Improving Quantum Efficiency and Thermal Stability in Blue-Emitting

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Improving Quantum Efficiency and Thermal Stability in BlueEmitting Ba2-xSrxSiO4:Ce3+ Phosphor via Solid Solution Xiaoyu Ji, Jilin Zhang, Ying Li, Shuzhen Liao, Xinguo Zhang, Zhiyu Yang, Zhengliang Wang, Zhongxian Qiu, Wenli Zhou, Liping Yu, and Shixun Lian Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01652 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 20, 2018

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Chemistry of Materials

Improving Quantum Efficiency and Thermal Stability in BlueEmitting Ba2-xSrxSiO4:Ce3+ Phosphor via Solid Solution Xiaoyu Ji,†,‡ Jilin Zhang,*,†,‡ Ying Li,†,‡ Shuzhen Liao,*,# Xinguo Zhang,¶ Zhiyu Yang,§ Zhengliang Wang,§ Zhongxian Qiu,†,‡ Wenli Zhou,†,‡ Liping Yu,†,‡ and Shixun Lian*,†,‡ †

Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education of China) ‡ and Key Laboratory of Sustainable Resources Processing and Advanced Materials of Hunan Province College, Hunan Normal University, Changsha 410081, China

#

School of Chemistry and Chemical Engineering, Hunan Institute of Engineering, Xiangtan 411104, China

¶

Guangdong Provincial Key Laboratory of New Drug Screening, School of Pharmaceutical Sciences, Southern Medical University, Guangzhou 510515, China

§

Key Laboratory of Comprehensive Utilization of Mineral Resource in Ethnic Regions, School of Chemistry & Environment, Yunnan Minzu University, Kunming 650500, China. ABSTRACT: Ba1.8-xSrxSiO4:0.1Ce3+,0.1Na+ (x = 0 - 1.8) phosphors were prepared by a high-temperature solid-state reaction. The emission peaks of Ba1.8-xSrxSiO4:0.1Ce3+,0.1Na+ shift from 391 nm to 411 nm with increasing Sr2+ content under excitation by a UV light at around 360 nm. Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ phosphor exhibits the best performance of luminescence, whose absolute quantum efficiency is 97.2% and the emission intensity at 150 °C remains 90% of that at room temperature. The effect of replacing Ba2+ by Sr2+ on the red shift of emission band and the increase of quantum efficiency (QE) and thermal stability (TS) was investigated in detail based on the Rietveld refinements, Raman spectra, thermoluminescence, and decay curves, etc. The performance of UV chip based pc-LEDs indicates that Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ can be a promising blue phosphor for white-emitting pc-LEDs.

1. Introduction White light-emitting diodes (w-LEDs) have recently received much attention as solid-state lighting devices due to the advantages such as high brightness, long lifetime, environmental friendly, and low power consumption.1-7 The most common method is the phosphor-converted LED (pc-LED) by combing a Y3Al5O12:Ce3+ (YAG:Ce3+) yellow phosphor with an InGaN blue LED chip.8 However, the color rendering index (CRI) of this device is low (< 80) due to the scarcity of red emitting component. There are several strategies to overcome this deficiency, for example, the combination of a blue LED chip with green/yellow and red phosphors,9-13 the combination of a nearultraviolet (n-UV) LED chip with blue, green, and red phosphors,14-15 the combination of a n-UV LED chip with a single-phase white-emitting phosphor,5, 16-21 etc. No matter what kind of combination is adopted, the phosphor used for pc-LEDs should have a high quantum efficiency (QE) and good thermal stability besides suitable excitation and emission spectra. The Ce3+ ion has only one electron in the 4f orbital, which is raised to a 5d level when the ion is excited. Both of excitation and emission processes between the 4f and

5d energy levels are parity and spin allowed. Therefore, the corresponding intensity is very high. The split of the 5d orbitals is greatly influenced by the coordination environment. Therefore, the energy difference between the lowest 5d excited state of Ce3+ ion and the 4f ground state varies from host to host. Modification of crystal structure is an efficient and convenient strategy to explore new Ce3+-doped phosphors. For example, the emission color of Ce3+-doped Y3Al5O12 can be tuned through partial replacement of the host atoms.22-26 Tunable emission of Ce3+ doped phosphors is realized in solid solutions with isotypic crystal structure, i.e. LaSr2AlO5 - Sr3SiO5,27 (Sr,Ba)3AlO4F - Sr3SiO528-30. Our group recently reported the improvement of quantum efficiency and thermal stability that accompanied with a red shift of emission through the adjustment of host composition in Li4Sr1+xCa1– 3+ 31 x(SiO4)2:Ce within a phase. As an important class of phosphors, Eu2+ or Ce3+ doped orthosilicate M2SiO4 (M = Ba, Sr, and Ca) phosphors have been widely studied for solid-state lighting.32-44 Xia et al. presented a ternary diagram of the phases depending on the chemical compositions with Ca2−xSrxSiO4, Ca2−yBaySiO4, and Sr2−zBazSiO4, and summarized the phases of three end members depending on temperature.45 There are several

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different kinds of phases for Ca2SiO4, therefore, it is hard for Ca2−xSrxSiO4 and Ca2−yBaySiO4 to realize continuous solid solutions within an isotypic crystal structure between the two end members. Ba2SiO4 has only a phase with an orthorhombic crystal structure. Sr2SiO4 exhibits only two phases β and α’H, and the latter is isotypic with that of Ba2SiO4. Furthermore, the addition of Ba2+ in Sr2SiO4 facilitates the formation of α’H phase.45 Therefore, continuous solid solutions can be expected in Sr2−xBaxSiO4. Denault et al. found that the intermediate composition (x = 0.9, ~46% Sr) for Ba2-xSrxSiO4:Eu2+ had the best performance when thermal stability was considered, which had a greater rigidity than others.38 To the best of our knowledge, the luminescence properties of Ce3+ in orthorhombic Ba2-xSrxSiO4 solid solutions have not been reported up to now, although there are several papers on Ce3+ doped Ba2SiO4 46, α’H-Sr2SiO4:Ce3+ 47-48 , and BaSrSiO4:Ce3+ 49. In this study, we focus on the evolution of crystal structure and luminescent performance of Ce3+-doped orthorhombic Ba2-xSrxSiO4. Improved thermal stability and high quantum efficiency are achieved in an intermediate composition. Relationship between crystal structure and luminescent properties are discussed in detail. The performance of pc-LEDs also indicates the potential of the phosphor as a candidate for white pc-LEDs.

2. Experimental 2.1 Materials and Synthesis Ba1.8-xSrxSiO4:0.1Ce3+,0.1Na+ (x = 0 - 1.8) powders were prepared by a high-temperature solid-state reaction. Typically, raw materials BaCO3 (A.R.), SrCO3 (A.R.), SiO2 (A.R.), CeO2 (99.99%), and Na2CO3 (A.R.) were thoroughly mixed together in an agate mortar according to different values of x, where Na+ acted as charge compensator. The mixed raw materials were then transferred to alumina crucibles, with subsequently firing at 1400 °C for 3 h in an electric tube furnace under a reducing atmosphere (N2/H2 = 95:5). Upon cooling down to room temperature naturally, final Ba1.8-xSrxSiO4:0.1Ce3+,0.1Na+ phosphors were obtained. The doping content of 0.1 for Ce3+ in the Ba1.8-xSrxSiO4:0.1Ce3+,0.1Na+ series is based on the best performance of Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ in Ba0.4Sr1.63+ + 2xSiO4:xCe ,xNa series, which is shown below.

2.2 Measurements and Characterization Measurement of X-ray powder diffraction (XRD) patterns was carried out by a PANalytical X’Pert Pro diffractometer (Cu Kα radiation, 40 kV, 40 mA). The Rietveld structure refinements were performed using the general structure analysis system (GSAS) program.50 Photoluminescence excitation (PLE), emission (PL) spectra, and temperature-dependent PL spectra were recorded on a Hitachi F4500 spectrophotometer (Japan) equipped with a high-temperature controller (TAP-02, Orient KOJI, China). The absolute quantum efficiencies (IQE) and decay curves were measured on an Edinburgh FLS980 Fluorescence Spectrometer with a 450 W xenon lamp and a nanosecond flash lamp. The temperature-dependent absorbance, external and internal quantum efficiencies

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(EQE and IQE) of the related phosphors have been measured on a QE-2100 spectrophotometer from Otsuka Photal Electronics, Japan. Raman spectra were collected on a LabRRM HR Evolution Raman spectrometer (Horiba Scientific, France) with a 532 nm laser. Thermoluminescence (TL) spectra were collected on a FJ-427A TL meter (Beijing Nuclear Instrument Factory) with a heating rate of 56 °C per minute after samples were exposed to the radiation of 254 and 365 nm UV light for 10 minutes. The measurements for pc-LED devices were achieved on a high accurate array spectrometer (HSP6000, HOPOO, China). All of the measurements were carried out at room temperature except the thermoluminescence spectra and temperature-dependent PL spectra.

3. Results and Discussion 3.1 Structure Characterization The XRD patterns of Ba1.8-xSrxSiO4:0.1Ce3+,0.1Na+ phosphors are shown in Figure 1. When Sr2+ concentrations are low, the XRD patterns match well with the standard card of Ba2SiO4 (JCPDS#70-2113) with an orthorhombic crystal structure. The XRD patterns of those with high Sr2+ concentrations match well with the standard card of Sr2SiO4 (JCPDS#76-1494, α’H-Sr2SiO4), which also has an orthorhombic structure). The magnified XRD patterns in Figure 1b exhibit a continuous shift of the diffraction peaks towards higher angle side with the increase of Sr2+ concentration, suggesting the existence of continuous solid solutions between Ba2SiO4 and Sr2SiO4. The shift of diffraction peaks is due to the replacement of Ba2+ ions by Sr2+ ions with a smaller radius. In order to get more information on crystal structure, Rietveld refinement of the powder XRD patterns has been done. The refinement results of three representative Ba1.83+ + xSrxSiO4:0.1Ce ,0.1Na samples are shown in Figure 2 with a 2 theta range of 15-60°. The refinement results for all the samples can be found in Figure S1 and S2 in Supporting Information. It should be noted that the refinement for samples with x = 0-0.6 are on the basis of Ba2SiO4 (ICSD#6246, Pmcn), while those for x = 0.8-1.8 are based on Sr1.9Ba0.1SiO4 (ICSD#36042, Pmnb). Figure 2a-c, S1, and S2 suggest that there are a few impurities existing. The contents of the main phase and impurities are listed in Table S1. Results suggest that the impurities are mainly BaSiO3 and BaAl2O4 for x = 0-1.0, while SrSiO3 and SiO2 for x = 1.2-1.8. Crystallographic data, refinement parameters, and average atomic displacement parameter (Uiso) are shown in Table 1. The data of cell edges and volume are also transformed to curves as shown in Figure 2d, which indicates the decrease of cell volume with the increase of Sr2+ content. Detailed atomic parameters of the three samples after refinement are listed in Table S2-4. Figure 3 shows the unit cell of Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ viewing along two different directions, the coordination environments of two Ba2+/Sr2+ sites, and the SiO4 tetrahedra. The two Ba2+/Sr2+ sites are surrounded by 9 and 11 nearest oxygen atoms, respectively. The Ce3+ and Na+ ions are much smaller than Ba2+ and Sr2+, therefore, it is expected that Ce3+ and Na+ ions tend to occupy the Ba/SrO9


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(MO9) sites. The result that Ce3+ occupies MO9 site has been verified by the PLE spectra monitored at different wavelengths and the calculated DFT total energies.44 The situation in Ba1.8-xSrxSiO4:0.1Ce3+,0.1Na+ samples is going to be verified below by the analyses of PL spectra excited at different wavelengths and decay curves. Table S5 lists the atomic distances in MO9 and SiO4 polyhedra in Ba1.83+ + xSrxSiO4:0.1Ce ,0.1Na (x = 0, 0.6, 1.2, 1.4, and 1.8). The average M-O distance in MO9 tends to decrease with the increase of Sr content. The variation of average Si-O distance in SiO4 is not in linear relation to Sr content (x). However, it should be noted that the SiO4 is smallest for x = 1.4.

3+

+

Figure 1. XRD patterns of (a) Ba1.8-xSrxSiO4:0.1Ce ,0.1Na with 2+ different Sr concentrations (x), (b) the magnified XRD patterns in 29-32° range, (c) in 26-29° range f0r x = 0-1.0, (d) in 30.4-30.9° range for x = 1.2-1.8. Standard cards for impurities: (1) BaAl2O4 (72-0387), (2) BaSiO3 (70-2112), (3) SrSiO3 (870474), (4) SiO2 (70-2540).

3+

+

Figure 2. Rietveld refinement of powder XRD patterns of Ba1.8-xSrxSiO4:0.1Ce ,0.1Na , (a) x = 0, (b) x = 1.4, (c) x = 1.8, and (d) cell parameters versus Sr content (x), cell edge b and c for x = 0 ~ 0.6 are exchanged for a better view of the variation of similar cell edge.

Table 1. Crystallographic data, structure refinement parameters of Ba1.8-xSrxSiO4:0.1Ce3+,0.1Na+ Unit cell

Cell volume

x=0

x = 1.4

x = 1.8

a = 5.8040(1) Å

a = 5.6712(1) Å

a = 5.6529(1) Å

b = 10.1821(1) Å

b = 7.2074(1) Å

b = 7.0886(1) Å

c = 7.5042(1) Å

c = 9.7670(1) Å

c = 9.7296(1) Å

V = 443.47(1) Å3

V = 399.22(1) Å3

V = 389.87(1) Å3

Space group

Pmcn (62)

Pmnb (62)

Pmnb (62)

Rwp

6.70%

5.87%

7.02%

Rp

4.78%

4.33%

4.86%

χ2

2.585

1.210

1.538

average Uiso

0.0282 Å2

0.0208 Å2

0.0290 Å2



3+

+

Figure 3. crystal structure of Ba0.4Sr1.4SiO4:0.1Ce ,0.1Na from refinement results, showing polyhedra of MO11, MO9, and SiO4. (a) view along nearly axis a, (b) view along axis b.

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3+

+

Figure 4. Raman spectra of Ba1.8-xSrxSiO4:0.1Ce ,0.1Na (x = 0-1 1.8) phosphors. Insert: Raman shift of the peak at ~830 cm versus Sr content.

3.2 Photoluminescent Properties The information about the internal vibrations of the chemical bonds can be obtained by analyzing the Raman spectra of Ba1.8-xSrxSiO4:0.1Ce3+,0.1Na+ (x = 0 - 1.8) phosphor as shown in Figure 4. The peaks in 300-600 cm-1 region relate to O-Si-O bending modes, while those in 800 1000 cm-1 region originate from Si-O stretching. This assignment is based on the comparison of IR and Raman spectra of orthosilicates.51-52 It is obvious that the wave number of the main peaks at around 800 cm-1 increases with the successive substation of Ba2+ ions by Sr2+ ions from x = 0 to 1.4, and then keeps nearly unchanged. The relationship between the internal vibration mode and the wave number of the related bonds can be expressed by the following equation53

ν =

1

k

2π c µ

(1)

where ν̅ , c, k, and µ are wave number, velocity of electromagnetic wave, force constant of chemical bond, and the reduced mass for the related two atoms, respectively. The increase of wave number for Si-O stretching is related to the increase of force constant, which suggests the decrease of Si-O bond length. This result is consistent with the refinement result as listed in Table S5. As indicated in our previous work, the decrease of Si-O and M-O bonds corresponds to the decrease of ΔR between the ground and excited state in configurational coordinate diagram, and an increase of Debye temperature, which in turn corresponds to the phosphor with a high thermal stability and QE value.31 Therefore, it is expected that Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ (x = 1.4) has the best performance in thermal stability and quantum efficiency.

The normalized excitation and emission spectra of Ba1.83+ + Sr x xSiO4:0.1Ce ,0.1Na are presented in Figure 5a. The PLE spectra exhibit a broad band from 250 to 400 nm peaking at around 360 nm, which can be attributed to 4f-5d transition of Ce3+ ions. When excited by the corresponding PLE peak wavelength, an asymmetric PL band is observed at around 400 nm. Both PLE and PL bands shift to longer wavelength side with the increase of Sr2+ content. Figure 5b shows clearly the evolution of PLE, PL intensity, and wavelength of PL peak with increasing Sr2+ content (x). The broad PL band shifts from 391 (x = 0) to 411 nm (x = 1.8) gradually. However, the PLE peak shows a red shift only in 0 – 1.4 range. Further increasing Sr2+ content exhibits a slight blue shift of the PLE peak. The PL intensity also increases with Sr2+ content (x) gradually until x = 1.4, and then decreases. The Gaussian fitting result of the PL profile of Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ excited at 360 nm is shown in Figure 5c. The energy difference between the two Gaussian bands is 1835.3 cm-1, which is due to the transitions from the lowest 5d excited state of Ce3+ to the 4f ground state levels 2F5/2 and 2F7/2. The PLE, PL data of selected phosphors are also listed in Table 2 together with Stokes shift values. It is obvious that the Stokes shift values of Ba1.8SiO4:0.1Ce3+,0.1Na+ and Sr1.8SiO4:0.1Ce3+,0.1Na+ are quite larger than others.


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Detailed discussion can be found in Supporting Information right after Figure S6. The excitation and emission processes of Ce3+-doped phosphors are mainly due to the transfer between 4f and 5d energy levels. The energy positions of 5d levels are affected greatly by the coordination environment. Generally, the effects can be explained in terms of two factors, namely, the centroid shift (εc) and the crystal field splitting (εcfs). Both of these factors greatly affect the 5d electrons of the Ce3+ ions, whereas the well-shield 4f electrons are not strongly affected. According to the formulas proposed by Dorenbos, the change of excitation and emission peaks due to centroid shift and crystal field splitting can be obtained.54-55 N

ε c = 1.79 × 1013 ∑ i =1

ε cfs = β poly R

α spi ( Ri − 0.6 ∆R ) 6

(2)

−2

(3) 3+

2-

where Ri is the distance between Ce and O and R is the average distance, ΔR is the difference between the radii of Ce3+ and the cation site replaced by Ce3+ ions, αspi is the spectroscopic polarizability of anion, and βpoly is a parameter depending on the coordination polyhedra. These two equations show that the centroid shift and the magnitude of crystal field splitting of 5d in Ba1.83+ + depend mainly on the distance xSrxSiO4:0.1Ce ,0.1Na between the Ce3+ ion and O2- surrounded. The distance between Ce3+ ion and O2- in MO9 site decreases with the increase of Sr2+ content as indicated in Table S5. Therefore, it can be concluded that the corresponding values of εc and εcfs increase with Sr2+ content. These effects lead to a decrease in the energy difference between the 5d and 4f energy levels, and the red shift of PLE and PL bands is reasonable. Table 2. PLE, PL peaks 1 and 2 by Gaussian fitting and Stokes shift (SS) of selected phosphors Figure 5. (a) Normalized PL and PLE spectra of Ba1.83+ + xSrxSiO4:0.1Ce ,0.1Na , (b) the dependence of PLE and PL 2+ peak, PL intensity on content of Sr , (c) Gaussian fitting 3+ + results of PL band for Ba0.4Sr1.4SiO4:0.1Ce ,0.1Na .

In order to see if the PLE and PL spectra of the phosphors are influenced by the impurities, BaSiO3:Ce3+,Na+, SrSiO3:Ce3+,Na+, and BaAl2O4:Ce3+,Na+ phosphors were synthesized. The XRD patterns, PLE and PL spectra of the samples are shown in Figure S3-5 in Supporting Information. XRD patterns indicated the formation of these phases. The profile and position of PLE and PL spectra for these three samples are different from those of the Ba13+ samples. These results suggest that the imxSrxSiO4:Ce purities have little influence on the PLE and PL properties of target orthosilicate phases. The influence of alkali metal ions on the luminescent properties of the phosphors was also concerned. Figure S6 shows the PL spectra three representative Ba1-xSrxSiO4:Ce3+ phosphors (x = 0, 1.4 and 1.8) without and with the addition of alkali metal ions (Li+, Na+, K+). Results suggest that sample with Na+ has the highest PL intensity in Ba0.4Sr1.4SiO4:0.1Ce3+,0.1A+ series.

0.1Ce3+,0.1Na+

Ba1.8-xSrxSiO4:

PLE peak (cm−1)

PL peak 1 (cm−1)

PL peak 2 (cm−1)

SS (cm−1)*

x=0

29291.2

24703.6

26357.4

2933.8

x = 0.6

28490.0

24307.2

25933.6

2556.4

x = 1.0

28089.9

23912.0

25536.3

2553.6

x = 1.4

27670.2

23277.5

24975.0

2695.2

x = 1.8

27777.8

23105.4

24850.9

2926.9

*SS: energy difference between PLE peak and PL peak 2.

Furthermore, Stokes shift can also influence the emission of Ce3+ doped phosphors. When configurational coordinate diagram is used to explain the Stokes shift, a smaller offset (ΔR) between the parabolas of excited and ground levels relates to a smaller Stokes shift. The offset is related to the rigidity of the crystal lattice, and the rigidity can be represented by Debye temperature (ΘD). A high ΘD value corresponds to a small average atomic displacement parameter (Uiso) according to the follow equation56-57



ΘD,i =

3h 2TN A Ai k BU iso,i

Page 6 of 14

nate from the different ion radii for Ce3+ and Eu2+, namely, the 10-coordianted Ba/Sr site may too large for Ce3+ with a much smaller radius to occupy.

(4)

A small Uiso relates to a structure with a high rigidity, and finally a small Stokes shift. Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ has smaller Uiso values than Ba1.8SiO4:0.1Ce3+,0.1Na+ and Sr1.8SiO4:0.1Ce3+,0.1Na+ as listed in Table 1 and Table S2-4. Therefore, it is understandable that Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ exhibits a smaller Stokes shift and higher emission intensity than the other two samples. Quantum efficiencies (QE) and absorbance are also measured for these phosphors, which are listed in Table 3. The QE values increase apparently from Ba1.8SiO4:0.1Ce3+,0.1Na+ to Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ with the increase of Sr2+ content, and then decreases slightly in Sr1.8SiO4:0.1Ce3+,0.1Na+. The highest QE value for Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ phosphor is consistent with its highest PL intensity and smallest Uiso value among these three phosphors. Detailed results of QE measurement in FLS980 are shown in Figure S7 in Supporting Information.

The lifetimes of the Ba1.8-xSrxSiO4:0.1Ce3+,0.1Na+ phosphors are listed in Figure 6, which tends to increase with Sr2+ contents until x = 1.4. It should be noticed that the doping content of Ce3+ keeps unchanged, while the host composition changes greatly in these samples. The decay curves of Ba0.4Sr1.6-2xSiO4:xCe3+,xNa+ with changing the doping content of Ce3+ are shown in Figure 7 for comparison. The curves can also be well fitted with a singleexponential equation. The lifetimes that are listed in Figure 7 show a slight increase with Ce3+ content, whose extent is smaller than those for Ba1.8-xSrxSiO4:0.1Ce3+,0.1Na+ ones. Figure S11 and S12 in Supporting Information show more information on the XRD patterns and PL bands with a slight red shift upon increasing Ce3+ content in Ba0.4Sr1.63+ + 2xSiO4:xCe ,xNa . The QE values of these phosphors are also listed in Supporting Information, which increase with Ce3+ content until maximizing at x = 0.1.

Table 3. IQE, EQE, and absorbance of the Ba1.83+ + xSrxSiO4:0.1Ce ,0.1Na phosphors

It is pointed out by Ronda that if the transition from the excited state to the ground state contains both radiative and nonradiative ones, the transition rate (k, in s-1) can be written as58

x

IQE (FLS980)

IQE

EQE

absorbance

0

66.02%

43.66%

35.46%

81.23%

0.6

77.81%

61.24%

48.83%

79.74%

1.0

94.24%

83.44%

69.09%

82.82%

1.4

97.19%

97.48%

80.56%

82.66%

1.8

86.42%

75.67%

63.13%

83.44%

The decay curves of the Ba1.8-xSrxSiO4:0.1Ce3+,0.1Na+ phosphors with host composition changed are shown in Figure 6. All the decay curves can be well fitted by the following equation with a single-exponential form It = I0 e(-t/τ)

(5)

where It and I0 are the luminescence intensities at times t and 0, respectively, τ is the lifetime. The singleexponential form of the decay curves suggests that Ce3+ ions occupy only one crystallographic site. Figure S8-10 in Supporting Information illustrate the PL and normalized PL spectra excited at different wavelengths for Ba1.8SiO4:0.1Ce3+,0.1Na+, Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+, and Sr1.8SiO4:0.1Ce3+,0.1Na+. The nearly overlapped normalized PL spectra for each phosphor suggest that the emission originates from Ce3+ at a single crystallographic site. The 9-coordinated site is preferred for Ce3+ ions when ion radii are concerned. The result that Ce3+ occupies MO9 site has been verified by the PLE spectra monitored at different wavelengths and the calculated DFT total energies.44 It should be noted that Eu2+ occupies two different Ba/Sr coordination environments in ref 36, which is deduced from the asymmetric PL band at 77 K and the biexponential fit and two lifetimes of the decay curves for Ba1.82+ samples. It is reasonable that the radius of xSrxSiO4:Eu 2+ Eu is close to Sr2+ and a little smaller than Ba2+ with a same coordination number. The difference in site occupancy between Ce3+ and Eu2+ doped samples may origi-

k = kr + knr

(6)

where kr is the transition rate of only emission process, and knr is the rate of nonradiative process. The existence of nonradiative process will increase the transition rate, namely, a shorter decay time (τ, equals 1/k). If the decay time (τ0) without nonradiative transitions is known, the QE of the phosphor can be expressed as QE = τ/τ0

(7)

The

increase of lifetime for both Ba1.83+ ,0.1Na+ and Ba0.4Sr1.6-2xSiO4:xCe3+,xNa+ series is accompanied by the increase of QE value. These results are accordance with equation (7). Therefore, the increase of lifetime in Ba0.4Sr1.6-2xSiO4:xCe3+,xNa+ with the increase of Ce3+ content should be due to the decrease of nonradiative rate. xSrxSiO4:0.1Ce

The thermal stability is an important parameter for the application of phosphors in pc-LEDs. Figure 8a shows the temperature-dependent emission spectra of Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ measured at temperatures ranging from 30 to 250 °C. The PL intensity of the sample decreases with increasing temperature due to thermal quenching effect. The probability of non-radiative transition from 5d to 4f increases upon increasing temperature. Figure 8b illustrates the curves of relative emission intensity versus temperature for Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ and other representative Ba1.8-xSrxSiO4:0.1Ce3+,0.1Na+ phosphors. It is found that the intermediate composition Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ shows best thermal stability among the samples when considering the PL intensity, whose PL intensity at 150 °C remains 90% of that at room temperature. The activation energy (Ea) for thermal quenching can be calculated by Arrhenius formula59 −E IT = [1 + D exp( a )]−1 I0 kT


(8)

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where IT and I0 are the emission intensity at a given testing temperature and 0 K, respectively. D is a constant depending on phosphor. k is Boltzmann constant (8.617 × 10-5 eV/K). According to the Arrhenius formula, the values of Ea were listed in Figure 8b. Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ has a largest Ea value among them. A higher Ea indicates a smaller non-radiative rate at the same temperature. Our previous work showed that Li4Sr1.4Ca0.57(SiO4)2:0.03Ce3+ had good thermal stability and high IQE with a thermal activation energy of 0.122 eV.31 Li et al. reported that the introduction of Al/Gd/B into α‑Ca1.65Sr0.35SiO4:Ce3+ resulted in lower thermal stability, however, the Ea values increase from 0.1358 eV to 0.1787, 0.2568, and 0.2020 eV, respectively, which are not in agreement with the thermal quenching behavior.35 Temperature-dependent PL spectra of the Ba1.8-xSrxSiO4:0.1Ce3+,0.1Na+ phosphors are shown as Supporting Information in Figure S13-17. The peak values of the PL band change little with increasing temperature, and the corresponding values of International Commission on Illumination (CIE) coordinates for Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ as a representative almost overlap with each other as shown in Figure 8c, which suggests good thermal stability of emission color versus temperature. Figure 8c also exhibits a photograph of the phosphor under a 365 nm UV lamp.

3+

+

Figure 7. Decay curves of the Ba0.4Sr1.6-2xSiO4:xCe ,xNa phosphors under excitation at 365 nm and monitored at 410 nm.

There are several reasons for thermal quenching, namely, the displacement between 4f and lowest 5d excited state in the configuration coordinate diagram, thermal ionization,60-61 and the degradation of absorbance induced by heat 62. TL spectra were collected after the samples exposed to both the radiation from 254 and 365 nm UV light. There are two different TL peaks at around 50 and 185 °C for Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ as shown in Figure 9a, which relate to two different trapping energy levels. The TL intensity of the former decreases with charging temperature and almost disappears when charging at above 175 °C. While the TL intensity of the latter keeps nearly unchanged when charging temperature increases from 25 to 75 °C, then increases with charging temperature from 75 to 150 °C, finally decreases above 150 °C. The TL peak at ~50 °C corresponds to a shallow trap, which can trap the electrons excited by 254 nm light to a higher excitation energy level, and then the trapped electrons can be released at lower temperatures. The TL peak at ~185 °C corresponds to a deep trap, which can trap the electrons excited by 365 nm light to the lower excitation energy level, and then the total release of the trapped electrons needs a high temperature. The trap depths E of these two traps can be estimated to be 0.775 and 1.113 eV, respectively, by the following equation63-65

 E  = s exp  −  kT  kTm 

βE

2 m

(9)

where heating rate β = 0.93 K/s, k is Boltzmann constant in eV/K, s = 1 × 1011 s-1, Tm is the TL peak in K.

3+

+

Figure 6. Decay curves of the Ba1.8-xSrxSiO4:0.1Ce ,0.1Na phosphors, with excitation (λex), monitoring wavelengths (λmo), and lifetimes (τ) listed.

The temperature-dependent PL spectra of the phosphor are measured under excitation at 365 nm. The TL peak at ~185 °C is not affected by thermal detrapping up to 150 °C. Therefore, the integrated TL intensity of this TL peak versus temperature from 25 to 150 °C is shown together with the integrated PL intensity versus temperature in Figure 9b. It is obvious that the TL intensity increase with increasing charging temperature, while the PL intensity decreases. This phenomenon strongly indicates that thermal ionization is responsible for thermal quenching of Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ at high temperature (T > 100 °C).60 As a comparison, the TL spectra of Ba1.8SiO4:0.1Ce3+,0.1Na+ and Sr1.8SiO4:0.1Ce3+,0.1Na+ are also measured, which are shown in Figure S18 in Supporting Information. Similar phenomena are found, which suggest that thermal ionization is also responsible for thermal quenching of Ba1.8SiO4:0.1Ce3+,0.1Na+ and 3+ + Sr1.8SiO4:0.1Ce ,0.1Na . It should be noted that the TL peaks for these two phosphors also situate at ~50 and 185 °C, which suggests the similar traps for these samples. The curves of PL intensity versus temperature in 100 – 250 °C range exhibit a similar tendency for these three phosphors with similar traps, which may supply some evidence for the thermal ionization induced thermal quenching at high temperature.



Page 8 of 14

Temperature-dependent absorbance, IQE, and EQE are collected by using the QE-2100 spectrophotometer from Otsuka Photal Electronics, which are shown in Figure 10. Results indicate that heating treatment really results in the degradation of absorbance that also contributes to thermal quenching. Therefore, the thermal quenching at high temperature (T > 100 °C) should originate from both thermal ionization and degradation of absorbance upon increasing temperature. However, the activation energy for thermal quenching is around 0.2 eV, which is obtained from the fitting of temperature-dependent PL intensity. This value is much smaller than the trap depth (1.113 eV). Therefore, we think other reasons for thermal quenching should not be ignored, such as the displacement between 4f and lowest 5d excited state in the configuration coordinate diagram, and the existence of quenching defects and impurities. The process of charging for TL measurement and thermal quenching induced by thermal ionization can by illustrated as the following diagram in Figure 11. Higher temperature benefits detrapping and thermal ionization.

3+

+

Figure 8. (a) 2D and 3D temperature-dependence PL spectra for Ba0.4Sr1.4SiO4:0.1Ce ,0.1Na , (b) thermal quenching behavior of 3+ + 3+ + representative Ba1.8-xSrxSiO4:0.1Ce ,0.1Na phosphors, and (c) CIE coordinates of Ba0.4Sr1.4SiO4:0.1Ce ,0.1Na at different temperatures and the digital photograph of the phosphor under a 365 nm UV lamp at room temperature.

3+

+

Figure 9. (a) Thermoluminescence (TL) of Ba0.4Sr1.4SiO4:0.1Ce ,0.1Na after exposing to the UV light at different temperatures. Integrated TL intensity of the Gaussian peak at 170 °C and PL intensity versus measuring temperature.


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Figure 10. (a) Temperature-dependent PL spectra for IQE and EQE, (b) IQE, EQE, and absorbance versus temperature.

Figure 11. Illustration of charging for TL measurement and thermal quenching induced by thermal ionization. The 254 and 365 nm radiation are turned on simultaneously. CB: conduction band.

Schematic configurational coordinate diagram is used to explain the phenomenon of evolution of luminescent properties with the change of host composition, which is shown in Figure 12. Refinement results indicate the reduction of the size of MO9 polyhedra, which leads to the decrease of energy difference between 5d and 4f levels. Average Uiso obtained from Rietveld refinement suggest the order of offset (∆R) between the parabolas. Furthermore, the decrease of MO9 size relates to a larger force constant (k) between M and O atoms, which narrows the parabolas. All of the above reasons lead to the sequence of PLE, PL peak, and activator energy (Ea) for thermal quenching as shown in Figure 12, which are also based on the data mention above. Finally, Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ phosphor with the best performance is used to fabricate pc-LEDs with an InGaN LED chip (λmax = ~365 nm) to further prove the potential application for w-LEDs. Figure 13a is the electroluminescence (EL) spectra of a pc-LED composed with a UV chip and the Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ phosphor under 50-350 mA forward bias current, whose intensity increase with the current. Figure 13b is the EL spectra of a warm white pc-LED by combining a UV LED chip with

Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+, commercial (Ba,Sr)2SiO4:Eu2+ and CaAlSiN3:Eu2+ phosphors under 50-350 mA forward bias current. The weight ratio of blue, yellow, red phosphor and silica gel is 7:2:1:50. The performance of the pcLEDs and a naked 365 chip (EL in Figure S19) are listed in Table 4. The warm white pc-LED exhibits an excellent CRI value (>90), and the correlated color temperature (CCT) value is 3042-3151 K. Figure 13c shows the CIE coordinates obtained from the EL spectra at different currents and the photographs of the two pc-LEDs. These results indicate that the Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ phosphor can be used as a promising blue phosphor for white-emitting pc-LEDs.



Page 10 of 14

Figure 12. Schematic configurational coordinate diagram for 3+ + Ba1.8-xSrxSiO4:0.1Ce ,0.1Na phosphors, showing the variation of the lowest 5d energy level relative to 4f level. Eex: excitation energy at PLE peak, Eem: emission energy at PL peak, ∆R: offset between the parabolas of excited and ground levels, Ea: activation energy for thermal quenching.

3+

+

Figure 13. EL spectra of (a) Ba0.4Sr1.4SiO4:0.1Ce ,0.1Na converted LED, (b) a warm white pc-LED combined with a ~365 nm UV 3+ + chip and Ba0.4Sr1.4SiO4:0.1Ce ,0.1Na , a commercial green phosphor, and a commercial red phosphor under 50 ~350 mA forward bias currents. (c) CIE coordinates obtained from the EL spectra at different currents and photographs of the two pc-LEDs.

Table 4. Performance of the two pc-LEDs and a naked 365 nm chip for comparison Current (mA) Blue pc-LED

CIE

CCT (K)

Ra

White pc-LED

350

0.1621, 0.0559

100000

-

1.9

50

0.4363, 0.4203

3140

94.5

26.3

100

0.4373, 0.4217

3134

94.1

23.9

150

0.4366, 0.4224

3151

93.9

21.3

Luminance Efficiency (lm/W)

200

0.4373, 0.4239

3150

93.2

19.0

300

0.4402, 0.4221

3088

92.0

12.8

350

0.4418, 0.4194

3042

91.5

12.2

100

-

-

-

0.3

50

0.1609, 0.0528

100000

-

0.8

100

0.1553, 0.0498

100000

-

1.5

150

0.1603, 0.0529

100000

-

2.3

200

0.1620, 0.0560

100000

-

2.2

250

0.1610, 0.0549

100000

-

1.9

300

0.1613, 0.0557

100000

-

2.0

365 nm chip

4. Conclusions


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Blue-emitting Ba1.8-xSrxSiO4:0.1Ce3+,0.1Na+ phosphors were synthesized by a high-temperature solid reaction method. Red shift was observed in the emission spectra when the composition changes from Ba1.8SiO4:0.1Ce3+,0.1Na+ to Sr1.8SiO4:0.1Ce3+,0.1Na+. However, Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ with intermediate host composition exhibits the best emission intensity, quantum efficiency, and thermal stability among them. The absolute quantum efficiency for Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ is 97.2%. The PL intensity of the phosphor at 150 °C remains 90% of that at room temperature. Rietveld refinement results indicate that the size of MO9 polyhedra decrease with increasing Sr2+ content, which support the red shift of emission band. Rietveld refinement results also suggest that the rigidity of Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ is the highest one among the Ba1.8-xSrxSiO4:0.1Ce3+,0.1Na+ phosphors, which is responsible for the highest quantum efficiency and thermal stability. Analyses indicate that thermal ionization, thermal induced degradation of absorbance, and resonance of the 4f and lowest 5d state should be responsible for the thermal quenching. The performance of the pc-LEDs based on the Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ phosphor suggests that it could be a candidate as a blue phosphor for white pc-LEDs.

ASSOCIATED CONTENT Supporting Information. Rietveld Refinement, Crystallographic Data, QE measurement, XRD, PL spectra, temperature-dependent PL spectra, TL spectra, EL spectra. (PDF) This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (J. Zhang). [email protected] (S. Liao). [email protected] (S. Lian).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is financially supported by the National Natural Science Foundation of China (Grant nos. 21505038, 51402105, 21601081, 21471055), National Key Research and Development Program (grant no. 2016YFB0302403). We thank Dr. Xuhui Xu and Mr. Xiaotong Fan (College of Materials Science and Engineering, Kunming University of Science and Technology, China) for their help in TL measurement.

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Improving Quantum Efficiency and Thermal Stability in Blue-Emitting

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