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Tunable luminescence of blue-green emitting NaBaBO3:Ce3+,Tb3+ phosphors for near-UV light emitting diodes
Journal of Luminescence 220 (2020) 116957
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Journal of Luminescence journal homepage: http://www.elsevier.com/locate/jlumin
Tunable luminescence of blue-green emitting NaBaBO3:Ce3þ,Tb3þ phosphors for near-UV light emitting diodes Menglong Xia , Weiren Zhao *, Jiyou Zhong , Peng Shi , Zifeng Liao , Xiang Liu , Jingzhou Song , Li Luo , Lin Ma , Zhaogang Nie School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Guangzhou, 510006, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Phosphors Ce3þ/Tb3þ co-doped Energy transfer Quantum efficiency Tunable luminescence White LEDs
A series of blue-green emitting NaBaBO3:Ce3þ, Tb3þ phosphors were synthesized via conventional high tem perature solid-state reaction method. The crystal structure, luminescence properties, energy transfer processes and the properties of the packing LED device were investigated. The phosphors present tunable luminescence in the blue-green region due to the energy transfer from Ce3þ to Tb3þ. The internal quantum efficiency (IQE) and external quantum efficiency (EQE) of the composition optimized NaBaBO3:0.01Ce3þ, 0.03 Tb3þ phosphor reached 57% and 36%, respectively. Using a 365 nm NUV LED chip combining with NaBaBO3: 0.01Ce3þ, 0.03 Tb3þ phosphor and CaAlSiN3:Eu2þ red phosphor, a high quality white light was obtained with CIE chromaticity coordinates of (0.316, 0.307), CCT of 6473 K, and CRI of 81.4. These results demonstrated that NaBaBO3:Ce3þ, Tb3þ phosphors is a promising blue-green phosphor for NUV white LEDs.
1. Introduction Unlike the commercial WLEDs using blue LED chips with yellow Y3Al5O12:Ce3þ (YAG:Ce3þ) phosphors, combining tricolor (red, green, blue) phosphors with near ultraviolet (NUV) LED chips can present more soft and uniform white light with tunable color properties [1–8]. Currently, the commercially available NUV excitable tricolor phosphors are Sr2Si5N8:Eu2þ red-emitting phosphor [9], β-SiAlON:Eu2þ green-emitting phosphor [10], and BaMgAl10O17:Eu2þ blue-emitting phosphor [11]. Generally, the nitrides phosphors are expensive due to its harsh synthesis conditions and expensive raw materials [12–16]. Therefore, it is attractive to develop oxides phosphors with low synthesis temperature and sufficient luminescence efficiency. In the design of phosphors, Tb3þ is always adopt as an activator of green-emitting materials due to the dominant 5D4-7F5 transition peak at about 545 nm. However, the intensities of absorption and emission peaks in the NUV region are very weak and narrow due to the forbidden 4f 4f transitions of Tb3þ [17–22]. As an effective sensitizer for Tb3þ ions, Ce3þ has been widely used in many hosts. Ce3þ-Tb3þ co-doped phosphors could enhance the luminescence intensity of Tb3þ by en ergy transfer from sensitizer to activator and enhance blue and green emission [23–29]. Recently, orthoborates phosphors have attracted much attention
among multitudinous phosphors because of their merits of low synthetic temperature, remarkable chemical stability and excellent luminescent properties [30,31]. Typically, in our previous work, we found a very interesting phenomenon that when the concentration of Ce3þ ions is different, we can obtain green-emitting and blue-emitting NaBaBO3: Ce3þ phosphors respectively. It is reported that the distinctive green emission in this borate may stem from Ce3þ occupying the Naþ crys tallographic sites whereas the more natural blue emission should stem from Ce3þ occupying the Ba2þ site. The internal quantum efficiency (IQE) of the blue-emitting monoclinic NaBaBO3:Ce3þ (λex ¼ 360 nm) reached 65% [32,33]. Additionally, Zheng et al., reported an intense green emission in NaBaBO3 phosphors singly doped with Tb3þ, indi cating that NaBaBO3 is an excellent host for lanthanide doping [22]. In this work, only blue emission was observed in NaBaBO3:Ce3þ, and basing on this, Ce3þ-Tb3þ co-doped NaBaBO3 phosphors were carefully researched. It is found that a tunable emission from blue to green can be realized through the change of Tb3þ doping content under NUV exci tation. With the internal quantum efficiency high as 57%, the phosphors show the potential application in the White LEDs with UNV chips. 2. Experimental Synthesis. NaBaBO3:0.01Ce3þ, xTb3þ (x ¼ 0, 0.01, 0.03, 0.05, 0.07,
* Corresponding author., E-mail address: [email protected] (W. Zhao). https://doi.org/10.1016/j.jlumin.2019.116957 Received 1 September 2019; Received in revised form 25 November 2019; Accepted 8 December 2019 Available online 17 December 2019 0022-2313/© 2019 Elsevier B.V. All rights reserved.
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Journal of Luminescence 220 (2020) 116957
Fig. 3. Diffuse reflectance spectra of NaBaBO3 host (a), NaBaBO3:0.01Ce3þ (b), and NaBaBO3:0.01Ce3þ,0.03 Tb3þ (c) phosphors. Fig. 1. XRD patterns of the NaBaBO3:0.01Ce3þ (a) and NaBaBO3:0.01Ce3þ, 0.03 Tb3þ (b) and the standard data of NaBaBO3 (ICSD 80110).
Fig. 4. The PL spectra (λex ¼ 355 nm) of NaBaBO3:0.01Ce3þ, xTb3þ (x ¼ 0, 0.01, 0.03, 0.05, 0.07, 0.09) phosphors.
Characterization. The X-ray diffraction (XRD) patterns of the samples were obtained by using an Ulitima Ⅳ X-ray diffractometer (XRD) with Cu Kα (λ ¼ 1.5406 Å) radiation at 36 kV and 21 mA. The diffuse reflection spectra were collected by Evolution 220 (Thermo Scientific) and used BaSO4 as a reference standard. The photoluminescence exci tation (PLE), photoluminescence (PL) spectra and its decay curves of the phosphors were recorded using an FLS980 spectrofluorimeter (Edin burgh Instruments, UK, Edinburgh). The Internal Quantum Efficiencies (IQEs) were measured by QE-2100 quantum yield measurement system (Otsuka Electronics Co., Ltd, Japan), with BaSO4 powder as a reference and a Xe lamp as an excitation source. The WLED device was fabricated using the as-prepared phosphor with a 365 nm UV LED chip, and its emission spectrum was obtained using an ATA-500 Sync-Skan color analyzer system under a forward bias of 20 mA.
Fig. 2. PLE (left) and PL (right) spectra of NaBaBO3:0.01Ce3þ (a), NaBaBO3:0.03 Tb3þ (b), and NaBaBO3:0.01Ce3þ, 0.03 Tb3þ (c).
and 0.09) phosphors were synthesized by a high-temperature solid-state reaction method. The raw materials were Na2CO3 (Aldrich, 99.95%), BaCO3 (Aldrich, 99.9%), H3BO3 (Aldrich, 99.5%), CeO2 (Aldrich, 99.99%), and Tb4O7 (Aldrich, 99.9%). The powers were weighed based on the stoichiometric proportion, except for H3BO3 with excess of 3 mol % to compensate for high-temperature evaporation. The materials were thoroughly grounded and mixed in an agate mortar for 40 min, then placed it into an alumina crucible. The mixture was first heated to 650 � C for 4 h in air to decompose BaCO3 and Na2CO3. The obtained powder was reground after cooled down to the room temperature, and then heated to 760 � C for 5 h in a flowing reducing atmosphere (5% H2/95% N2). Thus, the as-prepared phosphors were obtained.
3. Results and discussion Fig. 1 shows the XRD patterns of NaBaBO3:0.01Ce3þ and 2
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Journal of Luminescence 220 (2020) 116957
two Gaussian bands with peaks at 403 nm (24814 cm 1) and 435 nm (22989 cm 1). The energy difference is ~ 1825 cm 1, which is close to the theoretical difference of ~2000 cm 1 between the 2F5/2 and 2F7/2 levels [37]. Fig. 2b shows the PLE and PL spectra of NaBaBO3:0.03 Tb3þ. The excitation spectrum monitoring at 545 nm clearly showed two intense band ranging from 220 to 300 nm with maxima at 242 and 282 nm, along with some relatively weak narrow peaks in the range of 300–400 nm. The former intense bands can be assigned to the spin allowed 4f8-4f7 5d transitions of Tb3þ, and the weak peaks can be ascribed to the forbidden absorption f-f transition of Tb3þ [19,38,39]. The emission spectrum under the excitation of 245 nm is consisted of several peaks centered at 486, 545, 581, 622 nm due to the 5D4-7FJ (J ¼ 6, 5, 4, and 3) transitions of Tb3þ ions. Comparing Fig. 2a and b, it can be seen that there is partial overlap between the emission band of Ce3þ and the excitation band of Tb3þ, providing the probability of energy transfer between Ce3þ and Tb3þ ions through non-radiative energy transfer [40]. Fig. 2c showed the excitation and emission spectra of NaBaBO3:0.01Ce3þ, 0.03 Tb3þ phosphor. Comparing the PLE spectra monitored at 415 nm (Ce3þ 5d-4f emission) and that monitored at 545 nm (Tb3þ 5D4-7F5 emission), obviously, the excitation spectra monitored at 545 nm (Tb3þ emission) presents the typical absorption of Ce3þ, which demonstrate the occurrence of energy transfer from Ce3þ to Tb3þ. The PL spectra of NaBaBO3:0.01Ce3þ, 0.03 Tb3þ under 355 nm excita tion shows the emission characteristics of both Ce3þ and Tb3þ. Fig. 3 showed the diffuse reflectance spectra of NaBaBO3 host (a), NaBaBO3:0.01Ce3þ (b), and NaBaBO3:0.01Ce3þ,0.03 Tb3þ (c) phos phors. The NaBaBO3 host shows energy absorption at the wavelength that less than 245 nm, while having no absorption in the NUV region. However, Fig. 3b illustrated that NaBaBO3:0.01Ce3þ sample had three distinct broad absorption bands in the 250–400 nm NUV region, which can attribute to the 4f-5d transition of Ce3þ. It can also be seen from Fig. 3 that the absorption band of NaBaBO3:0.01Ce3þ,0.03 Tb3þ is quite similar with that of NaBaBO3:0.01Ce3þ except for the relative intensity. The peak position of absorption band is well coincided with that of the PLE spectrum (λem ¼ 545 nm) of NaBaBO3: 0.01Ce3þ, 0.03 Tb3þ in Fig. 2c. The strongest excitation wavelength of Ce3þ - Tb3þ co-doped NaBaBO3 well matches with NUV chips. Fig. 4 shows the PL spectra of NaBaBO3:0.01Ce3þ, xTb3þ (x ¼ 0, 0.01, 0.03, 0.05, 0.07. 0.09) phosphors under the excitation of 355 nm. As presented, a blue emission band for 5d-4f transition of the Ce3þ ions and a series of strong emission lines at 486, 545, 581, 622 nm due to the 5 D4-7FJ (J ¼ 6, 5, 4, and 3) transitions of Tb3þ ions can be observed. The emission intensity of Ce3þ gradually increases with the increasing of Tb3þ concentration and reach the maximum at x ¼ 0.03, then decreases due to the Tb3þ-Tb3þ internal concentration quench. The internal quantum efficiency (IQE) and external quantum efficiency (EQE) of NaBaBO3:0.01Ce3þ, 0.03 Tb3þ excited at 355 nm was 57% and 36%, respectively, lower than the blue-emitting NaBaBO3:Ce3þ (IQE ¼ 65%, EQE ¼ 46% under 355 nm excitation). The PL decay curves obtained by monitoring the Ce3þ emission at 415 nm in NaBaBO3:0.01Ce3þ, xTb3þ are presented in Fig. 5. It can be found that all decay curves can be well fitted by the single exponential function [41].
Fig. 5. Photoluminescence decays curves of Ce3þ by monitoring the Ce3þ emission at 415 nm in NaBaBO3:0.01Ce3þ, xTb3þ phosphors (x ¼ 0–0.09) upon excitation at 355 nm.
Fig. 6. The dependence of the Ce3þ emission, Tb3þ emission, and energy transfer efficiency of Ce3þ-Tb3þ on the concentration of Tb3þ for NaBaBO3:0.01Ce3þ, xTb3þ phosphors.
NaBaBO3:0.01Ce3þ, 0.03 Tb3þ samples. All diffraction peaks of the samples can be indexed to the standard NaBaBO3 card (ICSD No. 80110), indicating no significant impurities detected and no significant change in the host structure with the addition of dopant ions. The re ported lattice constants of NaBaBO3 are a ¼ 9.561 Å, b ¼ 5.557 Å, c ¼ 6.179 Å, V ¼ 324.4 Å3 and Z ¼ 4 [34]. Based on our XRD data, the lattice constants of NaBaBO3:0.01Ce3þ, 0.03 Tb3þ are a ¼ 9.558 Å, b ¼ 5.564 Å, c ¼ 6.181 Å, and V ¼ 324.7 Å3 calculated by the Jade program. The results show excellent agreement (<1% difference) with the reported data. Fig. 2a illustrates the PLE monitoring at 415 nm and PL spectra excited at 355 nm for NaBaBO3:0.01Ce3þ phosphor. The PLE spectrum shows two broad excitation bands with peaks located at 298 and 355 nm, respectively, which can be assigned to 4f-5d transition of Ce3þ [35]. The PL spectrum showed a blue emission band covering 365–500 nm with a peak at around 415 nm, which is a typical double-band emission of Ce3þ from the 5d excited state to the 2F5/2 and 2F7/2 ground states [36]. As showed in Fig. 2a, the emission band can be decomposed into
IðtÞ ¼ I0 þ Aeð
t=τÞ
(1)
where I(t) and I0 are the luminous intensity at time t and 0, A is a con stant, τ is the lifetime. The lifetime τ were calculated to be 34.53, 34.19, 34.03, 33.58, 32.86 and 32.74 ns for x ¼ 0, 0.01, 0.03, 0.05, 0.07 and 0.09 respectively. Obviously, lifetime τ decreases with the increase of Tb3þ concentration, which verify solidly the existence of the energy transfer from Ce3þ ions to Tb3þ ions. The energy transfer efficiency ηT can be calculated by the following equation [42]:
ηET ¼ 1
τS =τSO
(2)
where τs and τso stand for the PL lifetimes of the sensitizer Ce3þ ions with 3
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Journal of Luminescence 220 (2020) 116957
Fig. 7. The dependence of IS0/IS of Ce3þ on (a) C(Ce3þþTb3þ), (b) C(Ce3þþTb3þ)6/3, (c) C(Ce3þþTb3þ 8/3 and (d) C(Ce3þþTb3þ)10/3, the red line denotes the corresponding ) linear fit to the scattering data points.
and without the presence of the activator Tb3þ ions, respectively. The dependence of energy transfer efficiency ηT, together with the relative emission intensity of Ce3þ(4f-5d) and Tb3þ (5D4-7F5), on the concen tration of Tb3þ for NaBaBO3:0.01Ce3þ,xTb3þ phosphors are depicted in Fig. 6. The emission intensity of Ce3þ decreased monotonically due to the enhanced energy transfer from Ce3þ ions to Tb3þ ions, while the emission intensity of Tb3þ has a maximal value at x ¼ 0.03. The decease of Tb3þ emission when x > 0.03 can be ascribed to the Tb3þ-Tb3þ in ternal concentration quenching. The energy transfer efficiency ηT gradually increased with the increase of Tb3þ concentration. However, the ηT is only 5.2%, which is consistent with the relatively small spectra overlap between Ce3þ emission and Tb3þ excitation. The critical distance RCe-Tb between Ce3þ and Tb3þ with the con centration quenching method can be calculated by the following equa tion given by Blasse [43]: RCe
Tb
� �1=3 3V ¼2 4π xc N
calculated to be 12.46 Å when xc is about 0.08 according to eqn (3), which is much greater than 5 Å required for the exchange interaction. Therefore, the energy transfer from Ce3þ to Tb3þ ions in the NaBaBO3
(3)
V is the volume of the unit cell, N is the number of host cations in the unit cell. In the NaBaBO3 unit cell, the values of V and N are approxi mately 324.385 Å3 and 4. xc is the total concentration of Ce3þ and Tb3þ at which the luminescence intensity of Ce3þ reduces to half of that for the sample in the absence of Tb3þ. The critical distance RCe-Tb was Table 1 CIE chromaticity coordinates of NaBaBO3:xCe3þ,yTb3þ. Sample no.
Composition x, y
CIE (u, v)
1 2 3 4 5 6 7
0.01, 0 0.01, 0.01 0.01, 0.03 0.01, 0.05 0.01, 0.07 0.01, 0.09 0, 0.03
(0.157,0.064) (0.183,0.141) (0.213,0.223) (0.209,0.208) (0.179,0.122) (0.174,0.105) (0.316,0.584)
Fig. 8. Chromaticity coordinates of NaBaBO3:0.01Ce3þ, yTb3þ (y ¼ 0–0.09), NaBaBO3:0.03 Tb3þ, inset showed the digital photos of the samples under 360 nm UV lamp excitation. 4
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Journal of Luminescence 220 (2020) 116957
Fig. 9. (a) Electroluminescence spectrum of a white LED fabricated by a 365 nm GaN chip with NaBaBO3:0.01Ce3þ,0.03 Tb3þ and CaAlSiN3:Eu2þ; (b) the CIE coordinate of the fabricated white LED.
takes place via electric multipole interaction. According to Dexter’s energy transfer formula for multipolar in teractions, a relationship can be given as following [44,45]:
η0 IS0 n=3 � ∝C ηS IS ðCe3þ þTb3þ Þ
NUV, the emission color of the phosphors can be tuned from blue (0.157,0.064) to blue-greenish (0.213,0.223) through controlling Tb3þ concentrations. The IQE of NaBaBO3:0.01Ce3þ,0.03 Tb3þ reaches 57%. Taking this blue-green phosphor and CaAlSiN3:Eu2þ red phosphor, combining with a 365 nm NUV LED chip, A white LED was fabricated and shows the CIE chromaticity coordinates (0.316, 0.307) and a CCT 6473 K, a CRI 81.4. These results indicate that NaBaBO3:Ce3þ,Tb3þ phosphors have the potential to be used as blue-green phosphor for NUV excited white LEDs.
(4)
where η0 and ηS are the luminescence quantum efficiencies of Ce3þ in the absence and presence of Tb3þ, respectively, the values of ηS/η0 can be estimated approximately by the ratio of relative luminescence peak in 3þ 3þ tensity ratio (IS0/IS). C3þ and (CeþTb) is the total concentration of the Ce 3þ Tb ions. n ¼ 6, 8, and 10 corresponds to dipole-dipole, dipole-quad rupole, and quadrupole-quadrupole interactions, respectively. The re n/33þ 3þ 3þ lationships between IS0/IS and C3þ (CeþTb) (CeþTb) as well as IS0/IS and C are showed in Fig. 7(a-d), n ¼ 6 presented a higher fitting factor of R2 value, thus the energy transfer from Ce3þ to Tb3þ realizes through dipole dipole interaction.
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal re lationships which may be considered as potential competing interests:
The CIE chromaticity coordinates of NaBaBO3:0.01Ce3þ, xTb3þ phosphors with different dopant contents under 355 nm excitation were measured and listed in Table 1, as depicted in Fig. 8. As shown, the color can be tuned from blue (0.157,0.064) to blue-greenish (0.213,0.223) by controlling the concentration of Tb3þ. The digital photographs of NaBaBO3:0.01Ce3þ, xTb3þ phosphors under 360 nm UV lamp are depicted in the inset of Fig. 8. A white pc-LED was fabricated using a 365 nm NUV chip as the excitation source, combining with blue-green-emitting NaBaBO3:0.01Ce3þ,0.03 Tb3þ phosphor, and commercial red-emitting CaAlSiN3:Eu2þ phosphor. The emission spectrum of the device driven by a 20 mA current was shown in Fig. 9, together with the photograph of the device with and without current. As shown, the device generated white light emission with CIE color coordinates of (0.316, 0.307), CCT ¼ 6473 K, and Ra ¼ 81.4. These results indicate that the NaBaBO3:Ce3þ, Tb3þ phosphor may have promising potential applications for NUV pcWLEDs.
CRediT authorship contribution statement Menglong Xia: Writing - original draft, Investigation, Data curation, Writing - review & editing. Weiren Zhao: Conceptualization, Method ology, Resources, Funding acquisition. Jiyou Zhong: Writing - review & editing, Conceptualization. Peng Shi: Formal analysis. Zifeng Liao: Software, Project administration. Xiang Liu: Visualization, Software. Jingzhou Song: Investigation. Li Luo: Project administration, Re sources. Lin Ma: Supervision. Zhaogang Nie: Validation. Acknowledgments This work was financially supported by the Special Fund for Appli cation, Science and Technology Planning Projects of Guangdong Prov ince of China (2017B01021002). In addition, Major program for Cooperative Innovation of Production, Education ＆ Research of Guangzhou city (201704030106) also supported this work.
4. Conclusions
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In summary, NaBaBO3:Ce3þ,Tb3þ phosphors were synthesized by a high-temperature solid-state reaction method. The energy transfer from Ce3þ to Tb3þ was discovered. The critical distance for energy transfer was calculated to be 12.46 Å, indicating that the dipole-dipole interac tion was responsible for the energy transfer. Under the excitation of
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Journal of Luminescence journal homepage: http://www.elsevier.com/locate/jlumin
Tunable luminescence of blue-green emitting NaBaBO3:Ce3þ,Tb3þ phosphors for near-UV light emitting diodes Menglong Xia , Weiren Zhao *, Jiyou Zhong , Peng Shi , Zifeng Liao , Xiang Liu , Jingzhou Song , Li Luo , Lin Ma , Zhaogang Nie School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Guangzhou, 510006, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Phosphors Ce3þ/Tb3þ co-doped Energy transfer Quantum efficiency Tunable luminescence White LEDs
A series of blue-green emitting NaBaBO3:Ce3þ, Tb3þ phosphors were synthesized via conventional high tem perature solid-state reaction method. The crystal structure, luminescence properties, energy transfer processes and the properties of the packing LED device were investigated. The phosphors present tunable luminescence in the blue-green region due to the energy transfer from Ce3þ to Tb3þ. The internal quantum efficiency (IQE) and external quantum efficiency (EQE) of the composition optimized NaBaBO3:0.01Ce3þ, 0.03 Tb3þ phosphor reached 57% and 36%, respectively. Using a 365 nm NUV LED chip combining with NaBaBO3: 0.01Ce3þ, 0.03 Tb3þ phosphor and CaAlSiN3:Eu2þ red phosphor, a high quality white light was obtained with CIE chromaticity coordinates of (0.316, 0.307), CCT of 6473 K, and CRI of 81.4. These results demonstrated that NaBaBO3:Ce3þ, Tb3þ phosphors is a promising blue-green phosphor for NUV white LEDs.
1. Introduction Unlike the commercial WLEDs using blue LED chips with yellow Y3Al5O12:Ce3þ (YAG:Ce3þ) phosphors, combining tricolor (red, green, blue) phosphors with near ultraviolet (NUV) LED chips can present more soft and uniform white light with tunable color properties [1–8]. Currently, the commercially available NUV excitable tricolor phosphors are Sr2Si5N8:Eu2þ red-emitting phosphor [9], β-SiAlON:Eu2þ green-emitting phosphor [10], and BaMgAl10O17:Eu2þ blue-emitting phosphor [11]. Generally, the nitrides phosphors are expensive due to its harsh synthesis conditions and expensive raw materials [12–16]. Therefore, it is attractive to develop oxides phosphors with low synthesis temperature and sufficient luminescence efficiency. In the design of phosphors, Tb3þ is always adopt as an activator of green-emitting materials due to the dominant 5D4-7F5 transition peak at about 545 nm. However, the intensities of absorption and emission peaks in the NUV region are very weak and narrow due to the forbidden 4f 4f transitions of Tb3þ [17–22]. As an effective sensitizer for Tb3þ ions, Ce3þ has been widely used in many hosts. Ce3þ-Tb3þ co-doped phosphors could enhance the luminescence intensity of Tb3þ by en ergy transfer from sensitizer to activator and enhance blue and green emission [23–29]. Recently, orthoborates phosphors have attracted much attention
among multitudinous phosphors because of their merits of low synthetic temperature, remarkable chemical stability and excellent luminescent properties [30,31]. Typically, in our previous work, we found a very interesting phenomenon that when the concentration of Ce3þ ions is different, we can obtain green-emitting and blue-emitting NaBaBO3: Ce3þ phosphors respectively. It is reported that the distinctive green emission in this borate may stem from Ce3þ occupying the Naþ crys tallographic sites whereas the more natural blue emission should stem from Ce3þ occupying the Ba2þ site. The internal quantum efficiency (IQE) of the blue-emitting monoclinic NaBaBO3:Ce3þ (λex ¼ 360 nm) reached 65% [32,33]. Additionally, Zheng et al., reported an intense green emission in NaBaBO3 phosphors singly doped with Tb3þ, indi cating that NaBaBO3 is an excellent host for lanthanide doping [22]. In this work, only blue emission was observed in NaBaBO3:Ce3þ, and basing on this, Ce3þ-Tb3þ co-doped NaBaBO3 phosphors were carefully researched. It is found that a tunable emission from blue to green can be realized through the change of Tb3þ doping content under NUV exci tation. With the internal quantum efficiency high as 57%, the phosphors show the potential application in the White LEDs with UNV chips. 2. Experimental Synthesis. NaBaBO3:0.01Ce3þ, xTb3þ (x ¼ 0, 0.01, 0.03, 0.05, 0.07,
* Corresponding author., E-mail address: [email protected] (W. Zhao). https://doi.org/10.1016/j.jlumin.2019.116957 Received 1 September 2019; Received in revised form 25 November 2019; Accepted 8 December 2019 Available online 17 December 2019 0022-2313/© 2019 Elsevier B.V. All rights reserved.
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Fig. 3. Diffuse reflectance spectra of NaBaBO3 host (a), NaBaBO3:0.01Ce3þ (b), and NaBaBO3:0.01Ce3þ,0.03 Tb3þ (c) phosphors. Fig. 1. XRD patterns of the NaBaBO3:0.01Ce3þ (a) and NaBaBO3:0.01Ce3þ, 0.03 Tb3þ (b) and the standard data of NaBaBO3 (ICSD 80110).
Fig. 4. The PL spectra (λex ¼ 355 nm) of NaBaBO3:0.01Ce3þ, xTb3þ (x ¼ 0, 0.01, 0.03, 0.05, 0.07, 0.09) phosphors.
Characterization. The X-ray diffraction (XRD) patterns of the samples were obtained by using an Ulitima Ⅳ X-ray diffractometer (XRD) with Cu Kα (λ ¼ 1.5406 Å) radiation at 36 kV and 21 mA. The diffuse reflection spectra were collected by Evolution 220 (Thermo Scientific) and used BaSO4 as a reference standard. The photoluminescence exci tation (PLE), photoluminescence (PL) spectra and its decay curves of the phosphors were recorded using an FLS980 spectrofluorimeter (Edin burgh Instruments, UK, Edinburgh). The Internal Quantum Efficiencies (IQEs) were measured by QE-2100 quantum yield measurement system (Otsuka Electronics Co., Ltd, Japan), with BaSO4 powder as a reference and a Xe lamp as an excitation source. The WLED device was fabricated using the as-prepared phosphor with a 365 nm UV LED chip, and its emission spectrum was obtained using an ATA-500 Sync-Skan color analyzer system under a forward bias of 20 mA.
Fig. 2. PLE (left) and PL (right) spectra of NaBaBO3:0.01Ce3þ (a), NaBaBO3:0.03 Tb3þ (b), and NaBaBO3:0.01Ce3þ, 0.03 Tb3þ (c).
and 0.09) phosphors were synthesized by a high-temperature solid-state reaction method. The raw materials were Na2CO3 (Aldrich, 99.95%), BaCO3 (Aldrich, 99.9%), H3BO3 (Aldrich, 99.5%), CeO2 (Aldrich, 99.99%), and Tb4O7 (Aldrich, 99.9%). The powers were weighed based on the stoichiometric proportion, except for H3BO3 with excess of 3 mol % to compensate for high-temperature evaporation. The materials were thoroughly grounded and mixed in an agate mortar for 40 min, then placed it into an alumina crucible. The mixture was first heated to 650 � C for 4 h in air to decompose BaCO3 and Na2CO3. The obtained powder was reground after cooled down to the room temperature, and then heated to 760 � C for 5 h in a flowing reducing atmosphere (5% H2/95% N2). Thus, the as-prepared phosphors were obtained.
3. Results and discussion Fig. 1 shows the XRD patterns of NaBaBO3:0.01Ce3þ and 2
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two Gaussian bands with peaks at 403 nm (24814 cm 1) and 435 nm (22989 cm 1). The energy difference is ~ 1825 cm 1, which is close to the theoretical difference of ~2000 cm 1 between the 2F5/2 and 2F7/2 levels [37]. Fig. 2b shows the PLE and PL spectra of NaBaBO3:0.03 Tb3þ. The excitation spectrum monitoring at 545 nm clearly showed two intense band ranging from 220 to 300 nm with maxima at 242 and 282 nm, along with some relatively weak narrow peaks in the range of 300–400 nm. The former intense bands can be assigned to the spin allowed 4f8-4f7 5d transitions of Tb3þ, and the weak peaks can be ascribed to the forbidden absorption f-f transition of Tb3þ [19,38,39]. The emission spectrum under the excitation of 245 nm is consisted of several peaks centered at 486, 545, 581, 622 nm due to the 5D4-7FJ (J ¼ 6, 5, 4, and 3) transitions of Tb3þ ions. Comparing Fig. 2a and b, it can be seen that there is partial overlap between the emission band of Ce3þ and the excitation band of Tb3þ, providing the probability of energy transfer between Ce3þ and Tb3þ ions through non-radiative energy transfer [40]. Fig. 2c showed the excitation and emission spectra of NaBaBO3:0.01Ce3þ, 0.03 Tb3þ phosphor. Comparing the PLE spectra monitored at 415 nm (Ce3þ 5d-4f emission) and that monitored at 545 nm (Tb3þ 5D4-7F5 emission), obviously, the excitation spectra monitored at 545 nm (Tb3þ emission) presents the typical absorption of Ce3þ, which demonstrate the occurrence of energy transfer from Ce3þ to Tb3þ. The PL spectra of NaBaBO3:0.01Ce3þ, 0.03 Tb3þ under 355 nm excita tion shows the emission characteristics of both Ce3þ and Tb3þ. Fig. 3 showed the diffuse reflectance spectra of NaBaBO3 host (a), NaBaBO3:0.01Ce3þ (b), and NaBaBO3:0.01Ce3þ,0.03 Tb3þ (c) phos phors. The NaBaBO3 host shows energy absorption at the wavelength that less than 245 nm, while having no absorption in the NUV region. However, Fig. 3b illustrated that NaBaBO3:0.01Ce3þ sample had three distinct broad absorption bands in the 250–400 nm NUV region, which can attribute to the 4f-5d transition of Ce3þ. It can also be seen from Fig. 3 that the absorption band of NaBaBO3:0.01Ce3þ,0.03 Tb3þ is quite similar with that of NaBaBO3:0.01Ce3þ except for the relative intensity. The peak position of absorption band is well coincided with that of the PLE spectrum (λem ¼ 545 nm) of NaBaBO3: 0.01Ce3þ, 0.03 Tb3þ in Fig. 2c. The strongest excitation wavelength of Ce3þ - Tb3þ co-doped NaBaBO3 well matches with NUV chips. Fig. 4 shows the PL spectra of NaBaBO3:0.01Ce3þ, xTb3þ (x ¼ 0, 0.01, 0.03, 0.05, 0.07. 0.09) phosphors under the excitation of 355 nm. As presented, a blue emission band for 5d-4f transition of the Ce3þ ions and a series of strong emission lines at 486, 545, 581, 622 nm due to the 5 D4-7FJ (J ¼ 6, 5, 4, and 3) transitions of Tb3þ ions can be observed. The emission intensity of Ce3þ gradually increases with the increasing of Tb3þ concentration and reach the maximum at x ¼ 0.03, then decreases due to the Tb3þ-Tb3þ internal concentration quench. The internal quantum efficiency (IQE) and external quantum efficiency (EQE) of NaBaBO3:0.01Ce3þ, 0.03 Tb3þ excited at 355 nm was 57% and 36%, respectively, lower than the blue-emitting NaBaBO3:Ce3þ (IQE ¼ 65%, EQE ¼ 46% under 355 nm excitation). The PL decay curves obtained by monitoring the Ce3þ emission at 415 nm in NaBaBO3:0.01Ce3þ, xTb3þ are presented in Fig. 5. It can be found that all decay curves can be well fitted by the single exponential function [41].
Fig. 5. Photoluminescence decays curves of Ce3þ by monitoring the Ce3þ emission at 415 nm in NaBaBO3:0.01Ce3þ, xTb3þ phosphors (x ¼ 0–0.09) upon excitation at 355 nm.
Fig. 6. The dependence of the Ce3þ emission, Tb3þ emission, and energy transfer efficiency of Ce3þ-Tb3þ on the concentration of Tb3þ for NaBaBO3:0.01Ce3þ, xTb3þ phosphors.
NaBaBO3:0.01Ce3þ, 0.03 Tb3þ samples. All diffraction peaks of the samples can be indexed to the standard NaBaBO3 card (ICSD No. 80110), indicating no significant impurities detected and no significant change in the host structure with the addition of dopant ions. The re ported lattice constants of NaBaBO3 are a ¼ 9.561 Å, b ¼ 5.557 Å, c ¼ 6.179 Å, V ¼ 324.4 Å3 and Z ¼ 4 [34]. Based on our XRD data, the lattice constants of NaBaBO3:0.01Ce3þ, 0.03 Tb3þ are a ¼ 9.558 Å, b ¼ 5.564 Å, c ¼ 6.181 Å, and V ¼ 324.7 Å3 calculated by the Jade program. The results show excellent agreement (<1% difference) with the reported data. Fig. 2a illustrates the PLE monitoring at 415 nm and PL spectra excited at 355 nm for NaBaBO3:0.01Ce3þ phosphor. The PLE spectrum shows two broad excitation bands with peaks located at 298 and 355 nm, respectively, which can be assigned to 4f-5d transition of Ce3þ [35]. The PL spectrum showed a blue emission band covering 365–500 nm with a peak at around 415 nm, which is a typical double-band emission of Ce3þ from the 5d excited state to the 2F5/2 and 2F7/2 ground states [36]. As showed in Fig. 2a, the emission band can be decomposed into
IðtÞ ¼ I0 þ Aeð
t=τÞ
(1)
where I(t) and I0 are the luminous intensity at time t and 0, A is a con stant, τ is the lifetime. The lifetime τ were calculated to be 34.53, 34.19, 34.03, 33.58, 32.86 and 32.74 ns for x ¼ 0, 0.01, 0.03, 0.05, 0.07 and 0.09 respectively. Obviously, lifetime τ decreases with the increase of Tb3þ concentration, which verify solidly the existence of the energy transfer from Ce3þ ions to Tb3þ ions. The energy transfer efficiency ηT can be calculated by the following equation [42]:
ηET ¼ 1
τS =τSO
(2)
where τs and τso stand for the PL lifetimes of the sensitizer Ce3þ ions with 3
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Fig. 7. The dependence of IS0/IS of Ce3þ on (a) C(Ce3þþTb3þ), (b) C(Ce3þþTb3þ)6/3, (c) C(Ce3þþTb3þ 8/3 and (d) C(Ce3þþTb3þ)10/3, the red line denotes the corresponding ) linear fit to the scattering data points.
and without the presence of the activator Tb3þ ions, respectively. The dependence of energy transfer efficiency ηT, together with the relative emission intensity of Ce3þ(4f-5d) and Tb3þ (5D4-7F5), on the concen tration of Tb3þ for NaBaBO3:0.01Ce3þ,xTb3þ phosphors are depicted in Fig. 6. The emission intensity of Ce3þ decreased monotonically due to the enhanced energy transfer from Ce3þ ions to Tb3þ ions, while the emission intensity of Tb3þ has a maximal value at x ¼ 0.03. The decease of Tb3þ emission when x > 0.03 can be ascribed to the Tb3þ-Tb3þ in ternal concentration quenching. The energy transfer efficiency ηT gradually increased with the increase of Tb3þ concentration. However, the ηT is only 5.2%, which is consistent with the relatively small spectra overlap between Ce3þ emission and Tb3þ excitation. The critical distance RCe-Tb between Ce3þ and Tb3þ with the con centration quenching method can be calculated by the following equa tion given by Blasse [43]: RCe
Tb
� �1=3 3V ¼2 4π xc N
calculated to be 12.46 Å when xc is about 0.08 according to eqn (3), which is much greater than 5 Å required for the exchange interaction. Therefore, the energy transfer from Ce3þ to Tb3þ ions in the NaBaBO3
(3)
V is the volume of the unit cell, N is the number of host cations in the unit cell. In the NaBaBO3 unit cell, the values of V and N are approxi mately 324.385 Å3 and 4. xc is the total concentration of Ce3þ and Tb3þ at which the luminescence intensity of Ce3þ reduces to half of that for the sample in the absence of Tb3þ. The critical distance RCe-Tb was Table 1 CIE chromaticity coordinates of NaBaBO3:xCe3þ,yTb3þ. Sample no.
Composition x, y
CIE (u, v)
1 2 3 4 5 6 7
0.01, 0 0.01, 0.01 0.01, 0.03 0.01, 0.05 0.01, 0.07 0.01, 0.09 0, 0.03
(0.157,0.064) (0.183,0.141) (0.213,0.223) (0.209,0.208) (0.179,0.122) (0.174,0.105) (0.316,0.584)
Fig. 8. Chromaticity coordinates of NaBaBO3:0.01Ce3þ, yTb3þ (y ¼ 0–0.09), NaBaBO3:0.03 Tb3þ, inset showed the digital photos of the samples under 360 nm UV lamp excitation. 4
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Fig. 9. (a) Electroluminescence spectrum of a white LED fabricated by a 365 nm GaN chip with NaBaBO3:0.01Ce3þ,0.03 Tb3þ and CaAlSiN3:Eu2þ; (b) the CIE coordinate of the fabricated white LED.
takes place via electric multipole interaction. According to Dexter’s energy transfer formula for multipolar in teractions, a relationship can be given as following [44,45]:
η0 IS0 n=3 � ∝C ηS IS ðCe3þ þTb3þ Þ
NUV, the emission color of the phosphors can be tuned from blue (0.157,0.064) to blue-greenish (0.213,0.223) through controlling Tb3þ concentrations. The IQE of NaBaBO3:0.01Ce3þ,0.03 Tb3þ reaches 57%. Taking this blue-green phosphor and CaAlSiN3:Eu2þ red phosphor, combining with a 365 nm NUV LED chip, A white LED was fabricated and shows the CIE chromaticity coordinates (0.316, 0.307) and a CCT 6473 K, a CRI 81.4. These results indicate that NaBaBO3:Ce3þ,Tb3þ phosphors have the potential to be used as blue-green phosphor for NUV excited white LEDs.
(4)
where η0 and ηS are the luminescence quantum efficiencies of Ce3þ in the absence and presence of Tb3þ, respectively, the values of ηS/η0 can be estimated approximately by the ratio of relative luminescence peak in 3þ 3þ tensity ratio (IS0/IS). C3þ and (CeþTb) is the total concentration of the Ce 3þ Tb ions. n ¼ 6, 8, and 10 corresponds to dipole-dipole, dipole-quad rupole, and quadrupole-quadrupole interactions, respectively. The re n/33þ 3þ 3þ lationships between IS0/IS and C3þ (CeþTb) (CeþTb) as well as IS0/IS and C are showed in Fig. 7(a-d), n ¼ 6 presented a higher fitting factor of R2 value, thus the energy transfer from Ce3þ to Tb3þ realizes through dipole dipole interaction.
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal re lationships which may be considered as potential competing interests:
The CIE chromaticity coordinates of NaBaBO3:0.01Ce3þ, xTb3þ phosphors with different dopant contents under 355 nm excitation were measured and listed in Table 1, as depicted in Fig. 8. As shown, the color can be tuned from blue (0.157,0.064) to blue-greenish (0.213,0.223) by controlling the concentration of Tb3þ. The digital photographs of NaBaBO3:0.01Ce3þ, xTb3þ phosphors under 360 nm UV lamp are depicted in the inset of Fig. 8. A white pc-LED was fabricated using a 365 nm NUV chip as the excitation source, combining with blue-green-emitting NaBaBO3:0.01Ce3þ,0.03 Tb3þ phosphor, and commercial red-emitting CaAlSiN3:Eu2þ phosphor. The emission spectrum of the device driven by a 20 mA current was shown in Fig. 9, together with the photograph of the device with and without current. As shown, the device generated white light emission with CIE color coordinates of (0.316, 0.307), CCT ¼ 6473 K, and Ra ¼ 81.4. These results indicate that the NaBaBO3:Ce3þ, Tb3þ phosphor may have promising potential applications for NUV pcWLEDs.
CRediT authorship contribution statement Menglong Xia: Writing - original draft, Investigation, Data curation, Writing - review & editing. Weiren Zhao: Conceptualization, Method ology, Resources, Funding acquisition. Jiyou Zhong: Writing - review & editing, Conceptualization. Peng Shi: Formal analysis. Zifeng Liao: Software, Project administration. Xiang Liu: Visualization, Software. Jingzhou Song: Investigation. Li Luo: Project administration, Re sources. Lin Ma: Supervision. Zhaogang Nie: Validation. Acknowledgments This work was financially supported by the Special Fund for Appli cation, Science and Technology Planning Projects of Guangdong Prov ince of China (2017B01021002). In addition, Major program for Cooperative Innovation of Production, Education ＆ Research of Guangzhou city (201704030106) also supported this work.
4. Conclusions
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In summary, NaBaBO3:Ce3þ,Tb3þ phosphors were synthesized by a high-temperature solid-state reaction method. The energy transfer from Ce3þ to Tb3þ was discovered. The critical distance for energy transfer was calculated to be 12.46 Å, indicating that the dipole-dipole interac tion was responsible for the energy transfer. Under the excitation of
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