Article pubs.acs.org/IC
Evolution of Structure and Photoluminescence by Cation Cosubstitution in Eu2+-Doped (Ca1−xLix)(Al1−xSi1+x)N3 Solid Solutions Ting Wang,†,‡ Qianchuan Xiang,† Zhiguo Xia,† Jun Chen,‡ and Quanlin Liu*,† †
Beijing Key Laboratory for New Energy Materials and Technologies, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China ‡ Department of Physical Chemistry, School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China S Supporting Information *
ABSTRACT: Red-emitting nitride phosphors excited with blue light have great potential for the fabrication of warm white light-emitting diodes (WLEDs). Chemical composition and structural modification are generally adopted to optimize the photoluminescence behaviors of the targeted phosphors. Herein, on the basis of the famous CaAlSiN3 phosphors, Eu2+-doped (Ca1−xLix)(Al1−xSi1+x)N3 solid solutions via the cations’ cosubstitution of (CaAl)5+ pair by (LiSi)5+ pair are successfully synthesized by a solid state reaction, and the lattice parameters show a linear decrease with chemical compositions suggesting the formation of the isostructural phase relationship. Four types of coordinated structure models, corresponding to different coordination environments of Eu2+, are proposed over the course of structural evolution, which induces different structural rigidity and stability, and then they are responsible for three-stage changes of emission spectra of Eu2+ in (Ca1−xLix)(Al1−xSi1+x)N3 solid solution.
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shows a very interesting variation trend: first, a slight blue-shift from 669 to 663 nm, then an obvious red-shift from 663 to 738 nm, and finally an abrupt blue-shift to 600 nm when x = 1. By analyzing the changes of luminescence in detail, we reveal an evolution process of local structure around Eu 2+ in (Ca1−xLix)0.98(Al1−xSi1+x)N3:0.02Eu2+ which accounts for the interesting three-stage change of emission band. This work also demonstrates that Eu2+ ion is a good probe for local structure.
INTRODUCTION With the development of the white-light-emitting diodes (wLEDs), the need for new phosphors is increased. Nitridosilicates and oxynitridosilicates, in which the N3− is a soft Lewis base resulting in a high covalency, are sought through several strategies. The composition of nitride phosphors is normally focused on the alkaline earth silicon nitride and oxynitride, e.g., M2Si5N81,2 and MSi2O2N23,4 (M = Ca, Sr, Ba), as well as alkaline earth aluminum−silicon nitride and oxynitride, e.g., MAlSiN35,6 and Ca-α-SiAlON.7,8 Recently, some lithium silicon nitrides, e.g., LiSi2N3,9−11 Li2SiN2,12 and Li5SiN3, are also studied in the course of searching for novel nitride phosphors. LiSi2N3 achieves extensive attention because it is the most stable compound among above-mentioned lithium silicon nitrides, and the structure is isostructural with Si2N2O and Li2SiO3. With a wurzite-type structure (space group Cmc21), it is also interesting to find that LiSi2N3 has isotypic crystal structure with CaAlSiN3. Therefore, LiSi2N3 can be regarded as a complete substitution of the (CaAl)5+ pair by a (LiSi)5+ pair in CaAlSiN3. In other words, Li+ and Si4+ ions can be completely incorporated into the host of CaAlSiN3, and the charge is kept balanced by a cooperative substitution, i.e., Li replacing Ca with Al substituting for Si, which is similar to our recently reported chemical unit cosubstitution strategy for the design of phosphors.13,14 In the present work, we synthesized the solid solutions of (Ca1−xLix)0.98(Al1−xSi1+x)N3:0.02Eu2+. Depending on the variation of the chemical composition, the emission band of Eu2+ © XXXX American Chemical Society
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EXPERIMENTAL SECTION
Materials and Preparation. The binary nitride starting materials Ca3N2, Li3N, and EuN were prepared by firing the corresponding metals with high pressure N2 gas at 873, 573, and 1073 K, respectively. The raw materials of Ca3N2, Li3N, AlN, Si3N4, and EuN were then mixed in an agate mortar under N2 atmosphere in a glovebox to avoid oxidation. The compositions (Ca1−xLix)0.98(Al1−xSi1+x)N3:0.02Eu2+ (x = 0, 0.05, 0.1, 0.15, 0.2, 0.4, 0.6, 0.8, 0.84, 0.88, 0.92, 0.96, 1) were synthesized by heating the stoichiometric mixture in 1 MPa N2 atmosphere at 1873 K for 4 h. Then, the samples were slowly cooled to room temperature under the same atmosphere. The cooled samples were ground and prepared for measurements. Structure and Luminescence Property Characterization. Xray diffraction (XRD) data were collected using Rigaku TTR III diffractometer (Cu Kα radiation) at 40 kV, 200 mA. Photoluminescent spectra were measured by an Edinburgh Instrument FLSP920 with a 450 W xenon arc lamp used in continuous mode as the light source at room temperature.15 The photomultiplier used is Hamamatsu R928P which provides very low dark noise level. The emission and excitation Received: December 9, 2015
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DOI: 10.1021/acs.inorgchem.5b02845 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 1. (a) Full range (15−75°) XRD patterns and (b) the selected diffraction peaks near 32° of (Ca1−xLix)0.98(Al1−xSi1+x)N3:0.02Eu2+ (x = 0−1) samples. (c) The lattice parameters a, b, and c and the unit cell volume V of (Ca1−xLix)0.98(Al1−xSi1+x)N3:0.02Eu2+ (x = 0−1) samples depending on different x values.
Figure 2. (a) Normalized excitation spectra of (Ca1−xLix)0.98(Al1−xSi1+x)N3:0.02Eu2+ samples with x = 0−1. The normalized emission spectra of (Ca1−xLix)0.98(Al1−xSi1+x)N3:0.02Eu2+ samples for different ranges: (b) x = 0−0.2, (c) x = 0.3−0.9, (d) x = 0.8−1. (e) The wavelength of λc, λabs, λem, and λ0 as a function of x. (f) The Stokes shift as a function of x. spectra are all corrected according to the spectral instrumental response and the spectral output of the light source.
(x = 1) are in good agreement with standard card (XRD pattern of CaAlSiN3 in ref 16 and JCPDS cards 26-1186), and there is no detectable impurity phase presented. The result indicates that (Ca1−xLix)0.98(Al1−xSi1+x)N3:0.02Eu2+ retains a single phase with increasing x. A completely miscible solid solution can be realized by the cosubstitution of the (LiSi)5+ pair for the (CaAl)5+ pair in the CaAlSiN3 host, and this result is different from the early report by Li et al.10 We can also clearly find that the diffraction peaks of (Ca1−xLix)0.98(Al1−xSi1+x)N3:0.02Eu2+ shift to higher angles with respect to the position of CaAlSiN3:Eu2+ (Figure 1b) with increasing x
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RESULTS AND DISCUSSION The phase identification of (Ca 1−x Li x ) 0.98 (Al 1−x Si 1+x )N3:0.02Eu2+ (x = 0−1) phosphors characterized by X-ray diffraction (XRD) is shown in Figure 1a. (The phase identification of (Ca1−xLix)0.98(Al1−xSi1+x)N3:0.02Eu2+ (x = 0.1−0.9 and x = 0.8−1) phosphors are shown in detail in Figures S1 and S2 in the Supporting Information.) The powder diffraction patterns of CaAlSiN3:Eu2+ (x = 0) and LiSi2N3:Eu2+ B
DOI: 10.1021/acs.inorgchem.5b02845 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table 1. Average Electronegativity χav of the Cations and Data on the fd Spectroscopy of Eu2+ in the Compounds x
χav
λc (nm)
λabs (nm)
λem (nm)
λ0 (nm)
ΔS (cm−1)
fwhm (cm−1)
0 0.2 0.4 0.6 0.8 1
1.60 1.64 1.68 1.72 1.76 1.80
532 528 519 529 494 460
599 596 581 594 557 504
669 665 674 690 717 602
578 547 555 560 550 500
4707 6488 6362 6729 8470 6777
2086 2258 2636 2497 2960 3524
the d-manifold is smaller for Eu2+ in the solid solution with an increasing fraction of (LiSi)5+, causing a blue-shift of the excitation band and a more narrow excitation spectrum. A second origin of the blue-shift is the decreased centroid shift caused by the replacement of Si4+ with Al3+. On the basis of the model suggested by Morrison20 and its development by Dorenbos,21 the centroid shift εc (the lowering of the average of the five 5d levels) can be affected by the so-called spectroscopic polarizability αisp of anion Ni. Due to the optical property of Ce3+ being similar to that of Eu2+ in the same host, this estimated value of εc which was used to describe the characteristic of Ce3+ also pertains to Eu2+. In our previous work,22 a qualitative relationship between αsp and the electronegativity of the cations χ in Ce3+-doped nitride compounds was demonstrated as
values. The lattice parameters (a, b, and c) and the total lattice volume (V) decrease with x linearly (Figure 1c) due to the fact that the radius of Li+(0.76 Å, CN6) is smaller than that of Ca 2+ (1.00 Å, CN6). 17 The crystallographic data for (Ca1−xLix)0.98(Al1−xSi1+x)N3:0.02Eu2+ are obtained from the Rietveld refinement by using the Fullprof program18 (Figures S3−S8 and Tables S1−S2 in the Supporting Information). Figure 2a shows the normalized excitation spectra with x = 0−1. It can be observed that the dominant excitation bands of Eu2+ in the series of (Ca1−xLix)0.98(Al1−xSi1+x)N3:0.02Eu2+ (x = 0−1) compounds located at about 310 nm in the high energy side, which is probably associated with their small crystallographic site. Data on the fd spectroscopy of Eu2+ in the compounds have been analyzed and compiled in Table 1. λc refers to the centroid position of the excitation spectra. The wavelength where the integrated intensity of the spectra has reached to 50% of the total amount of intensity was used in column 3. λabs was used to express the first step of the characteristic “staircase” structure in the excitation spectra, and the wavelength where the band has risen to 20% of the maximum of the spectra on the low energy side was taken in column 4.19 The emission wavelengths are signed as λem and complied in column 5. Whenever both excitation and emission spectra are known, the wavelengths of the crossing which are adopted as the positions of zero-phonon energy have been determined and given in column 6. Since the fd absorption wavelengths of Eu2+ usually are resolved in the excitation spectra, the locations of the excitation wavelengths are difficult to estimate. To keep errors small, the Stokes shift (compiled in column 7) has been determined by twice the difference between λ0 and λem. With the increasing x values, the excitation spectra on the high energy side can be considered to stay invariable, while a blue-shift of the excitation spectra on the low energy side can be observed as shown as λabs in Table 1 and Figure 2e. This implies that the centroid position of the excitation band shifts to a higher energy region, which is also analyzed in Table 1 and plotted in Figure 2e. Moreover, more replacement results in a smaller split of 5d energy level and brings about narrower excitation bands. Because the average bond length of Li−N is much smaller than that of Ca−N, it is expected that the 5d excitation states of Eu2+ will shift to lower energy and split to a broader energy region. However, this blue-shift of the centroid position of the excitation bands is opposite to the expected behavior that smaller bond length generally results in a broad absorption band and the red-shift of the excitation peak due to stronger nephelauxetic effect and larger crystal field splitting. This unusual behavior may be caused by two effects. First, the Eu2+ coordinate environment in the (Ca1−xLix)(Al1−xSi1+x)N3 structure may stay unchanged when a small part of (CaAl)5+ is replaced by (LiSi)5+. To accommodate the smaller Li+ cations, the distances between Eu2+ and the anions may not decrease or even become slightly larger. Thus, the crystal field splitting of
αspN = 0.87 +
18.76 χav2
(1)
Thus, to some degree, the spectroscopic polarizability is in inverse proportion to the square of the average χav, which has been defined as χav =
1 Na
Nc
∑ i
ziχi γ
(2)
Equation 2 is obtained for the reason that a cation of formal charge +zi will bind on average with zi/γ anions of formal charge −γ. The summation is over all cations Nc in the formula of the compound, and Na is the number of anions in the formula. To apply eq 2 in the (Ca1−xLix)0.98(Al1−xSi1+x)N3:0.02Eu2+ (x = 0−1) compound, χav can be calculated with Pauling type23 electronegativity values χi from Allred24 with χCa = 1.00, χLi = 0.98, χAl = 1.61, and χSi = 1.90. It should be noticed that there is a small difference (0.02) between χCa and χLi, but a large difference (0.29) between χAl and χSi. With the substitution of (LiSi)5+ for (CaAl)5+, viz. Li+ → Ca2+ and Si4+ → Al3+ simultaneously, Si4+ → Al3+ may be the dominant reason to modify the luminescence properties of (Ca1−xLix)0.98(Al1−xSi1+x)N3:0.02Eu2+ (x = 0−1). The average electronegativity χav of the cations has been compiled in Table 1. With the increasing x, the average electronegativity of cations increases (see Table 1), which leads i to the decrease of the spectroscopic polarizability αsp . Therefore, the centroid of the 5d excitation band stays in a higher energy region with a larger x, which corresponds to the blue-shift of the excitation band. However, it should be noticed that it is a rough method to estimate the centroid position of 5d energy levels. As shown in Figure 2b−d, the normalized emission spectra of (Ca1−xLix)0.98(Al1−xSi1+x)N3:0.02Eu2+ in different composition ranges were demonstrated, which verified that they exhibit dark red emission with a broad band peaking at about 650−700 nm apart from the compound with x = 1, i.e., LiSi2N3:Eu2+. C
DOI: 10.1021/acs.inorgchem.5b02845 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
and are shown in Scheme 1. In CaAlSiN3, only M1 can be found in the coordination environments of the central atoms. Due to three [AlN 4 ] tetrahedrons and three [SiN 4 ] tetrahedrons in M1 being arranged alternatively, the structure of M1 is relatively rigid and stable. With the increase of concentration values, a [SiN4] tetrahedron will replace an [AlN4] tetrahedron, and M2, M3, and M4 appear successively. In three stages, different spectral shift behaviors should be related with the local structures of Eu2+ in the solid solutions. For stage I with x = 0−0.2, the slight blue-shift of the emission bands corresponds to the blue-shift of the 5d centroid (the average energy) of the excitation bands. The Stokes shift has an acute increase as can be seen in Figure 2f. As depicted in Scheme 1, in the beginning of the structural evolution, the coexistence of M1 and M2 breaks the relatively stable crystal structure and creates a less rigid crystal structure compared to the end-member compositions. Therefore, the appearance of the inhomogeneous ligand environments (M1 and M2) of the luminescence centers may account for the increase of Stokes shift and the full width in half-maximum of the emission bands (Table 1). However, because the cavity in M1 is looser than that in M2, the Eu2+ ions with a much larger size than Ca2+ and Li+ will occupy preferentially in M1 environments. Therefore, the emission bands show a very slight blue-shift which is consistent with the excitation bands. For the compounds with x from 0.2 to less than 1 in stage II, the red-shift of the emission band can be mainly attributed to the increasing Stokes shift (in Figure 2f) arising from the large structural relaxation. With the increase of x, the six-ring ligand of Eu2+ ions has two appearances as shown in Scheme 1: M2 and M3. The coexistence of M1, M2, and M3 is one of the dominant reasons for the larger Stokes shift. In (isostructural) alkaline earth compounds, the observation of the larger Stokes shift can be explained with the configurationally coordinate diagram. In the configurationally coordinate diagram, the equilibrium distance in the fd excited state of lanthanide ions can be changed by comparing with the equilibrium distance in the ground state. The change of the equilibrium distance can be affected significantly by the rigidness of the host lattice. As a result of the less rigid lattice structure caused by the presence of M2 and M3, the change of the equilibrium distance (due to the contraction) in the 4f65d1 excited state is increased. The increased change of the equilibrium distance brings about a larger shift of the parabolas in the configurationally coordinate diagram, which results in a larger Stokes shift in the luminescence spectra. The luminescence behavior of LiSi2N3:Eu2+ is quite surprising, which shows an unexpected emission wavelength peaking at about 600 nm. If it follows the red-shift tendency of the emission in the series of solid solutions, LiSi2N3:Eu2+ should have an emission band with a peak beyond 700 nm. Figure 2d shows more detailed work to study the abnormal luminescence behavior of LiSi2N3:Eu2+. (The completely miscible solid solutions with x = 0.8−1 can be obtained as a single phase, and the XRD patterns can be seen in Figure S2 in the Supporting Information.) As shown in Figure 2d, apart from the emission band of Eu2+ in LiSi2N3, a continuous red-shift of the emission peaks is observed with the increasing x. The jump point can be found exactly when x = 1. As can be seen in Figure 2a, a higher location of the lowest energy level of Eu2+ can be observed in the excitation spectrum of Eu2+ in LiSi2N3. The smaller fwhm of the excitation spectra also indicates that the crystal field
(However, the emission intensity for the as-measured emission spectra decreases with the increasing x, as shown in Figure S9.) From the normalized emission spectra, the spectral evolution depending on chemical compositions can be clearly observed. Three obvious changing stages can be found here: Stage I is a slight blue-shift of the emission bands in the compositions with x = 0−0.2 (Figure 2b). Stage II is a large red-shift in the compositions with x from 0.2 to less than 1 (Figure 2c). Stage III is an abrupt blue-shift when x is 1 (Figure 2d). The main emission band of LiSi2N3:Eu2+ is observed at about 600 nm, and the same result has been mentioned in several reports.10,11 These changes are very interesting and cannot be interpreted by the conventional isostructural solid solution model. Here, we proposed a coordination model to explain the three-stage changes. The model is proposed on the basis of the chemical unit cosubstitution strategy represented by structural evolution of the (Ca1−xLix)0.98(Al1−xSi1+x)N3:0.02Eu2+, from CaAlSiN3 (x = 0) to LiSi2N3 (x = 1) via the (LiSi)5+ substitution for (CaAl)5+ couple as shown in Scheme 1. When x is 0, CaAlSiN3 has a Scheme 1. Proposed Model on the Chemical Unit Cosubstitution Strategy Represented by Structural Evolution of (Ca1−xLix)0.98(Al1−xSi1+x)N3:0.02Eu2+a
a
From CaAlSiN3 (x = 0) to LiSi2N3 (x = 1) via the (LiSi)5+ substitution for (CaAl)5+ couple. It corresponds to three stages in the evolution of photoluminescence.
relatively stable and rigid crystal structure, and the coordinated tetrahedrons in the six-ring of Ca2+ ions contain three [AlN4] tetrahedrons and three [SiN4] tetrahedrons (labeled as M1). With the increasing fraction of (LiSi)5+, the local coordinated structures of the luminescence centers appear as the following three models: two [AlN4] tetrahedrons and four [SiN4] tetrahedrons (M2), one [AlN4] tetrahedron and five [SiN4] tetrahedrons (M3), and six [SiN4] tetrahedrons (M4) (when x = 1). M1, M2, M3, and M4 are all the possible local coordination environments for the luminescence center Eu2+. Although the compositions of (Ca1−xLix)0.98(Al1−xSi1+x)N3:0.02Eu2+ can be tuned continuously and correspondingly and the lattice parameters show continuous changes, the local environments of Eu2+ have only four types as mentioned above D
DOI: 10.1021/acs.inorgchem.5b02845 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry splitting of LiSi2N3:Eu2+ is much smaller than that of CaAlSiN3:Eu2+ which could be the dominant reason for such a sudden blue-shift. Compared to the sample with x = 0.8, a smaller Stokes shift of LiSi2N3:Eu2+ may be another reason for the sudden blue-shift. In the LiSi2N3 lattice, as the absence of Al3+, the skeleton structure is built only by [SiN4] tetrahedra, and the only coordination environment type is M4 (Scheme 1). This results in the most homogeneous environment of Eu2+ and a more rigid lattice, which accounts for the smaller Stokes shift. The slightly larger value of the Stokes shift of LiSi2N3:Eu2+ than that of (Ca1−xLix)0.98(Al1−xSi1+x)N3:0.02Eu2+ (x = 0−0.6) may be attributed to the serious mismatching size and valence of Eu2+ and Li+.
(5) Uheda, K.; Hirosaki, N.; Yamamoto, Y.; Naito, A.; Nakajima, T.; Yamamoto, H. Electrochem. Solid-State Lett. 2006, 9, H22−H25. (6) Li, J.; Watanabe, T.; Wada, H.; Setoyama, T.; Yoshimura, M. Chem. Mater. 2007, 19, 3592−3594. (7) Xie, R.; Mitomo, M.; Uheda, K.; Xu, F.; Akimune, Y. J. Am. Ceram. Soc. 2002, 85, 1229−1234. (8) van Krevel, J. W. H.; van Rutten, J. W. T.; Mandal, H.; Hintzen, H. T.; Metselaar, R. J. Solid State Chem. 2002, 165, 19−24. (9) Whitney, E. D.; Giese, R. F. Inorg. Chem. 1971, 10, 1090−1092. (10) Li, Y. Q.; Hirosaki, N.; Xie, R. J.; Takeka, T.; Mitomo, M. J. Solid State Chem. 2009, 182, 301−311. (11) Wu, Q.; Li, Y.; Wang, X.; Zhao, Z.; Wang, C.; Li, H.; Mao, A.; Wang, Y. RSC Adv. 2014, 4, 39030−39036. (12) Anderson, A. J.; Blair, R. G.; Hick, S. M.; Kaner, R. B. J. Mater. Chem. 2006, 16, 1318−1322. (13) Xia, Z. G.; Ma, C. G.; Molokeev, M. S.; Liu, Q. L.; Rickert, K.; Poeppelmeier, K. R. J. Am. Chem. Soc. 2015, 137, 12494−12497. (14) Xia, Z. G.; Liu, G. K.; Wen, J. G.; Mei, Z. G.; Balasubramanian, M.; Molokeev, M. S.; Peng, L. C.; Gu, L.; Miller, D. J.; Liu, Q. L.; Poeppelmeier, K. R. J. Am. Chem. Soc. 2016, 138, 1158. (15) Song, Z.; Zu, R.; Liu, X. L.; He, L. Z.; Liu, Q. L. Optik 2016, 127, 2798−2801. (16) Li, Y. Q.; Hirosaki, N.; Xie, R. J.; Takeda, T.; Mitomo, M. Chem. Mater. 2008, 20, 6704−6714. (17) Shannon, R. D. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, A32, 751−767. (18) Roisnel, T.; Rodríguez-Carvajal, J. Mater. Sci. Forum 2001, 118, 378−381. (19) Dorenbos, P. J. Phys.: Condens. Matter 2003, 15, 575−594. (20) Morrison, C. A. J. Chem. Phys. 1980, 72, 1001−1002. (21) Dorenbos, P. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 62, 15640−15649. (22) Wang, T.; Xia, Z. G.; Xiang, Q. C.; Qin, S. Q.; Liu, Q. L. J. Lumin. 2015, 166, 106−110. (23) Pauling, L. The Nature of the Chemical Bond; Cornell University Press: Ithaca, New York, 1960. (24) Allred, A. L. J. Inorg. Nucl. Chem. 1961, 17, 215−221.
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CONCLUSION In summary, completely miscible solid solutions of (Ca1−xLix)0.98(Al1−xSi1+x)N3:0.02Eu2+ have been designed and successfully prepared. With the increasing x, the excitation band on the low energy side shows a blue-shift, originating from the decreased centroid shift due to the main contribution of the replacement of Si4+ with Al3+. The emission bands of (Ca1−xLix)0.98(Al1−xSi1+x)N3:0.02Eu2+ indicated slightly blueshifted character, then significantly shifted from 663 to 738 nm, but there is a sudden shift to 600 nm when x is 1. Three stages can be divided in this process. A model on the chemical unit replacement strategy represented by structural evolution of the (Ca1−xLix)0.98(Al1−xSi1+x)N3:0.02Eu2+ is proposed. It will induce four-type coordinated structures over the course of evolution, which is responsible for three-stage spectral shift of the observed emission spectra mentioned above.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02845. Experimental details and refinement results, figures and tables showing results from (Ca1−xLix)0.98(Al1−xSi1+x)N3:0.02Eu2+ compounds (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This present work was supported by the National Natural Science Foundations of China (Grant No. 51272027, 51472028). T.W. is grateful to China Postdoctoral Science Foundation (Grant No. 2015M570927).
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REFERENCES
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DOI: 10.1021/acs.inorgchem.5b02845 Inorg. Chem. XXXX, XXX, XXX−XXX
