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Blue emission from Sr0.98Ga2B2O7: 0.01Bi3+, 0.01Dy3+ phosphor with high quantum yield
Journal of Alloys and Compounds 810 (2019) 151849
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Blue emission from Sr0.98Ga2B2O7: 0.01Bi3þ, 0.01Dy3þ phosphor with high quantum yield Shuai Yang a, Yannan Dai a, Yang Shen b, Chungang Duan b, c, Qunli Rao d, Hui Peng b, Fan Yang a, Yongkui Shan a, **, Qingbiao Zhao b, * a
School of Chemistry and Molecular Engineering, East China Normal University, Shanghai, 200241, China Key Laboratory of Polar Materials and Devices (MOE), Department of Optoelectronics, East China Normal University, Shanghai, 200241, China Collaborative Innovation Center of Extreme Optics, Shanxi University, Shanxi, 030006, China d Instrument Analysis Center, Shanghai Jiaotong University, Shanghai, 200240, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 June 2019 Received in revised form 10 August 2019 Accepted 12 August 2019 Available online 14 August 2019
Most of the blue phosphors are based on Eu2þ or Ce3þ doped compounds. The luminescent centers of Eu2þ or Ce3þ generally require reducing environment during synthesis. Also, Eu2þ and Ce3þ are prone to oxidation, causing instability of the phosphors for long-term use. Herein, we report Bi3þ and Dy3þ codoped SrGa2B2O7 blue phosphors with simpler synthesis conditions and more stable chemical property. In this phosphor, Bi3þ ions emit blue light and, while Dy3þ as sensitizer has no appreciable luminescence emission but can enhance the emission of Bi3þ. Co-doping with rare earth ions enhances the emission of Bi3þ, improving the efficiency and stability of the phosphor. The Sr0.98Ga2B2O7: 0.01Bi3þ, 0.01Dy3þ phosphor exhibits strong blue light emission with absolute quantum yield of 65.54%. Sr0.98Ga2B2O7: 0.01Bi3þ, 0.01Dy3þ has a broad excitation band (250e380 nm), emits bright blue light at 430 nm and has a narrow full width at half maxima (FWHM ¼ 64.6 nm) with a color purity of 94.2%. Furthermore, the Sr0.98Ga2B2O7: 0.01Bi3þ, 0.01Dy3þ phosphor retains strong blue light emission at high temperatures (95% at 100 C). Overall, Sr0.98Ga2B2O7: 0.01Bi3þ, 0.01Dy3þ is a promising blue emitting phosphor for n-UV WLED. © 2019 Elsevier B.V. All rights reserved.
Keywords: Phosphors Blue Bismuth Borate Quantum yield
1. Introduction LEDs are more and more widely used in lighting, due to advantages such as long service life and low power consumption [1]. There are roughly three schemes for obtaining white light by LED: (1) white lightning device composed of red, green, and blue emitting LEDs; (2) white lightning device combining blue LED with yellow emitting excited by blue light; (3) white lightning device combining UV-LED with red, green and blue emitting phosphors. However, green and yellow LEDs have relatively low efficiency and poor color rendering [2]. White light obtained by blue LED with yellow emitting phosphor lacks red light component, resulting in problems such as high correlated color temperature and poor color rendering. The white lighting device combining an UV-LED and
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Shan), [email protected] (Q. Zhao). https://doi.org/10.1016/j.jallcom.2019.151849 0925-8388/© 2019 Elsevier B.V. All rights reserved.
tricolor phosphors can solve these problems [2]. Presently, there are many tricolor phosphors used in UV-LEDs, most of which are solid compounds doped with rare earth ions. For most blue phosphors, the emission centers are Ce3þ and Eu2þ, because their 4f, 5d orbitals allow electric dipole transitions [3]. BaMgAl10O17: Eu2þ (BAM: Eu2þ) is a commercialized blue phosphor, which can be excited by near-ultraviolet light and emits blue light at ~450 nm [4]. It shall be noted that the amount of Eu, which is a relatively high cost element, in BAM is high (approximately 10% in molar ratio). Many blue phosphors doped with Ce3þ or Eu2þ have been reported, such as Sr5(PO4)3Cl: Eu2þ, KMg4(PO4)3: Eu2þ, Ba2Ca(PO4)2: Eu2þ, BaHfSi3O9: Eu2þ, Ba2Ca(BO3)2: Ce3þ, Ca4P2O9: Ce3þ, NaSrBO3: Ce3þ, Ca2YZr2Al3O12: Ce3þ, Ca3Zr2SiGa2O12: Ce3þ, Sr5(PO4)2SiO4: Ce3þ, Ca8La2(PO4)6O2: Ce3þ, Ca3Hf2SiAl2O12: Ce3þ [5e16]. The rare earths are relatively expensive elements. Moreover, the generation of the relatively low oxidation states of Ce3þ and Eu2þ typically requires reducing atmosphere, which adds to the cost. Furthermore, Ce3þ and Eu2þ are prone to the oxidation to Ce4þ and Eu3þ [17,18], resulting in weaker emission
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intensity and possible changes in emission color during long period of service. Alternatively, blue phosphors can be obtained by doping with Bi3þ. Bi3þ has 6s2 lone pairs, which can generate S-P transition [19]. The energy level difference between the s-orbital and p-orbitals of Bi3þ corresponds to the emission of blue light. Bi3þ has many advantages, such as high stability and low toxicity. Some phosphors doped with Bi3þ have been reported, such as GaBO3: Bi3þ, CaZrO3: Bi3þ, La2LiSbO6: Bi3þ, K2ZrSi2O7: Bi3þ, Ca4ZrGe3O12: Bi3þ, ZnGa2O4: Bi3þ, Sr3Y2Ge3O12: Bi3þ, BaAl2Si2O8: Bi3þ [20e27]. The emission of Bi3þ in different hosts varies, including yellow light, such as in Ca2MgWO6: Bi3þ [28], green light, such as in Y2O3: Bi3þ [29], and ultraviolet light, such as in YPO4: Bi3þ [30]. In some alkaline earth metal oxides, some rare earth ions can enhance the blue or green light emission of Eu2þ, such as Sr2MgSi2O7: Eu2þ, La3þ; CaAl2O4: Eu2þ, Nd3þ; Sr4Al14O25: Eu2þ, Dy3þ [31e33]. In this study, the blue phosphor Sr0.98Ga2B2O7: 0.01Bi3þ, 0.01Dy3þ was synthesized, which delivers high quantum yield and high thermal stability. Additionally, the oxidation states of Bi3þ and Dy3þ ensures higher redox stability compared to blue phosphors based on Eu2þ and Ce3þ. 2. Experimental section 2.1. Materials and synthetic procedures Phosphors of Sr1-xGa2B2O7: xBi and Sr1-x-yGa2B2O7: (xBi, yRE) were prepared by a conventional high temperature solid-state synthesis method. The raw materials were SrCO3 (Sigma-Aldrich, 99.9%), Ga2O3, H3BO3 (Sigma-Aldrich, 99.5%), Bi2O3 (Across, 99.9%) and rare earth oxides (Adamas Reagent Co., Ltd. 99.99%). The raw materials were weighed according to stoichiometric ratio, thoroughly ground and mixed in an agate mortar. A small amount of anhydrous ethanol (~2 mL) was added to aid grinding. All rare earth oxides were activated at 800 C for 24 h and Bi2O3 was activated at 600 C for 24 h before being used in the reaction. In order to compensate the loss of H3BO3 during grinding with ethanol and calcination at high temperature, the amount of H3BO3 was in an excess of 20%. The well-mixed raw materials were pressed into pellets under pressure of 10 MPa. The pellets were placed in an alumina crucible, heated to 850 C in a muffle furnace at a rate of 1 C/min, kept for 24 h, and naturally cooled. The calcination was repeated twice, and the powders were re-ground and pressed into pellets each time. After the reaction was completed, the products were ground and white powder samples were obtained.
Test and Calibration Technology Co., Ltd, Hangzhou) in the temperature range from room temperature to 200 C. The fluorescence lifetime was characterized with a FLS-980 fluorometer (Edinburgh Instruments, EI). The quantum yield was characterized with a FLS920 fluorometer (Edinburgh Instruments, EI).
3. Results and discussion 3.1. Structure refinement and description SrGa2B2O7 crystallizes in an orthorhombic crystal system with the space group of Cmcm, with the unit cell parameters of a ¼ 11.7260(7) Å, b ¼ 8.3745(4) Å, and c ¼ 5.7062(3) Å. The crystal structure is illustrated in Fig. 1. GaO4 tetrahedrons form twodimensional chains in the (001) direction by sharing vertices, two BO3 triangles form [B2O5] groups through common vertices, and are connected to the common vertices of four adjacent GaO4 tetrahedrons to form the three-dimensional skeleton of [B2O5-GaO4]∞. Sr2þ is filled in the voids formed by the skeleton with a coordination number of 8. The B2O5 group is planar and perpendicular to the [001] axis [34]. Fig. 1a and 1b shows the crystal structure of SrGa2B2O7 in different directions. Fig. 1c show the local structure in the SrGa2B2O7: the [GaO4] chain, the [GaO4] and [B2O5] connections and the [SrO8] polyhedron. As shown in Fig. 2, the XRD pattern of the synthesized SrGa2B2O7 substrate is consistent with that given in the literature [34]. Thus, the sample is a pure phase of SrGa2B2O7. As shown in the XRD patterns in Figs. S1eS3, SrGa2B2O7 doped with different ratios of Bi3þ and SrGa2B2O7 co-doped with rare earth ions and Bi3þ maintain the SrGa2B2O7 host structure, with no identifiable impurity peak. Fig. 2b shows the XRD Rietveld refinement results of SrGa2B2O7 host. The refined results show that the SrGa2B2O7 host
2.2. Characterization The crystal phases of the samples were characterized by X-ray powder diffraction (XRD), which was performed on a D8 DISCOVER diffractometer (Bruker AXS GmbH, Germany) with an increment of 0.01 in the 2q range from 5 to 70 with Cu Ka1 (l ¼ 1.540598 Å) radiation. The data for structural refinement was performed on an Ultima IV X-ray diffractometer (Rigaku, Japan) with an increment of 0.01 in the 2q range from 5 to 90 with Cu Ka radiation. XRD Rietveld refinements of the crystal structure were performed with Total Pattern Solution (TOPAS 3.0) software. UV-Vis diffuse reflectance spectra were collected using a Lambda-950 (PerkinElmer, U.S.A.). The photoluminescence excitation and emission spectra were collected with a LS-55 fluorometer (PerkinElmer, U.S.A.) equipped with a 50 Hz pulse xenon lamp as the excitation source. The steps of the excitation and emission spectra were 0.5 nm. The thermal stability of luminescence properties was characterized by EX-1000 phosphor thermal quenching analysis system (EVERFINE
Fig. 1. The crystal structure of SrGa2B2O7: (a, b) the crystal structure of SrGa2B2O7 in different directions; (c) the [GaO4] chain, the [GaO4], [B2O5] connections and the [SrO8] polyhedron shown in polyhedron and “ball and stick” models.
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Fig. 2. XRD pattern of SrGa2B2O7: (a) experimentally synthesized SrGa2B2O7 and Sr0.99Ga2B2O7: 0.01Bi3þ; (b) Rietveld refinement of SrGa2B2O7 host samples.
sample formed orthorhombic phase in space group Cmcm, a ¼ 11.7048(1) Å, b ¼ 8.3555(7) Å, c ¼ 5.6967(2) Å, V ¼ 557.1412(6) Å3. All the atomic positions and thermal vibrational parameters were refined and converged. The weighted profile R-factor (Rwp) is 8.28% and the profile R factor (Rp) is 5.70% (Supporting Materials, Tables S1 and S2). 3.2. UV-Vis diffuse reflection UV-Vis diffuse reflectance spectroscopy was performed on SrGa2B2O7, Sr0.99Ga2B2O7: 0.01Bi3þ and Sr0.98Ga2B2O7: 0.01Bi3þ-
0.01Dy3þ. The UV-Vis diffuse reflectance spectra is shown in Fig. 3. The band gaps were obtained by the following formula (1) [35]:
ahn ¼ k (hn - Eg)n
(1)
where Eg, a, h and n indicate band gap energy, absorbance, Planck's constant and photon frequency, respectively. The UV-Vis spectrum of SrGa2B2O7 substrate shows an absorption peak at ~5 eV. In addition to the absorption peak at ~5 eV, the doped samples, Sr0.99Ga2B2O7: 0.01Bi3þ and Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ, have significant absorption at ~4.4 eV. The
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Fig. 3. UV-Vis diffuse reflectance spectroscopy and optical band gap of (a) SrGa2B2O7; (b) Sr0.99Ga2B2O7: 0.01Bi3þ; (c) Sr0.98Ga2B2O7: 0.01Bi3þ - 0.01Dy3þ.
absorption at ~4.4 eV for Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ is more pronounced. The absorption at 4.4 eV corresponds to the 1S0/1P1 transition of Bi3þ. Co-doping with Dy3þ enhances the absorption of Bi3þ, hence the excitation energy can be more efficiently utilized. According to the diffuse reflectance spectrum, the optical band gaps of the SrGa2B2O7 host, Sr0.99Ga2B2O7: 0.01Bi3þ and Sr0.98Ga2B2O7: 0.01Bi3þ-0.01Dy3þ were estimated to be 4.61 eV, 3.97 eV and 4.11 eV, respectively.
3.3. Fluorescent properties 3.3.1. Fluorescent properties and luminescence mechanism of Sr0.99Ga2B2O7: Bi3þ We firstly discuss the fluorescent behaviors of SrGa2B2O7 solely doped with Bi3þ (without rare earth co-doping). For Bi3þ, the excitation wavelengths typically include three bands, i.e., UVA, UVB and UVC. The emission wavelengths are different in various hosts, which can be manifested as blue light [19], green light [29], yellow light [28], or in the ultraviolet band [30]. This indicates that the emission from Bi3þ is significantly affected by the lattice environment. The ground state of free Bi3þ ion is 1S0, and there are four excited states: 3P1, 3P2, 3P0 and 1P1. In general, the transitions of Bi3þ have three bands. A-band is an allowed transition due to the spineorbit coupling, whose corresponding transition is 1S0-3P1. Cband is a spin-allowed transition and the corresponding transition is 1S0-1P1. The corresponding transition of B-band is 1S0-3P2, which is in general forbidden, but can be induced by coupling with unsymmetrical lattice vibrational modes [36]. The transition 1S0-3P0 is strongly forbidden. In addition, there is a D-band transition that frequently occurs in the excitation spectrum of Bi3þ, which may be due to the charge transfer transition of Bi3þ [37]. As shown in Fig. 4a, Sr0.99Ga2B2O7: 0.01Bi3þ generates a single
Fig. 4. (a) Emission (l ex ¼ 330 nm) and excitation (l em ¼ 330 nm) spectra of Sr1xBi3þ; (b) The change in fluorescence intensity of Sr1-xGa2B2O7: xBi3þ and linear fitting of log (x) versus log (I/x) in various Sr1-xGa2B2O7: xBi3þ phosphors; (c) The energy transition and emission of Bi3þ ions.
xGa2B2O7:
broad peak emission with the 330 nm excitation. The peak wavelength is 430 nm, and the emission range is 340e600 nm. The excitation spectrum corresponding to the 430 nm emission shows two excitation bands, i. e., a strong excitation band with maximum at 330 nm and a weak excitation band at 276 nm. According to the excitation mechanism of Bi3, the excitation at 330 nm is attributed to the 1S0-3P1 transition of Bi3þ. The strong spin-orbit coupling interaction makes the excitation prominent, and the weak excitation band at 276 nm corresponds to the spin allowed 1S0-1P1
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transition. The emission spectrum shows only one broad peak, which is attributed to the 3P1-1S0 transition of Bi3þ, for which the electrons at the higher excitation level 1P1 lose part of the energy through non-radiative transition and transfer to the lower triplet state 3P1. The doping rate of Bi3þ was tuned to determine the optimum doping level for the strongest emission intensity. The phosphors with doping level of Bi3þ ranging from 0.5% to 9 mol% were synthesized and characterized. The emission intensity reached the maximum at the doping rate of 1 mol%, and the excitation spectrum exhibited the same tendency (Fig. 4a). The narrow full width at half maxima (FWHM) of Sr0.99Ga2B2O7: 0.01Bi3þ is 65.3 nm. Fig. 4b shows the trend of fluorescence intensity of Sr1-xGa2B2O7: xBi3þ as a function of doping ratio. With Bi3þ over 1 mol%, the emission intensity of Bi3þ decreases as the doping ratio increases. In SrGa2B2O7, only the Sr2þ site can be occupied by Bi3þ and rare earth ions. According to the study of energy transfer in oxide phosphors [38], the critical distance of Bi3þ in SrGa2B2O7 can be calculated by formula (2):
Rc ¼ 2
3V 4pxc N
1=3 (2)
where xc is the critical ratio. According to the crystal structure of SrGa2B2O7, N ¼ 4 and V ¼ 560.35 Å3. With the doping level of Bi3þ at 1mol%, the critical distance is calculated to be 29.91 Å, and the critical distance is 14.95 Å for the doping level of Bi3þ at 9mol%. According to the fluorescence spectrum of Sr1-xGa2B2O7: xBi3þ, the overlap of the excitation and emission spectra of Bi3þ is small. Thus, the probability of radiation reabsorption is small. The exchange coupling takes place only when the critical distance is short, i.e. ~5 Å. For Sr1-xGa2B2O7: xBi3þ, the calculated critical distance is as large as ~15 Å, even at higher of Bi3þ contents. Therefore, in Sr13þ xGa2B2O7: xBi , it is unlikely that the radiation reabsorption and exchange interaction between Bi3þ ions occur. The concentration quenching of Bi3þ is attributed to the electric multipole interaction and the energy transfer between adjacent ions. The relationship between emission intensity and doping level [39] is expressed by formula (3):
h i1 I ¼ k 1 þ bxq=3 x
(3)
where I represents the emission intensity, x is the ratio of Bi3þ, k and b denote the constants of the same interaction. The value of q indicates the type of interaction between the doping ions. q ¼ 3 means that the interaction is the energy transfer between adjacent ions, q ¼ 6 represents the dipole-dipole interaction, q ¼ 8 represents the electric dipole-electric quadrupole interaction, while q ¼ 10 represents electric quadrupole-electric quadrupole interaction. By fitting lg x with lg (I/x) (Fig. 4b), the obtained slope is 1.743. q ¼ 5.229 for Sr1-xGa2B2O7: xBi3þ, hence, the interaction between Bi3þ ions is composed of two parts: the dominant is dipole-dipole interaction and secondary is energy transfer between adjacent ions [40]. 3.3.2. Fluorescent properties and luminescence mechanism of SrGa2B2O7: Bi3þ- Dy3þ In order to obtain blue phosphors with stronger emission, various rare earth ions were co-doped with 1mol% Bi3þ in SrGa2B2O7. No impurity peak was observed in the XRD patterns of these phosphors (Supporting Information, Fig. S2). The fluorescence spectra of these phosphors are shown in Fig. S5. Among the various rare earth ions, the effect of Dy3þ in enhancing the emission from Bi3þ is exceptionally pronounced. With 1 mol% of Dy3þ and
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1mol% of Bi3þ co-doped in SrGa2B2O7, both the excitation and emission intensities were significantly increased. Thus, SrGa2B2O7 phosphors co-doped with different ratios of Dy3þ and 1mol% of Bi3þ were synthesized. As shown in the XRD pattern, SrGa2B2O7 phosphors co-doped with various ratios of Dy3þ and 1 mol% of Bi3þ are pure phases. The fluorescence emission of these phosphors are shown in Fig. 5a. To elucidate the mechanisms of the fluorescent emission in doped SrGa2B2O7, SrGa2B2O7 with solely rare earth doping (without Bi3þ) were also synthesized. For SrGa2B2O7 with solely rare earth doping at the level of 1 mol%, no appreciable emission in the visible wavelength range was observed with the excitation wavelength of 330 nm, except for Eu3þ. As 1 mol% of rare earth ions were doped with 1 mol% of Bi3þ, only Bi3þ emission was observed, and no characteristic emission of rare earth ions (except Eu3þ) was observed (Supporting Information, Fig. S5). Therefore, the rare earth ions function as sensitizers in this system, which is different from typical systems with Bi3þ as sensitizer and rare earth as emitting center. Fig. 5b shows the trend in luminescence intensity of Sr0.993þ 3þ 3þ content reaches 1 mol%, the yGa2B2O7: 0.01Bi - yDy . As the Dy strongest emission occurs at this doping level. The full width at half maxima (FWHM) of Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ is 64.6 nm. The color coordinates of the phosphor were calculated with the fluorescence spectra and they are shown in the 1931 CIE chromaticity diagram (Fig. 5b). The color coordinate of Sr0.99Ga2B2O7: 0.01Bi3þ under excitation of 330 nm are (0.156, 0.052), and the color coordinates of Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ are (0.155, 0.047). Based on the color coordinates, co-doping with Dy3þ essentially does not change the emitting color of Bi3þ. Notably, the color purity of Sr0.98Ga2B2O7: 0.01Bi3þ, 0.01Dy3þ phosphor is 94.2%. In order to verify whether the balanced charge can enhance the emission intensity, three alkali metal ions, Liþ, Naþ and Kþ, were codoped with Bi3þ and Dy3þ in SrGa2B2O7. As shown in Fig. 5c, the emission intensities of samples with charge compensation ions Liþ, Naþ and Kþ are comparable. The emission intensities with charge compensation ions are much weaker than the sample with Bi3þ and Dy3þ co-doping but without monovalent ions. 3.3.3. Quantum yield The SrGa2B2O7: Bi3þ phosphors can emit blue light in the range of 350e550 nm with the excitation of 330 nm ultraviolet light. In order to enhance the blue emission of Bi3þ, rare earth ions were codoped with Bi3þ in this host. The phosphor co-doped with 1 mol% Bi3þ and 1 mol% Dy3þ has the strongest emission intensity. Based on the quantum yield measurements, the absolute quantum yield of Sr0.99Ga2B2O7: 0.01Bi3þ is 32.48%, while that of Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ is as high as 64.54%. The quantum yield reached 92.2% of the commercialized blue phosphor BAM: Eu2þ, for which the external quantum yield is ~70% [4]. Compared with BAM: Eu2þ, SrGa2B2O7: Bi3þ- Dy3þ possesses several advantages, such as simpler reaction conditions (Eu2þ needs to be prepared from Eu3þ under reducing conditions), lower synthesis temperature, and lower doping ratio of luminescent center ions (The ratios of Bi3þ and Dy3þ are both 1 mol%, whereas the ratio of Eu2þ in BAM is ~10%). Moreover, the chemical properties are more stable (Both Bi3þ and Dy3þ are in stable oxidation state under ambient environment, whereas Eu2þ is prone to oxidation to Eu3þ). Table 1 shows the quantum yields of representative blue phosphors and their corresponding doping levels, excitation and emission wavelength. Most of the blue phosphors with relatively high quantum yield are doped with Ce3þ or Eu2þ, while the blue phosphors doped with Bi3þ generally have low quantum yields. For
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Fig. 5. (a) Emission (l ex ¼ 330 nm) and excitation (l em ¼ 430 nm) spectra of Sr0.99-yGa2B2O7: 0.01Bi3þ- yDy3þ (y ¼ 0e0.05); (b) The trend in luminescence intensity of Sr0.990.01Bi3þ- yDy3þ and the color coordinates of Sr0.99Ga2B2O7: 0.01Bi3þ and Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ; (c) Emission (l ex ¼ 330 nm) and excitation (l 3þ 3þ þ 3þ 3þ þ em ¼ 330 nm) spectra of Sr0.96Ga2B2O7: 0.01Bi - 0.01Dy - 0.02 M (M ¼ Li, Na, K); (d) The trend in luminescence intensity of Sr0.96Ga2B2O7: 0.01Bi - 0.01Dy - 0.02 M . (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
yGa2B2O7:
Table 1 The quantum yields of representative blue phosphors and their doping ratios, excitation and emission wavelength. Phosphor
Doping rate
Excitation wavelength (nm)
Emission wavelength (nm)
Quantum yield
Reference
BaMgAl10O17: Eu2þ CaPO4Cl: Eu2þ KMg4(PO4)3: Eu2þ BaHfSi3O9: Eu2þ Ca4P2O9: Ce3þ NaSrBO3: Ce3þ Ca2YZr2Al3O12: Ce3þ Ca3Zr2SiGa2O12: Ce3þ Sr5(PO4)2SiO4: Ce3þ Ca8La2(PO4)6O2: Ce3þ Ca4ZrGe3O12: Bi3þ SrGa2B2O7: Bi3þ- Dy3þ
10% 11% 0.75% 14% 0.50% 1% 3% 4% 3% 4% 2% 1%e1%
365 370 365 405 340 364 410 400 365 310 370 330
462 454 450 475 420 396, 426 495 478 425 415 435 430
69.60% 61.00% 50.44% 50.00% 64.80% 61.60% 56.00% 42.70% 46.00% 67.00% 37.40% 64.54%
[4] [41] [6] [8] [10] [11] [12] [13] [14] [15] [24] Present work
Sr0.98Ga2B2O7: 0.01Bi3þ, 0.01Dy3þ, a high quantum yield was obtained with a very low doping level. 3.3.4. Thermal stability In order to study the thermal stability of the phosphors, a variable temperature fluorescence spectrum test was performed on Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ, and the temperature ranged from room temperature to 200 C. Fig. 6 shows the temperature-
dependent spectra of Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ. In the temperature range of 26e100 C, the emission intensity does not change significantly, and the emission intensity at 50e75 C even slightly exceeds that at room temperature. As the temperature reaches 100 C, the peak intensity slightly decreases, but the half width become larger, and the total intensity remains at a level comparable to that emission at room temperature. As the temperature exceeds 100 C, the emission intensity decreases. At 100 C,
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Table 2 Thermal quenching activation energy and thermal quenching temperature of representative blue phosphors. Phosphor
Ea (eV)
Quenching temperature (K)
Reference
Ca4ZrGe3O12: Bi3þ CaZrO3: Bi3þ LaLiSbO6: Bi3þ K2ZrSi2O7: Bi3þ Sr5(PO4)2(SiO4): Ce3þ Ca3Hf2SiAl2O12: Ce3þ Ca3Zr2SiGa2O12: Ce3þ Ca2YZr2Al3O12: Ce3þ Ba2Ca(BO3)2: Ce3þ Ba2Ca(PO4)2: Eu2þ K2Al2B2O7: Eu2þ SrGa2B2O7: Bi3þ- Dy3þ
0.228 0.08 0.10 0.26 0.221 0.17163 0.39 0.22 0.06 0.30 0.30 0.41
298 N/A N/A 298 293 298 298 298 10 303 303 450.92
[24] [21] [22] [23] [14] [16] [13] [12] [9] [7] [43] present work
thermal quenching activation energy, resulting in relatively lower temperature for thermal quenching (mostly at room temperature). In comparison, the activation energy of Sr0.98Ga2B2O7: 0.01Bi3þ0.01Dy3þ is higher, leading to higher thermal quenching temperature. Therefore, compared with the conventional blue phosphors, the phosphor of Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ can be used at higher working temperature while maintaining relatively strong luminescence emission intensity.
Fig. 6. (a) Emission spectra and color coordinates of Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ in the temperature range of 26e200 C; (b) Relationship between emission intensity and temperature. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
the intensity is 95.53% of the strongest intensity, while at 200 C it is 47.50%. Based on these results, the temperature at which the intensity decays to half of the strongest was estimated to be 450.92 K (177.92 C). The color coordinates of Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ vary only slightly with temperatures. The color coordinates are (0.155, 0.047) at 26 C, (0.155, 0.053) at 100 C and (0.155, 0.065) at 200 C. As the temperature increases to 200 C, the color coordinates shift slightly toward the cyan region. According to the Arrhenius activation energy formula [42], ln(I0/ I-1) is plotted and fitted to 1/kT, showing the relationship between emission intensity and temperature, and the activation energy of emission intensity with temperature quenching was calculated. formula (4) is:
ln
I0 DEa 1 ¼ ln C I kT
(4)
where I0 is the maximum emission intensity, I represents the emission intensity at different temperature, k is the Boltzmann constant, T denotes the temperature, C is a constant, and DEa is the activation energy. With the linear fitting in Fig. 6b, the activation energy of Sr0.98Ga2B2O7: 0.01 Bi3þ- 0.01Dy3þ is calculated to be 0.41 eV. Table 2 shows the thermal quenching activation energy and thermal quenching temperature of the blue phosphors doped with Bi3þ, Ce3þ and Eu2þ. Most of the blue phosphors have lower
3.3.5. Fluorescence lifetime and possible fluorescent mechanism The measurements of time-resolved spectrum and absolute quantum yield were performed with the strongest blue phosphors solely doped with Bi3þ (Sr0.99Ga2B2O7: 0.01Bi3þ) and co-doped with Bi3þ- Dy3þ (Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ). As shown in Fig. 7, the fluorescence lifetime of SrGa2B2O7 doped with 1mol% of Bi3þ is 1.1864 ns. In comparison, the fluorescence lifetime of Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ is 972.6852 ns, which is longer by approximately 3 orders of magnitude. In the study of Sr4Al14O25 co-doped with Eu2þ and Dy3þ, the mechanism of electron traps was proposed [33]. When the trivalent Dy3þ ion is doped at the site of divalent Sr2þ, the site of the higher charge ion will produce a positively charged impurity defect, which becomes an electron trap capable of trapping electrons, and negatively charged Sr2þ vacancies become hole traps. The role of electron traps is to capture electrons and then release electrons. We found that the hypothesis of the electron traps-hole traps can explain the photoluminescence emission, emission with charge compensation and fluorescence lifetime of the present system well. The energy released when electrons and holes recombine can transfer to Bi3þ, which is more efficiently excited to cause a radiation transition. This process changes the pathway of energy, i. e. the electrons are easily captured by the electron traps and reduce the probability of Bi2þ- Bi4þ ion pairs, thereby significantly increasing the intensity of the emission. Fig. 7b illustrates the electron transfer process and energy transfer mechanism with [Dy·Sr] as electron trap. Both Bi3þ and Dy3þ are trivalent cations, thus doping in SrGa2B2O7 at the position of Sr2þ, which is replaced by an unequal charged impurity, forms impurity defects with a positive charge [Bi·Sr], [Dy·Sr]. No monovalent cation was added to get the equilibrium charge and, thus, a corresponding cation vacancy [VSr''] with two negative charges was produced. Among them, [Bi·Sr] does not appear to function as an electron trap and can directly receive the energy transmitted by the excitation source. [Dy·Sr] may capture electrons generated by the valence band as an electron trap, and [VSr''] can acts as a hole trap to capture holes generated by the conduction band. The formation of defects and traps, and the process of capturing and releasing electrons and holes are
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Fig. 7. (a) Time-resolved spectra of Sr0.99Ga2B2O7: 0.01Bi3þ and Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ; (b) The illustration of the electron transfer process and energy transfer mechanism with [Dy·Sr] as electron trap.
expressed by equations (5)e(9):
1 00 Bi3þ ! BiSr þ V Sr 2
(5)
1 00 Dy3þ ! DySr þ V Sr 2
(6)
BiSr þ e0 / BiSr , e0 / BiSr þ e0
(7)
DySr þ e0 / DySr , e0 / DySr þ e0
(8)
00 00 00 V Sr þ 2h / V Sr , 2h / V Sr þ 2h
As illustrated in Fig. 7b, the samples co-doped with Bi Dy3þ have undergone at least four processes:
(9) 3þ
and
(1) Excitation light is applied to the sample, and energy transfer to the valence band and the conduction band of the host lattice to generate electron-hole pairs; (2) Electron traps ([Dy·Sr]) capture electrons, and hole traps ([VSr'']) capture holes; (3) The traps release electrons and holes; (4) Electrons and holes recombine, releasing energy and transferring it to Bi3þ.
The Bi3þ ions that receives the energy undergoes the radiation transition and emit visible light. The electronic trap mechanism is consistent with the increase of fluorescence lifetime. According to Fig. 4c, for the sample without doping by Dy3þ, the excitation source directly transfers energy to Bi3þ. Since this process is very rapid, the fluorescence lifetime of the Sr0.99Ga2B2O7: 0.01Bi3þ is quite short. For SrGa2B2O7 co-doped with Bi3þ and Dy3þ, the excitation source does not directly transfer energy to Bi3þ, but first transfers energy to the host lattice energy band and generate electron-hole pairs. Electrons and holes are captured and released by the corresponding traps. When the electrons and holes recombine, the generated energy transfers to Bi3þ. The energy transferred to Bi3þ undergoes several steps, thus the fluorescence lifetime of the sample doped with Bi3þ and Dy3þ is much longer than that in the sample doped with Bi3þ. It has been reported that in the Sr4Al14O25 phosphors [33], indirect energy transfer to the luminescent center by the electron trap can enhance the steady-state fluorescence intensity of Eu2þ while increasing the fluorescence lifetime of Eu2þ. Similarly, Dy3þ, as sensitizer, enhancing the Bi3þ emission in the SrGa2B2O7 phosphors may also follow this energy transfer passway. As the Dy3þ content increases, the concentration of electron traps also increases, which can capture more electrons, then release and recombine with holes to transfer energy. As the Dy3þ content reaches a certain value (1 mol%), the number of electrons that can be captured is balanced with the number of electron traps. Thus, the strongest emission occurs at this doping level. As the doping level exceeds the optimum value, the concentration of electron traps in the phosphor exceeds the concentration of electrons that can be produced. Under such circumstances, no additional electrons can be trapped by empty electron traps. When those traps that capture electrons release electrons, these electrons can be captured again by the empty traps that result in reduced recombination efficiency of electrons and holes. Therefore, the energy transferred to Bi3þ is reduced, and the intensity of the emission decreases. To identify the appropriate doping levels for different rare earth ions when they are co-doped with 1% of Bi3þ, four doping rates, 0.005, 0.01, 0.05, and 0.1, were tested for each rare earth element. The optimal ratios of different rare earth ions are different. Fig. 8 shows the fluorescence spectra and fluorescence intensity changes of the SrGa2B2O7 phosphors co-doped with 1mol% Bi3þ and various rare earth ions (except Sc and Pm). By comparing the emission intensity of these samples with different rare earth ions doped at the relative optimum ratio, it was found that the fluorescence emission intensity of the samples doped with 1mol% Dy3þ is still the strongest. The appropriate contents of Y3þ, La3þ, Ce4þ (as the experiments were carried out in ambient atmosphere with no reducing gas, Ce is expected to be present in the form of Ce4þ), Pr3þ, Tb3þ, and Er3þ are all lower than 0.5mol%, those of Sm3þ and Dy3þ are 1mol%, Nd3þ, and those of Gd3þ, Tm3þ, Yb3þ, and Lu3þ are all higher than 5mol%. For Ho3þ, the fluorescence emission intensity is strongest at the doping level of 10mol%. Eu3þ strongly absorbs blueviolet light and emits red light in SrGa2B2O7, thus it is impossible to determine the effect of Eu3þ as electron trap in SrGa2B2O7. Sm3þ also emits red light at high ratios, and Tb3þ emits green light at high ratios. With charge compensation by introduction of alkali metal codoping on the Sr site, the emission intensity decreases rather than enhances (Fig. 5c & 5d). As the alkali metal ions are co-doped with Bi3þ and Dy3þ, the original hole traps ([VSr'']) become the negatively charged impurity defects (MSr'). Thus, it can be inferred that the Sr2þ vacancies enhance the emission intensity of the sample. Compared to the ([MSr']) hole traps, the Sr2þ vacancy is a more suitable hole trap that can capture holes more efficiently.
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9
Foundation of China (Grant No. 11404358). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.151849. References
Fig. 8. (a) Emission (l ex ¼ 330 nm) spectra of SGBO: 0.01Bi3þ- zRE (z ¼ 0e0.1); (b) The change in luminescence intensity of SGBO: 0.01Bi3þ- zRE.
4. Conclusions Bi3þ and rare earth ions co-doped SrGa2B2O7 blue phosphors were synthesized by high-temperature solid-state reaction. In these phosphors, Bi3þ ions emit blue light and co-doping with certain rare earth ions enhances the Bi3þ emission. The Sr0.98Ga2B2O7: 0.01Bi3þ, 0.01Dy3þ phosphor exhibits efficient blue light emission under 330 nm UV excitation, with a maximum around 430 nm and a narrow FWHM of 64.6 nm Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ phosphor delivers high quantum efficiency of 65.54%. The color purity of Sr0.98Ga2B2O7: 0.01Bi3þ, 0.01Dy3þ phosphor is 94.2%. Sr0.98Ga2B2O7: 0.01Bi3þ, 0.01Dy3þ phosphor possesses good thermal stability, with over 95% of the strongest intensity maintained at 100 C. Compared to blue phosphors based on doping by Eu2þ and Ce3þ, Sr0.98Ga2B2O7: 0.01Bi3þ, 0.01Dy3þ phosphor possesses redox stability. With the low doping ratio, high efficiency, high color purity, good thermal stability and excellent redox stability, Sr0.98Ga2B2O7: 0.01Bi3þ, 0.01Dy3þ phosphor is a promising blue emitting phosphor for n-UV WLED. Conflicts of interest The authors declare no competing financial interests. Acknowledgment The work was supported by National Natural Science
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Blue emission from Sr0.98Ga2B2O7: 0.01Bi3þ, 0.01Dy3þ phosphor with high quantum yield Shuai Yang a, Yannan Dai a, Yang Shen b, Chungang Duan b, c, Qunli Rao d, Hui Peng b, Fan Yang a, Yongkui Shan a, **, Qingbiao Zhao b, * a
School of Chemistry and Molecular Engineering, East China Normal University, Shanghai, 200241, China Key Laboratory of Polar Materials and Devices (MOE), Department of Optoelectronics, East China Normal University, Shanghai, 200241, China Collaborative Innovation Center of Extreme Optics, Shanxi University, Shanxi, 030006, China d Instrument Analysis Center, Shanghai Jiaotong University, Shanghai, 200240, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 June 2019 Received in revised form 10 August 2019 Accepted 12 August 2019 Available online 14 August 2019
Most of the blue phosphors are based on Eu2þ or Ce3þ doped compounds. The luminescent centers of Eu2þ or Ce3þ generally require reducing environment during synthesis. Also, Eu2þ and Ce3þ are prone to oxidation, causing instability of the phosphors for long-term use. Herein, we report Bi3þ and Dy3þ codoped SrGa2B2O7 blue phosphors with simpler synthesis conditions and more stable chemical property. In this phosphor, Bi3þ ions emit blue light and, while Dy3þ as sensitizer has no appreciable luminescence emission but can enhance the emission of Bi3þ. Co-doping with rare earth ions enhances the emission of Bi3þ, improving the efficiency and stability of the phosphor. The Sr0.98Ga2B2O7: 0.01Bi3þ, 0.01Dy3þ phosphor exhibits strong blue light emission with absolute quantum yield of 65.54%. Sr0.98Ga2B2O7: 0.01Bi3þ, 0.01Dy3þ has a broad excitation band (250e380 nm), emits bright blue light at 430 nm and has a narrow full width at half maxima (FWHM ¼ 64.6 nm) with a color purity of 94.2%. Furthermore, the Sr0.98Ga2B2O7: 0.01Bi3þ, 0.01Dy3þ phosphor retains strong blue light emission at high temperatures (95% at 100 C). Overall, Sr0.98Ga2B2O7: 0.01Bi3þ, 0.01Dy3þ is a promising blue emitting phosphor for n-UV WLED. © 2019 Elsevier B.V. All rights reserved.
Keywords: Phosphors Blue Bismuth Borate Quantum yield
1. Introduction LEDs are more and more widely used in lighting, due to advantages such as long service life and low power consumption [1]. There are roughly three schemes for obtaining white light by LED: (1) white lightning device composed of red, green, and blue emitting LEDs; (2) white lightning device combining blue LED with yellow emitting excited by blue light; (3) white lightning device combining UV-LED with red, green and blue emitting phosphors. However, green and yellow LEDs have relatively low efficiency and poor color rendering [2]. White light obtained by blue LED with yellow emitting phosphor lacks red light component, resulting in problems such as high correlated color temperature and poor color rendering. The white lighting device combining an UV-LED and
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Shan), [email protected] (Q. Zhao). https://doi.org/10.1016/j.jallcom.2019.151849 0925-8388/© 2019 Elsevier B.V. All rights reserved.
tricolor phosphors can solve these problems [2]. Presently, there are many tricolor phosphors used in UV-LEDs, most of which are solid compounds doped with rare earth ions. For most blue phosphors, the emission centers are Ce3þ and Eu2þ, because their 4f, 5d orbitals allow electric dipole transitions [3]. BaMgAl10O17: Eu2þ (BAM: Eu2þ) is a commercialized blue phosphor, which can be excited by near-ultraviolet light and emits blue light at ~450 nm [4]. It shall be noted that the amount of Eu, which is a relatively high cost element, in BAM is high (approximately 10% in molar ratio). Many blue phosphors doped with Ce3þ or Eu2þ have been reported, such as Sr5(PO4)3Cl: Eu2þ, KMg4(PO4)3: Eu2þ, Ba2Ca(PO4)2: Eu2þ, BaHfSi3O9: Eu2þ, Ba2Ca(BO3)2: Ce3þ, Ca4P2O9: Ce3þ, NaSrBO3: Ce3þ, Ca2YZr2Al3O12: Ce3þ, Ca3Zr2SiGa2O12: Ce3þ, Sr5(PO4)2SiO4: Ce3þ, Ca8La2(PO4)6O2: Ce3þ, Ca3Hf2SiAl2O12: Ce3þ [5e16]. The rare earths are relatively expensive elements. Moreover, the generation of the relatively low oxidation states of Ce3þ and Eu2þ typically requires reducing atmosphere, which adds to the cost. Furthermore, Ce3þ and Eu2þ are prone to the oxidation to Ce4þ and Eu3þ [17,18], resulting in weaker emission
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intensity and possible changes in emission color during long period of service. Alternatively, blue phosphors can be obtained by doping with Bi3þ. Bi3þ has 6s2 lone pairs, which can generate S-P transition [19]. The energy level difference between the s-orbital and p-orbitals of Bi3þ corresponds to the emission of blue light. Bi3þ has many advantages, such as high stability and low toxicity. Some phosphors doped with Bi3þ have been reported, such as GaBO3: Bi3þ, CaZrO3: Bi3þ, La2LiSbO6: Bi3þ, K2ZrSi2O7: Bi3þ, Ca4ZrGe3O12: Bi3þ, ZnGa2O4: Bi3þ, Sr3Y2Ge3O12: Bi3þ, BaAl2Si2O8: Bi3þ [20e27]. The emission of Bi3þ in different hosts varies, including yellow light, such as in Ca2MgWO6: Bi3þ [28], green light, such as in Y2O3: Bi3þ [29], and ultraviolet light, such as in YPO4: Bi3þ [30]. In some alkaline earth metal oxides, some rare earth ions can enhance the blue or green light emission of Eu2þ, such as Sr2MgSi2O7: Eu2þ, La3þ; CaAl2O4: Eu2þ, Nd3þ; Sr4Al14O25: Eu2þ, Dy3þ [31e33]. In this study, the blue phosphor Sr0.98Ga2B2O7: 0.01Bi3þ, 0.01Dy3þ was synthesized, which delivers high quantum yield and high thermal stability. Additionally, the oxidation states of Bi3þ and Dy3þ ensures higher redox stability compared to blue phosphors based on Eu2þ and Ce3þ. 2. Experimental section 2.1. Materials and synthetic procedures Phosphors of Sr1-xGa2B2O7: xBi and Sr1-x-yGa2B2O7: (xBi, yRE) were prepared by a conventional high temperature solid-state synthesis method. The raw materials were SrCO3 (Sigma-Aldrich, 99.9%), Ga2O3, H3BO3 (Sigma-Aldrich, 99.5%), Bi2O3 (Across, 99.9%) and rare earth oxides (Adamas Reagent Co., Ltd. 99.99%). The raw materials were weighed according to stoichiometric ratio, thoroughly ground and mixed in an agate mortar. A small amount of anhydrous ethanol (~2 mL) was added to aid grinding. All rare earth oxides were activated at 800 C for 24 h and Bi2O3 was activated at 600 C for 24 h before being used in the reaction. In order to compensate the loss of H3BO3 during grinding with ethanol and calcination at high temperature, the amount of H3BO3 was in an excess of 20%. The well-mixed raw materials were pressed into pellets under pressure of 10 MPa. The pellets were placed in an alumina crucible, heated to 850 C in a muffle furnace at a rate of 1 C/min, kept for 24 h, and naturally cooled. The calcination was repeated twice, and the powders were re-ground and pressed into pellets each time. After the reaction was completed, the products were ground and white powder samples were obtained.
Test and Calibration Technology Co., Ltd, Hangzhou) in the temperature range from room temperature to 200 C. The fluorescence lifetime was characterized with a FLS-980 fluorometer (Edinburgh Instruments, EI). The quantum yield was characterized with a FLS920 fluorometer (Edinburgh Instruments, EI).
3. Results and discussion 3.1. Structure refinement and description SrGa2B2O7 crystallizes in an orthorhombic crystal system with the space group of Cmcm, with the unit cell parameters of a ¼ 11.7260(7) Å, b ¼ 8.3745(4) Å, and c ¼ 5.7062(3) Å. The crystal structure is illustrated in Fig. 1. GaO4 tetrahedrons form twodimensional chains in the (001) direction by sharing vertices, two BO3 triangles form [B2O5] groups through common vertices, and are connected to the common vertices of four adjacent GaO4 tetrahedrons to form the three-dimensional skeleton of [B2O5-GaO4]∞. Sr2þ is filled in the voids formed by the skeleton with a coordination number of 8. The B2O5 group is planar and perpendicular to the [001] axis [34]. Fig. 1a and 1b shows the crystal structure of SrGa2B2O7 in different directions. Fig. 1c show the local structure in the SrGa2B2O7: the [GaO4] chain, the [GaO4] and [B2O5] connections and the [SrO8] polyhedron. As shown in Fig. 2, the XRD pattern of the synthesized SrGa2B2O7 substrate is consistent with that given in the literature [34]. Thus, the sample is a pure phase of SrGa2B2O7. As shown in the XRD patterns in Figs. S1eS3, SrGa2B2O7 doped with different ratios of Bi3þ and SrGa2B2O7 co-doped with rare earth ions and Bi3þ maintain the SrGa2B2O7 host structure, with no identifiable impurity peak. Fig. 2b shows the XRD Rietveld refinement results of SrGa2B2O7 host. The refined results show that the SrGa2B2O7 host
2.2. Characterization The crystal phases of the samples were characterized by X-ray powder diffraction (XRD), which was performed on a D8 DISCOVER diffractometer (Bruker AXS GmbH, Germany) with an increment of 0.01 in the 2q range from 5 to 70 with Cu Ka1 (l ¼ 1.540598 Å) radiation. The data for structural refinement was performed on an Ultima IV X-ray diffractometer (Rigaku, Japan) with an increment of 0.01 in the 2q range from 5 to 90 with Cu Ka radiation. XRD Rietveld refinements of the crystal structure were performed with Total Pattern Solution (TOPAS 3.0) software. UV-Vis diffuse reflectance spectra were collected using a Lambda-950 (PerkinElmer, U.S.A.). The photoluminescence excitation and emission spectra were collected with a LS-55 fluorometer (PerkinElmer, U.S.A.) equipped with a 50 Hz pulse xenon lamp as the excitation source. The steps of the excitation and emission spectra were 0.5 nm. The thermal stability of luminescence properties was characterized by EX-1000 phosphor thermal quenching analysis system (EVERFINE
Fig. 1. The crystal structure of SrGa2B2O7: (a, b) the crystal structure of SrGa2B2O7 in different directions; (c) the [GaO4] chain, the [GaO4], [B2O5] connections and the [SrO8] polyhedron shown in polyhedron and “ball and stick” models.
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Fig. 2. XRD pattern of SrGa2B2O7: (a) experimentally synthesized SrGa2B2O7 and Sr0.99Ga2B2O7: 0.01Bi3þ; (b) Rietveld refinement of SrGa2B2O7 host samples.
sample formed orthorhombic phase in space group Cmcm, a ¼ 11.7048(1) Å, b ¼ 8.3555(7) Å, c ¼ 5.6967(2) Å, V ¼ 557.1412(6) Å3. All the atomic positions and thermal vibrational parameters were refined and converged. The weighted profile R-factor (Rwp) is 8.28% and the profile R factor (Rp) is 5.70% (Supporting Materials, Tables S1 and S2). 3.2. UV-Vis diffuse reflection UV-Vis diffuse reflectance spectroscopy was performed on SrGa2B2O7, Sr0.99Ga2B2O7: 0.01Bi3þ and Sr0.98Ga2B2O7: 0.01Bi3þ-
0.01Dy3þ. The UV-Vis diffuse reflectance spectra is shown in Fig. 3. The band gaps were obtained by the following formula (1) [35]:
ahn ¼ k (hn - Eg)n
(1)
where Eg, a, h and n indicate band gap energy, absorbance, Planck's constant and photon frequency, respectively. The UV-Vis spectrum of SrGa2B2O7 substrate shows an absorption peak at ~5 eV. In addition to the absorption peak at ~5 eV, the doped samples, Sr0.99Ga2B2O7: 0.01Bi3þ and Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ, have significant absorption at ~4.4 eV. The
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Fig. 3. UV-Vis diffuse reflectance spectroscopy and optical band gap of (a) SrGa2B2O7; (b) Sr0.99Ga2B2O7: 0.01Bi3þ; (c) Sr0.98Ga2B2O7: 0.01Bi3þ - 0.01Dy3þ.
absorption at ~4.4 eV for Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ is more pronounced. The absorption at 4.4 eV corresponds to the 1S0/1P1 transition of Bi3þ. Co-doping with Dy3þ enhances the absorption of Bi3þ, hence the excitation energy can be more efficiently utilized. According to the diffuse reflectance spectrum, the optical band gaps of the SrGa2B2O7 host, Sr0.99Ga2B2O7: 0.01Bi3þ and Sr0.98Ga2B2O7: 0.01Bi3þ-0.01Dy3þ were estimated to be 4.61 eV, 3.97 eV and 4.11 eV, respectively.
3.3. Fluorescent properties 3.3.1. Fluorescent properties and luminescence mechanism of Sr0.99Ga2B2O7: Bi3þ We firstly discuss the fluorescent behaviors of SrGa2B2O7 solely doped with Bi3þ (without rare earth co-doping). For Bi3þ, the excitation wavelengths typically include three bands, i.e., UVA, UVB and UVC. The emission wavelengths are different in various hosts, which can be manifested as blue light [19], green light [29], yellow light [28], or in the ultraviolet band [30]. This indicates that the emission from Bi3þ is significantly affected by the lattice environment. The ground state of free Bi3þ ion is 1S0, and there are four excited states: 3P1, 3P2, 3P0 and 1P1. In general, the transitions of Bi3þ have three bands. A-band is an allowed transition due to the spineorbit coupling, whose corresponding transition is 1S0-3P1. Cband is a spin-allowed transition and the corresponding transition is 1S0-1P1. The corresponding transition of B-band is 1S0-3P2, which is in general forbidden, but can be induced by coupling with unsymmetrical lattice vibrational modes [36]. The transition 1S0-3P0 is strongly forbidden. In addition, there is a D-band transition that frequently occurs in the excitation spectrum of Bi3þ, which may be due to the charge transfer transition of Bi3þ [37]. As shown in Fig. 4a, Sr0.99Ga2B2O7: 0.01Bi3þ generates a single
Fig. 4. (a) Emission (l ex ¼ 330 nm) and excitation (l em ¼ 330 nm) spectra of Sr1xBi3þ; (b) The change in fluorescence intensity of Sr1-xGa2B2O7: xBi3þ and linear fitting of log (x) versus log (I/x) in various Sr1-xGa2B2O7: xBi3þ phosphors; (c) The energy transition and emission of Bi3þ ions.
xGa2B2O7:
broad peak emission with the 330 nm excitation. The peak wavelength is 430 nm, and the emission range is 340e600 nm. The excitation spectrum corresponding to the 430 nm emission shows two excitation bands, i. e., a strong excitation band with maximum at 330 nm and a weak excitation band at 276 nm. According to the excitation mechanism of Bi3, the excitation at 330 nm is attributed to the 1S0-3P1 transition of Bi3þ. The strong spin-orbit coupling interaction makes the excitation prominent, and the weak excitation band at 276 nm corresponds to the spin allowed 1S0-1P1
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transition. The emission spectrum shows only one broad peak, which is attributed to the 3P1-1S0 transition of Bi3þ, for which the electrons at the higher excitation level 1P1 lose part of the energy through non-radiative transition and transfer to the lower triplet state 3P1. The doping rate of Bi3þ was tuned to determine the optimum doping level for the strongest emission intensity. The phosphors with doping level of Bi3þ ranging from 0.5% to 9 mol% were synthesized and characterized. The emission intensity reached the maximum at the doping rate of 1 mol%, and the excitation spectrum exhibited the same tendency (Fig. 4a). The narrow full width at half maxima (FWHM) of Sr0.99Ga2B2O7: 0.01Bi3þ is 65.3 nm. Fig. 4b shows the trend of fluorescence intensity of Sr1-xGa2B2O7: xBi3þ as a function of doping ratio. With Bi3þ over 1 mol%, the emission intensity of Bi3þ decreases as the doping ratio increases. In SrGa2B2O7, only the Sr2þ site can be occupied by Bi3þ and rare earth ions. According to the study of energy transfer in oxide phosphors [38], the critical distance of Bi3þ in SrGa2B2O7 can be calculated by formula (2):
Rc ¼ 2
3V 4pxc N
1=3 (2)
where xc is the critical ratio. According to the crystal structure of SrGa2B2O7, N ¼ 4 and V ¼ 560.35 Å3. With the doping level of Bi3þ at 1mol%, the critical distance is calculated to be 29.91 Å, and the critical distance is 14.95 Å for the doping level of Bi3þ at 9mol%. According to the fluorescence spectrum of Sr1-xGa2B2O7: xBi3þ, the overlap of the excitation and emission spectra of Bi3þ is small. Thus, the probability of radiation reabsorption is small. The exchange coupling takes place only when the critical distance is short, i.e. ~5 Å. For Sr1-xGa2B2O7: xBi3þ, the calculated critical distance is as large as ~15 Å, even at higher of Bi3þ contents. Therefore, in Sr13þ xGa2B2O7: xBi , it is unlikely that the radiation reabsorption and exchange interaction between Bi3þ ions occur. The concentration quenching of Bi3þ is attributed to the electric multipole interaction and the energy transfer between adjacent ions. The relationship between emission intensity and doping level [39] is expressed by formula (3):
h i1 I ¼ k 1 þ bxq=3 x
(3)
where I represents the emission intensity, x is the ratio of Bi3þ, k and b denote the constants of the same interaction. The value of q indicates the type of interaction between the doping ions. q ¼ 3 means that the interaction is the energy transfer between adjacent ions, q ¼ 6 represents the dipole-dipole interaction, q ¼ 8 represents the electric dipole-electric quadrupole interaction, while q ¼ 10 represents electric quadrupole-electric quadrupole interaction. By fitting lg x with lg (I/x) (Fig. 4b), the obtained slope is 1.743. q ¼ 5.229 for Sr1-xGa2B2O7: xBi3þ, hence, the interaction between Bi3þ ions is composed of two parts: the dominant is dipole-dipole interaction and secondary is energy transfer between adjacent ions [40]. 3.3.2. Fluorescent properties and luminescence mechanism of SrGa2B2O7: Bi3þ- Dy3þ In order to obtain blue phosphors with stronger emission, various rare earth ions were co-doped with 1mol% Bi3þ in SrGa2B2O7. No impurity peak was observed in the XRD patterns of these phosphors (Supporting Information, Fig. S2). The fluorescence spectra of these phosphors are shown in Fig. S5. Among the various rare earth ions, the effect of Dy3þ in enhancing the emission from Bi3þ is exceptionally pronounced. With 1 mol% of Dy3þ and
5
1mol% of Bi3þ co-doped in SrGa2B2O7, both the excitation and emission intensities were significantly increased. Thus, SrGa2B2O7 phosphors co-doped with different ratios of Dy3þ and 1mol% of Bi3þ were synthesized. As shown in the XRD pattern, SrGa2B2O7 phosphors co-doped with various ratios of Dy3þ and 1 mol% of Bi3þ are pure phases. The fluorescence emission of these phosphors are shown in Fig. 5a. To elucidate the mechanisms of the fluorescent emission in doped SrGa2B2O7, SrGa2B2O7 with solely rare earth doping (without Bi3þ) were also synthesized. For SrGa2B2O7 with solely rare earth doping at the level of 1 mol%, no appreciable emission in the visible wavelength range was observed with the excitation wavelength of 330 nm, except for Eu3þ. As 1 mol% of rare earth ions were doped with 1 mol% of Bi3þ, only Bi3þ emission was observed, and no characteristic emission of rare earth ions (except Eu3þ) was observed (Supporting Information, Fig. S5). Therefore, the rare earth ions function as sensitizers in this system, which is different from typical systems with Bi3þ as sensitizer and rare earth as emitting center. Fig. 5b shows the trend in luminescence intensity of Sr0.993þ 3þ 3þ content reaches 1 mol%, the yGa2B2O7: 0.01Bi - yDy . As the Dy strongest emission occurs at this doping level. The full width at half maxima (FWHM) of Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ is 64.6 nm. The color coordinates of the phosphor were calculated with the fluorescence spectra and they are shown in the 1931 CIE chromaticity diagram (Fig. 5b). The color coordinate of Sr0.99Ga2B2O7: 0.01Bi3þ under excitation of 330 nm are (0.156, 0.052), and the color coordinates of Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ are (0.155, 0.047). Based on the color coordinates, co-doping with Dy3þ essentially does not change the emitting color of Bi3þ. Notably, the color purity of Sr0.98Ga2B2O7: 0.01Bi3þ, 0.01Dy3þ phosphor is 94.2%. In order to verify whether the balanced charge can enhance the emission intensity, three alkali metal ions, Liþ, Naþ and Kþ, were codoped with Bi3þ and Dy3þ in SrGa2B2O7. As shown in Fig. 5c, the emission intensities of samples with charge compensation ions Liþ, Naþ and Kþ are comparable. The emission intensities with charge compensation ions are much weaker than the sample with Bi3þ and Dy3þ co-doping but without monovalent ions. 3.3.3. Quantum yield The SrGa2B2O7: Bi3þ phosphors can emit blue light in the range of 350e550 nm with the excitation of 330 nm ultraviolet light. In order to enhance the blue emission of Bi3þ, rare earth ions were codoped with Bi3þ in this host. The phosphor co-doped with 1 mol% Bi3þ and 1 mol% Dy3þ has the strongest emission intensity. Based on the quantum yield measurements, the absolute quantum yield of Sr0.99Ga2B2O7: 0.01Bi3þ is 32.48%, while that of Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ is as high as 64.54%. The quantum yield reached 92.2% of the commercialized blue phosphor BAM: Eu2þ, for which the external quantum yield is ~70% [4]. Compared with BAM: Eu2þ, SrGa2B2O7: Bi3þ- Dy3þ possesses several advantages, such as simpler reaction conditions (Eu2þ needs to be prepared from Eu3þ under reducing conditions), lower synthesis temperature, and lower doping ratio of luminescent center ions (The ratios of Bi3þ and Dy3þ are both 1 mol%, whereas the ratio of Eu2þ in BAM is ~10%). Moreover, the chemical properties are more stable (Both Bi3þ and Dy3þ are in stable oxidation state under ambient environment, whereas Eu2þ is prone to oxidation to Eu3þ). Table 1 shows the quantum yields of representative blue phosphors and their corresponding doping levels, excitation and emission wavelength. Most of the blue phosphors with relatively high quantum yield are doped with Ce3þ or Eu2þ, while the blue phosphors doped with Bi3þ generally have low quantum yields. For
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Fig. 5. (a) Emission (l ex ¼ 330 nm) and excitation (l em ¼ 430 nm) spectra of Sr0.99-yGa2B2O7: 0.01Bi3þ- yDy3þ (y ¼ 0e0.05); (b) The trend in luminescence intensity of Sr0.990.01Bi3þ- yDy3þ and the color coordinates of Sr0.99Ga2B2O7: 0.01Bi3þ and Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ; (c) Emission (l ex ¼ 330 nm) and excitation (l 3þ 3þ þ 3þ 3þ þ em ¼ 330 nm) spectra of Sr0.96Ga2B2O7: 0.01Bi - 0.01Dy - 0.02 M (M ¼ Li, Na, K); (d) The trend in luminescence intensity of Sr0.96Ga2B2O7: 0.01Bi - 0.01Dy - 0.02 M . (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
yGa2B2O7:
Table 1 The quantum yields of representative blue phosphors and their doping ratios, excitation and emission wavelength. Phosphor
Doping rate
Excitation wavelength (nm)
Emission wavelength (nm)
Quantum yield
Reference
BaMgAl10O17: Eu2þ CaPO4Cl: Eu2þ KMg4(PO4)3: Eu2þ BaHfSi3O9: Eu2þ Ca4P2O9: Ce3þ NaSrBO3: Ce3þ Ca2YZr2Al3O12: Ce3þ Ca3Zr2SiGa2O12: Ce3þ Sr5(PO4)2SiO4: Ce3þ Ca8La2(PO4)6O2: Ce3þ Ca4ZrGe3O12: Bi3þ SrGa2B2O7: Bi3þ- Dy3þ
10% 11% 0.75% 14% 0.50% 1% 3% 4% 3% 4% 2% 1%e1%
365 370 365 405 340 364 410 400 365 310 370 330
462 454 450 475 420 396, 426 495 478 425 415 435 430
69.60% 61.00% 50.44% 50.00% 64.80% 61.60% 56.00% 42.70% 46.00% 67.00% 37.40% 64.54%
[4] [41] [6] [8] [10] [11] [12] [13] [14] [15] [24] Present work
Sr0.98Ga2B2O7: 0.01Bi3þ, 0.01Dy3þ, a high quantum yield was obtained with a very low doping level. 3.3.4. Thermal stability In order to study the thermal stability of the phosphors, a variable temperature fluorescence spectrum test was performed on Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ, and the temperature ranged from room temperature to 200 C. Fig. 6 shows the temperature-
dependent spectra of Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ. In the temperature range of 26e100 C, the emission intensity does not change significantly, and the emission intensity at 50e75 C even slightly exceeds that at room temperature. As the temperature reaches 100 C, the peak intensity slightly decreases, but the half width become larger, and the total intensity remains at a level comparable to that emission at room temperature. As the temperature exceeds 100 C, the emission intensity decreases. At 100 C,
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Table 2 Thermal quenching activation energy and thermal quenching temperature of representative blue phosphors. Phosphor
Ea (eV)
Quenching temperature (K)
Reference
Ca4ZrGe3O12: Bi3þ CaZrO3: Bi3þ LaLiSbO6: Bi3þ K2ZrSi2O7: Bi3þ Sr5(PO4)2(SiO4): Ce3þ Ca3Hf2SiAl2O12: Ce3þ Ca3Zr2SiGa2O12: Ce3þ Ca2YZr2Al3O12: Ce3þ Ba2Ca(BO3)2: Ce3þ Ba2Ca(PO4)2: Eu2þ K2Al2B2O7: Eu2þ SrGa2B2O7: Bi3þ- Dy3þ
0.228 0.08 0.10 0.26 0.221 0.17163 0.39 0.22 0.06 0.30 0.30 0.41
298 N/A N/A 298 293 298 298 298 10 303 303 450.92
[24] [21] [22] [23] [14] [16] [13] [12] [9] [7] [43] present work
thermal quenching activation energy, resulting in relatively lower temperature for thermal quenching (mostly at room temperature). In comparison, the activation energy of Sr0.98Ga2B2O7: 0.01Bi3þ0.01Dy3þ is higher, leading to higher thermal quenching temperature. Therefore, compared with the conventional blue phosphors, the phosphor of Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ can be used at higher working temperature while maintaining relatively strong luminescence emission intensity.
Fig. 6. (a) Emission spectra and color coordinates of Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ in the temperature range of 26e200 C; (b) Relationship between emission intensity and temperature. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
the intensity is 95.53% of the strongest intensity, while at 200 C it is 47.50%. Based on these results, the temperature at which the intensity decays to half of the strongest was estimated to be 450.92 K (177.92 C). The color coordinates of Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ vary only slightly with temperatures. The color coordinates are (0.155, 0.047) at 26 C, (0.155, 0.053) at 100 C and (0.155, 0.065) at 200 C. As the temperature increases to 200 C, the color coordinates shift slightly toward the cyan region. According to the Arrhenius activation energy formula [42], ln(I0/ I-1) is plotted and fitted to 1/kT, showing the relationship between emission intensity and temperature, and the activation energy of emission intensity with temperature quenching was calculated. formula (4) is:
ln
I0 DEa 1 ¼ ln C I kT
(4)
where I0 is the maximum emission intensity, I represents the emission intensity at different temperature, k is the Boltzmann constant, T denotes the temperature, C is a constant, and DEa is the activation energy. With the linear fitting in Fig. 6b, the activation energy of Sr0.98Ga2B2O7: 0.01 Bi3þ- 0.01Dy3þ is calculated to be 0.41 eV. Table 2 shows the thermal quenching activation energy and thermal quenching temperature of the blue phosphors doped with Bi3þ, Ce3þ and Eu2þ. Most of the blue phosphors have lower
3.3.5. Fluorescence lifetime and possible fluorescent mechanism The measurements of time-resolved spectrum and absolute quantum yield were performed with the strongest blue phosphors solely doped with Bi3þ (Sr0.99Ga2B2O7: 0.01Bi3þ) and co-doped with Bi3þ- Dy3þ (Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ). As shown in Fig. 7, the fluorescence lifetime of SrGa2B2O7 doped with 1mol% of Bi3þ is 1.1864 ns. In comparison, the fluorescence lifetime of Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ is 972.6852 ns, which is longer by approximately 3 orders of magnitude. In the study of Sr4Al14O25 co-doped with Eu2þ and Dy3þ, the mechanism of electron traps was proposed [33]. When the trivalent Dy3þ ion is doped at the site of divalent Sr2þ, the site of the higher charge ion will produce a positively charged impurity defect, which becomes an electron trap capable of trapping electrons, and negatively charged Sr2þ vacancies become hole traps. The role of electron traps is to capture electrons and then release electrons. We found that the hypothesis of the electron traps-hole traps can explain the photoluminescence emission, emission with charge compensation and fluorescence lifetime of the present system well. The energy released when electrons and holes recombine can transfer to Bi3þ, which is more efficiently excited to cause a radiation transition. This process changes the pathway of energy, i. e. the electrons are easily captured by the electron traps and reduce the probability of Bi2þ- Bi4þ ion pairs, thereby significantly increasing the intensity of the emission. Fig. 7b illustrates the electron transfer process and energy transfer mechanism with [Dy·Sr] as electron trap. Both Bi3þ and Dy3þ are trivalent cations, thus doping in SrGa2B2O7 at the position of Sr2þ, which is replaced by an unequal charged impurity, forms impurity defects with a positive charge [Bi·Sr], [Dy·Sr]. No monovalent cation was added to get the equilibrium charge and, thus, a corresponding cation vacancy [VSr''] with two negative charges was produced. Among them, [Bi·Sr] does not appear to function as an electron trap and can directly receive the energy transmitted by the excitation source. [Dy·Sr] may capture electrons generated by the valence band as an electron trap, and [VSr''] can acts as a hole trap to capture holes generated by the conduction band. The formation of defects and traps, and the process of capturing and releasing electrons and holes are
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Fig. 7. (a) Time-resolved spectra of Sr0.99Ga2B2O7: 0.01Bi3þ and Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ; (b) The illustration of the electron transfer process and energy transfer mechanism with [Dy·Sr] as electron trap.
expressed by equations (5)e(9):
1 00 Bi3þ ! BiSr þ V Sr 2
(5)
1 00 Dy3þ ! DySr þ V Sr 2
(6)
BiSr þ e0 / BiSr , e0 / BiSr þ e0
(7)
DySr þ e0 / DySr , e0 / DySr þ e0
(8)
00 00 00 V Sr þ 2h / V Sr , 2h / V Sr þ 2h
As illustrated in Fig. 7b, the samples co-doped with Bi Dy3þ have undergone at least four processes:
(9) 3þ
and
(1) Excitation light is applied to the sample, and energy transfer to the valence band and the conduction band of the host lattice to generate electron-hole pairs; (2) Electron traps ([Dy·Sr]) capture electrons, and hole traps ([VSr'']) capture holes; (3) The traps release electrons and holes; (4) Electrons and holes recombine, releasing energy and transferring it to Bi3þ.
The Bi3þ ions that receives the energy undergoes the radiation transition and emit visible light. The electronic trap mechanism is consistent with the increase of fluorescence lifetime. According to Fig. 4c, for the sample without doping by Dy3þ, the excitation source directly transfers energy to Bi3þ. Since this process is very rapid, the fluorescence lifetime of the Sr0.99Ga2B2O7: 0.01Bi3þ is quite short. For SrGa2B2O7 co-doped with Bi3þ and Dy3þ, the excitation source does not directly transfer energy to Bi3þ, but first transfers energy to the host lattice energy band and generate electron-hole pairs. Electrons and holes are captured and released by the corresponding traps. When the electrons and holes recombine, the generated energy transfers to Bi3þ. The energy transferred to Bi3þ undergoes several steps, thus the fluorescence lifetime of the sample doped with Bi3þ and Dy3þ is much longer than that in the sample doped with Bi3þ. It has been reported that in the Sr4Al14O25 phosphors [33], indirect energy transfer to the luminescent center by the electron trap can enhance the steady-state fluorescence intensity of Eu2þ while increasing the fluorescence lifetime of Eu2þ. Similarly, Dy3þ, as sensitizer, enhancing the Bi3þ emission in the SrGa2B2O7 phosphors may also follow this energy transfer passway. As the Dy3þ content increases, the concentration of electron traps also increases, which can capture more electrons, then release and recombine with holes to transfer energy. As the Dy3þ content reaches a certain value (1 mol%), the number of electrons that can be captured is balanced with the number of electron traps. Thus, the strongest emission occurs at this doping level. As the doping level exceeds the optimum value, the concentration of electron traps in the phosphor exceeds the concentration of electrons that can be produced. Under such circumstances, no additional electrons can be trapped by empty electron traps. When those traps that capture electrons release electrons, these electrons can be captured again by the empty traps that result in reduced recombination efficiency of electrons and holes. Therefore, the energy transferred to Bi3þ is reduced, and the intensity of the emission decreases. To identify the appropriate doping levels for different rare earth ions when they are co-doped with 1% of Bi3þ, four doping rates, 0.005, 0.01, 0.05, and 0.1, were tested for each rare earth element. The optimal ratios of different rare earth ions are different. Fig. 8 shows the fluorescence spectra and fluorescence intensity changes of the SrGa2B2O7 phosphors co-doped with 1mol% Bi3þ and various rare earth ions (except Sc and Pm). By comparing the emission intensity of these samples with different rare earth ions doped at the relative optimum ratio, it was found that the fluorescence emission intensity of the samples doped with 1mol% Dy3þ is still the strongest. The appropriate contents of Y3þ, La3þ, Ce4þ (as the experiments were carried out in ambient atmosphere with no reducing gas, Ce is expected to be present in the form of Ce4þ), Pr3þ, Tb3þ, and Er3þ are all lower than 0.5mol%, those of Sm3þ and Dy3þ are 1mol%, Nd3þ, and those of Gd3þ, Tm3þ, Yb3þ, and Lu3þ are all higher than 5mol%. For Ho3þ, the fluorescence emission intensity is strongest at the doping level of 10mol%. Eu3þ strongly absorbs blueviolet light and emits red light in SrGa2B2O7, thus it is impossible to determine the effect of Eu3þ as electron trap in SrGa2B2O7. Sm3þ also emits red light at high ratios, and Tb3þ emits green light at high ratios. With charge compensation by introduction of alkali metal codoping on the Sr site, the emission intensity decreases rather than enhances (Fig. 5c & 5d). As the alkali metal ions are co-doped with Bi3þ and Dy3þ, the original hole traps ([VSr'']) become the negatively charged impurity defects (MSr'). Thus, it can be inferred that the Sr2þ vacancies enhance the emission intensity of the sample. Compared to the ([MSr']) hole traps, the Sr2þ vacancy is a more suitable hole trap that can capture holes more efficiently.
S. Yang et al. / Journal of Alloys and Compounds 810 (2019) 151849
9
Foundation of China (Grant No. 11404358). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.151849. References
Fig. 8. (a) Emission (l ex ¼ 330 nm) spectra of SGBO: 0.01Bi3þ- zRE (z ¼ 0e0.1); (b) The change in luminescence intensity of SGBO: 0.01Bi3þ- zRE.
4. Conclusions Bi3þ and rare earth ions co-doped SrGa2B2O7 blue phosphors were synthesized by high-temperature solid-state reaction. In these phosphors, Bi3þ ions emit blue light and co-doping with certain rare earth ions enhances the Bi3þ emission. The Sr0.98Ga2B2O7: 0.01Bi3þ, 0.01Dy3þ phosphor exhibits efficient blue light emission under 330 nm UV excitation, with a maximum around 430 nm and a narrow FWHM of 64.6 nm Sr0.98Ga2B2O7: 0.01Bi3þ- 0.01Dy3þ phosphor delivers high quantum efficiency of 65.54%. The color purity of Sr0.98Ga2B2O7: 0.01Bi3þ, 0.01Dy3þ phosphor is 94.2%. Sr0.98Ga2B2O7: 0.01Bi3þ, 0.01Dy3þ phosphor possesses good thermal stability, with over 95% of the strongest intensity maintained at 100 C. Compared to blue phosphors based on doping by Eu2þ and Ce3þ, Sr0.98Ga2B2O7: 0.01Bi3þ, 0.01Dy3þ phosphor possesses redox stability. With the low doping ratio, high efficiency, high color purity, good thermal stability and excellent redox stability, Sr0.98Ga2B2O7: 0.01Bi3þ, 0.01Dy3þ phosphor is a promising blue emitting phosphor for n-UV WLED. Conflicts of interest The authors declare no competing financial interests. Acknowledgment The work was supported by National Natural Science
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