Understanding the Local and Electronic Structures toward Enhanced

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Understanding the local and electronic structures towards enhanced thermal stable luminescence of CaAlSiN3:Eu2+ Lei Chen, Mi Fei, Zhao Zhang, Yang Jiang, Shifu Chen, Yongqi Dong, Zhihu Sun, Zhi Zhao, Yibing Fu, Jinhua He, Can Li, and Zheng Jiang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02121 • Publication Date (Web): 10 Jul 2016 Downloaded from http://pubs.acs.org on July 10, 2016

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Chemistry of Materials

The shift of Fermi level, the change of Fermi-Dirac distribution of electrons on excited sub-states, and the thermal delocalization of electrons to conduction band are intrinsically responsible for thermal quenching luminescence of CaAlSiN3:Eu2+; and the thermal delocalization of electrons endow the phosphor with high heat conductivity. 240x149mm (150 x 150 DPI)

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Understanding the local and electronic structures towards enhanced thermal stable luminescence of CaAlSiN3:Eu2+ Lei Chen †, *, Mi Fei †, Zhao Zhang †, Yang Jiang †, Shifu Chen ‡, *, Yongqi Dong §, Zhihu Sun §, Zhi Zhao ⊥, Yibing Fu ||, Jinhua He ||, Can Li ▽, Zheng Jiang # †

School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China.

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Department of Chemistry, Anhui Science and Technology University, Fengyang 233100, China.

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National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230026, China.

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Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China. ||

Jiangsu Bree Optronics Co., Ltd., Nanjing 211103, China.

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Institute of Coordination Bond Metrology and Engineering, China Jiliang University, Hangzhou 310018, China.

#

Shanghai Synchrotron Radiation Facilities, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China. ABSTRACT: It is a great challenge to maintain thermally stable luminescence of red phosphors in white light-emitting diodes (LEDs), due to the large Stokes shift. For the purpose of overcoming this challenge, this work elucidates the intrinsic mechanism of the thermal quenching luminescence of CaAlSiN3:Eu2+. The empty 5d orbital of Eu2+ is partly filled with electrons upon Eu2+ increasing, as observed with using XANES; and the exceptional expansion of the local Eu-N bond length, the ratio of which is far larger than the volume expansion of crystal lattice brought by doping Eu2+, is measured using EXAFS. The shift of Fermi level predicted with the first-principles calculations is confirmed by the valence band spectra. Therefore, the changeable distribution of electrons on the excited sub-states and then thermal delocalization to the conduction band are the intrinsic mechanisms of thermal quenching luminescence of CaAlSiN3:Eu2+. The results provide a solid basis for exploring the methods to enhance the thermal stable luminescence of CaAlSiN3:Eu2+.

1. INTRODUCTION To provide comfortable living conditions for humans, the pursuit of high-quality white light with fresh color, warm temperature, and wide gamut is ongoing. Succeeding Edison’s invention of incandescent lamps and fluorescent lamps, the solid-state semiconductor devices of light-emitting diodes (LEDs) have led to a revolution in lighting and information displays, by introducing a new type of energy-saving and environmentally-friendly light source.1-2 In recognition of the importance of lighting, the developers of blue LEDs were awarded the Nobel Prize in Physics in 2014.2 However, nearly all white LEDs devices consist of a blue LED chip combined with one or more luminescent materials, which convert part of the blue light into other wavelengths. Thus, red phosphor is a vital raw material for fabricating high-colorrendering warm-white LEDs.3-4 However, maintaining thermally stable luminescence is a substantial challenge of red phosphors. The thermal stability of luminescence is critical to white LEDs, because a considerable amount of heat will be released with electron-hole recombination during the operation of LEDs, which not only gives rise to luminescence

quenching but also causes an emission color shift with increasing temperature.3-5 The emerging nitride red phosphor, CaAlSiN3:Eu2+, was determined to be the best available for fabricating high-color-rendering warm-white LEDs among the candidates of Sr2Sr5N8:Eu2+,3 Sr3SiO5:Eu2+,6 Sr4Al14O25:Mn4+,7 K2TiF6:Mn4+,8 CaS:Eu2+,9 etc., due to its high quantum efficiency, matchable excitation wavelength, desirable chromaticity, and chemical inertness.10-14 Nevertheless, the problems of thermal quenching luminescence and color shift with temperature variation still exist in CaAlSiN3:Eu2+. These problems are the key issues that white LEDs manufactures are addressing. Meanwhile, scientists have spent great effort to uncover the basic mechanisms and explore measures for issues.15 Unfortunately, a significant breakthrough has not yet been achieved. At the current technical level, the luminescence of CaAlSiN3:Eu2+ at 150 °C remains at approximately 83-89% of its initial room-temperature efficiency.10-12 One primary restriction to this breakthrough is the unclear mechanisms of thermally unstable luminescence. In classic thermodynamics, the mechanisms of thermal quenching luminescence are mainly interpreted as phonon scattering using the configuration-coordinate model, defects quenching,


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energy annihilation during energy transferring among different centers (which are concentration- or temperature-dependent), and auto-ionization.16-18 However, the strong Nephelauxetic effect (determined by covalence), which is used to produce the desirable red emission for application in white LEDs, makes CaAlSiN3:Eu2+ an intensive rigid structure that resists thermal vibrations.10-14 Thus, it is difficult to illustrate the thermal quenching luminescence of CaAlSiN3:Eu2+ with the configurationcoordination model. If phonon coupling is in effect, then the emission of CaAlSiN3:Eu2+ should red shift with increasing temperature; however, a slight blue shifts occurs instead.3,5 Moreover, the energy of annihilation during the electron migrating process could be excluded by ensuring Eu2+ remains below the critical concentration. In addition, the red shift of CaAlSiN3:Eu2+ with increasing Eu2+ concentration, where the expansion of crystal size will reduce crystal field strength, is inconsistent with the explanation of ligand-field theory. Although the synthesis, photoluminescence, thermal performance, crystal structure, and electronic structure of CaAlSiN3:Eu2+ have been intensively investigated, the real nature of its thermal quenching mechanism was never discerned.10-16, 19-28 The splitting of the Eu2+ 5d orbital into 5 sub-levels results in the 4f-5d excitation spectrum spreading over a large scale, which involves the distribution of electrons in different sub-states. In this case, the distribution and transition probability of electrons among different levels cannot be neglected; however, these factors were not considered in previous reports.10-28 The abnormal expansion of the crystal lattice, energy level shifts, and the redistribution of electrons to different sub-states affect the luminescence of CaAlSiN3:Eu2+, as discussed below, making the luminescence processes and mechanisms very complicated. Herein, the intrinsic mechanism of CaAlSiN3:Eu2+ thermal quenching luminescence was studied using the powerful experimental techniques of cryogenic spectroscopy, X-ray diffraction (XRD) combined with Rietveld refinement of the structure, X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS), time-correlated single-photon counting fluorescence lifetime, valence band spectroscopy, and infrared thermal imaging, assisted with theoretical calculations on the band structure, density of states, and charge deformation density. The results show that the increase of Eu, which has weak 4f orbital bonding ability due to shielding by the 5s, 5p and 5d orbitals, leads to crystal lattice expansion, band gap shrinkage, and energy level shifts, and therefore, changes the distribution of electrons on various sub-states, enabling thermal delocalization of electrons to the conduction band under the effect of temperature. 2. EXPERIMENTAL AND CALCULATION SECTION The samples of Ca1-xEuxAlSiN3 (x = 0.01%, 0.1%, 0.2%, 0.3%, and 0.5%) were synthesized with solid-state reaction at 1800 °C for 8 h from sources of Ca3N2 (Cerac, 99.0%), AlN (Tokuyama, H-grade), α-Si3N4 [Ube, SN-E10], and EuN (Cerac, 99.9%) under the protection of a high-pressure N2 atmosphere, using a gaspressure sintering furnace. The room-temperature excitation and emission spectra were measured using a Hitachi F-4600 spectrometer. The low-temperature photoluminescence spectra, over the temperature range from 20 to 300 K, were measured using a Fluorolog-3-Tau (Jobin Yvon) spectrometer equipped with a liquid-helium cycling accessory. The fluorescence lifetime was examined using a time-correlated single-photon counting spectrofluorometer. The X-ray diffraction (XRD) patterns were collected using a Rigaku D/max-IIIA diffractometer. The GSAS program29 was used to perform the Rietveld refinements to obtain the crystallographic parameters. Valence band spectra were measured using X-ray photoelectron spectroscopy (Thermo). X-ray absorption fine structure (XAFS) spectroscopy, including EXAFS and

XANES, was performed on beam line BL14W1 of the Shanghai Synchrotron Radiation Facility (SSRF) in Shanghai, China. Thermal delocalization of electrons was confirmed by measuring the surface temperature of the phosphors using a thermographic camera (Testo 882). The electronic structures, including band structure (BS), density of state (DOS), partial density of states (PDOS), and charger difference deformation, were calculated using the virtual-crystal approximation (VCA), based on the density functional theory (DFT) of periodic quantum chemistry and the crystal structure ICSD 16-179623. Al and Si plus Ca and Eu atoms arbitrarily occupy the same lattice position. Generalized gradient approximation (GGA)30 with Perdew-Burke-Ernzerhof (PBE) functional and norm-conserving pseudopotentials31 has been considered as the exchange-correlation functional. The 300 eV cutoff energy and 1×3×3 k-point sampling set were used to be converged. The max force and energy tolerances were set as the 0.03 eV/Å and 1.0×10-5 eV/atom, respectively; and the maximum displacement was set as 1.0×10-3 Å when running the geometry optimization. The Brillouin zone and the high-symmetry points corresponding to BS were exhibited Supplementary Fig. 1. 3. RESULTS AND DISCUSSION 3.1 Dependence of crystal structure and photoluminescence on Eu2+ concentration. Figure 1a presents the XRD patterns of Ca1-xEuxAlSiN3 (x = 0.01%, 0.1%, 0.2%, 0.3%, and 0.5%), in which all peaks can be indexed to standard ICSD 16-1796,23 suggesting the phosphors are of high-purity and ensuring all discussions in this work involve the Eu2+ activated CaAlSiN3 phosphor. The magnified XRD patterns in the 2θ range of 30.5-37.5 ° in Fig. 1b shows that the main diffraction peaks of Ca1-xEuxAlSiN3 blue shift with an increase of the x value, indicating the unit cell expands with increasing Eu2+ concentration. The experimental, calculated, and difference results of the XRD refinement of CaAlSiN3 doped with 0.01% and 0.5% Eu2+ are displayed in Figs. 1c and 1d, respectively. CaAlSiN3 crystallizes into the orthorhombic structure with a space group of Cmc21 (36).23 The inset in Fig. 1d presents the three-dimensional crystal structure of CaAlSiN3. The corresponding crystallographic data obtained by the XRD refinement, as shown in Supplementary Table 1, further confirm the expansion of the cell volume upon increasing Eu2+ concentration, because the radius of Eu2+ is larger than that of Ca2+. Thus, Eu2+ should replace the ideal site of Ca2+. The room-temperature spectra of Ca1-xEuxAlSiN3 displayed in Fig. 2 show that the emission peak red shifts from approximately 646 to 650 nm. Moreover, both the emission and excitation intensities enhance with Eu2+ increasing from 0.01% to 0.5%, although the intensities at 0.3% and 0.5% are nearly the same. Therefore, the concentration of Eu2+ studied here is less than the critical value; thus, the mechanism of energy annihilation caused by concentration quenching during transferring process could be dismissed. In addition, the relative excitation intensity of every sample at each wavelength (as indicated by the dashed line) is different, indicating that the configurations of excitation spectra change with Eu2+ concentration. 3.2 Spectral configuration changes with Eu2+ concentration and temperature. Figure 3 presents the low-temperature emission and excitation spectra of Ca1-xEuxAlSiN3 (x = 0.01%, 0.2%, and 0.5%). At any temperature, the sample activated by a higher concentration of Eu2+ emits a relatively longer wavelength. For more details regarding the variation of the emission peak wavelength, the full width at half maximum (FWHM) of the emission bands, and the thermal stability of luminescence as function of temperature, please refer to Supplementary Figs. 2 and 3, which show that the thermal stability of luminescence at x = 0.2% is better than that at 0.01% and 0.5%. Thus, it is very important to


Chemistry of Materials improve the thermal stability of luminescence by maintaining the

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Figure 2. Emission and excitation spectra of Ca1-xEuxAlSiN3 (x = 0.01%, 0.1%, 0.2%, 0.3%, and 0.5%) measured at room temperature. The excitation intensity of each sample in Figs. 3a, 3b, and 3c decreases rapidly as the temperature increases from 20 to 270 K, which can explain the luminescence decrease in Figs. 3d, 3e, and

3f. Besides depending on temperature, the comparison of Figs. 3b and 3c with 3a clearly shows that the excitation spectra configuration changes with varying Eu2+ concentration. The emission and excitation spectra in Fig. 3 were attributed to the 4f-5d transition of Eu2+. By virtue of charge and radius balance, Eu2+ will occupy the Ca2+ site in the CaAlSiN3 crystal lattice. However, there is only one site of Ca2+ in the crystal lattice of CaAlSiN3, which is 5-fold coordinated.20-23 When the nearest four N atoms are considered, the symmetry of Eu2+ that occupies the Ca2+ site could be processed approximately with tetrahedral Td symmetry.20-23 In Td symmetry, the 5 degenerate levels of the 5d orbital will split into the high-level triplet state T2g and the lowlevel doublet state Eg.10 However, the high-energy doublet states and the low-energy triplet states are observed in Fig. 3a, suggesting that the crystal field of Eu2+ experiencing at 0.01% low concentration is close to the octahedral (Oh) symmetry; but the highenergy triplet states and the low-energy doublet states are observed in Fig. 3b and 3c, suggesting that the crystal field of Eu2+ experiencing at 0.2% and 0.5% relative high concentration is close to the tetrahedral (Td) symmetry. Either in Oh or in Td symmetry, the 5d orbital comprises of 5 sub-states, i.e., 5d1, 5d2, 5d3, 5d4 and 5d5 in order of increasing energy.10 Moreover, the quintet states could be discriminated clearly from the 4f-5d excitation in Figs. 3a, 3b and 3c. Thus, the excitation spectra in the regions of 495-600, 416-495, 348-416, 297-348 and 255-297 nm are as-


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signed to the five sub-states 4f-5d1, 4f-5d2, 4f-5d3, 4f-5d4 and 4f5d5, respectively. Among them, the 5d2 sub-state in region of 415495 nm matches the 445-465 nm emission of blue LED chips well. Therefore, the red luminescence of CaAlSiN3:Eu2+ utilized in white LEDs is primarily based on the 4f-5d2 excitation. The change of the excitation spectral configuration provides a significant clue to understanding the mechanism of thermal quenching of the luminescence. Combining Fig. 2 with Fig. 3, we know that not only the excitation intensity (determined by electron population) but also the excitation wavelength (related with energy level) varies with Eu2+ concentration and temperature. The (a) (b)

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Figure 3. Emission and excitation spectra of Ca1-xEuxAlSiN3 (x = 0.01%, 0.2%, and 0.5%) measured at various low temperatures. 3.3 The shift of the energy levels predicted by theoretical calculations. Figure 4 gives the band structure and the DOS profiles of Ca1-xEuxAlSiN3 (x = 0, 0.5%, 1%, and 5%) in the neighborhood of the Fermi energy level, from which we can observe the following phenomena along with Eu2+ increasing: 1) the band gap decreases; 2) the relative energy levels of the valence band and conduction band shift towards the core level; and accordingly, 3) the Fermi energy level moves far from the valence band and approaches the conduction band. Similarly, the band gap shrinkage upon crystal lattice expansion was observed in the garnet rigid structure of Y3Al5O12.32 The smallest energy gap between valence and conduction bands occurs in the G point. The calculated band gaps for x = 0 and 0.5% are 3.636 and 3.613 eV, respectively. As Eu2+ increases to 1% and 5%, however, the Fermi level overlaps with the conduction band; thus, the band gap vanishes. In this situation, electrons will be automatically ionized from the excited state to the conduction band under the effect of temperature. Accordingly, the phosphor transforms from insulator to conductor. For more information about the band structure and DOS over the full energy scale, please refer to Supplementary Fig. 4. Without Eu2+ doping into CaAlSiN3, the DOS and PDOS in Fig. 4b show that the top of the valence band is mainly comprised of the p orbital, and the bottom of the conduction band mainly consists of p and d orbitals. However, as Eu2+ increases step-bystep, as shown in Figs. 4d, 4f and 4h, the s orbital has an increas-

ing contribution to both the valence band and the conduction band. The effect of s orbital on valence band could be clearly observed from the normalized valence band spectra of Ca1xEuxAlSiN3 (x = 0, 0.5%, 1% and 5%) in experiments, as displayed in supplementary Figure 6 (discussed below), mainly for the Si 3s orbital whose binding energy is about 9.0 eV. This point also confirms the availability of theoretical prediction. Due to the low content of Eu2+, the variation of the f orbital is difficult to discriminate from the PDOS in Fig. 4 The s, p, d, and f orbits in Figs. 4b, 4d, 4f and 4h comprise of all outer electrons of Al, Si, Ca, and Eu atoms, where the principal quantum numbers of them are indistinguishable from DOS and PDOS, because the Al and Si and the Ca and Eu atoms arbitrarily occupy the same lattice position. Supplementary Fig. 5 plots the partial density of states (PDOS) of the s, p, d and f orbitals, respectively, for x = 0, 0.5%, 1% and 5% side-by-side, which shows that the PDOS configuration of the s and d orbitals changes evidently with increasing Eu2+ concentration, but the shapes of the p and f orbitals remains nearly unchanged. Nevertheless, the energy level shifts, including the relative position of the Fermi level, is obviously observed in all orbitals. The variation of DOS and PDOS with varying Eu2+ concentration is conductive to understanding the change of the configuration of the excitation spectra in Figs. 3a, 3b and 3c.



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The band gap magnitude of our calculation agrees well with previously reported values, but is smaller than the experimental data because of the underestimation of the band gap in a DFT scheme.24-26 Jang’s25 results show that CaAlSiN3 has indirect band structure, with a band gap of approximately 3.37 eV. The calculations using the PBE method by Wang26 indicated that phase-I, phase-II and phase-III have indirect band gaps of 3.36, 3.36 and 3.4 eV, respectively, whereas the estimated value of phase-II using the hybrid exchange-correlation functional (HSE06) approach is approximately 4.76 eV. The first-principles calculation using the virtual chemical approximation by Mikami27 suggests that CaAlSiN3 has an indirect band gap at approximately 3.4 eV. However, the experimental results show that the peak of the excitation band is located at approximately 240 nm (5.17 eV) and the absorption edge is less than 300 nm (greater than 4.13 eV).10, 28 The first-principles or DFT calculations on the band structure and DOS of CaAlSiN3:Eu2+ have been performed by several scholars in the past; however, none paid attention to the energy level shifts and the transformation from insulator to conductor upon doping Eu2+. In this work, our focus is the effect of Eu2+ concentration on the band structure and the mechanism of thermal quenching luminescence. Perfectly, the theoretical prediction on the energy level shifts in this work is confirmed by the valence band spectra in the experiments (as below). Therefore, the DFT calculations could provide reasonable insights into the real nature of Eu2+ concentration and temperature on CaAlSiN3:Eu2+ luminescence.

Figure 5. Charge deformation density of Ca1-xEuxAlSiN3 (x = 0, 0.5%, 1% and 5%). 3.4 Verification of the theoretical prediction by the valence band spectra in experiments. Figure 6a gives the valence band spectra of Ca1-xEuxAlSiN3 (x = 0.01%, 0.2% and 0.5%). With Eu2+ increasing from 0.01% to 0.2% and finally to 0.5%, the binding energy of main absorption peak shifts from 23.46 to 24.90 and finally to 25.09 eV; the peak of valence band maximum (VBM) shifts from 4.36 to 5.03 and finally to 5.49 eV. The main absorption peak is attributed to the Ca 3p orbital, which has a standard binding energy of approximately 23.5 eV.33 This conclusion is consistent with the above-mentioned PDOS, indicating that the valence band is mainly comprised of the p orbital. The VBM may be comprised of Si 3p, Ca 3d and Eu 4f, with standard binding energies of 2.8, 5.0 and 6.9 eV, respectively.33 By calibrating the Ca 3p binding energy of all samples to 23.5 eV, the shift of the VBM far away from the Fermi energy level still could be found, as shown in Supplementary Fig. 6. Judging from the low-energy edge, the shift is mainly caused by the Ca 3d and Si 3s. The N 2s almost does not change, but a significant variation occurs to Eu 5p and the minor one is Eu 4f. In the experiments, the valence band spectra were measured in reference to the Fermi level, assuming Ef = 0. Thus, the shift of VBM far away from the Fermi energy level in Fig. 6a is equivalent to the movement of the Fermi energy level towards the bottom of conduction band in Fig. 4 because the band gap shrinks with Eu2+ increasing.

3.5 The local and electronic structures and the mechanism of the energy level shifts. To reveal the mechanism of energy level shifts with varying Eu2+ concentration in Ca1-xEuxAlSiN3, we turn to the crystal structure. Both the blue shift of the main diffraction peaks in the magnified XRD pattern in 2θ range of 30.5-37.5 ° in Fig. 1b and the refined cell volumes in Supplementary Table 1 confirm the expansion of the crystal lattice with Eu2+ increasing. With the doping of Eu2+, however, the expansion ratio of the local Eu-N bond length measured using EXAFS is far larger than that of the average volume of the crystal lattice resolved with XRD. As shown in Fig. 6b, the average distances of the firstneighbor coordination around Eu2+ are 1.99 and 2.06 Å for x = 0.01% and 0.5%, respectively. Evaluated from the average firstcoordination distance, the practical expansion ratio is approximately 3.5%. Nevertheless, the average distances of secondneighbor coordination around Eu2+ are 2.66 and 2.68 Å for x = 0.01% and 0.5%, respectively, the ratio of which is close to the volume expansion (i.e., approximately 0.09% from 281.185 Å3 to 281.447 Å3, as shown in Supplementary Table 1). Although the radius of Eu2+ (1.09 Å) is larger than Ca2+ (0.99 Å), the volume expansion in the rigid structure of CaAlSiN3 brought about by the addition of 0.5% Eu2+ is tiny. Interestingly, the analyses of the chemical bonds coordinated with the anion N, where two sites of N1 and N2 exist, show that the lengths of the N1/N2-Al and N1/N2Si bonds shorten, whereas the lengths of the N1/N2-Ca and N1/N2Eu bonds extend with increasing Eu2+ concentration (Supplementary Tables 3a and 3b). Moreover, the expansion of the N1-Eu bond is far larger than that of the N2-Eu bond (Supplementary Table 3c). It has to be emphasized that the difference between the cell volume expansion and the local expansion of the Eu-N neighboring coordination distance indicates that the distortion of crystal lattice and the movement of atoms will inevitably occur. Moreover, the excitation spectra in Fig. 3 show that the crystal field of Eu2+ experiencing at 0.01% low concentration is close to the octahedral (Oh) symmetry, but it is close to the tetrahedral (Td) symmetry at 0.2% and 0.5% relative high concentration of Eu2+. The transformation from Oh to Td symmetry indicates that the concentration of Eu2+ has significant impact on local structure, which will inevitably affect electronic structure and finally influences luminescence properties. For long time, there is some effort on predicting the f-d transition energies of Eu2+ and Ce3+ in inorganic compounds using the empirical relationships,34-35 but the predication of Eu2+ energy levels is far more difficult than that of Ce3+.36 Besides, the five 5d-state energies could be discerned in excitation spectra,37 but never all five levels were unambiguously assigned in the Ce3+ spectra.38 As for the reason for these differences, we think, the energy levels and the spectroscopic properties of Eu2+ influenced by local structure are far more intensively than that of Ce3+. Figure 6c presents the Eu L3-edge XANES of Ca1-xEuxAlSiN3 (x = 0.01% and 0.5%), in which the sharp line at approximately 6974 eV is caused by the transition from Eu 2p3/2 to the outer 5d orbital.39 Although the concentration increases 50 times from 0.01% to 0.5%, the absorption intensity of the Eu L3-edge does not increase; rather, it decreases slightly, indicating the outer 5d orbital is filled by a certain number of electrons, possibly caused by thermal delocalization (discussed below). It is basic knowledge that the electron configuration of Eu2+ is 2 2 6 2 6 10 2 6 10 7 2 6 1s 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p , but the observation of 5d empty orbit filled by electron at room temperature demonstrates that the electron configuration of Eu2+ in solid-state CaAlSiN3 compound is 1s22s22p63s23p63d104s24p64d104f7-δ5σ25p65dδ (δ is a variable quantity, mainly depending on the Eu2+ concentration of in CaAlSiN3 in addition to the temperature condition). The shield by the outer 5d, plus 5s and 5p orbitals will further reduce the



(a)

x = 0.01% (b) x = 0.2% x = 0.5%

Si 3s: 9.0 eV Si 3p: 2.8 eV Eu 5p:19.0 eV Ca 3d: 5.0 eV N 2s: 19.5 eV Eu 4f: 6.9 eV

25

20

15

10

VBM

E f

5

0

2.06 + 1.99 +

Magnitude of FT, a.u.

Counts, per second

Ca 3p 23.5 eV

30

in contrast to the Ca-N bond. This expansion is the basic reason for the energy level shifts when replacing Ca2+ with increasing amounts of Eu2+.

0

1

x = 0.01% x = 0.5%

6960

6980

7000

7020

Energy, eV

7040

7060

Natural logarithmic intensity, a.u.

(d) XANES Eu L3 - edge

++

R1st

2

Binding energy, eV

(c)

2.68 2.6 6

ability of the Eu 4f orbital to hybridize with the N 2p orbital to form a chemical bond, resulting in the exceptional expansion of local structure. Therefore, the local exceptional expansion of crystal lattice is mainly caused by the weak bond ability of Eu with N,

Normalized absorption, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 11

Eu L3 - edge EXAFS

x = 0.01% x = 0.5%

R2nd

R(Å)

3

4

I II

x = 0.01% x = 0.2% x = 0.5% 20

40

60

80

100

Lifetime, microsecond

Figure 6. (a) The valence band spectra, (b) the Fourier transformation of EXAFS in the R space, (c) the XANES Eu L3-edge, and (d) the natural logarithm fluorescence lifetime of Ca1-xEuxAlSiN3 (x = 0.01%, 0.2%, and 0.5%). 3.6 Mechanisms of thermal quenching luminescence. As discussed above, the splitting of the Eu2+ 5d orbital in CaAlSiN3 could be described with Figs. 7a and 7b. Because the luminescence used in white LEDs is mainly contributed by the 4f-5d2 excitation, visible light is radiated during the return of electrons from the lowest excited state 5d1 to the ground state 4f after relaxing from 5d2 to 5d1 (Fig. 7b). In the excitation process, however, electrons will not merely be excited to the 5d2 level; some of them will be excited to other sub-states. Electrons as well as holes have a 1/2 spin and obey the Pauli Exclusion Principle. At absolute zero temperature (T = 0 K), the energy levels are all filled up to a maximum energy, which is called the Fermi level, and no states above the Fermi level are filled. Regarding Eu2+ doped compounds, the 4f level of Eu2+ at absolute zero temperature (T = 0 K) is treated as the Fermi level in DFT theory. However, as the temperature increases, electrons could be filled into the empty states that are higher than the Fermi level. Since the fill of 5d empty orbital by electron upon increasing Eu2+ has been observed at room temperature in experiments (Fig. 6c), i.e., an existed fact, the distribution of electron populations on the 5d sub-states could be described by the Fermi-Dirac function as:

f ( x) =

1 1+ e

( Ei − E f ) / KT

(1)

where T is the absolute temperature, K is the Boltzmann's constant, Ei is the energy of a single-particle state i, and Ef is the Fermi level. The above results indicate that the band gap of CaAlSiN3 decreases and the relative positions of the valence band and conduction band shift towards the core level with increasing Eu2+ concentration. Therefore, the increase of Eu2+ concentration will not only reduce the energy barrier (∆E = Ei-Ef) between the ground and excited states but also decrease the barrier (Edc) of electrons ionized from excited sub-states to the conduction band (Figs. 7b and 7c). Thus, with both increasing temperature and Eu2+ content, an increasing number of electrons will be excited to increasingly high sub-states (such as 5d3, next step to 5d4, and finally to 5d5). Accordingly, the possibility of an electron being ionized from the 5d5 orbital to the conduction band will increase (Figs. 7b and 7c). When the Eu2+ concentration increases to 5%, the bottom of the conduction band overlaps with the Fermi level, and the highest 5d5 level has reached up to the conduction band (Fig. 7d). In this situation, electrons may thermally delocalize from the high-level sub-states to the conduction band. Therefore, the changeable distribution of electrons on various 5d sub-states


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and the thermal delocalization from the high-level 5d sub-states to the conduction band are the intrinsic thermal quenching mechanisms of CaAlSiN3:Eu2+ luminescence. The whole processes could be described by the schematic diagram in Fig. 7. CB e

TD

CB

Edc

splitting 5d Ex

TI 5d5 5d4 5d3 5d2 5d1

TD

Edc

e

VB

(a a) x= 0

x

) + A2 exp(−

x

τ2

) + y0

where fex(x) and fem(x) denote the wave functions of emission and excitation spectra, respectively.32 However, Supplementary Figs. 7a, 7b and 7c show that the overlapping area below the curves of the normalized emission and excitation spectra for each sample almost remain unchanged with temperature increasing from 20 to 270 K. The mechanism of energy annihilation during the electron migrating process has been excluded because the concentration of Eu2+ studied here is below the critical value and no concentrationdependent phenomenon is observed.

VB

(c c) x= 0.5%

(d d) x= 5%

3.7 Further confirmation of the thermal quenching model, with evidence on fluorescence lifetime and infrared thermal imaging. The multistage dynamic process of CaAlSiN3:Eu2+ luminescence involved in this model could be confirmed by the fluorescence lifetime, as presented in Fig. 6d. The lifetime fitted using the second-order exponential decay function14:

τ1

(3)

light

Figure 7. Schematic diagram of the thermal quenching mechanism of CaAlSiN3:Eu2+ luminescence. The relevant processes involving in excitation (Ex), emission (light), relaxation (RX), thermal ionization (TI), and thermal delocalization (TD). Four phenomena occurred with increasing Eu2+ concentration: 1) the shrinkage of the band gap, 2) the shift of the valence and conduction bands, 3) the expansion of the electron spread scale, and 4) the Fermi level rising close to the conduction band.

y = A1 exp(−

λ1

Eu 4f

VB

(b b) x= 0.2%

TI

light

Eu 4f

VB

e CB

Ex

light

Eu 4f

TD

TI Rx

Ex

Ef

CB

λ2

Eloss ∝ ∫ f ex ( x) f em ( x)dx

(2)

are approximately τ1 = 6.83, 7.63 and 10.10 µs and τ2 = 11.97, 17.12 and 15.95 µs (Supplementary Table 4) for x = 0.01%, 0.2% and 0.5%, respectively. The efficient luminescence is mainly determined by the relaxation from 5d2 to 5d1 (τ2) and the light emission from 5d1 to 4f (τ1). The lifetime τ1 increases continuously as Eu2+ increases from 0.01% to 0.5%. However, the lifetime τ2 increases from 0.01% to 0.2% but then decreases from 0.2% to 0.5%. Such a decrease should be caused by the thermal delocalization of electrons to the conduction band. Moreover, the thermal imaging pictures provide direct proof of the thermal delocalization of electrons involved in the luminescence process of CaAlSiN3:Eu2+. As shown in Figure 8, the phosphor activated by a high concentration of Eu2+ (0.5% and 0.2%) has a relatively higher temperature than that with a low concentration of Eu2+ (0.2% and 0.01%) and the difference between them intensifies with increasing temperature, because the thermal delocalization of electrons at high concentration of Eu2+ endows the phosphor with high heat conductivity. In addition, the mechanism of nonradiative transition could be excluded from the thermal quenching of CaAlSiN3:Eu2+. In the coordination-configuration model, the energy loss via nonradiative transition with electrons relaxed from excited state through the crossover of potential parabola curves to the ground state is proportional to the integral of normalized emission and excitation spectra, i.e.,

Figure 8. Infrared thermal imaging pictures of Ca1-xEuxAlSiN3 (x = 0.01%, 0.2%, and 0.5%). 3.8 Mechanism of emission color shift with varying Eu2+ concentration. In addition to thermal quenching, the energy level shifts could explain the red-shift of CaAlSiN3:Eu2+ emission with increasing Eu2+ concentration. The emission red-shift is originated from three factors: the centroid shift, crystal field splitting of 5d energy levels, and Stokes-shift.40-41 The above results demonstrate that the average bond length of Eu-N increases with increasing Eu2+ content in CaAlSiN3:Eu2+. According to the relationship between crystal field strength and ionic bond length:

Dq =

3 ze 2 r 4 5R 5

(4)

where the parameter Dq represents the crystal field stabilization energy, R is the average bond length, r is the mean size of a central ion, and Z is the charge of an ion; the longer bond length implies a weaker crystal field strength, and accordingly, the emission spectra should blue shift with increasing Eu2+ concentration. Herein, the simultaneously observed redshift of CaAlSiN3:Eu2+ emission and the expansion of crystal lattice size with increasing Eu2+ content rules out the explanation of crystal field theory. In fact, the crystal field effect on the 5d splitting could not be avoided. Probably, the shift of energy levels towards high energy direction exceeds the crystal field splitting towards low level, finally leading to the redshift of CaAlSiN3:Eu2+ emission with increasing Eu2+ concentration. Similarly, the red shift along with crystal lattice expansion was observed in Ref. 41, in addition to blue shift. 4. CONCLUSIONS In conclusion, the shift of Fermi level, the change of FermiDirac distribution of electrons on excited sub-states, and the thermal delocalization of electrons from excited sub-states to conduction band are intrinsically responsible for the thermal quenching luminescence of CaAlSiN3:Eu2+. The redshift of CaAlSiN3:Eu2+ emission with increasing Eu2+ content is also attributed to the energy level shifts caused by the weak Eu-N bonding and the exceptionally local expansion of the crystal lattice. As revealed by XANES, the empty 5d orbital of Eu2+ in CaAlSiN3 is filled with a certain amount of electrons. The shielding of 5d as


8


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well as the 5s and 5p orbitals reduces the bonding ability of Eu2+ with coordinated N3-, resulting in the exceptional expansion of the local Eu-N bond length. Meanwhile, the band gap of CaAlSiN3:Eu2+ decreases, the relative position of the valence band and conduction band shift toward the core level, and the Fermi level rises close to the conduction band. Accordingly, increasing numbers of electrons which obey the Fermi-Dirac distribution are excited to the high-level 5d sub-states of Eu2+. Finally, electrons possibly delocalize from the high-level sub-states to the conduction band. These basic mechanisms suggest that the future exploration on the methods to enhance the luminescence thermal stability of CaAlSiN3:Eu2+ should focus on stabilizing the Fermi level and suppressing the thermal delocalization of electrons. ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: The Brillouin zone and the high-symmetry points, luminescence properties in terms of the emission peak wavelength, FWHM, and relative intensity of Ca1-xEuxAlSiN3 (x = 0.01%, 0.2%, and 0.5%) as a function of temperature; the profiles of the band structure and the density of states over the full energy scale; normalized valence band spectra; refined crystallographic data, bonds length, and fitted fluorescence lifetime (PDF). AUTHOR INFORMATION Corresponding Author * [email protected] (L. C.), or [email protected] (S. C.). Author Contributions The manuscript was written through contributions of all the authors. L.C., Y.J., and S.C. proposed the original ideas on the thermal quenching mechanisms, conceived the experiments, and jointly supervised the work; M.F. and Z.Z.1 obtained the XRD data, room-temperature spectra, thermal imaging pictures, refined structure, and the 3D-structure; Y.Q. and C.L. performed the theoretical calculations; Z.Z.4 obtained the cryogenic excitation and emission spectra and fluorescence lifetimes; Y.F and J.H. synthesized the samples; Z.S. and Z.J. measured and analyzed the EXAFS and XANES spectra. All authors have given approval to the final version of the manuscript. Each of authors made an indispensable contribution to different aspects of this work. Funding Sources Financial support was provided by The National High-Tech R&D Program (863 program), the National Natural Science Foundation of China, the Science and Technology Program of Anhui Province of China, and the China Postdoctoral Science Foundation. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The work was financially supported by the National High-Tech R&D Program (863 program) (2013AA03A114), the joint funding of National Natural Science Foundation of China and the Chinese Academy of Sciences (U1332133), the Science and Technology Program of Anhui Province of China (1301022062 and 1301022067), the special fund for research and development of the Hefei institute (IMICZ2015112), and the fund of Beijing National Laboratory for Molecular Sciences (20140143). Moreover, we deeply thank Dr. Ronghui Liu, a senior scientist at the Grirem Advanced Materials Co. Ltd, for his assistance in measuring the absolute quantum efficiency and luminescence thermal stability of phosphors.

Page 10 of 11

ABBREVIATIONS LEDs, light-emitting diodes; LCD, liquid crystal display; EXAFS, extended X-ray absorption fine structure; XANES, X-ray absorption near edge structure; XRD, X-ray diffraction; ICSD, inorganic crystal structure database; FWHM, the full width at half maximum; DOS, density of states; PDOS, partial density of states (PDOS); VCR, virtual-crystal approximation; DFT, density functional theory; SSRF, Shanghai Synchrotron Radiation Facility.

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