A Nitride-based Red Phosphor with a Sharp Emission Line and Broad

Article pubs.acs.org/JPCC

α‑M3B2N4 (M = Ca, Sr):Eu3+: A Nitride-based Red Phosphor with a Sharp Emission Line and Broad Excitation Band Used for WLED Jianyan Ding, Quansheng Wu, Yanyan Li, Qiang Long, Yichao Wang, Xinlong Ma, and Yuhua Wang* Key Laboratory for Special Function Materials and Structural Design of the Ministry ofEducation, School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China ABSTRACT: For solving the problem that Eu3+-activated phosphors cannot be well excited by the near UV-LED chips, the charge transfer band (CTB) of Eu3+-N3− in α-M3B2N4 (M = Ca, Sr) has been investigated in this work. Compared with the sharp excitation peaks due to the f−f transitions of Eu3+, the charge transfer band is broad and the excitation band of α-M3B2N4 (M = Ca, Sr):Eu3+ centered at 370 or 350 nm lies in near-UV, implying a promising excitation. The electron and crystal structures of α-M3B2N4 (M = Ca, Sr) has been analyzed in detailed, indicating that they all crystallize in cubic phase with the space group (Im3̅m) and have similar band-gap structures. The double bond of [N = BN]3− combining with the face-shared (Ca, Sr)N6 octahedron forms the stable and compact crystal structure, and the band gap with the value about 4 eV is suitable for Eu3+ to form a luminescent center. Under near-UV excitation, the sharp red light that peaked at 613 and 593 nm resulting from the f−f transition of Eu3+ ions has been obtained. The optimal concentration of Eu3+ in the α-M3B2N4 (M = Ca, Sr) has been measured, and the decay curves of samples prove that the energy transfer happens between Eu3+ ions through the dipole−dipole interaction. The thermal stability of samples has been measured to access their application in the WLED.

1. INTRODUCTION In recent years, WLEDs have gradually replaced the conventional light sources to become next-generation solid-state light sources because of the their excellent advantages such as high brightness, long service lifetime, low energy consumption, environmental friendliness, and so on.1−4At present, there are two common ways to produce the white light: One way is the combination of a yellow-emitting phosphor Y3Al5O12:Ce3+ (YAG:Ce3+) and a blue InGaN LED chip, but this white light lacks red-emitting compound compared with sunlight. Therefore, this way suffers from the problem of a low color rendering index (CRI) and a high correlated color temperature.5,6 Another way is that the near-ultraviolet (NUV)-LED chips are assigned to the mixture of tricolor (blue, green, red) phosphors. Therefore, the redemitting phosphors play an important role in producing highquality white light. Recently, nitride-based red phosphors have received considerable attention due to their outstanding thermal and chemical stability as well as interesting photoluminescence (PL) properties. Some nitride-based phosphors have replaced sulfides to become new commercial red phosphors such as CaAlSiN3:Eu2+ 7,8 and Sr2Si5N8:Eu2+.9Compared with O, N ions show larger covalency, implying a higher nephelauxetic effect, and the covalent bond builds up a stable structure, leading to the larger crystal field splitting. In such coordination environment, Eu2+-activated nitride-based phosphors generally emit a red light due to the larger splitting of 5d level of Eu2+, but the broad emission band resulting from 4f−5d transitions of Eu2+ ions leads to a low color purity. Therefore, the shield of the crystal field effect is the main factor for obtaining the sharp emission light. Eu3+-activated phosphors have this advantage because the 4f orbital of Eu3+ ions © 2017 American Chemical Society

is well shielded by 5d energy level, and the sharp red emission line that peaks at 613 nm can be obtained due to the 5D0−7F2 transition of Eu3+ ions. The main problem that the sharp excitation peaks that result from the f−f transition of Eu3+ ions cannot be well excited by the NUV-LED chips limits their commercial application. To obtain the broad excitation band, the investigation is focused on the CTB of Eu3+ ions. Compared with the CTB of Eu3+−O2−, the CTB of Eu3+−N3− is expected to match NUV-LED chips due to the big covalency of N3− ions, indicating that Eu3+ ions can easily obtain the electron from the N3− ions. To the best of our knowledge, Eu3+-activated nitride-based phosphors have rarely been reported. In the previous work, we have reported the luminescence property of Li2SiN2:Eu3+ with the CTB of Eu3+−N3−centered at 350 nm.10 It is interesting to find that Eu3+ ions can exist in the phase of α-M3B2N4 (M = Ca, Sr). The phases of α-Sr3B2N4 and α-Ca3B2N4 have been reported by Häberlen,11 illustrating that α-M3B2N4 (M = Ca, Sr) has a stable crystal structure due to the [N = BN]3− and that there are two kinds of sites for rare earth to occupy. However, the luminescence properties of α-M3B2N4 (M = Ca, Sr) have never been reported. In this work, Eu3+-activated α-M3B2N4 (M = Ca, Sr) has been successfully synthesized. The PL and photoluminescence excitation (PLE) spectra of α-M3B2N4 (M = Ca, Sr):Eu3+combined with their crystal structure and electron structure have been investigated in detail. The morphology and the thermal stability of samples have been measured to assess their application in the WLED. Received: February 28, 2017 Revised: April 16, 2017 Published: April 19, 2017 10102

DOI: 10.1021/acs.jpcc.7b01945 J. Phys. Chem. C 2017, 121, 10102−10111

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The Journal of Physical Chemistry C

2. EXPERIMENTAL SECTION 2.1. Materials and Synthesis. A series of α-Sr3B2N4 and α-Ca3B2N4 phosphors have been synthesized through the sample-pressure sintering. The raw materials used for the prepared phosphors were Ca3N2 (Aldrich, > 99.50%), Sr3N2 (Aldrich, > 99.50%), BN (Aldrich, > 99.50%). Stoichiometric amounts of reactants were mixed and ground in an agate mortar, and the final mixture was compressed and then was placed in the Al2O3 crucible. The former procedures were finished in a glovebox. Finally, the mixture was annealed at 1050 °C in a N2 atmosphere for 8 h. After that, the samples were furnace-cooled to room temperature and ground again into powders for measurement. 2.2. Measurements and Characterization. The phase purity has been analyzed by the powder X-ray diffraction (XRD) using a Rigaku D/max−2400 X diffraction with Ni-filtered Cu Kα radiation, and the XRD data were collected in the range of 10−80° with the step size of 0.03° and count time of 0.1 s/step. The Rietveld refinement was carried out with general structure analysis system (GSAS) program,12 and the XRD data used for refinement is collected with the step size of 0.01° and count time of 0.6s/step. The morphology of the powder was displayed by scanning electron microscopy (SEM, S-340, Hitachi, Japan) and transmission electron microscopy (TEM). The electron structures of α-Sr3B2N4 andα-Ca3B2N4 were calculated using the density functional theory (DFT) and performed with GASTEP code on the basic of a single crystal structure. The local-density approximations (LDAs) based on DFT were chosen for the theoretical basis of the density function. The PL and PLE spectra of samples were measured by a FLS-920T fluorescence spectrophotometer equipped with a 450 W Xe light source and double-excitation monochromators. The PerkinElmer 950 spectrometer with BaSiO4 white plate used as the standard reference for reflection measurement was used to investigate the diffusion reflectance spectroscopy (DRS) ultraviolet−visible (UV−vis) absorption spectra of samples. All measurements were performed at room temperature. Thermal quenching was tested using a heating apparatus (TAP-02) in combination with PL equipment.

Figure 1. Rietveld refinement of (a) α-Sr3B2N4 and (b) α-Ca3B2N4 (experimental (cross), calculated (red solid line), their different (green line), and the Bragg diffraction positions). The inset shows the crystal structure.

respectively. The results also indicate that the impurity phase of B2O3 and CaO with the weight of about 4.7 and 5.2% can be ignored. The crystal data of α-Sr3B2N4 and α-Ca3B2N4 from the Rietveld refinement are displayed in Tables 1 and 2, respectively. Table 1. Crystal Data of α-Sr3B2N4 from the Rietveld Refinement formula crystal system space group cell parameters Z atom Sr1 Sr2 B N

3. RESULTS AND DISCUSSION 3.1. Crystal Structure and Phase Identification. Because the oxygen atoms would exist in the raw materials and enter into the samples in the process of the sintering, it is hard to avoid the fact that the samples are oxidized. As exhibited in Figure 1, the oxide impurity of CaO and B2O3 has been detected in α-Ca3B2N4 and α-Sr3B2N4, respectively. The luminescence properties of CaO:Eu3+ have been reported by Kang,13 indicating that CTB of CaO:Eu3+ lies in 250 nm, and it can be well distinguished from the PLE spectrum of α-Ca3B2N4:Eu3+. As for B2O3, there are no suitable sites for Eu3+ to form the luminescent center, and our experimental results also prove that B2O3:Eu3+ does not emit light. Therefore, it is reasonably deduced that the impurity phases have little influence on the luminescence properties of α-M3B2N4 (M = Ca, Sr):Eu3+. The Rietveld refinements of α-Sr3B2N4 and α-Ca3B2N4 have been studied using the GSAS program on the basic of XRD patterns. Figure 1a,b presents the observed and calculated Bragg positions and the difference patterns of α-Sr3B2N4 and α-Ca3B2N4, respectively. All of the XRD peaks satisfy the reflection condition, and the final reliability factors for the whole patterns are Rwp = 14.45 and 13.93%, Rp = 10.70 and 10.97%, and x2 = 3.389 and 1.603 for α-Sr3B2N4 and α-Ca3B2N4,

Sr3B2N4 cubic Im3m ̅ a = b = c = 7.6031(1) Å 2 x/a y/b z/c 0 0 0 0.25 0.25 0.25 0 0.5 0.5 0.31949 0 0

Wyck. 2a 8c 6b 12e

S.O.F. 0.4896 1 1 1

Table 2. Crystal data of α-Ca3B2N4 from the Rietveld Refinement formula crystal system space group cell parameters Z atom Ca1 Ca2 B N 10103

Ca3B2N4 cubic Im3m ̅ a = b = c = 7.2993 (1) Å 2 x/a y/b z/c 0 0 0 0.25 0.25 0.25 0 0.5 0.5 0.67834 0 0

Wyck. 2a 8c 6b 12e

S.O.F. 1 0.877 1 1


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Figure 2. XRD patterns of (a) α-Sr3B2N4:xEu3+ (0.01 ≤ x ≤ 0.06) and (b) α-Ca3B2N4:xEu3+ (0.01 ≤ x ≤ 0.06). SEM micrographs of (a) α-Sr3B2N4 and (b) α-Ca3B2N4. The inset exhibits particle distribution.

Figure 3. TEM images of (a) α-Sr3B2N4 and (b) α-Ca3B2N4. The inset shows the HRTEM images. EDX spectra of (a) α-Sr3B2N4 and (b) α-Ca3B2N4, respectively. 10104


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Figure 4. Total and partial densities of the states of (a) α-Sr3B2N4 and (b) α-Ca3B2N4. Brillouin zones of states of (c) α-Sr3B2N4 and (d) α-Ca3B2N4.

Figure 5. Band structures of (a) α-Sr3B2N4 and (b) α-Ca3B2N4. (c) DRS spectra of α-Sr3B2N4 and α-Sr3B2N4:0.02Eu3+. (d) DRS spectra of α-Ca3B2N4 and α-Ca3B2N4:0.02Eu3+. The inset shows plot of [F(R)hv]2 versus photoenergy hv.

As exhibited in the inset of Figure 1a,b, α-Sr3B2N4 and α-Ca3B2N4 both crystallize in the cubic phase with the space group (Im3̅m), and two kinds of M (Sr, Ca) sites connect to each

other through face-sharing, implying the compact host lattice. In general, two close cationic sites resulting from the facesharing would make the lattice unstable, but the rigid bond of 10105


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Figure 6. PL spectrum of Eu3+-activated α-Sr3B2N4 and α-Ca3B2N4 combing with the energy level diagram of Eu3+-activated α-Sr3B2N4 and α-Ca3B2N4.

[N = BN]3− in α-M3B2N4 (M = Ca, Sr) linking to alkaline earths forms a stable framework. Therefore, α-M3B2N4 (M = Ca, Sr) shows a compact and stable crystal structure, which is a promise matrix for phosphor. Two alkaline earths M (Sr, Ca) connect to six N atoms forming the (Sr, Ca)N6 octahedron: One alkaline earth locates at center of symmetry. Another alkaline earth coordinating with six N atoms forms the disorder (Sr, Ca)N6 octahedron. The Eu3+ ions are expected to occupy the Sr2+/Ca2+ sites due to their similar atom radii, and the symmetry of site determines the transition of Eu3+ ions discussed below. The series XRD patterns of α-Sr3B2N4:xEu3+ (0.01 ≤ x ≤ 0.06) and α-Ca3B2N4:xEu3+ (0.01 ≤ x ≤ 0.06) have been shown in Figure 2a,b, respectively. It can be observed that the weight of impurity phase (CaO and B2O3) does not have obviously change with the increase in Eu3+ content. Therefore, the influence resulting from impurity can be ignored. A close look at the diffraction peak (30−40°) reveals that the peaks move toward the larger 2θ degree with the increase in Eu3+ content, indicating a host shrinkage. As analyzed above, when the smaller radius of Eu3+ ions (coordination number (CN) = 6, R = 1.087 Å) replaces the larger radius of Sr2+ (CN = 6, R = 1.32 Å) and Ca2+ (CN = 6, R = 1.14 Å) ions,14 then the lattice would show a shrinkage, which is consistent with the experimental result. In fact, the impurity defects like vacancies also result in the shrinkage of the host lattice. According to the defect reaction equation, the trivalent europium ions occupying divalent alkaline earth sites would produce the vacancies, as shown in the following equations

2Eu 3 + → 2Eu•Ca + V″Ca

(1)

2Eu 3 + → 2Eu•Sr + V″Sr

(2)

where Eu•Ca and Eu•Sr show that the Eu3+ ions replace the Ca2+ and Sr2+ ions, resulting in the positive electricity of the sites. To keep the charge balance, the vacancies V″Ca and V″Sr would be produced, which leads to the shrinkage of the host lattice. The morphology is the important index for WLED, and the discrete particle is good for encapsulation. As exhibited in Figure 2c,d, the morphology of α-Sr3B2N4 and α-Ca3B2N4 investigated by the SEM shows a great dispersibility, and this irregular morphology results from the high-temperature solidstate reaction. The difference morphology between α-Sr3B2N4 and α-Ca3B2N4 can be observed, and the particles of α-Sr3B2N4 are larger and neater than those of α-Ca3B2N4. The distributions of the particles size for α-Sr3B2N4 and α-Ca3B2N4 investigated by the program of nanomeasurement have been depicted in the inset of Figure 2c,d, indicating that the average diameter of α-Sr3B2N4 and α-Ca3B2N4 is 5.08 and 1.64 μm, respectively, and this size reaches the requirement of WLED. The TEM images of α-Sr3B2N4 and α-Ca3B2N4 have been exhibited in Figure 3a,b, respectively, and the samples present an irregular morphology, which is consistent with the result of SEM. The inset of Figure 3a,b shows the HRTEM images of α-Sr3B2N4 and α-Ca3B2N4, respectively. In the phase of α-Sr3B2N4, the interplanar spacings are measured to be 0.241 and 0.263 nm, which match well with the (013) and (022) interplanar distances, respectively, and the zone axis [400] can be calculated. 10106


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The Journal of Physical Chemistry C In the phase of α-Ca3B2N4, the interplanar spacings are measured to be 0.261 and 0.294 nm, and this belongs to the (022) and (112) crystal face, respectively, with the zone axis [222]. As shown in Figure 3c,d, the EDX analysis spectrum confirms the presence of Sr, Ca, B, N, C, and Cu in the samples, and the C and Cu peaks are ascribed to the carbon film and copper grid supporting the TEM samples. 3.2. Luminescence Properties. The suitable band-gap structure is very important for the rare earth to form the luminescent center, so the investigation for electron structure of α-M3B2N4 (M = Ca, Sr) using the DFT calculations is necessary. As presented in Figure 4a,b, α-M3B2N4 (M = Ca, Sr) shows a similar electron structure due to their same crystal structure discussed above, and their partial densities can be divided into three energy regions: the first one is the lower energy part (−15 to −50 eV) formed by the alkaline earth M (M = Sr, Ca)-3s and 3p states. The second one closes to the Fermi level with the energy part (−15 to −3 eV), which is the valence band. In this region, N-2p states play a major role and hybridize with B-2p states forming the [N = B = N]3− bond. The final region is the conduction band with the energy part (0 to 10 eV). As can be seen, the conduction band of α-Ca3B2N4 is mainly composed of the Ca-3d states, and it is different from that of α-Sr3B2N4 formed by Sr-3p states. When the Eu3+ ions replace the Sr2+ ions or Ca2+ ions to hybridize with N3− ions, the part electron structure changes and the 4f energy level of Eu3+ forms the luminescent center in the band gap. The same Brillouin zones of α-Sr3B2N4 and α-Ca3B2N4 with the dodecahedron have been shown in Figure 4c,d, respectively, because they all crystallize in a cubic phase. Figure 5a,b exhibits the band-gap structure α-Sr3B2N4 and α-Ca3B2N4, respectively. The top of the valence of α-Sr3B2N4 and α-Ca3B2N4 is around the H point, but the bottom of the conduction band of α-Sr3B2N4 and α-Ca3B2N4 lies in the G point. So α-Sr3B2N4 and α-Ca3B2N4 can be regarded as the indirect band gap with the calculated band gap energy Eg about 1.8 and 2.0 eV, respectively. Figure 5c exhibits the DRS of α-Sr3B2N4 and α-Sr3B2N4:Eu3+. Clearly, the host lattice of α-Sr3B2N4 has a remarkable absorption around 340 nm caused by the electron transition of the host, and a white body color can be observed. With Eu3+ ions doped, the broad absorption band ranging from 350 to 450 nm resulting from the CTB of Eu3+-N3− can be detected, and the body color changes from white to yellow. The DRS of α-Ca3B2N4 and α-Ca3B2N4:Eu3+ is shown in Figure 5d. The absorption band of the host lattice lies at ∼300 nm, and the broad absorption band ranging from 350 to 550 nm is ascribed to the electron transition between the energy level of Eu3+ ions and valence band, which makes the body color become pink. According to the DRS of α-Sr3B2N4 and α-Ca3B2N4, the value of the band gap can be calculated through the plot [F(R)]2 versus photoenergy hv for α-Sr3B2N4 and α-Ca3B2N4, where R is the detected reflectance in the DRS and F(R) is the Kubelka−Munk function,15 where F(R) = (1 − R)2/2*R. As exhibited in the inset of Figure 5c,d, by adopting the methods proposed by Cao et al., the optical band gap energy of α-Sr3B2N4 and α-Ca3B2N4 is calculated to about 3.7 and 4.2 eV, respectively, by the extrapolating to F(R) = 0. The result is bigger than the calculated band-gap value due to the first-principles approaches based on the DFT approach. The sharp red emission line resulted from the f−f transition of Eu3+ ions has been obtained. As exhibited in Figure 6, α-Sr3B2N4:Eu3+ and α-Ca3B2N4:Eu3+ emit several sharp lines peaked at 593, 613, 672, and 735 nm for 5D0−7F1, 5D0−7F2,

Figure 7. (a) PLE spectrum of Eu3+-activated α-Sr3B2N4 and α-Ca3B2N4. (b) Coordination environment of Eu3+ in the phase of α-Sr3B2N4 and α-Ca3B2N4 with the electron cloud.

Table 3. Wavelengths of CTB of the Nitride- and Oxide-Based Phosphors formula

wavelength of CTB (nm)

K2GdZr(PO4)3:Eu3+ Y5Mo2O12:Eu3+ Ca2Gd(SiO4)6O2:Eu3+ SrB2O4:Eu3+ La3NbO7:Eu3+ BaB2O4:Eu 3+ α-Ca3B2N4:Eu3+ Li2SiN2:Eu3+ α-Sr3B2N4:Eu3+

250 260 275 280 293 300 340 350 370

ref U V ↓ N U V

18 19 20 21 22 23 this work 10 this work

D0−7F3, and 5D0−7F4 electrons transitions of Eu3+ ions, respectively. Among the several sharp lines, the emission line peaked at 613 nm is the strongest in the phase of Eu3+-doped α-Ca3B2N4, but α-Sr3B2N4:Eu3+ has two main emission lines peaked at 593 and 613 nm. In general, the electron dipole 5 D0−7F2 transition is a hypersensitive transition and the magnetic dipole 5D0−7F1 transition is regardless of the environment.16 When Eu3+ ions occupy the site with the symmetry, the 5D0−7F1 emission transition peaked at 593 nm is dominant in the emission spectrum. On the contrary, asymmetry site allows the 5 D0−7F2 transition of Eu3+. Therefore, the (5D0−7F2)/(5D0−7F1) 5

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Figure 8. PL spectra of (a) α-Sr3B2N4:xEu3+ (0.01 ≤ x ≤ 0.06) and (b) α-Ca3B2N4:xEu3+ (0.01 ≤ x ≤ 0.06). The relationship of Ln(I/x) versus Ln(x) for the (c) α-Sr3B2N4:Eu3+ and (d) α-Ca3B2N4:Eu3+.

emission intensity ratio (asymmetry ratio) can be used as an index to measure the site symmetry around Eu3+ ions. According to the above discussion, α-M3B2N4 (M = Ca, Sr) all have two alkaline earth sites with different symmetries for Eu3+ ions to occupy, and the asymmetry ratio of α-Ca 3 B2 N 4and α-Sr3B2N4:Eu3+ has been calculated to be about 14.5 and 1.02, respectively, implying that considerable Eu3+ ions occupy the symmetry site in the phase of α-Sr3B2N4. To well describe the process of light emitting resulted from the Eu3+ions, the energy level diagram of Eu3+-activated α-Sr3B2N4 and α-Ca3B2N4 has also been exhibited in Figure 6. According to the above experimental results, the position of 4f energy levels of Eu3+ ions in the band gap of α-M3B2N4 (M = Ca, Sr) has been roughly depicted, and the process of the light emitting resulting from the electron transition can be divided into three parts: first, the electrons transfer from the valence band to the lowest energy level of 4f7 along the way ①, and the excited energy determines the wavelength of CTB. In the way ②, the partial energy would be lost through the lattice vibration due to the spin−lattice relaxation of the electrons. Finally, the excited electrons will come back to the valence band along the way ③ with the light emitting. Because of the shield of the 5d level, emission spectra of Eu3+ ions are the sharp lines, and Eu3+-activated α-Sr3B2N4 and α-Ca3B2N4 have the same emission peaks except the intensity, whereas the CTB is determined by the coordination environment of Eu3+ ions, which is our main research object. As exhibited in Figure 7a, monitoring at 613 nm, α-M3B2N4 (M = Ca, Sr):Eu3+ has a broad excitation band resulting from the CTB of the Eu3+−N3−, which covers the sharp excitations ascribed to f−f transition of Eu3+ ions ranging from 300 to 400 nm, and the embossment peaked at 393 nm can be detected, which is attributed to 7F0−5L6 transition of Eu3+ ions. Clearly, the similar excitation peak pattern of α-M3B2N4 (M = Ca, Sr):Eu3+

has been observed because of their same crystal structure, but the wavelength of α-Ca3B2N4:Eu3+ is shorter than that of α-Sr3B2N4:Eu3+. The CTB describes the process that the Eu3+ ions obtain the electrons from the coordination anions like O2− or N3− ions. As exhibited in Figure 7b, Eu3+ ions not only connect with N3− ions but also coordinate with eight alkaline earths, and the electron cloud of Ca2+−N3− group is bigger than that of Sr2+−N3− group due to the higher electronegativity of Ca2+ ions. So Eu3+ ions obtaining the electrons from the Ca2+−N3− group need more energy compared with the Sr2+−N3− group, implying that the wavelength of the CTB of α-Ca3B2N4 is shorter than that of α-Sr3B2N4. The wavelengths of CTB of the Eu3+-activated oxide- and nitride-based phosphors have been listed in Table 3. It can be observed that the wavelengths of CTB of Eu3+-activated nitride-based phosphors are commonly longer than those of Eu3+-activated oxide-based phosphors. According to the equation proposed by the Jorgensen,17 the relationship between the energy of CTB and the optical electronegativity of the central and complex ions can be expressed by the following formula ECTB = [xopt(X ) − x uncorr(M )] × 30 × 103 cm−1

(3)

where ECTB is the energy of CTB and xopt(X) and xuncorr(M) are the optical electronegativities of the complex and central ions. To make the optical electronegativity be in direct proportion to Pauling electronegativity, Jorgensen introduced a constant 30 × 103 cm−1 and made a modification of ligand field, and the optical electronegativity of the O2+ ions is bigger than that of N3− ions, implying a higher energy of the ECTB of Eu3+−O2−. This well explains that the wavelengths of CTB of Eu3+-activated oxide-based phosphors are shorter than those of Eu3+-activated nitride-based phosphors. 10108


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The Journal of Physical Chemistry C Concentration quenching of α-M3B2N4 (M = Ca, Sr):Eu3+ is investigated and exhibited in Figure 8a,b. Clearly, with the increase in the Eu3+ content, the emission intensity increases obviously until the Eu3+ content reaches the quenching concentration, and the optimal Eu3+ content in α-Sr3B2N4 and α-Ca3B2N4 is 0.03 and 0.04, respectively. Furthermore, the critical distance (Rc) between Eu3+ ions can be calculated by the formula proposed by Blasse and Grabmaier as24 ⎡ ⎤1/3 3×V Rc = 2 × ⎢ ⎥ ⎣ 4 × π × xc × N ⎦

(4) 3+

where xc is the critical concentration of Eu ions, V is the volume of the unit cell, and N is the number of formula units per unit cell. In this work, the volume of α-Sr3B2N4 and α-Ca3B2N4 is 444.3 and 388.9 Å3, respectively; N = 6. So the critical distances of Eu3+ ions in α-Sr3B2N4 and α-Ca3B2N4 are calculated to be about 16.7 and 14.5 Å, respectively. Because the critical distances of α-Sr3B2N4 and α-Ca3B2N4 all are much longer than 4 Å, there is little possibility of concentration quenching via the exchange interaction mechanism.25 To further investigate the energy-transfer mechanism between the Eu3+ ions in the host, the relationship between the emission intensity and the doping concentration is discussed. According to the equation proposed by Van Uitert and Ozawa, the emission intensity (I) per activator ion follows the formula26,27 I /x = K × [1 + β × x θ /3] − 1

(5)

where I/x is the emission intensity (I) per doping concentration (x) and β and K are constants for the given host lattice. θ = 6, 8, 10 for dipole−dipole, dipole−quadrupole, quadrupole−quadrupole interaction, respectively. Equation 5 can simply be rearranged as follows ⎛I⎞ θ Ln⎜ ⎟ = K ′ − Ln(x)(K ′ = Ln K − Ln B) ⎝x⎠ 3

(6)

Figure 8c,d plots the relationship of Ln(I/x) versus Ln(x) for the α-Sr3B2N4:Eu3+ and α-Ca3B2N4:Eu3+, respectively. The results are linear with the slope of −1.9055 and −1.7605, and hence the value of θ for α-Sr3B2N4:Eu3+ and α-Ca3B2N4:Eu3+ is about 5.716 and 5.281, respectively, implying that the energy transfer happens between Eu3+ ions through the dipole−dipole interaction. To further investigate the energy transfer between Eu3+ ions, the decay curves of α-Sr3B2N4:xEu3+ (0.01 ≤ x ≤ 0.06) and α-Ca3B2N4:xEu3+ (0.01 ≤ x ≤ 0.06) have been measured and are shown in Figure 9a,b, respectively. The curves can be well-fitted by a second-order exponential decay curve using the following equation28 I = A1 exp(−t / τ1) +A 2 exp(−t / τ2)

Figure 9. Decay curves of (a) α-Sr3B2N4:xEu3+ (0.01 ≤ x ≤ 0.06) and (b) α-Ca3B2N4:xEu3+ (0.01 ≤ x ≤ 0.06).

becomes stronger leading to the concentration quenching. As for α-Ca3B2N4, the average lifetimes of τ are located around 10 ms and show similar regular to the α-Sr3B2N4. Figure 10a,b presents the temperature dependence of the PL spectra of α-Sr3B2N4:0.02Eu3+ and α-Ca3B2N4:0.02Eu3+. With the increase in the temperature, the emission intensity shows an obviously decrease. At 423.15 K, the emission intensity of α-Sr3B2N4:0.02Eu3+ and α-Ca3B2N4:0.02Eu3+ is ∼10% of the initial intensity at room temperature. As exhibited in Figure 10c, α-Sr3B2N4:0.02Eu3+ and α-Ca3B2N4:0.02Eu3+ present a bad thermal stability. To explain the phenomenon, the configurational coordinate diagram of Eu3+with different energy levels and CTB has been drawn and displayed in Figure 10d. In such mode, the excited electrons would return to the ground state through the following three ways: Most electrons process the way ① with the light emitting, and the partial electrons absorbing the thermal activation energy ΔE2 would go back to the ground state along the way ②, implying the nonradiative transition. In the phase of α-Sr3B2N4:0.02Eu3+ and α-Ca3B2N4:0.02Eu3+, the CTB provides a bridge for the electrons returning to ground state like the way ③, which is the reason for the low thermal stability of α-Sr3B2N4:0.02Eu3+ and α-Ca3B2N4:0.02Eu3+.

(7)

where A1 and A2 are fitting constants and τ1 and τ2 are short and long lifetimes for exponential components, respectively. So the average lifetime τ can be calculated by the formula as follows29 τ=

(A1τ12 + A 2 τ22) (A1τ1 + A 2 τ2)

(8)

According to the above equation, the average lifetimes of α-Sr3B2N4:xEu3+ (0.01 ≤ x ≤ 0.06) are calculated to be about 29, 28, 27, 27, 26, and 25 ms, respectively. Clearly, with the increase of in Eu3+ content, the average lifetimes of α-Sr3B2N4:xEu3+ (0.01 ≤ x ≤ 0.06) show an obvious decrease due to the energy transfer happening between Eu3+ ions, and nonradiative transition 10109


Article


Figure 10. Temperature dependence of the PL spectra of (a) α-Sr3B2N4:0.02Eu3+ and (b) α-Ca3B2N4:0.02Eu3+. (c) Temperature dependence of relative intensity of α-Sr3B2N4:0.02Eu3+ and α-Ca3B2N4:0.02Eu3+. (d) Configurational coordinate diagram of the ground states and the excited states of Eu3+ ions.

The typical samples excited by the 360 nm UV lamp have also been displayed and show a bright red light.

4. CONCLUSIONS Eu3+-activated α-Sr3B2N4 and α-Ca3B2N4 have been successfully synthesized. The PL and PLE spectra indicate that α-Sr3B2N4:Eu3+ and α-Ca3B2N4:Eu3+ not only emit a sharp red light but also have a broad excitation band. The same crystal structure of α-Sr3B2N4 and α-Ca3B2N4 implies the similar electron structure and excitation peak pattern, but the different coordination environment of Eu3+ in α-Sr3B2N4 and α-Ca3B2N4 leads to the different peak position of excitation band. Compared with α-Ca3B2N4:Eu3+, α-Sr3B2N4:Eu3+ has a longer excitation band centered at 370 nm, which matches well with NUV-LED chips. The energy transfer happening between the Eu3+ ions has been calculated to be through dipole−dipole interaction, and the decay curves provide the evidence. As for its commercial application, the poor thermal stability of α-Sr3B2N4:Eu3+ and α-Ca3B2N4:Eu3+ needs further improvement, but its CTB well solves the problem that Eu3+-activated phosphors suffer from. In conclusion, this work provides a good way to obtain the sharp red light with the broad excitation band.

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Figure 11. Chromaticity coordinates of α-Sr3B2N4:0.02Eu3+ and α-Ca3B2N4:0.02Eu3+.

The chromaticity coordinates of α-Sr3B2N4:0.02Eu3+ and α-Ca3B2N4:0.02Eu3+ have been shown in Figure 11, and the chromaticity coordinates of α-Sr3B2N4:0.02Eu3+ and α-Ca3B2N4:0.02Eu3+ are (0.646, 0.351) and (0.664, 0.329), respectively, which is close to the standard of NTSC (National Television Standards Committee) (0.67, 0.33) for red phosphor.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86-931-8913554. Tel: +86-9318912772. ORCID

Yuhua Wang: 0000-0002-5982-8799 10110


Article

The Journal of Physical Chemistry C Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Specialized Research Fund for the Doctoral Program of Higher Education (no. 20120211130003) and the National Natural Science Funds of China (Grant No. 51372105).

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A Nitride-based Red Phosphor with a Sharp Emission Line and Broad

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