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Luminescence properties of NaSrPO4: Tm3+ as novel blue emitting phosphors with high color purity
Optik - International Journal for Light and Electron Optics 169 (2018) 257–263
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Luminescence properties of NaSrPO4: Tm3+ as novel blue emitting phosphors with high color purity Yang Lia, Jianghui Zhengb, Zhen Lia, Xing Yanga, Jiachao Chena, Chao Chena, a b
T
⁎
College of Energy, Xiamen University, Xiamen 361005, China School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney 2052, Australia
A R T IC LE I N F O
ABS TRA CT
Keywords: Phosphors NaSrPO4: Tm3+ Blue emitting Color purity
Tm3+-doped NaSrPO4 phosphors were synthesized through solid-state sintering. Their crystal structure, micro-morphology, luminescent performance, fluorescent lifetime, and chromaticity property were studied. The synthesized NaSr1-xPO4: xTm3+ phosphors show an intense excitation peak at 357 nm near ultraviolet (NUV), and exhibit two blue emission peaks at 452 and 476 nm, corresponding to the transitions of 1D2 → 3F4 and 1G4 → 3H6 of Tm3+, respectively. Optimized doping concentration of Tm3+ in NaSr1-xPO4: xTm3+ is determined to be 0.02, while concentration quenching at higher Tm3+ doping concentrations is associated with the dipole-dipole interaction of Tm3+. The phosphor owns blue emission property with Commission International De L'Eclairage (i.e., CIE) chromaticity coordinate of (0.153, 0.043) and high color purity of 95%. All in all, the NaSrPO4: Tm3+ phosphors are a new kind of potential blue emitting phosphors for NUV white light-emitting diodes.
1. Introduction Nowadays, white light-emitting diodes (WLEDs) have emerged as the next generation solid-state lighting benefiting from their adjustable colors, high luminous efficiency, good stability, energy-saving and environmental protection [1–9]. Currently, commercial WLEDs are assembled with blue chips and yellow Y3Al5O12: Ce3+ phosphors, but the phosphors have some disadvantages such as poor rendering index and high color temperature caused by the scarcity of red emission [5]. By contrast, the WLEDs equipped with red/green/blue tricolor phosphors which are excited by near ultraviolet (NUV) chips (with emission wavelength of 350∼410 nm) possess higher efficiency and color rendering index [6,7]. Therefore, the tricolor phosphors-converted WLEDs are more competitive in the field of solid-state lighting. To synthesize red/green/blue tricolor phosphors, trivalent rare-earth ions (RE3+) activated inorganic compounds including aluminates [10,11], borates [12,13], tungstates [14,15], vanadates [16,17] and phosphates [8,18] are extensively studied. These inorganic compounds are chosen as host materials of tricolor phosphors, because of their broad charge transfer absorption bands in the NUV and the effective energy transferring between them and the RE3+ activator. In especial, the phosphates with chemical formula of ABPO4 (A represents alkaline metals and B represents alkaline-earth metals) and tetrahedral rigid three-dimensional matrix have been widely considered as an important family of luminescent host materials, benefiting from their excellent thermal stability and exceptional optical damage threshold as well as large band gap and strong absorption in the ultraviolet region [6,7,19]. NaSrPO4 is such a host material, which owns a standard monoclinic crystal structure with lattice parameters of a = 2.041 nm, b = 0.543 nm, c = 1.725 nm and angle β = 101.76° [8]. For RE3+-doped NaSrPO4 phosphors, component and doping concentration of
⁎
Corresponding author. E-mail address: [email protected] (C. Chen).
https://doi.org/10.1016/j.ijleo.2018.05.065 Received 27 March 2018; Accepted 16 May 2018 0030-4026/ © 2018 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 169 (2018) 257–263
Y. Li et al.
RE3+ play a critical role in the luminescent properties. A series of RE3+-doped NaSrPO4 phosphors, such as NaSrPO4: Tb3+, NaSrPO4: Sm3+ and NaSrPO4: Dy3+, were reported as green, orange red and white emitting phosphors for WLEDs [6–8]. However, Tm3+doped NaSrPO4 phosphor, to our best knowledge, has not been studied. Research on NaSrPO4: Tm3+ may be helpful for us to find novel phosphors with peculiar fluorescence performance. In the present work, Tm3+-doped NaSrPO4 phosphors with various Tm3+ doping concentrations were synthesized through solidstate sintering. Their physico-chemical characteristics such as crystal structure, micro-morphology and luminescence property were systematically investigated, and optimized doping concentration of Tm3+ was determined. The results demonstrated that NaSrPO4: Tm3+ phosphors could serve as a new kind of blue emitting phosphors with high color purity.
2. Experimental 2.1. Sample synthesis A series of Tm3+-doped NaSrPO4 phosphors were synthesized through solid-state sintering, and when doping concentration of Tm3+ is x mol/mol, the synthesized phosphor was noted as NaSr1-xPO4: xTm3+ (x value ranges from 0.01 to 0.09). Specifically, Na2CO3 (AR), SrCO3 (AR), NH4H2PO4 (AR) and Tm2O3 (99.99%) powders were weighted according to stoichiometric molar ratio and put into an agate mortar together. The above mixture was ground thoroughly and transferred into a quartz boat. Then, the quartz boat was placed in a box furnace and underwent sintering process (heat up from room temperature to 1100 °C with a heating rate of 10 °C/min and maintain at 1100 °C for 3 h) under air atmosphere to yield the proposed phosphors.
2.2. Characterization X-ray diffraction (XRD) analyzer (model: Ultima-IV) with a Cu Kα (λ = 1.5418 Å) radiation was used to analyze crystal structure of the synthesized phosphors. A scan range of 15–45° was applied. Scanning electron microscopy (SEM; model: Zeiss Supra55) was used to observe micro-morphologies. Energy dispersive spectroscopy (EDS) analysis was utilized to detect distribution state of Tm element inside phosphors. Photoluminescence (PL) measurements were performed using a spectrofluorometer (model: Hitachi F7000) equipped with a 150 W xenon lamp as the light source. The operation voltage was 700 V with a slit width of 2.5 nm for excitation and emission tests. The photoluminescence decays were recorded using a spectrometer (model: Edinburgh FLS920). Above measurements were carried out at room temperature.
3. Results and discussion 3.1. Structure and micro-morphologies Crystal structure and micro-morphologies of the synthesized NaSr1-xPO4: xTm3+ phosphors were firstly investigated. From XRD patterns in Fig. 1, we can see that the diffraction peaks of the as-synthesized samples fit well with the standard data card of NaSrPO4 (JCPDS#33-1282) and no impurity peaks are detected. Typical SEM image as displayed in Fig. 2 shows that the as-synthesized samples are composed of micro-particles, while EDS mapping reveals the existence and relatively uniform distribution of Tm element in the phosphors. Overall, XRD patterns and SEM observations clearly indicate that Tm3+ is doped into NaSrPO4 host, and meanwhile, Tm3+ doping does not observably change crystal structure of NaSrPO4.
Fig. 1. XRD patterns of NaSr1-xPO4: xTm3+ phosphors. 258
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Fig. 2. SEM image and EDS mapping of NaSr0.98PO4: 0.02Tm3+ phosphor.
3.2. Photoluminescence properties The PL excitation spectra at an emission light wavelength of 452 nm of the synthesized NaSr1-xPO4: xTm3+ phosphors exhibit a wide band in the NUV region, and the excitation peaks at around 357 nm corresponds to the 3H6 → 1D2 transition (Fig. 3a). The PL emission spectra at an excitation light wavelength of 357 nm of the NaSr1−xPO4: xTm3+ phosphors are given in Fig. 3b. Two major emission peaks appear at 452 and 456 nm, which are generated from the transition of 1D2→3F4. Meanwhile, several weak peaks appear at the wavelength of larger than 470 nm. Typically, the weak peak at 476 nm (amplified image is given as an inset in Fig. 3b) corresponds to the transition of 1G4 → 3H6. That all the emission peaks are in the blue-light region demonstrates our phosphors are blue emitting phosphors. For NaSr1−xPO4: xTm3+ phosphors, doping concentration of Tm3+ does not change the peak positions in emission spectra but changes the intensity of the emission peaks (Fig. 3b). The maximal emission intensity appears when the Tm3+ doping concentration is 0.02, and if doping concentration of Tm3+ is larger than 0.02, concentration quenching phenomenon occurs. Concentration quenching is often a result of energy transfer from one activator to another until the energy sink in the lattice is reached. Based on Blasse’s proposal [20], the critical transfer distance (Rc) between the activator ions in RE3+-doped phosphors is derived by Eq. (1), 1
3V ⎞ 3 Rc ≈ 2 × ⎛ ⎝ 4πx c N ⎠ ⎜
⎟
(1)
where V denotes the volume of the unit cell, N is the number of cations in the unit cell, and xc designates the critical concentration. As for NaSr0.98PO4: 0.02Tm3+ phosphor, V = 1871 (Å)3, N = 2 and xc = 0.02, thus Rc is calculated to be 44.7 Å. Such a large value of Rc proves that electric multipolar interactions are dominant in the energy transfer process [21,22]. The relationship between the luminescence intensity and the doping concentration has been investigated by Uitert [9], as
Fig. 3. (a) PL excitation spectra and (b) PL emission spectra of NaSr1-xPO4: xTm3+ phosphors. 259
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Fig. 4. Plot of log (I/x) vs log (x) for the NaSr1-xPO4: xTm3+ phosphors (λex = 357 nm).
expressed through Eq. (2),
I / x = K [1 + β (x )Q/3]−1
(2)
where I is the luminescent emission intensity, x is the activator concentration, Q is an indicator of multi-polar interaction, K and β are constants under the same excitation condition for the given host material. Based on this, we study the mechanism for concentration quenching at higher Tm3+ doping concentration (3–9 mol%) in NaSr1-xPO4: xTm3+ phosphors. According to Eq. (3),
Q log (I / x ) = A − ⎛ ⎞ logx ⎝3⎠
(3)
which is obtained by simplified Eq. (2), we can use a fitted line with slope of -1.742 to describe the relationship between log (I/x) and log x (Fig. 4). In this case, the fitted value of Q is close to 6, which means that dipole-dipole interaction is the major mechanism for concentration quenching at higher Tm3+ doping concentration in our NaSr1-xPO4: xTm3+ phosphors, according to previous research [9]. Representative time-resolved PL emission decay spectra of NaSr1-xPO4: xTm3+ (x = 0.01∼0.04) phosphors are further tested, as displayed in Fig. 5. They are fitted according to Eq. (4),
t t I (t ) = A + B1 exp(− ) + B2 exp(− ) τ2 τ1
(4)
where I(t) represents the luminescence intensity and t is the time. The values of A, B1, B2, τ1 and τ2 are obtained by fitting. Average fluorescent lifetime (τav) of corresponding phosphor is then calculated using Eq. (5), 2
τav =
∑ ΣBi τi2/ΣBi τi
(5)
i= 1
doping concentration (i.e., x) in NaSr1-xPO4: where B1, B2, τ1 and τ2 have the same meaning of those in Eq. (4) [23]. When Tm xTm3+ phosphors is 0.01, 0.02, 0.03 and 0.04, average fluorescent lifetime is 1.75, 2.03, 1.96 and 1.61 ns, respectively. Decreasing fluorescent lifetime at higher Tm3+ doping concentration confirms the existence of concentration quenching [24]. For an in-depth analysis, a simplified energy level diagram of the Tm3+ with the relevant optical transitions is given in Fig. 6. 3+
Fig. 5. Time-resolved PL emission decay spectra of NaSr1-xPO4: xTm3+ (x = 0.01∼0.04) phosphors. 260
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Fig. 6. Energy level diagram of Tm3+-doped NaSrPO4 phosphor.
Tm3+ has three emitting levels, including 3P0 (35,000 cm−1), 1D2 (27,770 cm−1) and 1G4 (21,200 cm−1) [25]. In Tm3+ dopedNaSrPO4 phosphors, the excited energy level of Tm3+ is at 28,011 cm−1. The excitation wavelength of 357 nm in Fig. 3a is assigned to 4f-4f transition of Tm3+ from the ground level 3H6 to the excited level 1D2; then transition from 1D2 to 3F4 via radiative transition generates emission peak at around 452 nm (i.e., two major emission peaks in Fig. 3b). Simultaneously, a little energy of 1D2 relaxes rapidly to the lowest emitting level 1G4 via nonradiative transition, followed by transition from 1G4 to the ground state with emission peak at 476 nm. However, when Tm3+ doping concentration in NaSr1-xPO4: xTm3+ phosphor increases, the average distance between neighboring Tm3+ decreases, causing deleterious non-radiative energy exchange between neighboring Tm3+. Under this condition, the dissipative processes lowering the fraction of excited Tm3+ lead to concentration quenching. The Commission International De L'Eclairage (CIE) color coordinate is an important indicator for evaluating phosphor performance. Chromaticity coordinate of (0.153, 0.043) is determined through calculating the emission spectrum under 357 nm NUV light excitation of NaSr0.98PO4: 0.02Tm3+ with the assist of CIE 1931 color matching functions (Fig. 7). This confirms that our NaSr1-xPO4: xTm3+ phosphor is a new kind of blue emitting phosphor. Furthermore, color purity of our NaSr1-xPO4: xTm3+, which is an essential feature for trichromatic phosphors, is also calculated based on Eq. (6) [26–28],
Color purity =
(x − x i )2 + (y − yi )2 (x d − x i )2 + (yd − yi )2
× 100% (6)
where (x, y) is the color coordinate of the light source, (xi, yi) is the CIE of an equal-energy illuminant with a value of (0.3333, 0.3333) and (xd, yd) is the chromaticity coordinate corresponding to the dominant wavelength of the light source. The calculation was performed using a color calculator software developed by Lu et al [29]. The calculated CIE chromaticity coordinate and the
Fig. 7. CIE chromaticity coordinates of our NaSr0.98PO4: 0.02Tm3+ phosphor and commercial BAM phosphor reported in literature [30]. 261
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Table 1 CIE chromaticity coordinate, dominant wavelength, and color purity of NaSr0.98PO4: 0.02Tm3+ phosphor and some representative blue phosphors. Sample
Chromaticity coordinate
Dominant wavelength (nm)
Color purity
Refs.
NaSrPO4: Tm3+ KMgBO3: Tm3+ BaMgAl10O7: Eu2+ NaSrBO3: Ce3+
(0.153, (0.146, (0.142, (0.158,
459.7 466.2 473.3 459.1
95% 94% 88% 93%
This work [12] [30] [30]
0.043) 0.062) 0.107) 0.049)
dominant wavelength point of each sample are summarized in Table 1. The result shows that our NaSr0.98PO4: 0.02Tm3+ phosphor is located in the blue region corresponding to the dominant wavelengths of 459.7 nm. Its color purity is as high as 95.0%, notably higher than that of commercial blue BaMgAl10O7: Eu2+ (BAM) phosphor and previously-reported blue phosphors of KMgBO3: Tm3+ and NaSrBO3: Ce3+ [12,30]. 4. Conclusions A new type of blue-emitting phosphors for WLEDs, i.e., Tm3+ doped-NaSrPO4, was studied. The phosphors were synthesized using a solid-state sintering method, and their components, micro-morphologies and luminescence properties were investigated. Both PL emission spectra and CIE color coordinate confirm that the NaSr1-xPO4: xTm3+ phosphors are suitable for blue emitting. When Tm3+ doping concentration in NaSr1-xPO4: xTm3+ is 0.02, the phosphor exhibits best luminescence performance, while when Tm3+ doping concentration is larger than 0.02, concentration quenching phenomenon occurs, which can be explained through dipole-dipole interaction mechanism. Moreover, the NaSrPO4: Tm3+ phosphors possess higher color purity with 95% than many other blue phosphors. Overall, the NaSrPO4: Tm3+ phosphors are a new kind of potential blue emitting phosphors for WLEDs. Acknowledgments The authors appreciate Dr. Haifeng Su (College of Chemistry and Chemical Engineering, Xiamen University) providing technical supports on the manuscript. References [1] R. Wang, J. Xu, C. Chen, Luminescent characteristics of Sr2B2O5: Tb3+, Li+ green phosphor, Mater. Lett. 68 (2012) 307–309. [2] Y.S. Tang, S.F. Hu, C.C. Lin, N.C. Bagkar, R.S. Liu, Thermally stable luminescence of KSrPO4: Eu2+ phosphor for white light UV light-emitting diodes, Appl. Phys. Lett. 90 (2007) 151108. [3] Z.C. 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Chang, The optimum sintering condition for KSrPO4:Eu3+ phosphors applied in WLEDs, Ceram. Int. 43 (2017) S682–S687. [19] R.Y. Yang, H.L. Lai, Microstructure, and luminescence properties of LiBaPO4:Dy3+ phosphors with various Dy3+ concentrations prepared by microwave assisted sintering, J. Lumin. 145 (2014) 49–54. [20] G.P.R.L. Blasse, Energy transfer in oxidic phosphors, Phys. Lett. A 24 (1968) 444–445. [21] J. Zheng, Q. Cheng, S. Wu, Z. Guo, Yi. Zhuang, Y. Lu, Y. Li, Chao Chen, An efficient blue-emitting Sr5(PO4)3Cl: Eu2+ phosphor for application in near-UV white light-emitting diodes, J. Mater. Chem. C 3 (2015) 11219–11227. [22] N. Zhang, C. Guo, J. Zheng, X. Su, J. Zhao, Synthesis, electronic structures and luminescent properties of Eu3+ doped KGdTiO4, J. Mater. Chem. C 2 (2014) 3988–3994. [23] M. Yu, J. Lin, J. Fang, Silica spheres coated with YVO4: Eu3+ layers via sol-gel process: a simple method to obtain spherical core-shell phosphors, Chem. Mater.
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263
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Luminescence properties of NaSrPO4: Tm3+ as novel blue emitting phosphors with high color purity Yang Lia, Jianghui Zhengb, Zhen Lia, Xing Yanga, Jiachao Chena, Chao Chena, a b
T
⁎
College of Energy, Xiamen University, Xiamen 361005, China School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney 2052, Australia
A R T IC LE I N F O
ABS TRA CT
Keywords: Phosphors NaSrPO4: Tm3+ Blue emitting Color purity
Tm3+-doped NaSrPO4 phosphors were synthesized through solid-state sintering. Their crystal structure, micro-morphology, luminescent performance, fluorescent lifetime, and chromaticity property were studied. The synthesized NaSr1-xPO4: xTm3+ phosphors show an intense excitation peak at 357 nm near ultraviolet (NUV), and exhibit two blue emission peaks at 452 and 476 nm, corresponding to the transitions of 1D2 → 3F4 and 1G4 → 3H6 of Tm3+, respectively. Optimized doping concentration of Tm3+ in NaSr1-xPO4: xTm3+ is determined to be 0.02, while concentration quenching at higher Tm3+ doping concentrations is associated with the dipole-dipole interaction of Tm3+. The phosphor owns blue emission property with Commission International De L'Eclairage (i.e., CIE) chromaticity coordinate of (0.153, 0.043) and high color purity of 95%. All in all, the NaSrPO4: Tm3+ phosphors are a new kind of potential blue emitting phosphors for NUV white light-emitting diodes.
1. Introduction Nowadays, white light-emitting diodes (WLEDs) have emerged as the next generation solid-state lighting benefiting from their adjustable colors, high luminous efficiency, good stability, energy-saving and environmental protection [1–9]. Currently, commercial WLEDs are assembled with blue chips and yellow Y3Al5O12: Ce3+ phosphors, but the phosphors have some disadvantages such as poor rendering index and high color temperature caused by the scarcity of red emission [5]. By contrast, the WLEDs equipped with red/green/blue tricolor phosphors which are excited by near ultraviolet (NUV) chips (with emission wavelength of 350∼410 nm) possess higher efficiency and color rendering index [6,7]. Therefore, the tricolor phosphors-converted WLEDs are more competitive in the field of solid-state lighting. To synthesize red/green/blue tricolor phosphors, trivalent rare-earth ions (RE3+) activated inorganic compounds including aluminates [10,11], borates [12,13], tungstates [14,15], vanadates [16,17] and phosphates [8,18] are extensively studied. These inorganic compounds are chosen as host materials of tricolor phosphors, because of their broad charge transfer absorption bands in the NUV and the effective energy transferring between them and the RE3+ activator. In especial, the phosphates with chemical formula of ABPO4 (A represents alkaline metals and B represents alkaline-earth metals) and tetrahedral rigid three-dimensional matrix have been widely considered as an important family of luminescent host materials, benefiting from their excellent thermal stability and exceptional optical damage threshold as well as large band gap and strong absorption in the ultraviolet region [6,7,19]. NaSrPO4 is such a host material, which owns a standard monoclinic crystal structure with lattice parameters of a = 2.041 nm, b = 0.543 nm, c = 1.725 nm and angle β = 101.76° [8]. For RE3+-doped NaSrPO4 phosphors, component and doping concentration of
⁎
Corresponding author. E-mail address: [email protected] (C. Chen).
https://doi.org/10.1016/j.ijleo.2018.05.065 Received 27 March 2018; Accepted 16 May 2018 0030-4026/ © 2018 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 169 (2018) 257–263
Y. Li et al.
RE3+ play a critical role in the luminescent properties. A series of RE3+-doped NaSrPO4 phosphors, such as NaSrPO4: Tb3+, NaSrPO4: Sm3+ and NaSrPO4: Dy3+, were reported as green, orange red and white emitting phosphors for WLEDs [6–8]. However, Tm3+doped NaSrPO4 phosphor, to our best knowledge, has not been studied. Research on NaSrPO4: Tm3+ may be helpful for us to find novel phosphors with peculiar fluorescence performance. In the present work, Tm3+-doped NaSrPO4 phosphors with various Tm3+ doping concentrations were synthesized through solidstate sintering. Their physico-chemical characteristics such as crystal structure, micro-morphology and luminescence property were systematically investigated, and optimized doping concentration of Tm3+ was determined. The results demonstrated that NaSrPO4: Tm3+ phosphors could serve as a new kind of blue emitting phosphors with high color purity.
2. Experimental 2.1. Sample synthesis A series of Tm3+-doped NaSrPO4 phosphors were synthesized through solid-state sintering, and when doping concentration of Tm3+ is x mol/mol, the synthesized phosphor was noted as NaSr1-xPO4: xTm3+ (x value ranges from 0.01 to 0.09). Specifically, Na2CO3 (AR), SrCO3 (AR), NH4H2PO4 (AR) and Tm2O3 (99.99%) powders were weighted according to stoichiometric molar ratio and put into an agate mortar together. The above mixture was ground thoroughly and transferred into a quartz boat. Then, the quartz boat was placed in a box furnace and underwent sintering process (heat up from room temperature to 1100 °C with a heating rate of 10 °C/min and maintain at 1100 °C for 3 h) under air atmosphere to yield the proposed phosphors.
2.2. Characterization X-ray diffraction (XRD) analyzer (model: Ultima-IV) with a Cu Kα (λ = 1.5418 Å) radiation was used to analyze crystal structure of the synthesized phosphors. A scan range of 15–45° was applied. Scanning electron microscopy (SEM; model: Zeiss Supra55) was used to observe micro-morphologies. Energy dispersive spectroscopy (EDS) analysis was utilized to detect distribution state of Tm element inside phosphors. Photoluminescence (PL) measurements were performed using a spectrofluorometer (model: Hitachi F7000) equipped with a 150 W xenon lamp as the light source. The operation voltage was 700 V with a slit width of 2.5 nm for excitation and emission tests. The photoluminescence decays were recorded using a spectrometer (model: Edinburgh FLS920). Above measurements were carried out at room temperature.
3. Results and discussion 3.1. Structure and micro-morphologies Crystal structure and micro-morphologies of the synthesized NaSr1-xPO4: xTm3+ phosphors were firstly investigated. From XRD patterns in Fig. 1, we can see that the diffraction peaks of the as-synthesized samples fit well with the standard data card of NaSrPO4 (JCPDS#33-1282) and no impurity peaks are detected. Typical SEM image as displayed in Fig. 2 shows that the as-synthesized samples are composed of micro-particles, while EDS mapping reveals the existence and relatively uniform distribution of Tm element in the phosphors. Overall, XRD patterns and SEM observations clearly indicate that Tm3+ is doped into NaSrPO4 host, and meanwhile, Tm3+ doping does not observably change crystal structure of NaSrPO4.
Fig. 1. XRD patterns of NaSr1-xPO4: xTm3+ phosphors. 258
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Fig. 2. SEM image and EDS mapping of NaSr0.98PO4: 0.02Tm3+ phosphor.
3.2. Photoluminescence properties The PL excitation spectra at an emission light wavelength of 452 nm of the synthesized NaSr1-xPO4: xTm3+ phosphors exhibit a wide band in the NUV region, and the excitation peaks at around 357 nm corresponds to the 3H6 → 1D2 transition (Fig. 3a). The PL emission spectra at an excitation light wavelength of 357 nm of the NaSr1−xPO4: xTm3+ phosphors are given in Fig. 3b. Two major emission peaks appear at 452 and 456 nm, which are generated from the transition of 1D2→3F4. Meanwhile, several weak peaks appear at the wavelength of larger than 470 nm. Typically, the weak peak at 476 nm (amplified image is given as an inset in Fig. 3b) corresponds to the transition of 1G4 → 3H6. That all the emission peaks are in the blue-light region demonstrates our phosphors are blue emitting phosphors. For NaSr1−xPO4: xTm3+ phosphors, doping concentration of Tm3+ does not change the peak positions in emission spectra but changes the intensity of the emission peaks (Fig. 3b). The maximal emission intensity appears when the Tm3+ doping concentration is 0.02, and if doping concentration of Tm3+ is larger than 0.02, concentration quenching phenomenon occurs. Concentration quenching is often a result of energy transfer from one activator to another until the energy sink in the lattice is reached. Based on Blasse’s proposal [20], the critical transfer distance (Rc) between the activator ions in RE3+-doped phosphors is derived by Eq. (1), 1
3V ⎞ 3 Rc ≈ 2 × ⎛ ⎝ 4πx c N ⎠ ⎜
⎟
(1)
where V denotes the volume of the unit cell, N is the number of cations in the unit cell, and xc designates the critical concentration. As for NaSr0.98PO4: 0.02Tm3+ phosphor, V = 1871 (Å)3, N = 2 and xc = 0.02, thus Rc is calculated to be 44.7 Å. Such a large value of Rc proves that electric multipolar interactions are dominant in the energy transfer process [21,22]. The relationship between the luminescence intensity and the doping concentration has been investigated by Uitert [9], as
Fig. 3. (a) PL excitation spectra and (b) PL emission spectra of NaSr1-xPO4: xTm3+ phosphors. 259
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Fig. 4. Plot of log (I/x) vs log (x) for the NaSr1-xPO4: xTm3+ phosphors (λex = 357 nm).
expressed through Eq. (2),
I / x = K [1 + β (x )Q/3]−1
(2)
where I is the luminescent emission intensity, x is the activator concentration, Q is an indicator of multi-polar interaction, K and β are constants under the same excitation condition for the given host material. Based on this, we study the mechanism for concentration quenching at higher Tm3+ doping concentration (3–9 mol%) in NaSr1-xPO4: xTm3+ phosphors. According to Eq. (3),
Q log (I / x ) = A − ⎛ ⎞ logx ⎝3⎠
(3)
which is obtained by simplified Eq. (2), we can use a fitted line with slope of -1.742 to describe the relationship between log (I/x) and log x (Fig. 4). In this case, the fitted value of Q is close to 6, which means that dipole-dipole interaction is the major mechanism for concentration quenching at higher Tm3+ doping concentration in our NaSr1-xPO4: xTm3+ phosphors, according to previous research [9]. Representative time-resolved PL emission decay spectra of NaSr1-xPO4: xTm3+ (x = 0.01∼0.04) phosphors are further tested, as displayed in Fig. 5. They are fitted according to Eq. (4),
t t I (t ) = A + B1 exp(− ) + B2 exp(− ) τ2 τ1
(4)
where I(t) represents the luminescence intensity and t is the time. The values of A, B1, B2, τ1 and τ2 are obtained by fitting. Average fluorescent lifetime (τav) of corresponding phosphor is then calculated using Eq. (5), 2
τav =
∑ ΣBi τi2/ΣBi τi
(5)
i= 1
doping concentration (i.e., x) in NaSr1-xPO4: where B1, B2, τ1 and τ2 have the same meaning of those in Eq. (4) [23]. When Tm xTm3+ phosphors is 0.01, 0.02, 0.03 and 0.04, average fluorescent lifetime is 1.75, 2.03, 1.96 and 1.61 ns, respectively. Decreasing fluorescent lifetime at higher Tm3+ doping concentration confirms the existence of concentration quenching [24]. For an in-depth analysis, a simplified energy level diagram of the Tm3+ with the relevant optical transitions is given in Fig. 6. 3+
Fig. 5. Time-resolved PL emission decay spectra of NaSr1-xPO4: xTm3+ (x = 0.01∼0.04) phosphors. 260
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Fig. 6. Energy level diagram of Tm3+-doped NaSrPO4 phosphor.
Tm3+ has three emitting levels, including 3P0 (35,000 cm−1), 1D2 (27,770 cm−1) and 1G4 (21,200 cm−1) [25]. In Tm3+ dopedNaSrPO4 phosphors, the excited energy level of Tm3+ is at 28,011 cm−1. The excitation wavelength of 357 nm in Fig. 3a is assigned to 4f-4f transition of Tm3+ from the ground level 3H6 to the excited level 1D2; then transition from 1D2 to 3F4 via radiative transition generates emission peak at around 452 nm (i.e., two major emission peaks in Fig. 3b). Simultaneously, a little energy of 1D2 relaxes rapidly to the lowest emitting level 1G4 via nonradiative transition, followed by transition from 1G4 to the ground state with emission peak at 476 nm. However, when Tm3+ doping concentration in NaSr1-xPO4: xTm3+ phosphor increases, the average distance between neighboring Tm3+ decreases, causing deleterious non-radiative energy exchange between neighboring Tm3+. Under this condition, the dissipative processes lowering the fraction of excited Tm3+ lead to concentration quenching. The Commission International De L'Eclairage (CIE) color coordinate is an important indicator for evaluating phosphor performance. Chromaticity coordinate of (0.153, 0.043) is determined through calculating the emission spectrum under 357 nm NUV light excitation of NaSr0.98PO4: 0.02Tm3+ with the assist of CIE 1931 color matching functions (Fig. 7). This confirms that our NaSr1-xPO4: xTm3+ phosphor is a new kind of blue emitting phosphor. Furthermore, color purity of our NaSr1-xPO4: xTm3+, which is an essential feature for trichromatic phosphors, is also calculated based on Eq. (6) [26–28],
Color purity =
(x − x i )2 + (y − yi )2 (x d − x i )2 + (yd − yi )2
× 100% (6)
where (x, y) is the color coordinate of the light source, (xi, yi) is the CIE of an equal-energy illuminant with a value of (0.3333, 0.3333) and (xd, yd) is the chromaticity coordinate corresponding to the dominant wavelength of the light source. The calculation was performed using a color calculator software developed by Lu et al [29]. The calculated CIE chromaticity coordinate and the
Fig. 7. CIE chromaticity coordinates of our NaSr0.98PO4: 0.02Tm3+ phosphor and commercial BAM phosphor reported in literature [30]. 261
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Table 1 CIE chromaticity coordinate, dominant wavelength, and color purity of NaSr0.98PO4: 0.02Tm3+ phosphor and some representative blue phosphors. Sample
Chromaticity coordinate
Dominant wavelength (nm)
Color purity
Refs.
NaSrPO4: Tm3+ KMgBO3: Tm3+ BaMgAl10O7: Eu2+ NaSrBO3: Ce3+
(0.153, (0.146, (0.142, (0.158,
459.7 466.2 473.3 459.1
95% 94% 88% 93%
This work [12] [30] [30]
0.043) 0.062) 0.107) 0.049)
dominant wavelength point of each sample are summarized in Table 1. The result shows that our NaSr0.98PO4: 0.02Tm3+ phosphor is located in the blue region corresponding to the dominant wavelengths of 459.7 nm. Its color purity is as high as 95.0%, notably higher than that of commercial blue BaMgAl10O7: Eu2+ (BAM) phosphor and previously-reported blue phosphors of KMgBO3: Tm3+ and NaSrBO3: Ce3+ [12,30]. 4. Conclusions A new type of blue-emitting phosphors for WLEDs, i.e., Tm3+ doped-NaSrPO4, was studied. The phosphors were synthesized using a solid-state sintering method, and their components, micro-morphologies and luminescence properties were investigated. Both PL emission spectra and CIE color coordinate confirm that the NaSr1-xPO4: xTm3+ phosphors are suitable for blue emitting. 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