Identifying the Emission Centers and Probing the Mechanism for

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C: Plasmonics, Optical Materials, and Hard Matter

Identifying the Emission Centers and Probing the Mechanism for Highly Efficient and Thermally Stable Luminescence in the LaSiN : Ce Phosphor 3

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Jiyou Zhong, Weiren Zhao, Fu Du, Jun Wen, Weidong Zhuang, Ronghui Liu, Chang-Kui Duan, Ligen Wang, and Kun Lin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00683 • Publication Date (Web): 15 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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The Journal of Physical Chemistry

Identifying the Emission Centers and Probing the Mechanism for HighHighly Efficient and Thermally Stable Lu Luminescence in the La3Si6N11: Ce3+ Phosphor Jiyou Zhong†, Weiren Zhao†*, Fu Du‡, Jun Wen§*, Weidong Zhuang‡*, Ronghui Liu‡, Chang-Kui Duan£, Ligen Wang∇ and Kun Lin∥ †

School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Guangzhou 510006, China National Engineering Research Center for Rare Earth Materials, General Research Institute for Nonferrous Metals, Grirem Advanced Materials Co., Ltd., Beijing 100088, China § School of Physics and Electronic Engineering, Anqing Normal University, Anqing 246133, China £ Department of Physics, University of Science and Technology of China, Hefei 230026, China ∇State Key Laboratory of Nonferrous Metals and Processes, General Research Institute for Nonferrous Metals, Beijing 100088, China ‡

∥

Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, China

ABSTRACT: Nitride La3Si6N11: Ce3+ is an important commercial phosphor for high-power white light-emitting diodes (WLEDs) due to its strong resisting toward thermal quenching and sufficient emission efficiency. However, the underlying mechanisms of this high performance is still in mystery. And the emission properties of Ce3+ in two kinds of crystallographic sites are currently in dispute. Here, we confirmed the yellow emission ascribed to CeLa(2) luminescence center and proposed blue emission owning to CeLa(1) luminescence center through both theoretical and experimental methods. Particularly, we find an unusual efficient and fast energy transfer from CeLa(1) to CeLa(2) due to a large spectral overlap between the emission of CeLa(1) and the absorption of CeLa(2), and efficient electron transfer from defects to 5d orbital at high temperature, which shows highly relevant to the highly efficient yellow emission and thermal stability of this material. This study presents a full and new understanding toward this special phosphor, and provides useful insights into design of highly efficient and thermally stable luminescent materials for future lighting.

INTRODUCTION Nowadays, nitride phosphors are widely utilized in WLEDs because of their superior chemical and thermal stabilities to those conventional sulfide and oxide phosphors.1-6 Among them, lanthanum silicate-based nitride phosphor La3Si6N11: Ce3+, was considered as the best available phosphor for fabricating high-power WLEDs,7 since it exhibits particularly high thermal stability and sufficient emission efficiency, typically, more than 95% of initial emission intensity can be kept when temperature heated from 25 o C up to 200 oC;8 and the external quantum efficiency (EQE) can reach to ~80% under 450 nm excitation at room temperature.9 However, the previous research of this phosphor mainly focused on improvement of emission efficiency and adjustment of the emission wavelengths by synthesis optimization and cationic substitutions,9-13 respectively. For example, Suehiro et al. improve the EQE up to 42.4% from 32.2% by Ca2+ incorporation, mainly due

to the reduction of impurity phases LaN and LaSi3N5.10 With further optimization of preparing technology, in 2017, Du et al. synthesized the pure phase of La3Si6N11: Ce3+ and promoted the EQE to 78.2% under 450 nm excitation.9 Besides, Du et al successfully adjusted the emission spectrum shifting toward longer wavelength to meet the demand of high color rendering index (CRI) application by Y3+ partially substituting La3+ and Al3+ partially substituting Si4+, respectively,11,12 but unfortunately, the emission efficiency decreases with the increase of substitution ratio. One primary restriction to these luminescence tunings is the unclear emission properties of Ce3+ in two kinds of crystallographic sites. For instance, George et al. believed that Ce3+ on either La3+ site exhibits similar emission properties for their identical La/Ce-N coordination numbers and similar bond lengths,8 while Du et al. considered that CeLa(1) should present longer emission wavelength for its lower site symmetry than CeLa(2).11,12 Additionally, Jia et al. proposed that the yellow emission spectrum should be ascribed to CeLa(2), and no luminescence in CeLa(1) for its 5d spreading into conduction band via constrained DFT calculation.14 Generally, it is possible to identify the emission centers experimentally by introducing low concentration activator to host, in which the interactions between luminescence centers are very weak and the luminescence centers can be approximately considered as independent luminescence centers. Coincidentally, in 2015, Park et al. prepared La3Si6N11:Ce3+ and Lu3Al5O12: Ce3+ sample with low Ce3+ doping concentration (1%) to compare their decay behavior, and observed a blue emission ranging from 400 to 475 nm under 355 nm excitation in the emission spectrum of La3Si6N11:Ce3+ sample, and they ascribed this blue emission to the impurity LaSi3N5:Ce3+ phase.15 Therefore, identifying the emission centers experimentally of this phosphor is mainly restricted by synthesis of La3Si6N11:Ce3+ pure phase. Recently, wave function-based embedded cluster ab-initio calculations was successfully used to calculate the energies and relative oscillator strengths of the 4f→5d transitions of Ce3+ ions, base on which the theoretical absorption spectra of Ce3+ at different sites can be simulated.16-20 Thus, this method is succeed in identifying different types of luminescence centers independent of experiment.

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Although the La3Si6N11: Ce3+ phosphor is currently commercial available, the mechanism of highly efficient and thermally stable luminescence of this material is still in mystery. Revealing the underlying mechanisms of this high performance may be useful to design of promising phosphors with highly efficient and thermal stabilities for future lighting. According to the traditional theory, there are two models that can explain the thermal quenching behavior, which are crossing relaxation and thermal ionization.21 The crossing relaxation can be characterized by the well known activation energy for thermal quenching Ea, which can be obtained experimentally by fitting the Arrhenius equation.22 The thermal ionization can be characterized by Edc, the energy difference between 5d and the bottom of conduction band.23 Generally, the host of a phosphor with large band gap is beneficial to reducing thermal ionization, while the band gap of La3Si6N11 is unavailable in literature so far, the theoretical calculated values are ~3 eV and ~4 eV by using the GGA-PBE and HSE06 functional, respectively.8,14 The values are really not large compared with that of YAG (6.5 eV),24 but the La3Si6N11: Ce3+ phosphor do exhibit extremely excellent thermal stability, which is difficult to understand. Recently, some unusual thermal quenching behaviors in phosphors are reported. For instance, Wu et al. reported a thermal stability enhancement in SrBi2B2O7: Sm3+, Eu3+ phosphor due to the crystal structure becoming more compact at high temperature than that at room temperature, which is possible to resist the access of high- energy phonons at high temperature.25 Kim et al. observed zero-thermal-quenching in Na3Sc2(PO4)3:Eu2+ phosphor, this behavior is attributed to the energy transfer from traps to the 5d of Eu2+,26 similar situation was observed in KxCs12+ phosphor reported by Zhang et al.27 These provide xAlSi2O6:Eu clues for us to explore the mechanism for the abnormal thermal quenching behavior in La3Si6N11: Ce3+ phosphor. Generally, the emission efficiency of a phosphor is mainly determined by crystalline morphology and some inherent natures. The inherent natures are highly related to the structural rigidity, which can be characterized by Debby temperature (ΘD).28-30 A large ΘD value indicates that high-energy phonon modes are inaccessible, therefore decreases the number of nonradiative relaxation pathways, leading to highly efficient emission. Additionally, the activators accepting energy from sensitizers or host matrix is another way to promote the emission efficiency of luminescence centers, which is common in many commercial phosphors, such as CeMgAl11O19:Tb3+, (La,Ce)PO4:Tb3+, BaMgAl10O17:Eu2+,Mn2+, Y2O3: Bi3+,Eu3+, etc.31-34 And nearly all these efficient energy transfer occur between different ion species due to their spectral overlap. In this work, to clarify the luminescent properties of Ce3+ in two kinds of luminescence centers, the pure phase La3Si6N11:Ce3+ phosphors with low Ce3+ doping concentration were designed and prepared experimentally, and we also performed wave functionbased embedded cluster ab-initio calculations to confirmed the experimental results. To reveal the intrinsic mechanism of highly efficient and thermally stable luminescence of this phosphor, we redetermined the band gap of La3Si6N11 by theoretical and experimental investigation. Additionally, thermal behavior of crystal structure, temperature dependent photoluminescence, thermoluminescence, fluorescence decay, X-ray photoelectron spectroscopy, first-principles calculations, etc. were adopted to probing the mechanisms.

METHODOLOGY Synthesis. The samples (La1-xCex)3Si6N11 (x= 0, 0.1%, 0.5%, 1%, and 5%) were prepared by using a gas pressure sintering method. Highly pure LaN, Si3N4, CeN, etc., were adopt as raw materials, and were mixed in a nitrogen-filled glove box according to stoi-

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chiometric ratio. The mixture were put into crucibles and fired at 1500-1800 °C for several hours under nitrogen atmosphere with several interval grindings. The as-prepared products were washed with dilute acid and deionized water, and finally dried at an appropriate temperature. Characterizations. The room temperature X-ray diffraction (XRD) patterns were obtained using an X-ray powder diffractometer (Rigaku, Japan) with Co-Ka radiation (λ = 0.178892 nm). High temperature X-ray diffraction (XRD) (X’Pert3 Powder, PANalytical B.V., Almelo, The Netherlands, Cu Kα, λ = 1.5406 Å) data were collected using an Anton-Paar TTK450 chamber. The heating rate was 10 °C/min, and the sample was held for 10 min at the specified temperature to reach heat equilibrium. Highresolution transmission electron microscope (HRTEM) images were acquired by using a TecnaiG2F30 (FEI, U.S.A.) microscope with an accelerated voltage of 300 kV. Scanning electron microscopy (SEM) image was tested on Hitachi-S4800 (Japan). The photoluminescence spectra and thermal quenching were measured by a spectrofluorometer (Fluoromax-4, Edison, U.S.A.), which is composed of a Xe high-pressure arc lamp, a photomultiplier tube and a heating apparatus. The decay curves of Ce3+ lifetime values were measured using a FLS-980 fluorescence spectrophotometer (Edinburgh Instruments) equipped with a xenon lamp (450 W, Osram) as the excitation source. Quantum efficiency was measured using the integrating sphere on the QE-2100 quantum yield measurement system (Otsuka Electronics Co., Ltd., Japan), and a Xe lamp was used as an excitation source and white BaSO4 powder as a reference. Thermoluminescence (TL) curves were collected by a thermoluminescence meter (SL08-L, GuangzhouRadiation Science and Technology Co. Ltd) with the heating rate 1 °C/s after fully excited by 254 nm light source for two minutes. The X-ray Rietveld profile refinements in this work were performed with the General Structure Analysis System (GSAS) software. The X-ray photoelectron spectroscopy (XPS) was measured on ESCALAB 250Xi (Thermo Fisher, U.S.A.), using the Al Kα line as an X-ray source with a minimum resolution of 0.45 eV (Ag 3d5/2). The luminous efficiency of the w-LED device fabricated using the as-prepared phosphor with a 450 nm InGaN blue LED chip was obtained using a ATA-500 Sync-Skan color analyser system under a forward bias of 3 mA. Computational Methods. Structural optimization, elastic constants and dielectric functions calculations of La3Si6N11 were performed by using the density functional theory (DFT) method with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional,35 as implemented in the VASP package.36,37 The electronic properties of La3Si6N11 were calculated based on the optimized geometries using the hybrid HSE06 functional, and the electronic properties of Ce3+-doped La3Si6N11 by using the standard PBE0 functional, which contains 25% of exact Hartree-Fock exchange and 75% of PBE exchange, and 100% of PBE correlation energy. The La3Si6N11 host crystal containing 40 atoms was used as the computational model. The La 5s25p65d16s2, Si 3s23p2, N 2s22p3 and Ce 5s25p64f15d16s2 electrons were treated as the valence electrons, whose interactions with the ion cores are treated with the projected augmented wave (PAW) method.38 The geometric structures were fully relaxed with the convergence criteria of 10−6 eV used for the change in the total energy and of 0.01 eV/Å used for Hellman–Feynman forces on atoms. The cutoff energy of 550 eV was used for the basis set of the plane waves. The Brillouin zone integrations was sampled using a 6×6×12 Monkhorst-Pack k-point mesh. The energies and relative oscillator strengths of the 4f→5d transitions of Ce3+ ions were calculated basing on optimized crystal structures, constructing Ce3+-center defect clusters (CeLa(n)N8)21(n = 1, 2) embedded into the La3Si6N11 host and then perform wave function-based embedded cluster ab-initio calculations (as


RESULTS AND DISCUSSION To eliminate the influence of concentration quenching, the Ce3+ doing concentration was designed within 6%.8,9 The solid solutions (La1-xCex)3Si6N11 (x= 0, 0.1%, 0.5%, 1%, and 5%) with highly pure phase and highly crystalline nature were successfully prepared via using the same synthesis technique provided by Du et al.,9,11,12 which are confirmed by the powder X-ray diffraction (PXRD) patterns, Rietveld structural refinement, scanning electron microscope (SEM) image, and high resolution transmission electron microscope (HRTEM) image presented in Figure 1. The sample (La1-xCex)3Si6N11 (x→ 0) is chosen to study the band gap of La3Si6N11 since the band gap of host matrix is essential to luminescent property of a phosphor.28,47 The band gap of La3Si6N11 was calculated to be 4.16 eV (shown in Figure 2a) through electronic structure analysis obtained by using HSE06 functional, which is consistent with experimental 4.17 eV (presented in Figure 2b) from optical band gap calculations. The value is really not large compared with that of YAG (experimental value of 6.5 eV).16 The partial density of states (PDOS) of La(1) and La(2) demonstrated in Figure 2a are completely different in 4f and 5d states distribution, indicating large differences exist in the two distinct crystallographic sites. To further study the difference of the two sites, we calculated the 4f1 and 5d1 energy levels (shown in Table S1, Supporting Information) of Ce3+ at the two different sites by performing ab-initio model potential (AIMP) embedded cluster calculations using the (CeLa(n)N8)21- (n = 1, 2) as clusters.48 The schematic diagram for the calculated energy and relative 5.0%

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Figure 1. (a) The powder X-ray diffraction (PXRD) patterns of (La1-xCex)3Si6N11 (x= 0, 0.1%, 0.5%, 1%, and 5%) compared with standard card of Ce3Si6N11; (b) Rietveld structural refinement of (La1-xCex)3Si6N11 (x= 0%) at 25 oC; (c) scanning electron microscope (SEM) image of (La1-xCex)3Si6N11 (x= 5%); and (d) high resolution transmission electron microscope (HRTEM) image of

(La1-xCex)3Si6N11 (x= 5%), insets show the TEM image and the fast Fourier transforms of the HRTEM image.

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implemented in the MOLCAS program).39 The accurate quantum chemical ab-initio calculations were used to treat the valence electrons of the atoms in the defect clusters, whose immediate lattice environments were represented by the embedding ab-initio model potentials (AIMPs) located at the host lattice sites within a sphere of the radius 10.0 Å.40 The point charges, which were situated at the lattice sites within a sphere of the radius 50.0 Å, were used to represent the lattice environments outside the AIMPs. And the CASSCF/CASPT2/RASSI–SO methods utilized in the embedded cluster calculations include the effects of the spin–orbit coupling (SOC) and second-order perturbation correction and thus accurately give the energies and wave functions of 4f and 5d states of Ce3+ ion.41-46

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Figure 2. (a) Electronic structure of La3Si6N11 obtained by using HSE06 functional; (b) diffuse reflection spectrum of (La1xCex)3Si6N11 (x= 0, and 0.5%) and calculated band gap of La3Si6N11; (c) schematic diagram for the calculated energy and relative oscillator strengths of 4f→5d transitions of Ce3+ in two kind of sites. oscillator strengths of 4f→5d transitions of Ce3+ presented in Figure 2c shows that CeLa(2) has larger energy level splitting than CeLa(1), indicating CeLa(2) is expected to exhibits a longer emission wavelength than CeLa(1). This also suggests that average Ce-N bond length (shown in Table S2, Supporting Information) determines the luminescent properties in this phosphor, ascribing to the large covalency of this material since the average value of static dielectric constant of La3Si6N11 is as high as 5.54 (dielectric functions are shown in Figure S 1, Supporting Information), which indicates large covalency or nephelauxetic effect.49 So, in this material the covalency, rather than symmetry, dominates the luminescent properties. Figure 3a presents spectral properties of the sample (La13+ doping concentraxCex)3Si6N11 (x→ 0) with extremely low Ce tion (< 0.001%), in which the interactions between luminescence centers are very weak, and they can be approximately considered as independent luminescence centers. As shown, this sample exhibits yellow emission peaking at 530 nm under 450 nm excitation, while an unreported blue emission peaking at 410 nm are mainly presented when excited by 360 nm. Both of the two emission spectra can fit well with two Gaussian emissions (shown as Figure S2, Supporting Information) separated by ~1830 cm−1 and ~1730 cm−1, respectively, consistent with the energy difference between 2F5/2 and 2F7/2 of Ce3+ 4f levels in one kind of emission center,50 indicating the two emission spectra should be ascribed to two different type of Ce3+ emission centers. Moreover, the excitation spectrum monitored at 530 nm shows two excitation bands with peaks at 455 nm and 370 nm in the region of 300-500 nm, and the excitation spectrum monitored at 410 nm shows two excitation bands with peaks at 375 nm and 350 nm in the region of 300-400 nm. Obviously, these excitation spectra match the typical energy levels splitting of Ce3+ under eight-coordinated environment, similar to Ce3+ activated garnet phosphors.51,52 Additionally, considering comprehensively the diffuse reflection spectrum (shown in Figure 2b) and theoretically calculated 4f→5d transitions (shown in Figure 2c), which is somewhat deviate from actual value due to the larger charge number of selected clusters (Ce21(n = 1, 2), we can deduce that the yellow emission La(n)N8) should be ascribed to CeLa(2), and the blue emission should be ascribed to CeLa(1).


The Journal of Physical Chemistry therefore decreases the number of nonradiative relaxation path ways.8,28 The ΘD of La3Si6N11 is calculated to be 747 K (the related parameters calculated by DFT are shown in Table S3 and 6

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Figure 3a-c, presents the spectral variations with the increase of Ce3+ concentration. As shown, the yellow emission spectrum under 450 nm excitation exhibits less changes except for emission intensity due to increased number of luminescence centers and slightly red-shift due to enhanced crystal field splitting induced by a-b plane shrinking,8 indicating that there is very limited change occurred around the CeLa(2) luminescent center. However, the excitation spectrum monitored at 530 nm shows significant variations with increasing Ce3+ concentration (excitation spectra are shown in Figure S3, Supporting Information), especially in the region of 300-400 nm, which seemingly induced by incorporating the excitation spectrum monitored at 410 nm. Additionally, as shown in the emission spectrum under 360 nm excitation, the blue emission gradually weakens, while the yellow emission enhances rapidly, and even exceeding the emission intensity excited by 450 nm at high Ce3+ doping concentration. Generally, the variations of excitation spectrum is ascribed to changes in energy level distribution or energy transfer. Obviously, an efficient energy transfer from CeLa(1) to CeLa(2) should be responsible for this situation since the Ce3+ concentration variation is unable to induce such large changes in energy levels distributions. Generally, an efficient energy transfer always occurs between different ion species, such as Ce3+-Tb3+, Eu2+-Mn2+, Bi3+-Eu3+, etc.,31-34 but rarely reported in the same ion species. The behavior of energy transfer is further supported by room temperature decay curves of Ce3+ presented in Figure 4. The decay curves of Ce3+ are fitted by single exponential function, due to the independent emission center.53 As shown, the life times monitored at 410 nm and 530 nm excited by 360 nm rapidly decrease and slightly increase, respectively, with increasing Ce3+ doping concentration. While the lifetimes monitored at 530 nm excited by 450 nm gradually decease with the increasing Ce3+ doping concentration due to the enhanced probability of nonradiative transitions,54 and the values of life times are 8-10 ns shorter than those monitored at 530 nm excited by 360 nm. The time difference is mainly consumed in process of energy transfer. These results indicate an efficient and fast energy transfer from CeLa(1) to CeLa(2) indeed occurs, mainly ascribing to the following two reasons. Firstly, a large spectral overlap (400-480 nm) between the emission of CeLa(1) and the absorption of CeLa(2) can be observed in Figure 3a-c, suggesting that efficient resonant energy transfer can occur from the CeLa(1) to CeLa(2) according to the Dexter's equation.55,56 Secondly, the distance between CeLa(1) and CeLa(2) (3.65 Å) is much shorter than that between CeLa(1) and CeLa(1) (5.16 Å), which means the interaction between CeLa(1) and CeLa(2) is stronger than that between CeLa(1) and CeLa(1). Thus, the CeLa(1) with high energy is easy to transfer its energy to CeLa(2), and the energy transfer between CeLa(1) and CeLa(1) is neglectable. Actually, due to the highly efficient energy transfer, the CeLa(1) even can be regarded as sensitizer at high Ce3+ doping concentration under 360 nm ultraviolet excitation. In this case, there’s only CeLa(2) that can be excited under 450 nm blue light excitation, which means two thirds of Ce3+ remain in the ground state and are unable to accept energy from excited CeLa(2). Therefore, the excited CeLa(2) has two ways to consume its energy, i.e. photon emitting and nonradiative transition. And nonradiative transition is always caused by concentration quenching and the interaction between activator and host matrix. The concentration quenching is actually the interaction between activators (CeLa(2)), basing on energy transfer, which depends on the distance between the activators. Obviously, even in Ce3Si6N11 the distance between CeLa(2) is as large as 7.04 Å, implying that the concentration quenching is very weak. The interaction between activator and host matrix is highly related to the structural rigidity, which can be characterized by Debby temperature (ΘD). A large ΘD value indicates that high-energy phonon modes are inaccessible,

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Figure 3. Emission spectrum excited by 360 nm and 450 nm, and excitation spectrum monitored at 410 nm and 530 nm of (La1xCex)3Si6N11 samples with (a) x→ 0.0%, (b) x= 0.5%, and (c) x= 5%; the digital photographs of corresponding phosphors under 365 nm UV lamp are inserted. λex = 360 nm λem = 410 nm

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Figure 4. Decay curves of Ce3+ and life times (a) monitored at 410 nm and excited by 360 nm, (b) monitored at 530 nm and excited by 360 nm, and (c) monitored at 530 nm and excited by 450 nm in (La1-xCex)3Si6N11 (x= 0.5%, 1%, and 5%). Table S4, Supporting Information), higher than that of YAG (726 K)28, indicating that this material has a highly structural rigidity. Thus most of the energy absorb by CeLa(2) is converted into pho-


700

Figure 5. Temperature dependent PL emission spectra of (La1o xCex)3Si6N11 (x= 5%) excited by 450 nm, ranging from 25 C to o 300 C. Wavelength (nm) Intensity (a.u.) Intensity (a.u.)

tons. And this may be the main reason for high emission efficiency of this phosphor under 450 nm blue light excitation. The temperature dependent emission spectra of (La1-xCex)3Si6N11 (x= 5%) sample excited by 450 nm in the range of 25-300 oC are shown in Figure 5, the PL intensity as a function of temperature are presented in Figure 6a. As shown, the emission intensity first increases up to a temperature of 75 oC and then decreases slightly. The emission intensity increase is usually related to the formation of defect levels in phosphor,26,27 the intense thermoluminescence (TL) of sample (La1-xCex)3Si6N11 (x= 5%) presented in Figure 6b confirmed that high density of defects exist in this phosphor. The main peaks of TL curve suggest that traps with the activation energy of ~0.73 eV and ~0.95 eV corresponding to temperature of 90 oC and 200 oC, respectively, are formed, which can capture electrons, and then release them with increasing temperature, causing the increase of PL intensity. As Figure 6a presented, the PL intensity within temperature range of 25-225 oC is higher than that at room temperature (25 oC), indicating that the energy provided by releasing electrons from traps is more than that consumed in thermal quenching process by crossing relaxation or thermal ionization. Therefore, the defect of this material play an important role in stabilizing thermal stability. When the temperature acceding 225 oC, this phosphor shows normal thermal quenching property without interference of defects. The formation of defects should be ascribed to the existence of O2- and unreduced Ce4+ in the lattice, which can be detected from XPS of O 1s and Ce 3d (shown in Figure S4, Supporting Information). Additionally, the temperature dependent relative emission intensity of CeLa(1) and CeLa(2) in sample (La1-xCex)3Si6N11 (x= 0.5%) under 360 nm excitation (shown in Figure S5, Supporting Information) are investigated to compare their thermal stabilities. As shown, the emission intensity of CeLa(2) also presents first increase and then decrease, while the emission intensity of CeLa(1) decreases dramatically as the increase of temperature. The emission intensity increase of CeLa(2) may not only be related to the formation of defect levels, but also the increased possibility to accept the energy transferred from CeLa(1) at high temperature. To clarify the mechanism of thermal quenching, crossing relaxation or thermal ionization.21 The activation energy for thermal quenching Ea, and the energy barrier Eb between 5d and the bottom of conduction band for thermal ionization are investigated. The calculated Ea of (La1-xCex)3Si6N11 (x= 5%) phosphor is ~0.75 eV (Arrhenius equation fitting is shown in Figure S6, Supporting Information),22 which is similar to that of YAG: Ce3+ (~0.77 eV).57 The energy barrier Eb is calculated to be ~0.76 eV (Figure S7 and S8, Supporting Information), which is very close to the value of activation energy Ea, indicating that thermal ionization is the main reason for thermal quenching in this phosphor under blue light excitation at high temperature due to its relatively narrow band gap.

120 (a) 80

PL Intensity Fitted curve

40 0 900 (b)

TL Intensity

600 300 0

(c) PL Wavelength Fitted line

550 540 530

1.002

(d)

a/a0 c/c0

50

600 0.25

0.00

500 150

200 o

250

-1

-6

100

150

200

-1

250

300

Figure 6. (a) Temperature dependent PL intensity excited by 450 nm; (b) TL intensity of sample (La1-xCex)3Si6N11 (x= 5%); (c) Temperature dependent PL emission wavelength excited by 450 nm; and (d) n/n0 (n= lattice parameters a, and c) of sample (La1xCex)3Si6N11 (x= 0%) as a function of temperature, ranging from 25 oC to 300 oC. Figure 6c shows the temperature dependent PL emission wavelength of sample (La1-xCex)3Si6N11 (x= 5%) under 450 nm excitation. As presented, the emission wavelength shifting toward longer wavelength as increase of temperature is significant. The spectral redshift of phosphor is always the reflect of local microstructural variations including reduction of average bond length between luminescence center and coordinate anions, and distortion of sites occupied by luminescence centers, leading to larger spiting of crystal field. To clarify this, we measured the PXRD of (La1-xCex)3Si6N11 (x→ 0%) at different temperature ranging from 25 oC to 300 oC (the structural refinements results are shown in Figure 1b, Figure S9 and Table S5, Supporting Information). As Figure 6d presented, the sample shows normal thermal expansion, but the a-b plane has a larger expansion coefficient than the c-axis direction. The bond length difference of the two kinds of CeLa(2)– N (Table S6, Supporting Information) enlarges as increasing temperature, leading to increasing CeLa(2) site distortion, while the average CeLa(2)–N bond length is not significantly changed with increase of temperature. The result indicates that the effect of site distortion causing larger splitting of 5d or 4f of CeLa(2), namely the

0.50

100

-6

αa = 5.47×10 °C

Temperature (°C)

0.75

50

-1

αc = 3.6×10 °C

1.000

650

550

-5

αV = 1.46×10 °C

Fitted line Fitted line

1.00

Wavelength (nm)

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


n/n0

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300

Temperature ( C)



Figure 7. Emission spectra of a w-LED lamp fabricated via the combination of a 450 nm blue LED chip and (La1-xCex)3Si6N11 (x= 5%) phosphor with the driven current of 5 mA. The inset shows digital images of the LED package without and with power input. John-Teller effect, is dominant in this phosphor. This effect should be responsible for the redshift of emission wavelength as increase of temperature.58 Finally, the photoluminescence efficiency and application performance of the selected sample (La1-xCex)3Si6N11 (x= 5%) were investigated. The external and inner quantum efficiency was measured to be and 58.4% and 81.1%, respectively, under 450 nmexcitation. The emission spectra of the w-LED lamp fabricated via the combination of a 450 nm blue LED chip and (La1xCex)3Si6N11 (x= 5%) phosphor with the driven current of 3 mA, and digital images of the LED package are shown in Figure 7. The obtained CIE color coordinates, correlated color temperature (CCT), color rendering index (Ra) and luminous efficacy are measured to be (0.27, 0.31), 9498 K, 75.0 and 110.6 lm/W, respectively. The luminous efficacy of this device is high enough for applications, while the relatively high CCT and low CRI due to the lack of red emission of this phosphor need to be improved by adding the red-emitting phosphor or red-shifting the emission spectra of this phosphor.

CONCLUSIONS In summary, the luminescent properties of two kinds of emission centers in La3Si6N11: Ce3+ phosphor are clarified by both experimental and theoretical investigations. A fast and highly efficient energy transfer from CeLa(1) to CeLa(2) occurs under ultraviolet excitation, making the CeLa(1) nearly act as sensitizer at high Ce3+ doping concentration. Under blue light excitation, only CeLa(2) can be excited, and less concentration quenching can be occurred due to the long distance between the CeLa(2) centers, and the high rigidity of this material reduces the probability of high-energy phonon accessing lattice, resulting in the high emission efficiency. The thermally stable luminescence of this material is mainly ascribed to the efficient electron transfer from defects to 5d orbitals at high temperature. The mechanism of thermal quenching is dominated by thermal ionization. The redshift of emission spectra as increasing temperature should be ascribed to CeLa(2) sites distortions. This study presents a full understanding toward this highly efficient and thermally stable phosphor, which is useful to research, design and explore phosphor with high performance for solid state lighting.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Related spectral information, calculated dielectric function, elastic constant, electronic structure, etc., and energy level diagram. (PDF)

AUTHOR INFORMATION Corresponding Corresponding Author *

[email protected] [email protected] * [email protected] *

Author Contributions †

These authors contributed equally in this work.

Page 6 of 8

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This present work was financially supported by the National Basic Research Program of China (2014CB643801); the National Natural Science Foundation of China (Nos. 51702057); China Postdoctoral Science Foundation (174815); Major Program for Cooperative Innovation of Production, Education & Research of Guangzhou City (201704030106; 201604016029); and Science and Technology Planning Projects of Guangdong Province of China (2014B050505020; 2016201604030035).

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TOC Graphic

8 ACS Paragon Plus Environment

Identifying the Emission Centers and Probing the Mechanism for

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