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Eu3+ co-doped oxide glasses
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Energy transfer and white luminescence in Bi3þ/Eu3þ co-doped oxide glasses Olga Giraldo Giraldo, Mingzhi Fei, Rongfei Wei, Liming Teng, Zhigang Zheng, Hai Guo * Department of Physics, Zhejiang Normal University, Jinhua, Zhejiang, 321004, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Luminescent materials White light glasses Bi3þ Eu3þ Energy transfer
Bi3þ/Eu3þ-doped SiO2–Al2O3–CaO–B2O3–La2O3–K2O glasses were fabricated and the luminescent properties, energy transfer as well as thermal stability were systematically investigated. Under 333 nm light excitation, bluish-green luminescence centered at 480 nm is observed in the obtained Bi3þ-doped glass. By co-doped with Eu3þ, highly efficient energy transfer process happens from Bi3þ to Eu3þ via a quadrupole-quadrupole interac tion. The energy transfer efficiency can reach 86% and the concentration of Eu3þ is not quenched in here. With increasing Eu3þ concentration, tunable luminescence from bluish-green via white to orange-red is realized. Significantly, pure white light emission with CIE coordinates (0.348, 0.333) is obtained when excited by 333 nm light. Temperature-dependent emission spectra present superior color stability in operating temperature lower than 423 K for the prepared glasses. These results indicate that these glasses are potential white light emitting materials for organic-resin-free W-LEDs.
1. Introduction Studies on white light emission have brought a tremendous progress in solid state lighting technology. Nowadays, white light emitting diodes (W-LEDs) are a fundamental light source due to their energy-saving, high efficiency, low cost, environmental friendliness and technological advantages [1–6]. In addition, W-LEDs have a huge field of applications such as devices indicators, backlights, automobile headlights and gen eral illuminations, etc [3,7–10]. One of the popular means to obtain W-LEDs is through the combi nation of UV LEDs with white light-emitting phosphors [6,11–13]. However, the deterioration and yellowing of organic resins when working at the high current are its main drawbacks, which lead to degradation, shift of chromaticity and reduction of long-term reliability of W-LEDs [8,14,15]. To solve this problem, the substitution of phos phors byluminescence glasses was developed. Luminescence glasses exhibit excellent optical properties, good mechanical resistance and cheap production. And more importantly, they show an organic-resin free assembly process [8,16]. Bi3þ is a famous activator ion in luminescent materials. Its emission comes from 3P1→1S0 transition (that is 6sp to 6s2 transition). Due to its outer 6s2 and 6sp electronic configurations, Bi3þ exhibits luminescence in different colors, which strongly depends on the type of host [17,18].
Besides, Bi3þ can be used as sensitizer also because of the spectral overlap of emission spectra of Bi3þ and excitation spectra of other ac tivators. Eu3þ ion is a popular activator in red luminescent phosphors. And it can be sensitized by Bi3þ ion due to energy transfer process [19, 20]. Many investigations on energy transfer in Bi3þ/Eu3þ co-doped phosphors have been reported [9,21,22]. However, to the best of our knowledge, there were few researches on the Bi3þ/Eu3þ co-doped glasses [8,23]. Herein, oxide glass with the composition of SiO2 –Al2O3–CaO–B2O3–La2O3–K2O was chosen as host glass due to its high transparence, good solubility for doping, as well as physical and chemical stability [24]. The luminescence behaviors, energy transfer from Bi3þ to Eu3þ and thermal quenching property in the prepared Bi3þ/Eu3þ-doped glasses were studied in detail. The spectra present that Bi3þ-doped glass emits bluish-green light upon 333 nm light irradiation. Via introducing Eu3þ, tunable emission from bluish-green to orange-red is acquired due to the energy transfer from Bi3þ to Eu3þ. The mechanism was confirmed to be quadrupole-quadrupole interaction. Especially, pure white light emission with CIE coordinates (0.348, 0.333) is real ized. And the excellent color stability was identified by the temperature-dependent emission spectra.
* Corresponding author. E-mail address: [email protected] (H. Guo). https://doi.org/10.1016/j.jlumin.2019.116918 Received 19 July 2019; Received in revised form 8 November 2019; Accepted 23 November 2019 Available online 25 November 2019 0022-2313/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Olga Giraldo Giraldo, Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2019.116918
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and ground state of Bi3þ get closer, which produce the red-shift of the absorption edge. Besides, the doping of Bi2O3 may effectively expand the glass network, which results in more nonbridged oxygen ions and thus gives rise to the red-shift of the absorption edge [8]. Fig. 1(b) shows the excitation and emission spectra of Bx glasses. There is no emission from Host glass (not shown here). Under the excitation of 333 nm, Bx glasses exhibit broadband bluish-green emis sion from 380 to 700 nm with a maximum at 480 nm, which is origi nated from 3P1→1S0 transition of Bi3þ. With increasing Bi3þ concentration, the emission intensity of Bx glasses firstly increases until reach maximum at x ¼ 2, and then decreases due to the concentration quenching effect. The excitation spectra (λem ¼ 480 nm) of Bx glasses in Fig. 1(b) exhibits broadband from 250 to 410 nm with a maximum at 333 nm, which is attributed to 1S0→3P1 transition of Bi3þ. With growing Bi3þ content, the excitation bands show an obvious red-shift, which is consistent with the red-shift of absorption spectra in Fig. 1(a).
2. Experimental Glass with nominal composition of 20SiO2 –5Al2O3–20CaO–30B2O3–15La2O3–10K2O (in mol%) was prepared by melt-quenching method and denoted as Host. Host doped with xBi2O3 (x ¼ 0.5, 1.0, 1.5, 2.0, 2.5), 1.0Eu2O3 as well as 2.0Bi2O3 and yEu2O3 (y ¼ 0.2, 0.3, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5) were named as Bx, E1.0, and BEy, respectively. Here, CaCO3 and K2CO3 are the sources of CaO and K2O, respectively. Raw materials containing SiO2, Al2O3, CaCO3 and K2CO3 (A.R., Sinopharm Chemical reagents), B2O3 (A.R., Macklin Biochemical reagents), La2O3 and Eu2O3 (99.99%, Shanghai Yuelong Nonferrous Metals), and Bi2O3 (99.99%, Aladdin reagents) were weighted according to the molar composition of glasses and then they were mixed for 40 min in an agate mortar. Subsequently, they were melted at 1450 � C for 1 h in a corundum crucible in air atmosphere. The glasses were obtained by quenching the melt on a 300 � C preheated brass plate. At last, all glasses were sliced and polished with a thickness of 1.5 mm for further optical measurements. Transmittance spectra were obtained using a Hitachi U-3900 ultra violet–visible (UV–Vis) spectrophotometer. Photoluminescence emis sion and excitation spectra were measured using an Edinburgh FS5 spectrofluorometer (Livingston, UK) equipped with a continuous wave 150 W Xe lamp. TCB1402C temperature controller (China) was added to FS5 spectrofluorometer to measure the temperature dependent emission spectra from 299 to 540 K. The decay lifetime curves were measured in a double monochromator (Jobin-Yvon HRD1) under the excitation of tunable laser (Opolette system) with wavelength tuning range of 410–2200 nm, spectral linewidth of 4–7 cm 1, pulse duration of 7 ns and repetition rate of 20 Hz, finally it was recorded by a TDS2024 digital oscilloscope (Tektronix). Except the temperature dependent spectra, all the measurements were carried out at room temperature.
3.2. Photoluminescent properties of E1.0 glass Fig. 2(a) displays the excitation and emission spectra of E1.0 glass in the range from 200 to 750 nm. The excitation spectra (λem ¼ 613 nm) of Eu3þ are characterized for six sharp excitation peaks centered at 363, 382, 393, 413, 464 and 529 nm, arising from 7F0 to 5D4, 5G4, 5L6, 5D3, 5 D2 and 5D1 transitions, respectively. And a broad band centered at around 270 nm is related to the Eu3þ-O2- charge transfer. The emission spectra of Eu3þ exhibit five red characteristic peaks at 580, 591, 613, 653, and 703 nm, which are attributed to the intrinsic 4f-4f transition, that is, 5D0→7FJ (J ¼ 0–4) transitions of Eu3þ. Fig. 2(b) shows the existence of a strong overlap between the emis sion spectra of B2.0 glass and the excitation spectra of E1.0 glass. The spectral overlap between 350 and 550 nm strongly suggests that energy transfer from Bi3þ to Eu3þ can occur easily.
3. Results and discussion 3.1. Photoluminescent properties of Bx glasses
3.3. Photoluminescent properties of BEy glasses
In order to investigate the luminescent behavior of Bi3þ ions, the host is doped with different concentrations of Bi3þ. Fig. 1(a) gives the transmittance spectra of Bx (x ¼ 0.5, 1.0, 1.5, 2.0, 2.5) glasses. All samples are highly transparent (over 80%) in the visible region. High transparency is beneficial to practical applications. Compared to host glass, Bx glasses present a clear red-shift from 320 to 360 nm in ab sorption edge, which comes from characteristic 1S0→3P1 transition of Bi3þ ions [25,26]. With augmenting Bi3þ content, it is obviously observed that the absorption edge moves to a longer wavelength. This red-shift may be interpreted as follows [27]: As Bi3þ concentration grows, the distance between Bi3þ ions is compressed and then the overlap of the excited state of Bi3þ is increased, making the excited state
With the purpose to investigate the energy transfer process and to generate white light emission, Eu3þ was introduced in B2.0 glass as a red emitter. Luminescent behavior of BEy samples shows that the efficiency of energy transfer from Bi3þ to Eu3þ highly depends on Eu3þ concentration. Fig. 3(a) presents the emission spectra (λex ¼ 333 nm) of B2.0, E1.0 and BEy glasses. As shown in Figs. 1(b) and Figure 2 (b), Bi3þ can be excited efficiently under the excitation of 333 nm, but Eu3þ cannot. However, emission spectra of BEy glasses not just show bluish-green emission from Bi3þ, but also exhibit red emission from Eu3þ. Then, with increasing Eu3þ content, the emission intensity of Bi3þ decreases while the emission intensity of Eu3þ increases. Such behavior suggests that Eu3þ can be excited through energy transfer process from Bi3þ to
Fig. 1. (a) UV–Vis transmission spectra of Bx glasses and Host; (b) Emission spectra (λex ¼ 333 nm) and excitation spectra (λem ¼ 480 nm) of Bx glasses. 2
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Fig. 2. (a) Excitation spectra (λem ¼ 613 nm) and emission spectra (λex ¼ 393 nm) of E1.0 glass; (b) Overlap between excitation spectra (λem ¼ 613 nm) of E1.0 glass and emission spectra (λex ¼ 333 nm) of B2.0 glass.
Fig. 3. (a) Emission spectra (λex ¼ 333 nm) of BEy, E1.0 and B2.0 glasses, (b) excitation spectra (λem ¼ 613 nm) of BEy, E1.0 glasses and excitation spectra (λem ¼ 480 nm) of B2.0 glass; (c) Luminescence decay curves at 480 nm emission of Bi3þ in B2.0 and BEy glasses. (d) Energy level scheme of Bi3þ and Eu3þ, and energy transfer process.
Eu3þ. The quenching concentration of Eu3þ is not found in this work. Additionally, it is noticed that the inset spectra in Fig. 3(a) shows an obvious dip located at 464 nm in the emission band of Bi3þ, which is attributed to the re-absorption of Eu3þ. Fig. 3(b) gives the excitation spectra (λem ¼ 613 nm) of BEy, E1.0 glasses and excitation spectra (λem ¼ 480 nm) of B2.0 glass. Compared with Eu3þ-doped glass, an additional excitation at 325–355 nm is found in Bi3þ/Eu3þ co-doped glasses, which can be assigned to the absorption of Bi3þ (1S0→3P1 transition). With augmenting Eu3þ content, the exci tation intensity at 333 nm increases, indicating that the energy transfer efficiency increases. Such phenomena are direct proof of energy transfer from Bi3þ to Eu3þ. The luminescence decay curves of Bi3þ emission at 480 nm present one more direct proof of energy transfer, which is evident from the
visual lifetime reduction (see Fig. 3(c)). The decay process is charac terized by the average lifetime τ, which can be calculated by Refs. [28–30], Z
,Z
∞
τ¼
IðtÞtdt 0
∞
IðtÞdt 0
(1)
I(t) is the emission intensity at time t. The average lifetimes obtained are 0.87, 0.87, 0.86, 0.84, 0.81, 0.80, 0.77, 0.75, 0.74, 0.71 μs for B2.0 and BEy (y ¼ 0.2, 0.3, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5) glasses, respec tively. It is assumed that the increase of Eu3þ concentration causes a shorter luminescent lifetime of Bi3þ emission. It agrees with the lumi nescent behavior of Bi3þ presented in Fig. 3(a) and it can be attributed to the energy transfer from Bi3þ to Eu3þ. 3
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The energy transfer efficiency η from Bi3þ to Eu3þ is calculated by using the following equation [14,31], � η ¼ 1 Iy Io (2) where I0 and Iy are the intensities of Bi3þ-doped and Bi3þ/Eu3þ co-doped glasses, respectively. The energy transfer efficiencies obtained are 31, 34, 43, 55, 67, 77, 81, 83, 86% for BEy (y ¼ 0.2, 0.3, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5) samples, respectively. The increase of Eu3þ concentration leads to the increase of energy transfer efficiency. 3.4. Analysis of energy transfer Usually, energy transfer mechanism can be identified using the Dexter’s theory, as displayed in the following equation [5],
η0 n3 ∝C η
(3)
η and η0 are the luminescence quantum efficiencies of Bi3þ in glasses
with and without Eu3þ ions, respectively, and C is the sum of all doped ions concentration. The type of interaction is characterized by the parameter n, n ¼ 6, 8 and 10 correspond to the interactions of dipoledipole (d-d), dipole-quadrupole (d-q), and quadrupole-quadrupole (qq), respectively. In general, the ratio η0/η can be changed for the related integrated emission intensity I0I with a good approximation. Following this reasoning, the relationship of I0/I vs Cn/3 is plotted in Fig. 4. When n ¼ 10, the best linear fit to the data is exhibited, implying that energy transfer from Bi3þ to Eu3þ in the prepared Bi3þ/Eu3þ co-doped glasses happens via quadrupole-quadrupole interaction. Fig. 3(d) presents the energy level scheme of Bi3þ and Eu3þ, and the possible energy transfer process from Bi3þ to Eu3þ in the obtained Bi3þ/ Eu3þ co-doped glasses. Initially, Bi3þ ions are excited from 1S0 ground state to 3P1 excited state under 333 nm light irradiation. Then, they turn back to 1S0 ground state to emit bluish-green luminescence peaked at 480 nm in Bi3þ-doped glass (see Fig. 1(b)). Due to the 3P1 excited level of Bi3þ is close to the 5D2 as well as other energy levels of Eu3þ. Therefore, excited electrons of Bi3þ relax to 1S0 ground state nonradiatively and transfers their energy to a nearby Eu3þ, facilitating the electrons on the excited states of Eu3þ going through multiphonon relaxation and then radiatively transfer to ground state, leading to red emission of Eu3þ in Bi3þ/Eu3þ co-doped glasses [9].
Fig. 5. CIE coordinates related to the emission (λex ¼ 333 nm) of B2.0 and BEy glasses and a series of digital photographs of BEy (y ¼ 0.3, 1.5, 3.5). Samples with y ¼ 0, 0.2, 0.3, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5 are marked as No. 1–10.
boosting Eu3þ content, the CIE color coordinates depict the obvious change of the emission color from bluish-green (0.265, 0.332) to orangered (0.595, 0.352), which correspond to B2.0 and BE3.5, respectively. This variation also be clearly observed from the photographs of BEy (y ¼ 0.3, 1.5, 3.5) glasses. Excitingly, pure white light emission (0.348, 0.333) is realized when irradiated by 333 nm light. Such behavior infers that the prepared glasses may be promising candidates for utilization in UV-converted W-LEDs. 3.6. Thermal quenching properties of BE0.3 glass It is vital to evaluate the thermal stability of luminescent glass at higher temperature, which has a considerable influence in the light output and color rendering index [32]. In Fig. 6(a), temperature dependent emission spectra of BE0.3 glass excited by 333 nm light are
3.5. CIE coordinates Fig. 5 displays the CIE coordinates of B2.0 and BEy (y ¼ 0.2, 0.3, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5) glasses under 333 nm light excitation. With
Fig. 4. Dependence of (I0/I) values versus (a) C6/3, (b) C8/3, (c) C10/3 in BEy (y ¼ 0.2, 0.3, 0.5, 1.0, 1.5) glasses. 4
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Fig. 6. (a) Temperature-dependent emission spectra (λex ¼ 333 nm) of BE0.3 glass, the inset presents the CIE coordinates at different temperatures; (b) Relative integrated emission intensity and chromaticity shift (ΔE) versus temperature for the BE0.3 glass from 299 to 540 K. (c) Plot of Ln (I0/IT-1) versus 1/kT for the BE0.3 glass and the linear fit of the data through equation (4).
interaction. By increasing Eu3þ concentration, tunable emission from bluish-green via white to orange-red is obtained under 333 nm light irradiation. Significantly, pure white light with CIE coordinates (0.348, 0.333) is realized. And the glass can maintain excellent color stability even at 423 K. All these above characteristics strongly suggest that Eu3þ/Bi3þ co-doped samples may be efficiently used in applications of W-LEDs pumped by UV-LED chip.
displayed in the temperature range from 299 to 540 K. With rising the temperature, the luminescence intensities of both Eu3þ and Bi3þ de creases properly because of the thermal quenching effect. Meanwhile, the positions of emission peaks are not changed. Fig. 6(b) shows the integrated intensity versus temperature in the range of 299–540 K. It can be noticed that at the temperature of 423 K, the integrated intensity still remains 45.8% of its initial value. The activation energy is evaluated by the Arrhenius equation [33], IðTÞ ¼ Io ½1 þ c expðΔEa =kTÞ�
1
Acknowledgments
(4)
This work was supported by NSFC (Grant No. 11974315 and No. 51802285) and the Natural Science Foundation of Zhejiang Province (Grant No. LZ20E020002).
where I0 is the initial intensity, T is the temperature in Kelvin, c is a fitting constant and k is the Boltzmann constant (8.629 � 10 5 eV K 1). From the slope of Ln (I0/IT-1) versus 1/kT as presented in Fig. 6(c), the activation energy calculated is about 0.254 eV. Besides, the color sta bility can be quantifiably depicted by the chromaticity shift (ΔE) using the next equation [34], qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ΔE ¼ ðut ’ u0 ’ Þ2 ðvt ’ v0 ’ Þ2 þ ðwt ’ w0 ’ Þ2 (5)
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jlumin.2019.116918. References
where u’ ¼ 4x/(3–2xþ12y), v’ ¼ 9y/(3–2xþ12y), and w’ ¼ 1-u’-v’. x and y are the chromaticity coordinates. The BE0.3 glass shows a relatively tiny chromaticity shift about 4.31 � 10 2 at 423 K. The CIE coordinates when excited by 333 nm light at different temperatures are exhibited in the inset of Fig. 6(a). Obviously, the luminescence color of the BE0.3 glass in the temperature from 299 to 423 K is located in white area. When the temperature further increases, the luminescence color grad ually moves to yellow region. This is assigned to strongly thermal quenching of the luminescence of Bi3þ. We can further improve the thermal stability of the prepared Bi3þ/Eu3þ-doped glass through opti mizing the glass composition.
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4. Conclusions The luminescent properties of Bi3þ/Eu3þ-doped glasses were sys tematically investigated. Emission and excitation spectra behavior as well as decay lifetime curves are strong evidences of energy transfer from Bi3þ to Eu3þ. The energy transfer efficiency reaches 86% and the probable mechanism is identified to be a quadrupole-quadrupole 5
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Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: http://www.elsevier.com/locate/jlumin
Energy transfer and white luminescence in Bi3þ/Eu3þ co-doped oxide glasses Olga Giraldo Giraldo, Mingzhi Fei, Rongfei Wei, Liming Teng, Zhigang Zheng, Hai Guo * Department of Physics, Zhejiang Normal University, Jinhua, Zhejiang, 321004, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Luminescent materials White light glasses Bi3þ Eu3þ Energy transfer
Bi3þ/Eu3þ-doped SiO2–Al2O3–CaO–B2O3–La2O3–K2O glasses were fabricated and the luminescent properties, energy transfer as well as thermal stability were systematically investigated. Under 333 nm light excitation, bluish-green luminescence centered at 480 nm is observed in the obtained Bi3þ-doped glass. By co-doped with Eu3þ, highly efficient energy transfer process happens from Bi3þ to Eu3þ via a quadrupole-quadrupole interac tion. The energy transfer efficiency can reach 86% and the concentration of Eu3þ is not quenched in here. With increasing Eu3þ concentration, tunable luminescence from bluish-green via white to orange-red is realized. Significantly, pure white light emission with CIE coordinates (0.348, 0.333) is obtained when excited by 333 nm light. Temperature-dependent emission spectra present superior color stability in operating temperature lower than 423 K for the prepared glasses. These results indicate that these glasses are potential white light emitting materials for organic-resin-free W-LEDs.
1. Introduction Studies on white light emission have brought a tremendous progress in solid state lighting technology. Nowadays, white light emitting diodes (W-LEDs) are a fundamental light source due to their energy-saving, high efficiency, low cost, environmental friendliness and technological advantages [1–6]. In addition, W-LEDs have a huge field of applications such as devices indicators, backlights, automobile headlights and gen eral illuminations, etc [3,7–10]. One of the popular means to obtain W-LEDs is through the combi nation of UV LEDs with white light-emitting phosphors [6,11–13]. However, the deterioration and yellowing of organic resins when working at the high current are its main drawbacks, which lead to degradation, shift of chromaticity and reduction of long-term reliability of W-LEDs [8,14,15]. To solve this problem, the substitution of phos phors byluminescence glasses was developed. Luminescence glasses exhibit excellent optical properties, good mechanical resistance and cheap production. And more importantly, they show an organic-resin free assembly process [8,16]. Bi3þ is a famous activator ion in luminescent materials. Its emission comes from 3P1→1S0 transition (that is 6sp to 6s2 transition). Due to its outer 6s2 and 6sp electronic configurations, Bi3þ exhibits luminescence in different colors, which strongly depends on the type of host [17,18].
Besides, Bi3þ can be used as sensitizer also because of the spectral overlap of emission spectra of Bi3þ and excitation spectra of other ac tivators. Eu3þ ion is a popular activator in red luminescent phosphors. And it can be sensitized by Bi3þ ion due to energy transfer process [19, 20]. Many investigations on energy transfer in Bi3þ/Eu3þ co-doped phosphors have been reported [9,21,22]. However, to the best of our knowledge, there were few researches on the Bi3þ/Eu3þ co-doped glasses [8,23]. Herein, oxide glass with the composition of SiO2 –Al2O3–CaO–B2O3–La2O3–K2O was chosen as host glass due to its high transparence, good solubility for doping, as well as physical and chemical stability [24]. The luminescence behaviors, energy transfer from Bi3þ to Eu3þ and thermal quenching property in the prepared Bi3þ/Eu3þ-doped glasses were studied in detail. The spectra present that Bi3þ-doped glass emits bluish-green light upon 333 nm light irradiation. Via introducing Eu3þ, tunable emission from bluish-green to orange-red is acquired due to the energy transfer from Bi3þ to Eu3þ. The mechanism was confirmed to be quadrupole-quadrupole interaction. Especially, pure white light emission with CIE coordinates (0.348, 0.333) is real ized. And the excellent color stability was identified by the temperature-dependent emission spectra.
* Corresponding author. E-mail address: [email protected] (H. Guo). https://doi.org/10.1016/j.jlumin.2019.116918 Received 19 July 2019; Received in revised form 8 November 2019; Accepted 23 November 2019 Available online 25 November 2019 0022-2313/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Olga Giraldo Giraldo, Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2019.116918
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and ground state of Bi3þ get closer, which produce the red-shift of the absorption edge. Besides, the doping of Bi2O3 may effectively expand the glass network, which results in more nonbridged oxygen ions and thus gives rise to the red-shift of the absorption edge [8]. Fig. 1(b) shows the excitation and emission spectra of Bx glasses. There is no emission from Host glass (not shown here). Under the excitation of 333 nm, Bx glasses exhibit broadband bluish-green emis sion from 380 to 700 nm with a maximum at 480 nm, which is origi nated from 3P1→1S0 transition of Bi3þ. With increasing Bi3þ concentration, the emission intensity of Bx glasses firstly increases until reach maximum at x ¼ 2, and then decreases due to the concentration quenching effect. The excitation spectra (λem ¼ 480 nm) of Bx glasses in Fig. 1(b) exhibits broadband from 250 to 410 nm with a maximum at 333 nm, which is attributed to 1S0→3P1 transition of Bi3þ. With growing Bi3þ content, the excitation bands show an obvious red-shift, which is consistent with the red-shift of absorption spectra in Fig. 1(a).
2. Experimental Glass with nominal composition of 20SiO2 –5Al2O3–20CaO–30B2O3–15La2O3–10K2O (in mol%) was prepared by melt-quenching method and denoted as Host. Host doped with xBi2O3 (x ¼ 0.5, 1.0, 1.5, 2.0, 2.5), 1.0Eu2O3 as well as 2.0Bi2O3 and yEu2O3 (y ¼ 0.2, 0.3, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5) were named as Bx, E1.0, and BEy, respectively. Here, CaCO3 and K2CO3 are the sources of CaO and K2O, respectively. Raw materials containing SiO2, Al2O3, CaCO3 and K2CO3 (A.R., Sinopharm Chemical reagents), B2O3 (A.R., Macklin Biochemical reagents), La2O3 and Eu2O3 (99.99%, Shanghai Yuelong Nonferrous Metals), and Bi2O3 (99.99%, Aladdin reagents) were weighted according to the molar composition of glasses and then they were mixed for 40 min in an agate mortar. Subsequently, they were melted at 1450 � C for 1 h in a corundum crucible in air atmosphere. The glasses were obtained by quenching the melt on a 300 � C preheated brass plate. At last, all glasses were sliced and polished with a thickness of 1.5 mm for further optical measurements. Transmittance spectra were obtained using a Hitachi U-3900 ultra violet–visible (UV–Vis) spectrophotometer. Photoluminescence emis sion and excitation spectra were measured using an Edinburgh FS5 spectrofluorometer (Livingston, UK) equipped with a continuous wave 150 W Xe lamp. TCB1402C temperature controller (China) was added to FS5 spectrofluorometer to measure the temperature dependent emission spectra from 299 to 540 K. The decay lifetime curves were measured in a double monochromator (Jobin-Yvon HRD1) under the excitation of tunable laser (Opolette system) with wavelength tuning range of 410–2200 nm, spectral linewidth of 4–7 cm 1, pulse duration of 7 ns and repetition rate of 20 Hz, finally it was recorded by a TDS2024 digital oscilloscope (Tektronix). Except the temperature dependent spectra, all the measurements were carried out at room temperature.
3.2. Photoluminescent properties of E1.0 glass Fig. 2(a) displays the excitation and emission spectra of E1.0 glass in the range from 200 to 750 nm. The excitation spectra (λem ¼ 613 nm) of Eu3þ are characterized for six sharp excitation peaks centered at 363, 382, 393, 413, 464 and 529 nm, arising from 7F0 to 5D4, 5G4, 5L6, 5D3, 5 D2 and 5D1 transitions, respectively. And a broad band centered at around 270 nm is related to the Eu3þ-O2- charge transfer. The emission spectra of Eu3þ exhibit five red characteristic peaks at 580, 591, 613, 653, and 703 nm, which are attributed to the intrinsic 4f-4f transition, that is, 5D0→7FJ (J ¼ 0–4) transitions of Eu3þ. Fig. 2(b) shows the existence of a strong overlap between the emis sion spectra of B2.0 glass and the excitation spectra of E1.0 glass. The spectral overlap between 350 and 550 nm strongly suggests that energy transfer from Bi3þ to Eu3þ can occur easily.
3. Results and discussion 3.1. Photoluminescent properties of Bx glasses
3.3. Photoluminescent properties of BEy glasses
In order to investigate the luminescent behavior of Bi3þ ions, the host is doped with different concentrations of Bi3þ. Fig. 1(a) gives the transmittance spectra of Bx (x ¼ 0.5, 1.0, 1.5, 2.0, 2.5) glasses. All samples are highly transparent (over 80%) in the visible region. High transparency is beneficial to practical applications. Compared to host glass, Bx glasses present a clear red-shift from 320 to 360 nm in ab sorption edge, which comes from characteristic 1S0→3P1 transition of Bi3þ ions [25,26]. With augmenting Bi3þ content, it is obviously observed that the absorption edge moves to a longer wavelength. This red-shift may be interpreted as follows [27]: As Bi3þ concentration grows, the distance between Bi3þ ions is compressed and then the overlap of the excited state of Bi3þ is increased, making the excited state
With the purpose to investigate the energy transfer process and to generate white light emission, Eu3þ was introduced in B2.0 glass as a red emitter. Luminescent behavior of BEy samples shows that the efficiency of energy transfer from Bi3þ to Eu3þ highly depends on Eu3þ concentration. Fig. 3(a) presents the emission spectra (λex ¼ 333 nm) of B2.0, E1.0 and BEy glasses. As shown in Figs. 1(b) and Figure 2 (b), Bi3þ can be excited efficiently under the excitation of 333 nm, but Eu3þ cannot. However, emission spectra of BEy glasses not just show bluish-green emission from Bi3þ, but also exhibit red emission from Eu3þ. Then, with increasing Eu3þ content, the emission intensity of Bi3þ decreases while the emission intensity of Eu3þ increases. Such behavior suggests that Eu3þ can be excited through energy transfer process from Bi3þ to
Fig. 1. (a) UV–Vis transmission spectra of Bx glasses and Host; (b) Emission spectra (λex ¼ 333 nm) and excitation spectra (λem ¼ 480 nm) of Bx glasses. 2
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Fig. 2. (a) Excitation spectra (λem ¼ 613 nm) and emission spectra (λex ¼ 393 nm) of E1.0 glass; (b) Overlap between excitation spectra (λem ¼ 613 nm) of E1.0 glass and emission spectra (λex ¼ 333 nm) of B2.0 glass.
Fig. 3. (a) Emission spectra (λex ¼ 333 nm) of BEy, E1.0 and B2.0 glasses, (b) excitation spectra (λem ¼ 613 nm) of BEy, E1.0 glasses and excitation spectra (λem ¼ 480 nm) of B2.0 glass; (c) Luminescence decay curves at 480 nm emission of Bi3þ in B2.0 and BEy glasses. (d) Energy level scheme of Bi3þ and Eu3þ, and energy transfer process.
Eu3þ. The quenching concentration of Eu3þ is not found in this work. Additionally, it is noticed that the inset spectra in Fig. 3(a) shows an obvious dip located at 464 nm in the emission band of Bi3þ, which is attributed to the re-absorption of Eu3þ. Fig. 3(b) gives the excitation spectra (λem ¼ 613 nm) of BEy, E1.0 glasses and excitation spectra (λem ¼ 480 nm) of B2.0 glass. Compared with Eu3þ-doped glass, an additional excitation at 325–355 nm is found in Bi3þ/Eu3þ co-doped glasses, which can be assigned to the absorption of Bi3þ (1S0→3P1 transition). With augmenting Eu3þ content, the exci tation intensity at 333 nm increases, indicating that the energy transfer efficiency increases. Such phenomena are direct proof of energy transfer from Bi3þ to Eu3þ. The luminescence decay curves of Bi3þ emission at 480 nm present one more direct proof of energy transfer, which is evident from the
visual lifetime reduction (see Fig. 3(c)). The decay process is charac terized by the average lifetime τ, which can be calculated by Refs. [28–30], Z
,Z
∞
τ¼
IðtÞtdt 0
∞
IðtÞdt 0
(1)
I(t) is the emission intensity at time t. The average lifetimes obtained are 0.87, 0.87, 0.86, 0.84, 0.81, 0.80, 0.77, 0.75, 0.74, 0.71 μs for B2.0 and BEy (y ¼ 0.2, 0.3, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5) glasses, respec tively. It is assumed that the increase of Eu3þ concentration causes a shorter luminescent lifetime of Bi3þ emission. It agrees with the lumi nescent behavior of Bi3þ presented in Fig. 3(a) and it can be attributed to the energy transfer from Bi3þ to Eu3þ. 3
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The energy transfer efficiency η from Bi3þ to Eu3þ is calculated by using the following equation [14,31], � η ¼ 1 Iy Io (2) where I0 and Iy are the intensities of Bi3þ-doped and Bi3þ/Eu3þ co-doped glasses, respectively. The energy transfer efficiencies obtained are 31, 34, 43, 55, 67, 77, 81, 83, 86% for BEy (y ¼ 0.2, 0.3, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5) samples, respectively. The increase of Eu3þ concentration leads to the increase of energy transfer efficiency. 3.4. Analysis of energy transfer Usually, energy transfer mechanism can be identified using the Dexter’s theory, as displayed in the following equation [5],
η0 n3 ∝C η
(3)
η and η0 are the luminescence quantum efficiencies of Bi3þ in glasses
with and without Eu3þ ions, respectively, and C is the sum of all doped ions concentration. The type of interaction is characterized by the parameter n, n ¼ 6, 8 and 10 correspond to the interactions of dipoledipole (d-d), dipole-quadrupole (d-q), and quadrupole-quadrupole (qq), respectively. In general, the ratio η0/η can be changed for the related integrated emission intensity I0I with a good approximation. Following this reasoning, the relationship of I0/I vs Cn/3 is plotted in Fig. 4. When n ¼ 10, the best linear fit to the data is exhibited, implying that energy transfer from Bi3þ to Eu3þ in the prepared Bi3þ/Eu3þ co-doped glasses happens via quadrupole-quadrupole interaction. Fig. 3(d) presents the energy level scheme of Bi3þ and Eu3þ, and the possible energy transfer process from Bi3þ to Eu3þ in the obtained Bi3þ/ Eu3þ co-doped glasses. Initially, Bi3þ ions are excited from 1S0 ground state to 3P1 excited state under 333 nm light irradiation. Then, they turn back to 1S0 ground state to emit bluish-green luminescence peaked at 480 nm in Bi3þ-doped glass (see Fig. 1(b)). Due to the 3P1 excited level of Bi3þ is close to the 5D2 as well as other energy levels of Eu3þ. Therefore, excited electrons of Bi3þ relax to 1S0 ground state nonradiatively and transfers their energy to a nearby Eu3þ, facilitating the electrons on the excited states of Eu3þ going through multiphonon relaxation and then radiatively transfer to ground state, leading to red emission of Eu3þ in Bi3þ/Eu3þ co-doped glasses [9].
Fig. 5. CIE coordinates related to the emission (λex ¼ 333 nm) of B2.0 and BEy glasses and a series of digital photographs of BEy (y ¼ 0.3, 1.5, 3.5). Samples with y ¼ 0, 0.2, 0.3, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5 are marked as No. 1–10.
boosting Eu3þ content, the CIE color coordinates depict the obvious change of the emission color from bluish-green (0.265, 0.332) to orangered (0.595, 0.352), which correspond to B2.0 and BE3.5, respectively. This variation also be clearly observed from the photographs of BEy (y ¼ 0.3, 1.5, 3.5) glasses. Excitingly, pure white light emission (0.348, 0.333) is realized when irradiated by 333 nm light. Such behavior infers that the prepared glasses may be promising candidates for utilization in UV-converted W-LEDs. 3.6. Thermal quenching properties of BE0.3 glass It is vital to evaluate the thermal stability of luminescent glass at higher temperature, which has a considerable influence in the light output and color rendering index [32]. In Fig. 6(a), temperature dependent emission spectra of BE0.3 glass excited by 333 nm light are
3.5. CIE coordinates Fig. 5 displays the CIE coordinates of B2.0 and BEy (y ¼ 0.2, 0.3, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5) glasses under 333 nm light excitation. With
Fig. 4. Dependence of (I0/I) values versus (a) C6/3, (b) C8/3, (c) C10/3 in BEy (y ¼ 0.2, 0.3, 0.5, 1.0, 1.5) glasses. 4
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Fig. 6. (a) Temperature-dependent emission spectra (λex ¼ 333 nm) of BE0.3 glass, the inset presents the CIE coordinates at different temperatures; (b) Relative integrated emission intensity and chromaticity shift (ΔE) versus temperature for the BE0.3 glass from 299 to 540 K. (c) Plot of Ln (I0/IT-1) versus 1/kT for the BE0.3 glass and the linear fit of the data through equation (4).
interaction. By increasing Eu3þ concentration, tunable emission from bluish-green via white to orange-red is obtained under 333 nm light irradiation. Significantly, pure white light with CIE coordinates (0.348, 0.333) is realized. And the glass can maintain excellent color stability even at 423 K. All these above characteristics strongly suggest that Eu3þ/Bi3þ co-doped samples may be efficiently used in applications of W-LEDs pumped by UV-LED chip.
displayed in the temperature range from 299 to 540 K. With rising the temperature, the luminescence intensities of both Eu3þ and Bi3þ de creases properly because of the thermal quenching effect. Meanwhile, the positions of emission peaks are not changed. Fig. 6(b) shows the integrated intensity versus temperature in the range of 299–540 K. It can be noticed that at the temperature of 423 K, the integrated intensity still remains 45.8% of its initial value. The activation energy is evaluated by the Arrhenius equation [33], IðTÞ ¼ Io ½1 þ c expðΔEa =kTÞ�
1
Acknowledgments
(4)
This work was supported by NSFC (Grant No. 11974315 and No. 51802285) and the Natural Science Foundation of Zhejiang Province (Grant No. LZ20E020002).
where I0 is the initial intensity, T is the temperature in Kelvin, c is a fitting constant and k is the Boltzmann constant (8.629 � 10 5 eV K 1). From the slope of Ln (I0/IT-1) versus 1/kT as presented in Fig. 6(c), the activation energy calculated is about 0.254 eV. Besides, the color sta bility can be quantifiably depicted by the chromaticity shift (ΔE) using the next equation [34], qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ΔE ¼ ðut ’ u0 ’ Þ2 ðvt ’ v0 ’ Þ2 þ ðwt ’ w0 ’ Þ2 (5)
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jlumin.2019.116918. References
where u’ ¼ 4x/(3–2xþ12y), v’ ¼ 9y/(3–2xþ12y), and w’ ¼ 1-u’-v’. x and y are the chromaticity coordinates. The BE0.3 glass shows a relatively tiny chromaticity shift about 4.31 � 10 2 at 423 K. The CIE coordinates when excited by 333 nm light at different temperatures are exhibited in the inset of Fig. 6(a). Obviously, the luminescence color of the BE0.3 glass in the temperature from 299 to 423 K is located in white area. When the temperature further increases, the luminescence color grad ually moves to yellow region. This is assigned to strongly thermal quenching of the luminescence of Bi3þ. We can further improve the thermal stability of the prepared Bi3þ/Eu3þ-doped glass through opti mizing the glass composition.
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4. Conclusions The luminescent properties of Bi3þ/Eu3þ-doped glasses were sys tematically investigated. Emission and excitation spectra behavior as well as decay lifetime curves are strong evidences of energy transfer from Bi3þ to Eu3þ. The energy transfer efficiency reaches 86% and the probable mechanism is identified to be a quadrupole-quadrupole 5
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