1. Introduction
White light-emitting diodes (LEDs) have been developing rapidly over the past decade, since its advanced properties such as long life time, high efficiency, and environmentally friendliness without use of mercury. The application of white LEDs is expanding into extensive fields such as residential lighting, medical lighting, mobile, back lights, traffic lights, emotional lighting and so on. General white LEDs are composed of a blue LED chip and yellow phosphor such as Y3Al5O12:Ce3+ [1]. Such combination certainly achieves generation of artificial white light; however, it is not preferable in respect of the high color temperature and the low color rendering index value (CRI < 75) owing to the lack of red and green components. In case of residential lighting, CRI value should satisfy over 80 [2]. In order to realize such a high CRI value for the white LED, highly efficient bluegreen (470 - 510 nm) and red (650 nm) emitting phosphors capable of excitation by blue or NUV LEDs are demanded.
Eu2+ ions are widely used as activators in phosphors since the emission from Eu2+ attributed to the electron transition 4f65d1 → 4f7 is strongly affected by its surrounding environment, i.e. symmetry, covalence, bond length, crystal-field strength. In other words, the emission wavelengths from Eu2+ ions are able to be tuned from blue to red region with the selection of suitable materials as the hosts. Oxynitrides are regarded as suitable hosts since excitation and emission bands at longer wavelength are expected from the nephelauxetic effect owing to the larger covalence nature for M-N (M: metal) bonds than that for M-O [3,4]. It results in the extensive research for the Eu2+-doped oxynitrides particularly silicon-contained oxynitrides such as β-SiAlON:Eu2+ [5], MSi2O2N2:Eu2+ (M = Ca, Sr, and Ba) [6-8] and Ba3Si6O12N2:Eu2+ [9]. Development of new oxynitride phosphors is an important research topic to enrich the phosphor library with various excitation and emission properties. Sun et al. have reported the synthesis of solid solutions between CaAl4O7 and Ca3Al8Si4O17N4 [10]. Their research has attracted the authors’ interest in investigation of photoluminescence properties of Eu2+-activated CaAl4−xSixO7−xNx solid solutions.
On the other hand, homogeneous distribution of the Eu2+ activators in given host materials is one of the important factors in order to achieve high luminescence efficiency. Solution-based processes are potential methods to achieve homogeneous distribution of the activators [11-18]. Our research group recently has succeeded in improvements of emission intensities for oxynitride phosphors, Na1–xMxAlSiO4–xNx:Eu2+ (M = Mg2+, Ca2+, Sr2+) [13] and Ba3Si6O12N2:Eu2+ [14], by the combined synthesis methods composed of the preparation of oxide precursors by solution-based method and its subsequent nitridation under ammonium atmosphere.
Based on the background described above, the synthesis of oxynitride solid solutions CaAl4−xSixO7−xNx (x = 0 - 4) by the SSR method and investigation of their photoluminescence properties with the Eu2+ activation were examined in the present study. In addition, the combined method involving the solution-based process and the subsequent nitridation of oxide precursor was applied for the synthesis of Ca3Al8Si4O17N4:Eu2+.
2. Experimental
Two series of synthesis were examined for the CaAl4−xSixO7−xNx:Eu2+ solid solutions by a conventional solid state reaction (SSR) method as summarized in Table 1. Raw materials of CaCO3, Eu2O3, α-Al2O3, SiO2, AlN and Si3N4 were weighed and mixed thoroughly in an agate mortal with a pestle according to the ratios listed in Table 1. 2 mol% of europium was substituted for calcium. The mixed powder was heat-treated at 1673 K for 4 h in a H2(4%)-Ar stream. For the x = 4/3 sample corresponding to Ca3Al8Si4O17N4, the combined synthesis method involving the preparation of an oxide precursor by an amorphous metal complex (AMC) method, one of solution-based methods, and its subsequent nitridation under ammonia atmosphere was also examined [13,14]. The oxide precursor having a composition of (Ca, Eu):Al:Si = 3:8:4 was prepared by the AMC method. After dissolving CaCO3 in an aqueous citric acid solution, aqueous solutions of 1 M Eu(NO3)3 and 1 M Al(NO3)3∙9H2O were added. Then, an aqueous solution of propylene glycol-modified silane (PGMS) was added.
PGMS was obtained by an alkoxy group exchange reaction for tetraethoxysilane with propylene glycol at 353 K in the presence of hydrochloric acid as a catalyst [11,15]. The mixed solutions were heated on a heating plate operated at 393 K with stirring to promote polymerization. The obtained polymer gel was pyrolyzed at 723 K for 3 h, and subsequently at 823 K for 10 h to remove organic compounds gradually. The obtained oxide precursor, finally, was heat-treated at 1673 K for 4 h under an ammonia stream (50 ml/min).
The X-ray diffraction (XRD; Bruker AXS: D2 Phaser) was used for the phase identification. Photoluminescence spectra were measured using a fluorescence spectrometer (Hitachi: F-4500) at room temperature. Internal quantum efficiency was measured using another fluorescence spectrometer (Jasco: FP-6500) equipped with an integrating sphere.
3. Results and Discussion
Figure 1 shows XRD patterns of the samples in the sersies A using Si3N4 as a nitrogen source as listed in Table 1. CaAl4O7 (x = 0) was successfully obtained without any impurities. The x = 0.5 sample contained CaAl4O7 and Ca2Al2SiO7 whereas the x = 1 sample was the mixture of Ca2Al2SiO7, Ca3Al8Si4O17N4, Al2O3, and CaAl4O7. The x = 4/3 sample whose nominal composition was equal to Ca3Al8Si4O17N4 of the known Ca-Al-Si-O-N compound was also crystallized in the multiphase of Ca3Al8Si4O17N4, Ca2Al2SiO7, and Al2O3 although there is a report on the synthesis of Ca3Al8Si4O17N4 in a single phase [10]. The x = 2 sample showed a diffraction pattern similar to that for the x =3/4 sample. The x = 3 sample was composed of Ca2Al2SiO7 and unknown phase while the x = 4 sample was mixture of Ca3Si3O9, Si2N2O, and Si3N4. Thus, no known Ca-Al-Si-O-N phases were formed in the samples of x = 3 and 4. The results in the synthesis of the samples in the series A implied the difficulties in the synthesis of the CaAl4–xSixO7–xNx samples using Si3N4 as the nitrogen source even for the known