To find out efficient red phosphors used for white light-emitting diodes (LEDs), a new Ba2Gd2Si4O13:Eu3+ phosphor was prepared by conventional solid-state reaction method. The effect of Li2CO3 flux and Eu3+ doping concentrations on structural and luminescent properties of Ba2Gd2Si4O13 phosphors was studied in detail. The phosphors show intense absorption in near ultraviolet-blue region and exhibit intense red emissions with CIE coordinates of (0.66, 0.34) under 393 nm excitation. The integrated emission intensity of Ba2(Gd0.4Eu0.6)2Si4O13 excited at 393 nm, 362 nm and 464 nm is about 3.5, 4.0 and 3.1 times as that of Y2O3:Eu3+ commercial phosphors, respectively. The excellent luminescent properties and good color saturation make it a promising red phosphor for white LEDs.
© 2011 OSA
Over the past few years, white light-emitting diodes (LEDs) has become an interesting field for many excellent characteristics, such as high efficiency, long lifetime, reliability, toxicity-free and energy-saving, etc [1–3]. In addition to three primary colors mixing emissions from three individual LED, white light can also be produced by coupling a blue or a near-ultraviolet (NUV) LED with a down-converting phosphor, much in the same way a fluorescent light bulb works. At present, the latter way has attracted more and more attention for its easy fabrication, low cost, and high brightness [3–5].
Nevertheless, commercial white LEDs based on the combination of a blue LED chip and a yellow-emitting phosphor Y3Al5O12:Ce3+ (YAG:Ce3+) are poor in the color rendering index (CRI) because of color deficiency in red region [6,7], which is not suitable for applications requiring high CRI properties, such as residential and medical lighting. Some red phosphors such as Y2O2S:Eu3+ and (Ca,Sr)S:Eu2+ were used to make up for this shortcoming. Unfortunately, these sulfide-based red phosphors are undesirable because of low chemical stability [8–10]. On the other hand, with the remarkable development of NUV diodes, the combination of an NUV chip with red, green and blue phosphors pave a valid way to generate warm white light in recent years [11–13]. But the efficiency of red phosphor is much lower than that of green and blue phosphors [14–16]. Therefore, the problem is still open as to develop novel red phosphors with high brightness for white LEDs.
Of many rare-earth ions (REI), Eu3+ is an excellent red activator in many classic phosphors, such as Y2O3:Eu3+ and (Y,Gd)BO3:Eu3+, etc [17,18]. Additionally, silicates are usually good choice for many luminescent materials due to their good physical and chemical stability and excellent optical properties. The luminescent properties of REI-doped silicates phosphors, such as Sr2SiO4:Eu2+ , have been extensively investigated because of their potential applications in white LEDs. More interesting, our previous work had demonstrated that Ce3+,Tb3+ codoped Ba2Gd2Si4O13 (BGSO:Ce3+,Tb3+) can emit white light with high quantum efficiency under NUV excitation .
In this paper, we synthesized Eu3+-doped Ba2Gd2Si4O13 (BGSO:Eu3+) powders by solid-state reaction method using Li2CO3 as flux. The influence of Li2CO3 flux on structural and luminescent properties of BGSO:Eu3+ was studied in detail. Furthermore, the integrated emission intensity was compared with the commercial Y2O3:Eu3+ phosphors. Results indicate that BGSO:Eu3+ phosphors can be effectively excited by blue and (or) NUV LEDs and exhibit satisfactory red emissions.
Powder samples of BGSO:Eu3+ were synthesized by a conventional solid-state method. Gd2O3 (99.99%) and Eu2O3 (99.99%) were purchased from Shanghai YueLong Nonferrous Metals Co., Ltd., China. BaCO3, SiO2, H3BO3, NH4F, NH4Cl, Li2CO3 (all with purity of A.R.) were purchased from Sinopharm Chemical Reagent Co., Ltd., China.
To facilitate the reaction and to improve the crystallinity of luminescent materials, flux agents or molten salts are often added to provide a more interactive medium for solid-state reaction [5,19]. The use of an interaction medium often results in lower reaction temperatures and allows for the optimization of grain size of luminophors being synthesized.
In this process, Li2CO3 was selected as flux by comparison with NH4F, NH4Cl and H3BO3. Stoichiometric amounts of reactants were first weighed out and well ground, then fired at 1100 °C for 4 h in air atmosphere. The resulting samples were cooled to room temperature and pulverized for further characterizations. For comparison, BGSO:Eu3+ phosphors fired at 1100 °C, 1200 °C and 1300 °C were also prepared without Li2CO3 flux.
The crystalline structures of the prepared powders were investigated by X-ray diffraction (XRD) on a Philips X’Pert PRO SUPER X-ray diffraction apparatus with Cu Kα radiation (λ = 0.154056 nm) as the incident radiation. Excitation spectra and emission spectra were measured by using an FS920 spectrofluorometer (Edinburgh Instruments) with a CW Xe lamp (450 W) as the excitation light source and an RR928P photomultiplier for signal detection. All the measurements were carried out at room temperature.
3. Results and discussion
BGSO is a new silicate structure that first reported by Wierzbicka et al. in 2010 . It has a monoclinic structure with space group C2/c and lattice constants of a = 12.896(3) Å, b = 5.212(1) Å, c = 17.549(4) Å, β = 104.08(3) o, V = 1144.1(5) Å3, and Z = 4. The ionic radii of Ba2+ (CN = 8), Gd3+ (CN = 6), Si4+ (CN = 4), Eu3+ (CN = 6) are 1.42, 0.94, 0.26, and 0.95 Å, respectively. In view of the effective ionic radii of cations and different coordination numbers, Eu3+ dopants were expected to replace Gd3+ sites in BGSO.
Figure 1 gives the XRD patterns of BGSO:Eu3+ (60 mol%) powders (a) fired at different temperatures ranged from 1100 °C to 1300 °C without flux and (b) fired at 1100 °C with various Li2CO3 flux amounts, as well as the calculated patterns from ICSD file of BGSO as a reference.
Li2CO3 flux plays an important effect on the structure of BGSO. Without Li2CO3 flux, BaSiO3 and Gd2O3 impurities were observed in XRD patterns for samples annealed below 1200 °C. Pure BGSO can be obtained for heating temperature above 1300 °C only. More interesting, for samples prepared at relative low temperature (1100 °C) by using Li2CO3 as a flux, the diffraction peaks match quite well with the calculated patterns, indicating that Li2CO3 flux reduced the firing temperature about 200 °C successfully. Besides, XRD patterns indicate that the optimal dosage of Li2CO3 flux is about 2 wt% for the preparation of pure BGSO and excess flux leads to the formation of an unintended LiGdSiO4 impurity (weak sharp peak at 31.6 o for samples prepared with 3, 5 wt% flux) during the reaction process.
Figure 2 portrays the emission spectra of BGSO:Eu3+ phosphors (a) fired at 1100 °C with Li2CO3 flux (2 wt%) and (b) fired at 1300 °C without flux. Upon a NUV irradiation of 393 nm, both samples show characteristic red emissions of Eu3+ which can be assigned to the 5D0 to 7FJ (J = 0-4) transitions. It is well-known that 590 nm emission is associated with 5D0-7F1 magnetic dipole transition and 612 nm emission corresponds to 5D0-7F2 electric dipole transition. In this work, the dominant emission peak of BGSO:Eu3+ locate at 612 nm (5D0-7F2), indicating that Eu3+ ions occupied the sites of non-inversion symmetry in BGSO matrix. Consequently, the phosphor exhibits a red light with high color purity, which can be used to improve the color rendering property of white LEDs.
In addition, the detailed integrated emission intensity of BGSO:Eu3+ powers fired at 1100 °C with various Li2CO3 flux amounts (0 wt% to 5 wt%.), as well as the sample fired at 1300 °C without flux are shown directly in the inset of Fig. 2. The luminescent intensity increases with the content of Li2CO3 and reaches a maximum when the content of Li2CO3 is 2 wt%. With the help of Li2CO3 flux, compared with the sample synthesized at 1300 °C, the sample prepared at 1100 °C has a higher luminescent intensity. Such phenomena indicate that Li2CO3 flux not only reduced the firing temperature about 200 °C, but also improved the luminescent performance. The fluorescence enhancement also proves that Li2CO3 flux is helpful to enhance the crystallization degree and to decrease surface defects, agreeing well with the results obtained from XRD patterns.
To investigate the contribution of Eu3+ ions towards the luminescent properties of BGSO:Eu3+ phosphors, we synthesized BGSO powders with 2 wt% Li2CO3 flux by varying the concentration of Eu3+. The doping content of Eu3+ is labeled as x (x = 0.1-0.8). Luminescent properties are highly dependent on the activator concentration, and brightness tends to increase with increasing activator concentration. Figure 3 presents the emission intensity as a function of Eu3+ concentration. When amount of Li2CO3 flux is fixed, with Eu3+ increasing, the intensity of Eu3+ emission increases rapidly first and reaches a maximum (x = 0.6) and then remarkably decreases when Eu3+ content is further increased.
This phenomenon is due to pairing or aggregation of activators at high concentration led to efficient resonant energy transfer between Eu3+ ions and a fraction of energy migration to distant luminescent killer or quencher followed by the appearance of quenching effect. According to Blasse’s energy transfer mechanism in oxide phosphors , the critical transfer distance (Rc) can be calculated from the concentration quenching data by using the following equation:
To probe the luminescent properties of BGSO:Eu3+ phosphors, optimized composition Ba2(Gd0.4Eu0.6)2Si4O13 phosphor was compared with commercial Y2O3:Eu3+ red phosphors by excitation and emission spectra.
Excitation spectra of BGSO:Eu3+ and commercial Y2O3:Eu3+ phosphors monitored at 612 nm are presented in Fig. 4 . It can be clearly seen that both excitation spectra consist of two parts: one broad band at 255 nm and several sharp peaks in 280-500 nm region. Obviously, the first one is caused by the well-known O2−-Eu3+ charge transfer band (CTB), and the other sharp peaks are related to the 4f-4f transitions of Eu3+ ions . Compared with commercial Y2O3, the 4f-4f transitions of Eu3+ ions in BGSO are much stronger in the range of 280-500 nm, which fit well with the emission wavelength of commercial chips. This character makes these phosphors suitable for application in white LEDs combined with both blue and NUV light.
Figure 5 shows the emission spectra of BGSO:Eu3+ and commercial Y2O3:Eu3+ phosphors (λex = 393 nm). Both samples show characteristic red emissions of Eu3+ which can be assigned to the 5D0 to 7FJ (J = 0-4) transitions. Obviously, the emission intensity of BGSO:Eu3+ phosphor is much stronger than that of Y2O3:Eu3+ phosphor. The integrated emission intensity of BGSO:Eu3+ phosphor is about 3.5 times stronger than that of Y2O3 excited at 393 nm, whereas 4 and 3.1 times better than that of Y2O3:Eu3+ excited at 362 and 464 nm, respectively (as shown in inset of Fig. 5). Hence, it is believed that the emission efficiency of BGSO:Eu3+ have meet the commercial requirement in solid-state lighting.
The luminescence colors of as-prepared BGSO:Eu3+ powers are characterized by Commission International de I’Eclairage (CIE) chromaticity coordinates. The chromaticity coordinate of the BGSO:Eu3+ and commercial Y2O3:Eu3+ phosphors are (0.66, 0.34) and (0.65, 0.35), respectively. The chromaticity coordinates of BGSO:Eu3+ phosphor are even closer to the standard of the National Television System Committee (NTSC) for red phosphor (0.67, 0.33) than that of Y2O3:Eu3+, indicating that the phosphor has a high color purity. The result proves that this phosphor can effectively improve the CRI of white LEDs.
A red-emitting phosphor Ba2Gd2Si4O13:Eu3+ was successfully prepared by solid-state reaction method by using Li2CO3 as flux. Li2CO3 flux can reduce the firing temperature about 200 °C, maintain the pure phase structure and enhance the luminescent performance. The integrated emission intensity of Ba2Gd2Si4O13:Eu3+ excited at 393 nm, 362 nm and 464 nm is about 3.5, 4.0 and 3.1 times as that of commercial red phosphor (Y2O3:Eu3+), respectively. The Ba2Gd2Si4O13:Eu3+ phosphors, with relative low synthesis temperature, good color saturation, strong absorption in both NUV and blue region, and excellent luminescent properties, are promising red emitting candidates for phosphor-converted white LEDs.
This work was supported by the National Natural Science Foundation of China (No. 10904131).
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