1. Introduction

Recently, there has been a dramatic proliferation in research concerned white light-emitting diodes (wLEDs) because of their merits of long operation lifetime, energy-saving feature, and environmentally friendly characteristics [1,2]. Consequently, these superior characteristics may lead to the replacement of as-discharge fluorescent lamps by wLEDs in the near future. In general, there are several approaches to generate white light by using of the LEDs and phosphors. The first one is to generate white light is by using blue-emitting LED chips coated with yellow-emitting phosphor Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺). However, this typical type of white light suffered from poor color rendering index (CRI) caused by the deficiency of red emission in the visible spectrum. During the past few years, white LEDs fabricated using a near-ultraviolet (n-UV) LED (370-420nm) coupled with red, green, and blue-emitting phosphors have attracted much attention due to excellent CRI and good color stability. Therefore, there is an urgent need to develop new phosphors that can be effectively excited in the near ultraviolet range.

Nitride/oxynitride compounds, such as M₂Si₅N₈ (M = Ca, Sr, Ba)] [3,4], MAlSiN₃ (M = Ca, Sr) [5,6], and MSi₂O₂N₂ (M = Ca, Sr, Ba) [7,8], produce a strong nephelauxetic effect or large crystal-field splitting, which leads to the unique luminescence such as broad band excitation, long-wavelength emission, high quantum efficiency, and good thermal stability. However, some of these nitride phosphors are hard to synthesize in atmosphere pressure, which causing the high cost. Thus, new nitride-based phosphors for normal pressure are worth to develop. The crystal structure of Ca₂BN₂F was first reported by Nesper et al. [9]. Li et al. reported the electronic structure and photoluminescence properties of Ca₂BN₂F:Eu²⁺ [10]. Nonetheless, few reports have been published on Ca₂BN₂F:Eu²⁺ being applied to n-UV LEDs as well as thermal quenching behavior for solid state lighting.

In this paper, we report on the luminescence properties, thermal stability as well as applications of blue-emitting Ca₂BN₂F:Eu²⁺ phosphors in the fabrication of a wLED by combination with an n-UV LED. High-quality white light was generated from the white LEDs fabricated from Ca₂BN₂F:Eu²⁺. Ca₂BN₂F:Eu²⁺ is thus a promising blue-emitting phosphor for n-UV LEDs.

2. Experimental

In study, powder samples of Ca₂BN₂F:Eu²⁺ were prepared by a solid-state reaction in where the constituent raw materials Ca₃N₂ (95%), BN (98%), EuCl₂ (99.99%), CaF₂(99.99%) (all from Aldrich Chemicals, Milwaukee, WI, U.S.A) were weighed in stoichiometric proportions and intimately ground in the glove box. The powder mixtures were sintered under a reducing atmosphere (15%H₂/85%N₂) at 1000°C for 8 h with one intermittent regrinding to prevent the possibility of incomplete reaction. The products were then cooled to room temperature in the furnace, ground, and pulverized for further measurements. The phase purity of the as-prepared samples were identified by powder X-ray diffraction (XRD) using a Bruker AXS D8 advanced automatic diffractometer with Cu-K_α radiation (λ = 1.5418 Å) operating at 40 kV and 30 mA. The XRD profiles were collected in the range of 10° < 2θ < 80°. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra were measured at room temperature by a Spex Fluorolog-3 spectrofluorometer (Instruments S.A., N.J., U.S.A) equipped with a 450W Xe light source and double excitation monochromators. The powder samples were compacted and excited and emitted fluorescence was detected by a Hamamatsu Photonics R928 type photomultiplier perpendicular to the excitation beam. The spectral response of the measurement system is calibrated automatically on start up. To eliminate the second-order emission of the source radiation, a cut-off filter was used in the measurements. The Commission International de I’Eclairage (CIE) chromaticity coordinates for all samples were determined by using a Laiko DT-100 color analyzer equipped with a CCD detector (Laiko Co., Tokyo, Japan).

3. Results and discussion

3.1. XRD analysis and crystal structure of as-synthesized Ca₂BN₂F:Eu²⁺

Figure 1 shows the XRD patterns of as-synthesized Ca₂BN₂F:Eu²⁺ and JCPDS standard pattern. The XRD pattern of as-synthesized Ca₂BN₂F:Eu²⁺ agrees with that of JCPDS file No. 89-4591. The result indicates that doping of Eu²⁺ into Ca₂BN₂F does not generate any impurity phase. According to the single crystal X-ray data reported by Nesper [9], Ca₂BN₂F crystallizes in the orthorhombic system with space group Pnma and with four formula units per unit cell. The dimensions of the unit cell are a = 9.182 Ǻ, b = 3.649 Ǻ, and c = 9.966 Ǻ. The crystal structure consists of tri-BN₂^3- anions and tetra-Ca1(Ca2)₃F units. There are two different crystallographic sites for the Ca²⁺ ions. The Ca1 site is coordinated by five nitrogen ions and one fluorine ion, and another Ca2 site is coordinated by three nitrogen and three fluorine ions. Based on the effective ionic radii (r) of cations with different coordination numbers (CN) as reported by Shannon [11], the ionic radius of Eu²⁺ (r = 1.17 Å when CN = 6) is close to that of Ca²⁺ (r = 1.00 Å when CN = 6). Due to size considerations, Eu²⁺ was expected to occupy the Ca²⁺ sites and not B³⁺ sites, which are too small. The lattice constants of undoped Ca₂BN₂F and Ca₂BN₂F:5%Eu²⁺ were determined to be a = 9.15 Ǻ, b = 3.66 Ǻ, c = 9.92 Ǻ and a = 9.18 Ǻ, b = 3.67 Ǻ and c = 9.96 Ǻ, respectively. The lattice constants of a and c slight increased with the increasing the content of Eu²⁺ concentration.

 

Fig. 1 XRD patterns of Ca₂BN₂F from JCPDS 89-4591 and as-synthesized Ca₂BN₂F:Eu²⁺ sample. Inset: Crystal structure of Ca₂BN₂F.

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3.2. Luminescence properties of Ca₂BN₂F:Eu²⁺ phosphors

Figure 2 shows the PL and PLE spectra of Ca₂BN₂F:1%Eu²⁺. The excitation spectrum consisted of four absorption bands peaking at 337, 352, 365, and 376 nm, which correspond to the transition from the ground state of Eu²⁺ to its field-splitting levels at the 5d¹ state. The sample exhibited a blue-emitting band peaking at 423 nm under optimal excitation at 336 nm. The Stokes shift is the difference between positions of the band maxima of the excitation and emission spectra, and it was estimated to be 6121 cm⁻¹ in Ca₂BN₂F:1%Eu²⁺. A broad asymmetric band was observed in the emission spectrum for the wavelength range of 390–530 nm; this can be assigned to the allowed 4f⁶5d¹ → 4f⁷ electronic transitions of Eu²⁺. Since Eu²⁺ ions occupied two different Ca²⁺ sites, the emission band deconvoluted into two individual emission bands centered at 415 nm and 435 nm, which were contributed to CaN₃F₃ and CaN₅F, respectively. The inset of Fig. 2 demonstrates the phosphor excited at 365 nm in the UV box, which gives an intense blue color. The CIE chromaticity coordinate of the Ca₂BN₂F phosphor is shown in Fig. 3 . The chromaticity index (x, y) was found to be locate at (0.17, 0.09) for the composition with 1 mol%.

 

Fig. 2 Excitation and emission spectra of as-synthesized Ca₂BN₂F:1%Eu²⁺ phosphor. The inset shows the phosphor under 365 nm excitation in a UV box.

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Fig. 3 CIE chromaticity diagram for Ca₂BN₂F:1%Eu²⁺ excited at 336 nm.

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Figure 4 displays the PL intensity of Ca₂BN₂F as a function of doped Eu²⁺ content. The optimal doping concentration was observed at 1 mol%. The PL intensity of Ca₂BN₂F:Eu²⁺ was found to decline dramatically as the content of Eu²⁺ exceeded 1 mol% due to concentration quenching. The phenomenon of concentration quenching is mainly caused by energy transfer among Eu²⁺ ions and the possibility increases with the Eu²⁺ concentration. In addition, with the increasing the Eu content, the ratios of PL intensity ratio 415 to 435 nm were almost the same. The result indicates that no preferential occupation of Eu²⁺ on the two cites of CaN₃F₃ and CaN₅F in the Ca₂BN₂F host. The result was in consistent with previous work done by Y. Q. Li et al [10]. The quantum efficiencies and UV-absorption of Ca₂BN₂F:1%Eu²⁺ and commercial blue-emitting phosphor-BaMgAl₁₀O₁₇:Eu²⁺ are measured to be 15.8% and 48.8% for Ca₂BN₂F:1%Eu²⁺ and 78.5% and 68.6% for BaMgAl₁₀O₁₇:Eu²⁺, respectively, by using integrated sphere under 336 nm excitation. It is believed the quantum efficiency of Ca₂BN₂F:1%Eu²⁺ could be enhanced by processes optimization. The PL/PLE spectra of Ca₂BN₂F:Eu²⁺ phosphor exhibited no significant change in the wavelength and band shape with increasing Eu²⁺ concentration, which indicated that the crystal field strength experienced by the activator does not change. In other words, the doping into the lattice site does not cause expansion or shrinkage of the unit cell in accordance with the XRD data.

 

Fig. 4 Excitation and emission spectra of Ca₂BN₂F:x%Eu²⁺ phosphors with varying Eu²⁺ concentrations.

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Blasse [12] pointed out that the critical transfer distance (R_c) is approximately equal to twice the radius of a sphere with the volume of the unit cell:

Several phenomena can contribute to non-radiative energy transfer, such as exchange interaction, radiation reabsorption, or electric multipolar interaction. Exchange interaction is observed in the forbidden transitions, and it requires a large direct or indirect overlap of the wavefunctions of the donor and acceptor. The critical distance for the exchange interaction is approximately 5 Ǻ [13]. The exchange mechanism plays no role in the energy transfer within the Ca₂BN₂F:Eu²⁺ phosphors because the 4f⁷→ 4f⁶5d¹ transition of Eu²⁺ is allowed transition. The mechanism of radiation reabsorption is only effective when the fluorescence spectra are broadly overlapping. Therefore, radiation reabsorption does not occur in this case. The process of energy transfer between Eu²⁺ ions in the Ca₂BN₂F:Eu²⁺ phosphor was ascribed to the electric multipolar interaction, as suggested by Dexter [13].

The temperature dependence of the PL intensity of Ca₂BN₂F:Eu²⁺ under excitation at 336 nm and above room temperature is shown in Fig. 5 . The activation energy (E_a) can be expressed by

 

Fig. 5 Thermal quenching of Ca₂BN₂F:1%Eu²⁺ excited at 336 nm. Inset: Normalized PL intensity of Ca₂BN₂F:1%Eu²⁺ as a function of temperature.

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3.3 LED package tests

The white LED was successfully fabricated by combining the n-UV chip (370 nm) and the Ca₂BN₂F:Eu²⁺, (Ba,Sr)₂SiO₄:Eu²⁺ and CaAlSiN₃:Eu²⁺ phosphors. The electroluminescent (EL) spectrum and the corresponding LED image are shown in Fig. 6 . The CIE color coordinates and correlated color temperature (Tc) of the white LED were found to be (0.338, 0.337) and 5235 K, respectively. The average color-rendering index Ra was determined to be as high as 90.2, which was excellent for lighting applications. The inset of Fig. 6 shows the appearance of a well-packaged LED lamp in operation. Ca₂BN₂F:Eu²⁺ is a suitable blue-emitting phosphor for use in white LEDs.

 

Fig. 6 EL spectrum of the white LED composed of GaN-based n-UV-LED (370 nm) and Ca₂BN₂F:Eu²⁺ (blue), (Ba,Sr)₂SiO₄:Eu²⁺ (green) and CaAlSiN₃:Eu²⁺ (red) phosphors driven by a 350-mA current.

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4. Conclusions

In summary, a blue-emitting Ca₂BN₂F:Eu²⁺ phosphor is reported. The energy transfer between Eu²⁺ ions in Ca₂BN₂F occurred via electric multipolar interaction, and the critical transfer distance was 25.11 Å. The white LEDs fabricated with an n-UV chip, green/red phosphors and Ca₂BN₂F:Eu²⁺ generate white light with a high color rendering index (R_a = 90.2). Eu²⁺-activated phosphors can be used for n-UV convertible blue-emitting phosphors.