Recently, the generation of white light through a combination of an ultraviolet or blue-emitting chip and phosphors as a solidstate lighting source has attracted considerable interest [1,2]. These phosphors and chip-based light-emitting diodes (LEDs) have several advantages over conventional incandescent and fluorescent lamps, in terms of power efficiency, long lifetime, absence of pollution, and design flexibility [2,3]. White lightemitting diodes (W-LEDs) have been the subject of increasing interest, due to their advantages of low energy consumption, long life span, and lack of pollutants such as Hg, and to their potential applications in indicators, backlights, automobile headlights and general illumination [4]. White light-emitting diodes (LEDs) are in high demand in solid-state lighting (SSL) technology, because of their most challenging application as a replacement for conventional incandescent and fluorescent lamps. Therefore, they are considered the next generation of solid-state lighting. Without doubt, SSL is a pivotal emerging technology that promises to alter lighting in the future. With the development of materials science and technology, soft-chemical synthesis methods such as sol-gel [5], co-precipitation [6] and hydrothermal synthesis [7,8] have been successfully applied to synthesize phosphatewhite LED phosphors. All of these methods use liquid components, which can be accurately controlled, and thoroughly mixed. Combustion synthesis is characterized by more complete reaction, increased reaction efficiency, fast reaction, and convenience.

The aim of this work is to investigate the effect of Ca_xSr_2-x and activator on the structural and luminescent properties of green-emitting Ca_xSr_2-xSiO₄:Eu²⁺ nano phosphor. For small spherical particles with smooth and round surfaces, Ca_xSr_2-xSiO₄:Eu²⁺ phosphors were synthesized. Using urea as fuel, and ammonium nitrate as oxidizer, Ca_xSr_2-xSiO₄:Eu²⁺ was successfully synthesized, using a combustion method. The influence of X(Ca_xSr_2-x) content on the crystalline structure of the produced powders of Ca_xSr_2-xSiO₄:Eu²⁺ phosphors was investigated. The results of characterization showed that the phosphors particles are nanosize. The phosphors exhibit a green emission spectrum for near uv excitation. The material has application as a fluorescent material for ultraviolet light-emitting diodes (UVLEDs).

In this work, we synthesize a Ca_xSr_2-xSiO₄:Eu²⁺ nano phosphor by a combustion method. Also, we investigate the effect of Ca_xSr_2-xSiO₄:Eu_y on the structural transformation and luminescent properties of green-emitting Ca_xSr_2-xSiO₄:Eu²⁺ nano phosphor.

In this study, CaSrSiO₄:Eu²⁺ phosphors were prepared using a combustion method. Ca(NO₃)₂ (99.997%, Aldrich), Sr(NO₃)₂ (99.995%, Aldrich), SiO₂ (99.9%, Aldrich), and Eu₂O₃ (99.999%, Aldrich) were used as the starting materials. The CaSrSiO₄ phosphors were doped by Eu²⁺, with the molecular formula of Ca_xSr_2-xSiO₄:Eu_y ²⁺. Ca(NO₃)₂, Sr(NO₃)₂, SiO₂ and Eu₂O₃ were mixed together by mol ratio, and then distilled water was added. Urea was used as fuel, and ammonium nitrate served as the oxidizer. The parameters were measured, and are shown in Table 1. A flowchart of the preparation of the phosphor powders is shown in Fig. 1. The urea and ammonium nitrate solution was heated to 80℃, and continuously stirred, using a magnetic bar. The metal solution was dropped into the fuel, and the heating was continued for 30 minutes at 80℃. The solution was then transferred to a pre-heated furnace set to 500℃. After heating, different samples of the mixture were sintered in a furnace for 3 hours, at 600℃~1,400℃. Crystalline development of the resulting samples was checked by X-ray diffraction (XRD, model D/MAX-2200), using CuKα-radiation in the range of 2 θ = 20~80°. Measurement of the photoluminescence (PL) spectra was carried out by 150 W Xe lamp (spectrofluorometer, FP-6200, JASCO). The morphology and size of the prepared particles were investigated by fieldemission scanning electron microscopy (FE-SEM, model S-4700, HITACHI).

Figure 2 shows XRD patterns of the Sr₂SiO₄:Eu²⁺(X=0) sintered at different temperatures. At the different temperatures, the XRD patterns show pure Sr₂SiO₄ phase. In this structure, Sr²⁺ ions locate at two kinds of nonequivalent lattice sites, and their coordination numbers are nine and ten, respectively. When the Eu is introduced into the Sr₂SiO₄ structure, it takes the place of the Sr²⁺. In the case of precursor, not only the Sr₂SiO₄ phase was observed, but also impurities peaks, due to material not having been synthesized. At 1,000℃, the diffraction peaks became sharper and stronger. The Sr₂SiO₄ phase crystallized with results that are in good agreement. However, at 1,200℃, the Sr₂SiO₄ peaks were markedly weakened. Figure 3 shows XRD patterns of the Ca_xSr_2-xSiO₄:Eu²⁺ phosphors (x = 0.2, 0.5, 1.0, and 1.5) produced by the combustion method at 1,000℃. The structure of Ca₂SiO₄ is monoclinic, which is different from that of Sr₂SiO₄ (orthorhombic structure). Therefore, substitution of the Ca sites with Sr²⁺ ions might cause a monoclinic to orthorhombic structural transformation [9]. According to JCPDS card 72- 2260, pure CaSrSiO₄ has a orthorhombic crystal structure with Pna2₁(33) space group, and lattice parameters of a = 20.87 Å, b = 9.496 Å, and c = 5.6 Å. In the case of Sr substitution with Ca at a concentration ratio of 1.5, a single phase was identified by XRD, without any extraneous phase. Moreover, the Ca doped ions had a slight influence on the CaSrSiO₄ orthorhombic crystal structure. A comparison of JCPDS No. 72-2260 with the phosphor with Sr concentration ratio of 1.5 showed that the main diffraction peaks were shifted to a lower angle. Therefore, the addition of Sr increased the lattice parameters of the phosphor, because the ionic radius of Ca²⁺ (0.10 nm) is slightly lower than that of Sr²⁺ (0.118 nm) [6]. The main diffraction peaks were shifted to a smaller angle, as the concentration ratio of Ca²⁺ ions in the host lattice was increased from 0.2 to 1.5. Figure 4 shows SEM imagery of CaSrSiO₄:Eu²⁺ synthesized at various firing temperatures. The surface morphologies of the phosphor at 600℃ were rough. With increased sintering temperature, the phosphor particles became spherical, which occurred because the particles condense at higher temperatures [10]. But at 1,200℃, particles with agglomerates were observed. At over 1,200℃, the particles became cohesive and rough, due to the high sintering temperature. At this point, we determined that the characteristics of the phosphor powders were improved by increasing the sintering temperature; however, defects form in the phosphor, when it is heated beyond a critical temperature. Figure 5 shows the excitation and emission spectra of the Ca_1.5Sr_0.5SiO₄:_0.05Eu²⁺. The emission spectrum for 360nm excitation showed a single band, with a peak at 490 nm, which is a green emission. The main emission peak is at about 490 nm, which is ascribed to the transition between 5d and 4f of Eu²⁺. The 5d state of Eu²⁺ is greatly affected by the crystal field, and it can be spitted by different crystal fields. This makes Eu²⁺ emit different light when the crystal fields change [11]. Figure 6 shows the PL emission spectra of the Ca_1.5Sr_0.5SiO₄:Eu²⁺ with various concentrations of Eu²⁺. The emission intensity of green emission increased with increasing Eu²⁺ concentration to 0.05. Moreover, when the concentration of Eu²⁺ was above 0.05, a decrease in the emission intensity was observed and this may be attributed to non-radiative energy transfer between the Eu²⁺ ions. This non-radiative energy transfer may be attributed to the distance dependent multipole-multipole interaction between Eu²⁺ ions [12,13]. As the concentration increases, the distance between dopant ions decreases, and this may lead to non-radiative energy transition. The emission spectra showed the presence of a broad band, whose broadness indicates the existence of an interaction between the host and activator. This was attributed to the presence of excited electrons in the outer shell of the Eu²⁺ ions. The probability of energy transfer between Eu²⁺ ions increased with increasing Eu²⁺ concentration. Figure 7 shows the PL emission spectra of the CaSrSiO₄:Eu²⁺ phosphor synthesized at various firing temperatures under 360 nm excitation. The excitation band must originate from the 4 f → 5 d transition of doped Eu²⁺ ions, because the host Ca_1.5Sr_0.5SiO₄: Eu²⁺ barely showed any absorption between 250 and 450 nm [14].

In the present research, the Ca_xSr_2-xSiO₄:Eu²⁺ phosphors with different concentrations of Eu²⁺ were prepared by a combustion method. The particles were found to be small and spherical with round surfaces. With increased sintering temperature, the phosphor particles became spherical, which occurs because the particles condense at higher temperatures. We determined that the characteristics of the phosphor powders were improved by increasing the sintering temperature. The Ca_xSr_2-xSiO₄:Eu²⁺ emission spectrum for 360 nm excitation showed a single band, with a peak at 490 nm, which is a green emission. The luminescent properties of the Ca_xSr_2-xSiO₄:Eu²⁺ phosphor were optimized by changing the Eu²⁺ content. As a result, the highest luminous intensity was at 1,000℃, which was obtained when the Eu²⁺ content (y) was 0.05. Increase of the Eu²⁺ concentration played a key role in enhancing the luminous intensity. In conclusion, these optimized phosphors are expected to have potential application in ultraviolet light-emitting diodes (UV-LEDs) as fluorescent material.