The green-emitting phosphor Ca3Si2O4N2:Eu2+ was synthesized using a solid-state reaction. The luminescence properties, diffuse reflection spectrum, and thermal quenching were firstly studied, and a white light-emitting diode (wLED) was fabricated using the Eu2+-activated Ca3Si2O4N2 phosphor. Eu2+-doped Ca3Si2O4N2 exhibited a broad green emission band centered between 510 and 550 nm depending on the concentration of Eu2+. The optimal doping concentration of Eu2+ in Ca3Si2O4N2 was 1 mol%. The energy transfer between Eu2+ ions proceeds by an electric multipolar interaction mechanism, with a critical transfer distance of approximately 30.08 Å. A wLED with an color-rendering index Ra of 88.25 at a correlated color temperature of 6029 K was obtained by combining a GaN-based n-UV LED (380 nm) with the blue-emitting BaMgAl10O17:Eu2+, green-emitting Ca3Si2O4N2:Eu2+, and red-emitting CaAlSiN3:Eu2+ phosphors. The results present Ca3Si2O4N2:Eu2+ as an attractive candidate for use as a conversion phosphor for wLED applications.
© 2011 OSA
In recent years, there has been considerable interest in white light-emitting diodes (wLEDs) owing to their long operation lifetime, low energy consumption, high material stability, and environmentally friendly characteristics [1–4]. These desirable characteristics may ultimately lead to the replacement of gas-discharge fluorescent lamps by wLEDs in the near future. In general, white light can be generated by a combination of blue LED chips coated with the yellow-emitting phosphor Y3Al5O12:Ce3+ (YAG:Ce3+). Nevertheless, the drawbacks of this method include a low-rendering index and a high color temperature due to the deficiency of red emission in the visible spectrum. The alternative approach to generating white light involves combining a near ultraviolet (n-UV) LED (380–420nm) with RGB (red, green, blue) phosphors. Consequently, the development of new phosphors that can be effectively excited in the near ultraviolet range is an important prospect that requires prompt attention.
Nitride/oxynitride hosts such as M2Si5N8 (M = Ca, Sr, Ba) [5–9], MAlSiN3 (M = Ca, Sr) [10–14], MYSi4N7 (M = Sr, Ba) [15,16], MgSiN2 , MSi2O2N2 (M = Ca, Sr, Ba) [18,19], α-SiAlON [20,21], and β-SiAlON , which present intense luminescence when activated with Eu2+, are good candidates for host materials owing to several merits such as their high chemical and physical stability and low thermal quenching. The powder X-ray data of Ca3Si2O4N2 was first reported by Huang et al.  Sharafat later reported the crystal structure of Ca3-xSi2O4+2xN2-2x . The luminescence properties of Ca3Si2O4N2, however, have not been previously investigated, to the best of our knowledge. In the present study, the luminescence properties, thermal stability, and application of the green-emitting Ca3Si2O4N2:Eu2+ phosphors in the fabrication of a wLED by combination with a n-UV LED are investigated. The results show that Ca3Si2O4N2:Eu2+ has good thermal stability, and the white LEDs fabricated using Ca3Si2O4N2:Eu2+ exhibit high CRI. The green-emitting phosphor based on Ca3Si2O4N2:Eu2+ is a promising candidate for solid-state lighting applications.
The polycrystalline phosphors composed of Ca3Si2O4N2:Eu2+ were prepared by a solid-state reaction in which the constituent raw materials CaCO3 (99.99%), α-Si3N4 (99.99%), SiO2 (99.6%), and Eu2O3 (99.99%) (all from Aldrich Chemicals, Milwaukee, WI, U.S.A) were weighed in stoichiometric proportions. The powder mixtures were sintered under a reducing atmosphere (15%H2/85%N2) at 1400°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. Powder X-ray diffraction (XRD) of the samples was performed using a Bruker AXS D8 advanced automatic diffractometer with Cu-Kα 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 using a Spex Fluorolog-3 spectrofluorometer (Instruments S.A., N.J., U.S.A) equipped with a 450 W Xe light source and double excitation monochromators. The quantum efficiency (QE) was measured by an integrating sphere whose inner face was coated with Spectralon equipped with a spectrofluorometer (Horiba Jobin-Yvon Fluorolog 3–22 Tau-3). The measurement procedures and correlation theorem were described previously by Liu et al.  The Commission International de I’Eclairage (CIE) chromaticity coordinates for all samples were determined using a Laiko DT-100 color analyzer equipped with a CCD detector (Laiko Co., Tokyo, Japan). The diffuse reflection spectra were recorded in the range of 200–800 nm with a Hitachi 3010 double-beam UV-VIS spectrometer (Hitachi Co., Tokyo, Japan) equipped with a Ø60 mm integrating sphere whose inner face was coated with BaSO4 or Spectralon and PTFE.
3. Results and discussion
3.1. Crystal Structure of Ca3Si2O4N2
Figure 1 shows the X-ray powder diffraction (XRD) pattern of the as-synthesized Ca3Si2O4N2:9%Eu2+ powder. The as-prepared Ca3Si2O4N2 material was obtained as a single phase compound that matches well with JCPDS file No. 38-0944. From the single crystal X-ray data reported by Sharafat , Ca2.88Si2O4.23N1.77 crystallizes in the cubic space group Pa with unit cell dimensions of a = b = c = 15.07Ǻ and with twenty-four formula units per unit cell. The crystal structure of Ca3Si2O4N2, which is similar to that of Ca3Al2O6 , is shown in Fig. 2 . The crystal structure of Ca3Si2O4N2 consists of isolated 12-membered rings composed of Si12(O,N)36. Seven different crystallographic sites are available for the Ca2+ ions, which are located in the voids of the packed 12-membered rings. The effective cationic radii (r) reported by Shannon  indicate that the ionic radius of Eu2+ is close to that of Ca2+. Based on size considerations, it was expected that upon doping, the Eu2+ cation would occupy the Ca2+ lattice site given that the Si4+ site is too small for Eu2+ to occupy.
3.2. Photoluminescence Properties of Ca3Si2O4N2:Eu2+
Figure 3 shows the excitation and emission spectra of Ca3Si2O4N2:Eu2+ for different Eu2+ concentrations. The excitation bands of the samples doped with varying concentrations of Eu2+ had peaks at ~289 nm, ~328 nm, ~368 nm, and ~405 nm, which consisted mainly of unresolved bands of the 4f 65d1 multiplets of the Eu2+ excited states. The sample exhibited a green emission band with maximum emission at 510 nm, under optimal excitation at 330 nm. Although the excitation maximum was obtained at 330 nm, the samples could be efficiently excited in the range of 350–400 nm. Consequently, an n-UV chip (380 nm) was selected for use in the subsequent fabrication of the white LEDs. A broad, asymmetric band was observed in the emission spectrum in the wavelength range of 450–650 nm, which corresponds to the allowed 4f 65d1 → 4f 7 electronic transitions of Eu2+. The broad excitation band was attributed to the high covalency of the CaEu-N bond and a large crystal-field splitting effect. The crystal-field splittings of Eu2+ shown in Table 1 were estimated to be 18730~21050 cm−1 in Ca3Si2O4N2. The Stokes shifts of Ca3Si2O4N2:x%Eu2+ (x = 0.0025–0.09) were estimated to be 10973 cm−1–12500 cm−1. The emission band shifts toward longer wavelength as the concentration of the Eu2+ dopant increases. This bathochromic shift is ascribed to the change in the crystal-field splitting of Eu2+. Consequently, the phenomenon can be explained in terms of energy transfer from Eu2+ ions at the higher 5d levels to those at the lower levels. This causes the emission energy from the 5d excited state to the 4f ground state to become lower, and therefore, the emission shifts to longer wavelength.
Figure 4 shows the PL intensity of Ca3Si2O4N2:Eu2+ as a function of doped Eu2+ content. The optimal doping concentration was observed at 1 mol%, the PL intensity was found to decline dramatically as the content of Eu2+ exceeds 1 mol% due to concentration quenching. Concentration quenching is mainly caused by energy transfer among Eu2+ ions, the possibility of which increases as the concentration of Eu2+ increases. Blasse  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:
Non-radiative energy transfer from one Eu2+ ion to another Eu2+ ion may take place via an exchange interaction, radiation reabsorption, or an electric multipolar interaction. The exchange interaction requires a large direct or indirect overlap of the wavefunctions of the donor and acceptor, and this mechanism is responsible for energy transfer in the case of forbidden transitions. The critical distance for the exchange interaction is approximately 5 Ǻ.24 The 4f 7→ 4f 65d1 transition of Eu2+ is allowed; hence, the exchange mechanism plays no role in the energy transfer within the Ca3Si2O4N2:Eu2+ phosphors. The mechanism of radiation reabsorption is only effective when the fluorescence and absorption spectra are broadly overlapping. Therefore, radiation reabsorption does not occur in this case. The process of energy transfer between Eu2+ ions in the Ca3Si2O4N2:Eu2+ phosphor is attributed to the electric multipolar interaction, as suggested by Dexter .Fig. 4 illustrates the I/x dependence on x on a logarithmic scale. The dependence of log (I/x) on log (x) was found to be relatively linear, and the slope was determined to be −1.15. The value of θ was found to be approximately 6, indicating that the concentration quenching mechanism of Eu2+ emission was dominated by the dipole–dipole interaction.
The CIE chromaticity coordinates of the Ca3Si2O4N2 phosphors with different Eu2+ dopant contents are summarized in Table 1 and also shown in Fig. 5 . The chromaticity index varies from (0.25, 0.50) for the composition with 0.25 mol% to (0.42, 0.52) for the composition with 9 mol%. The internal () and external () quantum efficiencies (QEs) were calculated based on the equations reported by Hirosaki et al. previously . The internal quantum efficiency of Ca3Si2O4N2:Eu2+ and Ba2SiO4:Eu2+ phosphor were found to be 23.8% and 89.3% and the corresponding external quantum efficiency is 17.1% and 74.8%, respectively, at the excitation wavelength of 400 nm.
Figure 6 shows the diffuse reflectance spectra for the parent and the Eu2+-doped Ca3Si2O4N2 phosphors. The daylight color of the undoped Ca3Si2O4N2 is white. Hence, the Ca3Si2O4N2 exhibited high reflectance in the UV to visible range (250–700 nm) and a decrease in reflectance in the 200–250 nm range, which corresponds to the host lattice absorption. Accordingly, the absorption edge of the undoped materials was estimated to be 245 nm (5.06 eV). The diffuse reflectance spectrum of the Eu2+-doped Ca3Si2O4N2 shows an absorption band from 250 to 500 nm, which was attributed to the 4f → 5d transition of Eu2+ in the Ca3Si2O4N2 matrix and which was consistent with the excitation spectrum.
3.3 Thermal Quenching Properties of Ca3Si2O4N2:Eu2+
The thermal stability of phosphor is one of the major considerations for its application in high-power LEDs. The temperature dependence of the PL intensity of Ca3Si2O4N2:Eu2+ under excitation at 380 nm and above room temperature is shown in Fig. 7 . The activation energy (Ea) can be expressed by:Fig. 7, inset) shows that the thermal quenching of Ca3Si2O4N2:Eu2+ was superior to that of Ba2SiO4:Eu2+. The results also indicate that Ca3Si2O4N2:Eu2+ could be a promising phosphor for high-power LED applications.
3.4 Electroluminescent Properties of Ca3Si2O4N2:Eu2+
Figure 8 shows the spectrua of the white LED composed of an n-UV chip (380 nm) and blue (BaMgAl10O17:Eu2+), green (Ca3Si2O4N2:Eu2+) and red (CaAlSiN3:Eu2+) phosphors. An emitter-type LED package was chosen for the fabrication of the LED device on the basis of its high light extraction efficiency. The CIE color coordinates and correlated color temperature (CCT) of the white LED were found to be (0.322, 0.330) and 6029 K, respectively. The average color-rendering index, Ra, was determined to be 88.25, which was considered to be fitted for lighting applications. The luminous efficiency of this white LED is 20.1 lm/W at 350 mA. The inset of Fig. 8 shows the appearance of a well-packaged LED lamp in operation. These results show that Ca3Si2O4N2:Eu2+ could be a potential green-emitting phosphor for applications of display and illumination.
Eu2+-doped nitridosilicate phosphors, Ca3Si2O4N2:Eu2+, for application in white LEDs have been successfully synthesized by a solid-state reaction. The Ca3Si2O4N2:Eu2+ phosphor exhibited excellent luminescent properties and good thermal stability. The critical transfer distance between Eu2+ ions in Ca3Si2O4N2 was estimated to be 30.08 Å. Furthermore, the white LEDs developed in this study exhibit a high color-rendering index of 88.25. Therefore, Ca3Si2O4N2:Eu2+ is proposed to be a promising candidate for use in white LEDs.
The authors would like to thank for the financial support from Industrial Technology Research Institute under contract no. A301AR4150, the NSC (contract no. 97-2113-M-002-012-MY3 and 97-3114-M-002-005), and the Economic Affair (contract no. 97-EC-17-A-07-S1-043).
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