1. Introduction

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 [21]. 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 [22]. 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.

2. Experimental

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 Ba_1-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 Ba_0.994SO₄:0.006Bi²⁺ 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 Ba_0.994SO₄:0.006Bi²⁺ 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 SO₄^2- 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 ų and z = 4 [27]. Each Ba²⁺ consists of twelve oxygen atoms. In the BaSO₄ host the Bi²⁺ ion substitutes for the Ba²⁺ ion.

 

Fig. 1 a. XRD profiles of BaSO₄, Ba_0.994SO₄:0.006Bi²⁺ and JCPDS Card No. 76-213 (BaSO₄); b. Unit-cell representation of the BaSO₄.

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Figure 2 shows PLE and PL spectra of the BaSO₄:Bi²⁺ phosphor sintered at 800°C for 2h in air (λ_ex = 452 nm, λ_em = 627 nm) and pictures of the Ba_0.994SO₄:0.006Bi²⁺ phosphor under daytime (left) and 254 nm UV lamp (right), respectively. Three excitation bands and one emission band can be observed at 260 nm (²P_1/2 → ²S_1/2), 452 nm (²P_1/2 → ²P_3/2(2)), 592 nm (²P_1/2 → ²P_3/2(1)), and 627 nm (²P_3/2(1) → ²P_1/2), 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 [23]. The electronic configuration of Bi²⁺ is [Xe]4f¹⁴5d¹⁰6s²6p¹. The ground state is ²P_1/2, the first excited state ²P_3/2 can be split into two sublevels ²P_3/2(1) and ²P_3/2(2). All these states derive from 6s²6p¹. The excited level ²S_1/2 derives from 6s²7s¹. Normally, ²P_1/2 → ²P_3/2 is parity forbidden and ²P_1/2 → ²S_1/2 is strongly allowed. However, when incorporated in a crystal lattice vibrations electron-phonon coupling will admix the ²P_1/2, ²S_1/2, and ²P_3/2 wave functions. Then the parity selection rule will be broken and ²P_1/2 → ²P_3/2 will be allowed. The inset pictures in Fig. 2 show the photographs of Ba_0.994SO₄:0.006Bi²⁺ phosphor under daytime (left) and 254 nm UV lamp (right). The CIE chromaticity coordinates of the Ba_0.994SO₄:0.006Bi²⁺ phosphor is (x = 0.6712; y = 0.3225).

 

Fig. 2 PLE and PL spectra of the BaSO₄:Bi²⁺ phosphor sintered at 800°C for 2h in air (λ_ex = 452 nm, λ_em = 627 nm). The inset: photographs of the Ba_0.994SO₄:0.006Bi²⁺ phosphor under daytime (left) and 254 nm UV lamp (right), respectively.

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Table 1. Excitation (λ_ex in nm), emission (λ_em in nm) and fluorescence lifetime (τBi²⁺ in μs) of MSO₄: Bi²⁺ (M = Ca²⁺, Sr²⁺ and Ba²⁺).

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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⁺ (λ_ex = 452 nm; λ_em = 627 nm), Sr²⁺ (λ_ex = 452 nm; λ_em = 610 nm) and Ca²⁺ (λ_ex = 395 nm; λ_em = 580 nm)) at room temperature. Three excitation bands corresponding to ²P_1/2 → ²S_1/2, ²P_1/2 → ²P_3/2(2), and ²P_1/2 → ²P_3/2(1) transition were detected in MSO₄:Bi²⁺ (M = Ca²⁺, Sr²⁺ and Ba²⁺) and the UV excitation bands corresponding to ²P_1/2 → ²S_1/2 transition were first detected in MSO₄: Bi²⁺ (M = Ca²⁺ and Sr²⁺). Emission band occurs via ²P_3/2(1) → ²P_1/2 transition. The excitation band of ²P_1/2 → ²S_1/2 transition lies between 240 and 280nm, and the two excitation bands of ²P_1/2 → ²P_3/2(2) and ²P_1/2 → ²P_3/2(1) transition appear in the spectral range of 380 to 600 nm, respectively. The emission band of ²P_3/2(1) → ²P_1/2 transition appears in the spectral range of 570 to 640 nm. Similar to those observed in other Bi²⁺-doped alkaline earth borophosphate phosphors [22], 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 Ba_1-xSO₄:xBi²⁺ (x = 0.6 mol%) is three times magnitude of those of Sr_1-xSO₄:xBi²⁺ (x = 0.2 mol%), and ten times magnitude of those of Ca_1-xSO₄:xBi²⁺ (x = 0.05 mol%).

 

Fig. 3 a. Simplified energy level diagrams of MSO₄: Bi²⁺ (M = Ca²⁺, Sr²⁺ and Ba²⁺) (NR: non-radiative relaxation process); b. PLE and PL spectra of the MSO₄: Bi²⁺ phosphors sintered at 800°C for 2h in air (M = Ba²⁺ (λ_ex = 452 nm; λ_em = 627 nm), Sr²⁺ (λ_ex = 452 nm; λ_em = 610 nm) and Ca²⁺ (λ_ex = 395 nm; λ_em = 580 nm)) at room temperature. The spectrum of CaSO₄:Bi²⁺ in the range of 230 to 280 nm and 530 to 650 nm were amplified 10 times for clarity.

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Figure 4(a) shows PL spectra of Ba_1-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 Ba_1-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%.

 

Fig. 4 a. PL spectra of Ba_1-xSO₄:xBi²⁺ phosphors sintered at 800°C for 2h in air (x = 0.03, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2 mol%) with excitation at 452 nm. The inset: Dependence of PL intensity on Bi doped concentration in Ba_1-xSO₄:xBi²⁺ phosphors (x = 0.03 ~1.2 mol%) with excitation at 452 and 592 nm, respectively. b. 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). The red curve is a fit of the experimental data to a first order exponential decay equation. The inset: Dependence of fluorescent lifetime on Bi²⁺ doped concentration in Ba_1-xSO₄: xBi²⁺ phosphors (x = 0.03 ~1.2 mol%).

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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 Ba_1-xSO₄:xBi²⁺ phosphors (x = 0.03 ~1.2 mol%). The luminescence decay curve can be well fitted by a first-order exponential function [28].

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 Ba_0.994SO₄:0.006Bi²⁺ 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.

 

Fig. 5 a 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 Ba_0.994SO₄:0.006Bi²⁺ phosphor sintered at 800°C for 2h in air. b. Time resolved luminescence spectra of Ba_0.994SO₄:0.006Bi²⁺ phosphor sintered at 800°C for 2h in air. Excitation wavelength is 452 nm. Delay times are listed in legend.

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Figure 5(b) shows time resolved luminescence spectra of Ba_0.994SO₄:0.006Bi²⁺ 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 Ba_0.994SO₄:0.006Bi²⁺ 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.

 

Fig. 6 EL spectrum of the W-LEDs using phosphors blend of Ba_0.994SO₄:0.006Bi²⁺ and YAG:Ce³⁺ phosphors pumped with a 455 nm blue LEDs chip. The inset: a. Luminescence photograph of the W-LEDs lamp (3.2V); b. CIE chromaticity coordinates of the W-LEDs.

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4. Conclusions

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 Ba_0.994SO₄:0.006Bi²⁺ 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.