed at 5% with respect to Sr in all compositions. The mixtures were fired in a gas-pressure sintering furnace (FVPHR-R-10, FRET-40) at 1500 oC for 2 h in 1.0 MPa N2 atmosphere. The fired samples were then used for the screening of small single crystals. For the scale-up synthesis, powder samples of Sr3Si82+ were prepared at the same condition. The x xAlxO7+xN8-x:yEu value (i.e., the number of Al-O bond) and the y value (i.e., the Eu concentration) were varied in the range of x = 1.5 - 3.5 and y = 1 10%, respectively. Single-Particle Structural Analysis. XRD data of the single particle were collected in all directions using a SMART APEX II Ultra diffractometer with Mo Kα radiation (λ = 0.71073 Å) and multilayer mirrors as monochromator, operated at 50 kV and 50 mA. Applied absorption corrections were done using the multiscan procedure SADABS.38 The structure was solved by direct methods implemented in SHELSX-97.39 Refinement of crystal structure was carried out with anisotropic displacement parameters for all atoms by full-matrix least-squares calculation on F2 in SHELXL-97.40 Single-Particle Chemical Composition. The elemental analysis of the new single crystal was carried out on carbon-coated particles using a scanning electron microscopy (SU1510) equipped with an energy dispersive spectroscopy (XFlash SDD) operated at 15 kV. Single-Particle Luminescence. The photoluminescence spectra were recorded by a multichannel photo detector (MCPD7700, Otsuka electronics) through an optical bundle fiber attached to an optical microscope (BX51M, objective lens 10×-50×, Olympus).35 Powder X-ray Diffraction. The phase purity of the powder phosphor was identified by a powder X-ray diffraction (XRD) (Ultima III, Rigaku, Japan), using Cu Kα radiation. The Rietveld refinement analysis was carried out by using the program RIETAN-FP.41 Luminescence of Phosphor Powders. The photoluminescence spectra were measured at room temperature with a fluorescent spectrophotometer (Hitachi, F-4500) equipped with a 200W Xe lamp as an excitation source. Quantum Efficiency and Absorption efficiency. The external (η0 ) and internal (ηi ) quantum efficiencies (QEs) were calculated by using the following equations42:

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λ · P(λ) dλ
λ · P(λ) dλ ;η= (1)
λ · E(λ) dλ i
λE(λ) - R(λ) dλ where E(λ)/hυ, R(λ)/hυ and P(λ)/hυ are the number of photons in the excitation, reflectance and emission spectra of the phosphor, respectively. The luminescence spectra for QE measurements and the absorption efficiency were recorded using an intensified multichannel spectrometer (MCPD-7000, Otsuka Electronics, Japan). The reflection spectrum of spectralon diffusive white standard was used for calibration. Decay Time. Fluorescence lifetime data were collected by using a time-correlated single-photon counting fluorometer (TemPro, Horiba Jobin-Yvon) equipped with a Nano LED-370 nm with the pulse duration full width at half-maximum of ~1ns. FTIR Spectroscopy. The Fourier transform infrared spectroscopy (FTIR) spectra were recorded using a KBr pellet with a Spectrum BX II spectrometer (Thermos cientific, 4700). Temperature-dependent Luminescence. The low temperature-dependent luminescence was measured at 4-298 K by an intensified multichannel spectrometer (MCPD-9800, Otsuka Electronics, Japan). The phosphor powder was cooled at a cooling rate of 5 oC/min, and held at each temperature for 30 min. The high temperature-dependent luminescence was measured using an intensified multichannel spectrometer (MCPD-7000, Otsuka Electronics, Japan). The phosphor powder was heated up to 573 K in 25 K interval at a heating rate of 100 oC/min, and held at each temperature for 5 min. UV-VIS Spectroscopy. The diffusive reflection spectrum was recorded by using an UV-VIS spectrophotometer with an integrating sphere (JASCO, Ubest V-560). The spectrum was measured between 220 and 780 nm with 5 nm step size. η0 =

RESULTS AND DISCUSSION Distinguish a New Single Crystal from Powder Mixtures. Using the aforementioned approach described in our previous work,35 a new blue single particle was discovered with a chemical composition of Sr2.86Eu0.14Si8-xAlxO7+xN8-x (x=2.5). The discovery process is briefly described as follow: The synthesised powder mixture was firstly visualized under the 365 nm UV-LED excitation by a digital optical microscope. Among luminescent particles, a triangle blue single particle with a size about 15 µm x 11 µm x 4 µm was distinguished and assembled on a fine glass fiber, as shown in Figure 1. The lattice parameters of the single particle were preliminarily measured using a single-crystal X-ray diffractometer, and checked in available databases such as the Inorganic Crystal Structure Database. It was unable to find the matched XRD pattern in the database, suggesting that the single crystal was a new compound that was not reported before.

Figure 1. Emission color and morphology of a new single particle.

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Structure Determination. Crystal structure of the novel single crystal was further investigated in detail. The single crystal was refined in monoclinic space group C2/c (no.15) with a = 18.1828 (13) Å, b = 4.9721 (4) Å, c = 15.9557 (12) Å, β = 115.994 (10)ο, and Z = 2, as summarized in Table 1. Both the structure and EDS analyses determine the chemical composition of the new compound as Sr2.86Eu0.14Si5.5Al2.5O9.5N5.5. Atomic coordinates, isotropic displacement parameters, occupancies and anisotropic displacement parameters are given in Table 2 and Table S1.

Table 1. Crystal data and refinement details of Sr2.86Eu0.14Si5.5Al2.5O9.5N5.5. crystal system

Monoclinic

space group

C 2/c (No. 15)

cell parameters/Å

a = 18.1828 (13), b = 4.9721 (4) c =15.9557 (12)

β /° V /Å

The crystal structure of Sr2.86Eu0.14Si5.5Al2.5O9.5N5.5 is built up by vertex-sharing (Si,Al)(O,N)4 tetrahedra (Figure 2), which is the typical structural feature for oxonitridosilicates and oxonitridoaluminosilcates.7 This leads to a condensation degree k = n(Si,Al): n(O,N) of 0.533. The (Si,Al)-(O,N) bond lengths range from 1.609 to 1.780 Å with an average of 1.708 Å, shorter than that of SrSi9Al19ON31 (~1.870 Å).43 The (Si,Al)(O,N)4 tetrahedra form two types of channels along the [010] direction (Figure 2a). Sr atoms locate in the channels, coordinated to 8 (Sr1) and 6 (Sr2) O/N atoms, forming a cubic antiprism and octahedron respectively (Figure 3). The Sr-(O,N) bond distance is in the range of 2.514-3.097 Å for Sr1 with an average distance of 2.797 Å, and in the range of 2.572-2.741 Å for Sr2 with an average distance of 2.648 Å (Table 3). Similar to other SiAlON phosphors, activator ions Eu2+ enter the structure by replacing Sr atoms. In addition, there are 8 anion sites in the structure, all of which are occupied by 37% N and 63% O.

115.994 (10) 3

1296.579

Z

2

radiation type

Mo Kα

absorption coefficient

13.12

Cryatal size/μm

15 x 11 x 4

diffractometer

Three-cycle diffratometer

abs correction

Multiscan (SADABS)

measured reflections

7498

independent reflections

3075

observed reflections

1576

Rint

0.041 2

2

R[F > 2σ(F )]

Single-Particle Luminescence. Photoluminescence spectra of the Sr2.86Eu0.14Si5.5Al2.5O9.5N5.5 single particle were measured by a home-built single-particle fluorescence spectroscopy,35 as shown in Figure 4.

0.033

2

wR(F )

0.075

S Δρmax, Δρmin/eÅ

Figure 3. Coordination polyhedra of (Sr1)(O,N)8 and (Sr2)(O,N)6.

1.15 -3

1.05, -0.87

Figure 4. Excitation (red) and emission (blue) spectra of the Sr2.86Eu0.14Si5.5Al2.5O9.5N5.5 single particle.

Figure 2. Crystal structures of Sr2.86Eu0.14Si5.5Al2.5O9.5N5.5. Sr1, red; Sr2, green; and (Si,Al)(O,N)4, orange tetrahedron.

Due to the absorption of the lens glass and the sensitivity of the detector, the excitation spectrum was measured from 335 nm, which displays a band with a tail extending to 440 nm. Upon excitation at 355 nm, the single particle gives a blue emission peaking at 460 nm with full width at a half maximum (fwhm) of about 73 nm. This emission band can be well deconvoluted into two subbands (455 and 493 nm) by Gaussian fitting, corresponding to the emissions from two different crystallographic sites of Eu2+ ions

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Table 2. Atomic positions of the Sr2.86Eu0.14Si5.5Al2.5O9.5N5.5 phase. Atom

x

y

z

Uiso*/Ueq

occupied

Sr1

0.386860 (13)

0.48963 (9)

0.17574 (2)

0.01593 (11)

0.965

Eu1

0.386860 (13)

0.48963 (9)

0.17574 (2)

0.01593 (11)

0.035

Sr2

0

0

0

0.02087 (18)

0.965

Eu2

0

0

0

0.02087 (18)

0.035

Al1

0.35161 (6)

0.0233 (2)

0.50692 (6)

0.0084 (3)

0.3125

Si1

0.35161 (6)

0.0233 (2)

0.50692 (6)

0.0084 (3)

0.6875

Al2

0.22779 (5)

0.0239 (2)

0.09854 (6)

0.0069 (3)

0.3125

Si2

0.22779 (5)

0.0239 (2)

0.09854 (6)

0.0069 (3)

0.6875

Al3

0.02003 (5)

0.4932 (2)

0.16502 (6)

0.0067 (3)

0.3125

Si3

0.02003 (5)

0.4932 (2)

0.16502 (6)

0.0067 (3)

0.6875

Al4

0.29922 (6)

0.0425 (2)

0.30470 (6)

0.0071 (3)

0.3125

Si4

0.29922 (6)

0.0425 (2)

0.30470 (6)

0.0071 (3)

0.6875

N1

0.31449 (15)

0.0301 (7)

0.08569 (17)

0.0160 (9)

0.3667

O1

0.31449 (15)

0.0301 (7)

0.08569 (17)

0.0160 (9)

0.6333

N2

0.39373 (16)

0.1368 (6)

0.3219 (2)

0.0139 (10)

0.3667

O2

0.39373 (16)

0.1368 (6)

0.3219 (2)

0.0139 (10)

0.6333

N3

0.14985 (16)

0.1347 (6)

0.51004 (18)

0.0135 (10)

0.3667

O3

0.14985 (16)

0.1347 (6)

0.51004 (18)

0.0135 (10)

0.6333

N4

0.02119 (16)

0.1697 (5)

0.16518 (18)

0.0093 (9)

0.3667

O4

0.02119 (16)

0.1697 (5)

0.16518 (18)

0.0093 (9)

0.6333

N5

0.44986 (15)

0.0954 (6)

0.06129 (18)

0.0122 (9)

0.3667

O5

0.44986 (15)

0.0954 (6)

0.06129 (18)

0.0122 (9)

0.6333

N6

0

0.6234 (8)

0.25

0.0137 (14)

0.3667

O6

0

0.6234 (8)

0.25

0.0137 (14)

0.6333

N7

0.20228 (19)

0.3617 (6)

0.1019 (2)

0.0202 (11)

0.3667

O7

0.20228 (19)

0.3617 (6)

0.1019 (2)

0.0202 (11)

0.6333

N8

0.24676 (18)

0.3508 (6)

0.2966 (2)

0.0190 (11)

0.3667

O8

0.24676 (18)

0.3508 (6)

0.2966 (2)

0.0190 (11)

0.6333

Table 3. Bond distances Sr2.86Eu0.14Si5.5Al2.5O9.5N5.5. Sr1-

(O,N)1

3.095

(O,N)1 (O,N)2

Sr2-

(Å)

of

Sr-(O,N)

(O,N)3

2.741

2.713

(O,N)3

2.741

2.877

(O,N)4

2.630

(O,N)3

2.832

(O,N)4

2.630

(O,N)4

2.675

(O,N)5

2.572

(O,N)4

2.514

(O,N)5

2.572

(O,N)6

2.611

(O,N)7

3.097

in

(Figure 4). The high-energy emission band (455 nm) assigns to the Eu2+ occupied Sr1 site (CN = 8, dSr-(N,O) = 2.797 Å) with a weaker crystal field strength, while the low-energy emission band (493 nm) is resulted from Eu2+ on the Sr2 site (CN = 6, dSr-(N,O) = 2.648 Å) with a stronger crystal field strength. Obviously, the PL emission intensity contributed by Eu1 is higher than that of Eu2,

which leads to a blue emission color of the single particle. The photoluminescence spectra indicate that Eu2+-doped Sr3Si5.5Al2.5O9.5N5.5 could be used for ultraviolet white LEDs. It is recognized that the luminescence of Eu2+ ions in a host material has an extremely close relation with the structural parameters, such as coordinated number, the size of the substituted cation, bond length, covalence, and etc. These parameters determine the crystal field strength and the nephelauxitic effect impacting on the activators. The emission color of Sr2.86Eu0.14Si5.5Al2.5O9.5N5.5 can be simply estimated by using an empirical equation8,44: E = Q× 1-[V/4]1/V 10-(cn.ea.r)/8  (2) where E is the position in energy of the lower d-band edge for Eu2+ (in cm-1); Q is the position in energy of the lower d-band edge for free Eu2+ (34,000 cm-1); V is the valence of the activator; cn is the coordination number; ea is the electron affinity of the atoms that form anions, here it can be roughly determined as 2.0 eV; and r is the radius of the host cation replaced by the activator. In the present case, Eu2+ ions occupy two distinct Sr sites with coordination numbers of 8 (Sr1) and 6 (Sr2), the corresponding

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Table 4. Structural parameters and emission maxima of several Sr-oxonitridoaluminosilicate phosphors. Phosphor SrAlSi4N7

cn 6

Crystal structure (space group) Pna 21 (no. 33)

8 Sr3Si5.5Al2.5O9.5N5.5

6

Average bond length/Å

(Si,Al)/(O,N)

Measured

Ref.

2.713

0.71

639

22, 36

0.533

460

This work

λem/nm

2.871 C 2/c (no. 15)

8

2.648 2.797

Sr-α-sialon

7

P31c (no. 159)

2.618

0.75

575

46

SrSiAl2O3N2

9

P212121 (no. 19)

2.862

0.60

475

47, 48

Sr5Al5+xSi21-xO2+xN35-x

9

Pmn 21 (no. 31)

2.740

0.71

510

49

Sr3Si13Al3O2N21

10

Pna 21 (no. 33)

3.000

0.70

515

33

SrSi9Al19ON31

12

R-3 (no. 148)

3.000

0.875

445

43, 50

2+

radii of Sr are 0.126 nm (Sr1) and 0.118 nm (Sr2), respectively.45 The emission maxima of Eu1 and Eu2 are thus calculated to be around 487 and 555 nm, respectively. In comparison to the experimental results, the estimated ones are a little bit longer but still acceptable. In system Sr-Al-Si-O-N, several Eu2+-doped Sroxonitridoaluminosilicate phosphors have been reported, and their structure parameters and emission colors are summarized in Table 4. It is seen that the smaller coordination number usually gives a longer emissions, due to the strong crystal-field splitting. However, it is not true for the phosphor investigated. For example, the coordination numbers of Sr atoms in Sr3Si5.5Al2.5O9.5N5.5:Eu2+ and SrAlSi4N7:Eu2+ are the same, but the emission colors are quite different from each other. It thus indicates that the nature of chemical bond between Eu and the anion ligand also plays a key role in luminescence. In SrAlSi4N7, Eu is exclusively coordinated to nitrogen atoms, whereas Eu is mostly bonded to oxygen atoms in Sr3Si5.5Al2.5O9.5N5.5. Therefore, the covalence of the Eu-(O,N) bonds in the title phosphor is weaker than that of the Eu-N ones in SrAlSi4N7, resulting in a smaller nephelauxitic effect and thus a higher position of 5d energy level of Eu2+. Additionally, we also can conclude that the weak covalence of bond in Sr3Si5.5Al2.5O9.5N5.5 is the major reason for its blue emission color. Scale-up of Powder Synthesis. Sr3Si8-xAlxO7+xN8-x:Eu2+ phosphor powders were prepared following the chemical composition of the single particle. The x value corresponds to the number of the Al3+-O2- bond substituted for the Si4+-N3- bond, which has a great influence on the phase purity of the powder phosphor. X-ray powder diffraction patterns show that the pure phase of Sr3Si82+ can only be obtained in the range of x = 1.5 xAlxO7+xN8-x:Eu 3.5 (Figure S1a), indicating that the large-quantity synthesis of the single crystal composition was successful. It thus enables to evaluate the photoluminescence properties (e.g., absorption efficiency, quantum efficiency, thermal stability, concentration quenching, low-temperature luminescence, and etc.) that are hard to measure for small single crystals correctly and precisely. Furthermore, the powder sample rather than a small single crystal makes it possible to testify its application in lighting or display devices. The diffraction peaks of the synthesized materials slightly shift to lower angle side as the x value increases, which is caused by the substitution of the larger Al3+-O2- (1.75 Å ) bond for the shorter Si4+-N3- (1.70 Å ) bond. In addition, the lattice parameters a, b, and c present a linear shift because of the volume expansion by the Al3+-O2- substitution (Figure S1b). The Rietveld refinement of powder X-ray diffraction patterns of Sr3Si8-xAlxO7+xN8-x:5%Eu2+ (x = 2.2) also confirms a single phase of the title phosphor having the same crystal structure with the single particle, as shown in

Figure 5. The crystallographic data are given in Table 5. The lattice parameters of the powder sample are a = 18.1906 (5) Å, b = 4.97494 (7) Å, c = 15.9631 (5) Å, β = 115.995 (2)ο, and cell volume = 1298.46 (6) Å3, suggesting that there are no big differences in structure between the powder and the single crystal.

Figure 5. X-ray Rietveld refinement pattern of Sr3Si82+ xAlxO7+xN8-x: 5%Eu (x = 2.2).

Table 5. Crystallographic data of Sr3Si8-xAlxO7+xN8-x: 5%Eu2+ (x = 2.2) phosphor powder. crystal system

Monoclinic

space group

C 2/c (No. 15)

cell parameters/Å

a = 18.1906 (5) b = 4.97494 (7) c = 15.9631 (5)

β /° V /Å

115.995 (2) 3

1298.46 (6)

Z

2

radiation type

Cu Kα1

Rwp

11.31 %

RB

6.16 %

RF

3.27 %

S

1.78

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Luminescence of Powder Samples. Excitation and emission spectra of Sr3Si8-xAlxO7+xN8-x:5%Eu2+ (x = 2.5) are presented in Figure 6 (a). The excitation spectrum shows two maxima at 315 and 355 nm, which can be decomposed into three Gaussian profiles peaked at 311 nm, 371 nm, and 408 nm. The crystal filed splitting is then roughly estimated as 7,600 cm-1. With excitation at 355 nm, the sample gives a blue emission band peaked at 469 nm with a fwhm of 68 nm. By calculating the wavenumber difference between the lowest-energy excitation band (408 nm) obtained by Gaussian fitting and the peak emission wavelength (469 nm), the Stokes shift is estimated to be 3,200 cm-1, which is close to that calculated by using the mirror-image relationship (3,990 cm-1). The emission band can be fitted into two sub-bands centered at 468 and 506 nm, corresponding to two different crystallographic sites of Eu2+. Moreover, when monitored at 468 and 506 nm, the shape and position of excitation spectra remain unchanged (Figure S2), indicative of a similar local environment of Eu2+ in two different sites. In comparison to the single particle, the emission spectrum of the Sr3Si8-xAlxO7+xN8-x:Eu2+ powder at the high-energy side is red shifted by about 2 nm, which is attributed to the reabsorption between particles in the powder phosphor. The reabsorption also enhances the low-energy wing of the excitation band. This difference again highlights the necessity of the scale-up synthesis of powder.

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The chemical composition (i.e., the x value) has a great effect on the photoluminescence property of sample. As shown in the inset of Figure 6 (a), the luminescence intensity and the peak emission wavelength of samples are largely dependent on the x value. As the Eu concentration is fixed at 5 mol%, the strongest luminescence is achieved at about x = 2.2. The emission wavelength shows a blue shift as the x value increases, which is due to the decrease in covalence of the Eu-(O,N) bonds caused by the substitution of Si4+-N3- by Al3+-O2-. In order to confirm the Si-N bond is substituted by the Al-O bond, the FTIR spectra of Sr3Si8-xAlxO7+xN8-x:5%Eu2+ were performed, as shown in Figure 6(b). As we can see, the positions of absorption bands of all samples are similar, indicating no other chemical bonds were introduced after the substitution. The bands from 700 to 1200 cm-1 are attributed to the Si-O stretching vibration, overlapping with that of the Si-N stretching vibration (1045 cm-1, 853 cm-1) of [Si(O,N)4]-tetrahedra. Absorption peaks at 660, 540, 468, 460, and 420 cm-1 are ascribed to the Al-N, Si-O, Si-O, Si-N, and Si-N bonds, respectively.51,52 The vibration band at around 600-630 cm-1 is attributed to the Al-O bond. As we can see, the absorption intensity of Al-O increases whereas Si-N decreases as the x value increases, indicating a partial replacement of the SiN bond by the Al-O bond in the material. Moreover, the oxygenrelated bonds of the short wavenumber region exhibit a slight shift to a smaller wavenumber, implying the incorporation of oxygen in the host lattice. All of these results evidence the successful substitution of Si-N by Al-O in Sr3Si8-xAlxO7+xN8-x. Concentration quenching was also investigated for the powder sample. As shown in the Figure S3, the optimal doping concentration of Eu2+ is about 2 mol%. For this optimized composition, the absorption efficiency, internal quantum efficiency and external quantum efficiency are 83, 44.9 and 37.4%, respectively (Figure S4). Decay Time of Powder Sample. To further confirm the presence of two Eu2+ sites in Sr3Si8-xAlxO7+xN8-x, decay curves were monitored at 468 and 506 nm respectively, under the excitation of 370 nm, as given in Figure 7.

Figure 7. Luminescence decay curves of Sr3Si8-xAlxO7+xN82+ x:Eu monitored at 468 nm and 506 nm, the excitation wavelength is 370 nm. Figure 6. (a) Excitation (red), and emission (blue) spectra of 2+ Sr3Si8-xAlxO7+xN8-x:5%Eu (x = 2.5) phosphor, the inset is the PL intensity and peak emission wavelength of Sr3Si82+ with different x value; (b) FTIR spectra xAlxO7+xN8-x: 5%Eu 2+ of Sr3Si8-xAlxO7+xN8-x: 5%Eu with different x value.

The lifetime was calculated by the following equation: τ = T1 [ B1 ⁄(B1 +B2 )] + T2 [ B2 ⁄(B1 +B2 )] (3) where τ is the calculated lifetime, T1 and T2 are two components obtained by fitting the measured decay curve, B1 and B2 are the intensities contributed by the two components, respectively. The lifetimes were calculated to be 0.38 and 0.44 µs monitored at 468 and 506 nm, respectively. The difference in lifetime indicates two kinds of Eu2+ luminescent centers.53 The lifetime of Eu(Sr1) is

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slightly shorter than that of Eu(Sr2), because the emission from Eu1 site might be partially quenched by the energy transfer between Eu1 and Eu2. In addition, the lifetime of the title phosphor is shorter than other Eu2+ doped blue phosphors (~0.60 µs),54 one reason is the small energy difference between the 5d energy level of Eu2+ and the bottom of the conduction band of the host, which will be discussed in the following. Abnormal Thermal Quenching Behavior. Low temperaturedependent and high temperature-dependent luminescence properties of Sr3Si8-xAlxO7+xN8-x:Eu2+ were carried out from 4 K to 573 K, as shown in Figure 8. The emission intensity decreases significantly with raising the temperature, indicating a large thermal quenching of the material. Differing from other interesting LED phosphors,55,56 the title phosphor does not have its luminescence intensity to be remain unchanged from 4 K to the room temperature. It leads to a small thermal quenching temperature (at which the luminescence intensity reduces by 50%) of ~ 370 K, indicative of an abnormal thermal quenching behavior. The thermal quenching mechanism is usually interpreted by two possible reasons: Stokes shift and photoionization. As mentioned above, the Stokes shift of Sr3Si8-xAlxO7+xN8-x:Eu2+ is small (3,200 cm-1), comparable to that of the blue AlN:Eu2+ phosphor (4,400 cm-1) that has an excellent thermal stability.16 Moreover, the activation energy for thermal quenching (∆E) was calculated to be 0.294 eV (Figure 8), larger than other (oxy)nitride phosphor, such as Sr-α-sialon:Eu2+ (0.202 eV) and Sr2Si5N8:Eu2+ (0.200 eV).57,58 Thus, the Stokes shift mechanism is not considered as a major reason for the larger thermal quenching.

Figure 8. Temperature-dependent luminescence intensity of 2+ Sr3Si8-xAlxO7+xN8-x:Eu , the inset is the emission spectra measured at different temperatures.

Energy level scheme was constructed to further elucidate the thermal quenching behavior of the Sr3Si8-xAlxO7+xN8-x:Eu2+ phosphor, via several specific optical data59: (i) the band gap; (ii) the charge transfer band of Sm3+ ECT ) ; and (iii) the lowest 4f→5d Sm3+

transition of Eu2+ Efd . The optical band gap was estimated as Eu2+ 5.39 eV from the UV-VIS spectrum (Figure S5). The exciton creation energy (Eex) would be located at about 0.1 eV higher than the optical band gap measured at room temperature, if the spectrum is measured at the temperature of 10 K.60 Thus, the optical band gap at the low temperature is about 5.49 eV. The exciton binding energy, i.e., the energy to create the exciton, is about 8% higher than the optical band gap of 5.49 eV,60 which is calculated as 5.93 eV. The ECT Sm3+ is 4.51 eV, deduced from the excitation spectrum of Sr3Si8-xAlxO7+xN8-x:Sm3+ (Figure S6), which is used CT3+ to calculate EEu . Dorenbos59 reported the energy difference beCT3+ CT3+ CT3+ tween ESm and EEu is about 1.45 eV. Therefore, EEu of Sr3Si8xAlxO7+xN8-x is 3.06 eV, which can be considered as the energy difference between the top of valence band and the 4f ground state of Eu2+. Thus, the position of 4f energy level of Eu2+ is deterfd mined. In addition, EEu2+ is estimated as 2.85 eV (435 nm) from its excitation spectrum, which is taken at the point where the excitation energy drops to 15-20 % intensity at the low energy side of the maximum. With these optical data, the energy level scheme was constructed and shown in Figure 9. As we can see, the lowest 5d level of Eu2+ is very close to the bottom of the conduction, the energy difference between them is only 0.02 eV, which is rather small (Sr2Si5N8:Eu2+: 0.9 eV).61 The crystal field splitting energy (CFS) of Sr3Si8-xAlxO7+xN8-x:Eu2+ is about 0.95 eV, calculated from its excitation spectrum, and thus other 5d energy levels of Eu2+ are trapped in the conduction band, as shown in Figure 9. In this case, the luminescence is easily quenched by photoionization under a high temperature. The thermal quenching process is exhibited in the constructed energy level scheme, and described as follow: Generally, under the UV light excitation, the 4f electrons of Eu2+ are preferentially raised to the lowest 5d excitation level, and then, the excited electrons return to the 4f state by radiative transition, giving blue light emission. However, under a high temperature, numbers of the excited electrons will be thermally activated from the lowest 5d level to the conduction band because of the narrow energy difference between them in Sr3Si8-xAlxO7+xN82+ x:Eu . This will result a recombination of the excited electrons with holes somewhere else, causing a nonrdiative transition. Therefore, the photoionization is the main reason for the large thermal quenching. The investigation of the energy level scheme not only uncovers the reason for the larger thermal quenching behaviour, but also guides the improvement path of the thermal stability, by adjusting the band gap. The thermal stability of the new phosphor might be improved by the combinatorial composition spread technique, for example, the cationic substitution strategy. Additionally, as mentioned above, the lifetime of the Sr3Si82+ phosphor is very short, which also can be exxAlxO7+xN8-x:Eu plained by the constructed energy level scheme. Due to the small energy difference between the 5d energy level of Eu2+ and the bottom of the conduction band, the energy transfer possibility among them increases, which increases the possible nonradiative transition and leads to a short lifetime in such a phosphor.

CONCLUSION In summary, a new blue oxynitride phosphor, Sr3Si82+ , was discovered by the single particle diagnosis approach. Its crystal structure is analogue to that of La3Si6.5Al1.5N9.5O5.5.62 Phase-pure powder samples were obtained as the x value is between 1.5 and 3.5. Under the UV light excitation, the title phosphor gives a blue emission peaking at ~ 465 nm xAlxO7+xN8-x:Eu

2+

Figure 9. Energy level scheme of Sr3Si8-xAlxO7+xN8-x:Eu .

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with full width at a half maximum of about ~ 70 nm. The absorption efficiency, internal quantum efficiency and external quantum efficiency are 83, 44.9 and 37.4% for the sample with 2 mol % Eu. The quite low thermal quenching temperature (~ 370 K) of the title phosphor indicates the abnormal thermal quenching behavior. Using the computed energy level scheme, the thermal quenching mechanism is interpreted by photoionization due to a small energy difference between the 5d energy level of Eu2+ and the bottom of the conduction band of the host. For potential applications in white pc-LEDs, it is still required to enhance both the luminescence efficiency (e.g., internal quantum efficiency) and the thermal stability by optimizing the chemical composition and processing conditions, or modifying the band gap with cation/anion substitutions.

ASSOCIATED CONTENT Supporting Information Anisotropic displacement parameters of Sr3Si8-xAlxO7+xN82+ 2+ x:Eu ; CIF files of Sr3Si8-xAlxO7+xN8-x:Eu ; XRD patterns of 2+ Sr3Si8-xAlxO7+xN8-x:5%Eu (x = 1.5-3.5) powder samples; Exci2+ tation spectra of Sr3Si8-xAlxO7+xN8-x:5%Eu (x = 2.5) monitored at 468 and 506 nm; Emission spectra, absorption efficiency, internal quantum efficiency and external quantum 2+ efficiency of Sr3Si8-xAlxO7+xN8-x:yEu (x = 2.2, y = 0.5 - 10%); 2+ UV-VIS spectrum of Sr3Si8-xAlxO7+xN8-x:Eu (x = 2.2); Excita3+ tion and emission spectra of Sr3Si8-xAlxO7+xN8-x:Sm (x = 2.2). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported in part by Grants-in-Aid for Scientific Research from KAKENHI (No. 15K06448), National Natural Science Foundation of China (No.61177050 and No. 51272259).

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Chemistry of Materials

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