Bi2+-doped BaSO4 phosphor was synthesized in air via solid state reaction method. Three excitation bands and one emission band were observed at 260 nm (2P1/2 → 2S1/2), 452 nm (2P1/2 → 2P3/2(2)), 592 nm (2P1/2 → 2P3/2(1)), and 627 nm (2P3/2(1) → 2P1/2), respectively. W-LEDs were demonstrated by using a blend composition of BaSO4:Bi2+ and YAG:Ce3+ phosphors pumped with a 455 nm blue LEDs chip. The results indicate that BaSO4:Bi2+ phosphor is suitable as potential red phosphor for application in W-LEDs excited with blue LEDs chip.
© 2012 OSA
Energy-efficient solid-state lighting and high-power white light-emitting diodes (W-LEDs) have been subjects of significant research efforts for application in mobile, traffic signal, as well as daily life in the past decades [1–5]. The current two-color W-LEDs, fabricated with Ce3+-doped yttrium aluminum garnet (YAG:Ce3+) phosphor and blue LEDs chip, exhibits a poor color rendering index (CRI) due to deficiency of red color in luminescence [6, 7]. To overcome this weakness, various red emission phosphors have been researched such as Eu3+, Sm3+, Pr3+ and Mn4+ doped phosphors and Eu2+ and Ce3+ doped nitride phosphors etc [8–15]. The applications of red emitting phosphors doped with Eu3+, Sm3+ and Pr3+ ions have some limits in current W-LEDs due to their sharp excitation band in ultraviolet (UV) and blue region. Red emitting phosphors doped with Mn4+ ions are also limited in W-LEDs excitation with blue LED chip due to poor excitation band in blue region. Red emitting nitride phosphors are appropriate for W-LEDs excitation with blue LEDs chip but nitride phosphors require the severe synthesis conditions, such as elevated pressure, atmosphere and temperature etc. Therefore, it is imperative to develop a novel red emitting phosphor for W-LEDs excited with blue LEDs chip.
Bismuth exhibits a broad variety of optoelectronic properties and potential applications due to the large number of possible valence states [16–20]. Bi3+ ion in the Bi-dopant exhibits a [Xe]4f145d106s2 electronic configuration and photoluminescence in the soft UV to blue spectral range, even green luminescence . Bi+, Bi0 and other lower state species, which show photoluminescence in the near infrared (NIR) region, have also been discussed extensively [17–20]. Bi2+ ion, which exhibits a [Xe]4f145d106s26p1 electronic configuration, shows absorption band in the UV and visible region and photoluminescence in the orange or red spectral range with excitation at UV or blue wavelength region . Optical properties of Bi2+ ion depend strongly on ligand field strength in the employed host material. Due to difficulties in stabilizing Bi2+ ion over its Bi3+ ion, Bi2+-doped luminescence materials are rather limited. The most suitable host sites for incorporating Bi2+ ions usually are Ba2+ and Sr2+ ions vacancies within a relatively highly polymerized lattice such as alkaline earth borates, sulfates, phosphates and borophosphates [23–26].
In the investigation, MSO4:Bi2+ phosphors (M = Ca2+, Sr2+ and Ba2+) were synthesized via solid state reaction method at 800°C in air. The luminescence properties of BaSO4:Bi2+ phosphors were discussed. W-LEDs were fabricated by using a blend composition of BaSO4:Bi2+ and YAG:Ce3+ phosphors pumped with a 455 nm blue LEDs chip. The experimental results indicate that BaSO4:Bi2+ phosphors are suitable as potential red phosphors for application in W-LEDs.
Doped and un-doped samples of BaSO4 were prepared via solid state reaction method in air. Analytical grade reagents BaCl2, (NH4)2SO4 and Bi2O3 were used as raw materials. The stoichiometric amount of raw materials were weighed according to the Ba1-xSO4:xBi2+ (x = 0.03, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2 mol%), well ground in an agate mortar, then filled in alumina crucibles and sintered at 300°C for 3h and subsequently 800°C for 2h in air. Repeated grindings were performed between two sintering processes. For comparison, MSO4:Bi2+ phosphors (M = Ca2+ and Sr2+) were also prepared via solid state reaction method under the same synthesis condition. W-LEDs were fabricated by using a blend composition of BaSO4:Bi2+ and YAG:Ce3+ phosphors pumped with a 455 nm blue LEDs chip (3.2V, 16mW).
The crystal structures of the BaSO4 and BaSO4:Bi2+ sintered at 800°C for 2h in air were characterized by using X-ray diffractometer (XRD) (Philips Model PW1830) with Cu-Kα radiation at 40 kV and 40 mA. The XRD data were collected in the range 2θ = 10 - 90° in 0.02° steps at room temperature. Luminescence properties of MSO4:Bi2+ phosphors (M = Ca2+, Sr2+ and Ba2+) were characterized using FLS920 spectrofluorimeter (Edinburgh) at room temperature. The Commission International de I’Eclairage (CIE) chromaticity coordinates for all samples were measured by a Laiko DT-101 color analyzer equipped with a CCD detector (Laiko Co., Tokyo, Japan).
3. Results and discussion
Figure 1 shows the XRD profiles of BaSO4 and Ba0.994SO4:0.006Bi2+ sintered at 800°C for 2h in air, the standard card JCPDS Card No: 76-213 (BaSO4) and unit-cell representation of the BaSO4. The XRD profiles of BaSO4 and Ba0.994SO4:0.006Bi2+ match well with the standard data in JCPDS Card No. 76-213 and the doping of Bi2+ ions do not cause any significant change in the BaSO4 host structure. The single crystalline phase of BaSO4 is orthorhombic crystal structure (2/m 2/m 2/m) with space-group Pnma (no. 62) and has four SO42- ions in per unit cell and their lattice parameters a = 8.848 Å, b = 5.441 Å, c = 7.132 Å, cell volume = 348.44 Å3 and z = 4 . Each Ba2+ consists of twelve oxygen atoms. In the BaSO4 host the Bi2+ ion substitutes for the Ba2+ ion.
Figure 2 shows PLE and PL spectra of the BaSO4:Bi2+ phosphor sintered at 800°C for 2h in air (λex = 452 nm, λem = 627 nm) and pictures of the Ba0.994SO4:0.006Bi2+ phosphor under daytime (left) and 254 nm UV lamp (right), respectively. Three excitation bands and one emission band can be observed at 260 nm (2P1/2 → 2S1/2), 452 nm (2P1/2 → 2P3/2(2)), 592 nm (2P1/2 → 2P3/2(1)), and 627 nm (2P3/2(1) → 2P1/2), respectively. Excitation at 260, 452 and 592 nm always leads to the same emission at 627 nm. All the bands are consistent with Hamstra’s observations . The electronic configuration of Bi2+ is [Xe]4f145d106s26p1. The ground state is 2P1/2, the first excited state 2P3/2 can be split into two sublevels 2P3/2(1) and 2P3/2(2). All these states derive from 6s26p1. The excited level 2S1/2 derives from 6s27s1. Normally, 2P1/2 → 2P3/2 is parity forbidden and 2P1/2 → 2S1/2 is strongly allowed. However, when incorporated in a crystal lattice vibrations electron-phonon coupling will admix the 2P1/2, 2S1/2, and 2P3/2 wave functions. Then the parity selection rule will be broken and 2P1/2 → 2P3/2 will be allowed. The inset pictures in Fig. 2 show the photographs of Ba0.994SO4:0.006Bi2+ phosphor under daytime (left) and 254 nm UV lamp (right). The CIE chromaticity coordinates of the Ba0.994SO4:0.006Bi2+ phosphor is (x = 0.6712; y = 0.3225).
Figure 3 shows simplified energy level diagrams and PLE and PL spectra of the MSO4: Bi2+ phosphors annealed at 800°C for 2h in air (M = Ba2+ (λex = 452 nm; λem = 627 nm), Sr2+ (λex = 452 nm; λem = 610 nm) and Ca2+ (λex = 395 nm; λem = 580 nm)) at room temperature. Three excitation bands corresponding to 2P1/2 → 2S1/2, 2P1/2 → 2P3/2(2), and 2P1/2 → 2P3/2(1) transition were detected in MSO4:Bi2+ (M = Ca2+, Sr2+ and Ba2+) and the UV excitation bands corresponding to 2P1/2 → 2S1/2 transition were first detected in MSO4: Bi2+ (M = Ca2+ and Sr2+). Emission band occurs via 2P3/2(1) → 2P1/2 transition. The excitation band of 2P1/2 → 2S1/2 transition lies between 240 and 280nm, and the two excitation bands of 2P1/2 → 2P3/2(2) and 2P1/2 → 2P3/2(1) transition appear in the spectral range of 380 to 600 nm, respectively. The emission band of 2P3/2(1) → 2P1/2 transition appears in the spectral range of 570 to 640 nm. Similar to those observed in other Bi2+-doped alkaline earth borophosphate phosphors , the excitation and emission bands show a red-shift, which may be caused by different ligand field effect on Bi2+ ion. Lifetimes at 627 nm of MSO4:Bi2+ (M = Ca2+, Sr2+ and Ba2+) are ~6.21, 9.86 and 10.82 μs, respectively. The optimized PL intensity of Ba1-xSO4:xBi2+ (x = 0.6 mol%) is three times magnitude of those of Sr1-xSO4:xBi2+ (x = 0.2 mol%), and ten times magnitude of those of Ca1-xSO4:xBi2+ (x = 0.05 mol%).
Figure 4(a) shows PL spectra of Ba1-xSO4:xBi2+ phosphors sintered at 800°C for 2h in air (x = 0.03 ~1.2 mol%) with excitation at 452 nm and dependence of PL intensity on Bi doped concentration in Ba1-xSO4:xBi2+ phosphors (x = 0.03 ~1.2 mol%) with excitation at 452 and 592 nm, respectively. The PL spectral shapes and luminescence center wavelength were not changed appreciably with changing Bi doping concentration from 0.03 to 1.2 mol%. According to the inset in Fig. 4(a), the optimized Bi doping concentration for PL intensity excited at 452 and 592 nm is 0.6 ± 0.05 mol%.
Figure 4(b) shows luminescence decay curve of BaSO4:Bi2+ phosphor sintered at 800°C for 2h in air (The monitoring wavelength is at 627 nm with 452 nm excitation) and dependence of lifetime on Bi2+ doped concentration in Ba1-xSO4:xBi2+ phosphors (x = 0.03 ~1.2 mol%). The luminescence decay curve can be well fitted by a first-order exponential function .Fig. 4(b) shows that fluorescent lifetime of BaSO4:Bi2+ phosphors almost remains unchanged (10 ± 1µs) with increasing Bi doping concentration from 0.03 to 0.8 mol% and then decreases with further increasing Bi doping concentration due to concentration quenching.
Figure 5 a shows dependence of integrated lifetime (The monitoring wavelength is at 627 nm with 452 nm excitation) and PL intensity with excitation at 452 and 592 nm, respectively, in the temperature range of 10 to 300 K, in Ba0.994SO4:0.006Bi2+ phosphor sintered at 800°C for 2h in air. The fluorescent lifetime at 627 nm and PL intensity excitation at 452 and 592 nm all show little change with increasing temperature from10 to 300 K. These indicate that the phosphors have a good thermal stability.
Figure 5(b) shows time resolved luminescence spectra of Ba0.994SO4:0.006Bi2+ phosphor sintered at 800°C for 2h in air excited with 452 nm at different delay times (room temperature). Emission intensity monotonically decreases while the line shape remains unchanged with increasing delay time. Combining with the data in Fig. 4(b), it is confirmed that only a single type of Bi2+ center exists in BaSO4:Bi2+ phosphor.
Figure 6 shows electroluminescence (EL) spectrum of the W-LEDs lamp fabricated by using phosphors blend of the Ba0.994SO4:0.006Bi2+ and YAG:Ce3+ phosphors pumped with a 455 nm blue LEDs chip (3.2V, 16 mW). The white light generated shows CIE chromaticity coordinates (x = 0.3086, y = 0.2902), CRI value 78 and CCT (7256 K). Compared with W-LEDs fabricated by only YAG:Ce3+ phosphors pumped with a 455 nm blue LEDs chip, the CRI index has been improved from 66 to 78. Further investigations on the quantum efficiency aw\s well as thermal and photo stability of the phosphor under high power LED excitation will be carried out.
In summary, MSO4:Bi2+ phosphors (M = Ba, Sr and Ca) were synthesized at 800°C in air via a solid state reaction method. Bi2+ ions substitute for the M2+ ions in the host structure. Among three types of phosphors BaSO4: Bi2+ phosphor exhibits superior luminous property and strong excitation band in the blue spectral region. W-LEDs lamp was fabricated by using phosphors blend of the Ba0.994SO4:0.006Bi2+ and YAG:Ce3+ phosphors pumped with a 455 nm blue LEDs chip. Bi2+ shows a red emission with a relatively small full width at half maximum, which could have significant advantages over traditional red Eu2+ LED phosphors that have a significant amount of near-IR emission. Our experimental results indicate that BaSO4:Bi2+ phosphors would be a promising candidate as red phosphor in the field of blue-based W-LEDs.
This work was financially supported by the National Natural Science Foundation of China (Grants no. 51072054, 51072060, 51132004, 51102096), Fundamental Research Funds for the Central Universities (Grants no. 2011ZZ0001, 2011ZB0001, 2011ZP0002), Guangdong Natural Science Foundation (Grant no. S2011030001349, 1045106410104887), Fok Ying Tong Education Foundation (Grant No. 132004), Chinese Program for New Century Excellent Talents in University (Grant no. NCET-11-0158) and National Basic Research Program of China (2011CB808100).
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