Luminescent Characteristics of Blue Phosphor CaAl2Si2O8:Eu2+ in White LED Applications by Solid State Method

Using the solid state reaction method, we synthesize the Ca0.88Al2Si2O8:Eu0.12 phosphor powders at different sintering temperatures ranging from 1100 to 1500 C. From XRay diffraction patterns, we confirmed that all the samples acquired mixed CaAl2Si2O8 (triclinic), CaAlO4 (Monoclinic) and SiO8 (Hexagonal) phases. For the Ca0.88Al2Si2O8:Eu0.12 phosphors, the excitation spectra exhibited an unresolved broad band in the range from 200 to 420 nm and maximum intensity at 350 nm. The emission spectra of CaAl2Si2O8:Eu show broad band ranging from 400 to 560 nm and maximum wavelength is found to be at around 440nm, which can be ascribed to the electric-dipole-allowed 4f5d S7/2(4f) transition of the Eu ions. The Commission International De I-Eclairage 1931 (CIE 1931) chromaticity coordinates for Ca0.88Al2Si2O8:Eu0.12 phosphors are calculated and represented at (0.168, 0.144).

Eu 2+ have been widely used for activators in phosphor materials [7][8].Much attention has been paid towards the luminescent properties of Eu 2+ ions activated compounds and the reduction processes of Eu 3+ to Eu 2+ in phosphor preparations.The Eu 2+ ion shows broad emission bands excited by ultraviolet (UV) light (320 -400 nm), arising from interconfigurational 4f 6 5d 1 (excitation state) -4f 7  (ground state) allowed transition [9][10].This transition is strongly dependent on the crystal fields of the host lattices since 5d orbitals are more sensitive to the ligand field.Accordingly, the Eu 2+ becomes a very useful activator in phosphors for applications in displays, lamps and luminescent paintings [11].In general, the reduction of ionic state from Eu 3+ to Eu 2+ needs sintering process in reduce atmosphere such as H 2 /N 2 or carbon.If the reduction of Eu 3+ to Eu 2+ can be realized in air condition, it would highly reduce the cost and increase the safety in preparing the Eu 2+ -activated phosphor materials [12][13][14].
Aluminum silicate is well known as a long-persistent phosphor host with high chemical stability and water resistance.The alkaline-earth materials of MAl 2 Si 2 O 8 (M = Ca, Sr, Ba) have drawn much attention for their good luminescence properties [15][16][17].These compounds exhibit strong 4f 6 5d 1 -4f 7 luminescence on doping Eu 2+ .A series of solid solutions containing isostructural members with slightly different size cations as hosts, such as MAl 2 Si 2 O 8 (M = Ca, Sr, Ba) would be an ideal system to study the effect of crystal chemical variation on the luminescence of Eu 2+ ions, and may offer possibilities for commercial application [18,19].
In this paper, study of the solid state reaction, sintering temperature dependent photoluminescence characteristics of CaAl 2 Si 2 O 8 :Eu 2+ , is reported.Also, lifetime and CIE chromaticity coordinates of CaAl 2 Si 2 O 8 :Eu 2+ have been be discussed with PL spectra behaviour.

Experiment and measurement
The powder of the Ca0.88Al2Si2O8:Eu 2+ 0.12 phosphor was synthesized using a solid state reaction method.At first, CaAl 2 Si 2 O 8 :Eu 2+ precursor powder was prepared by taking stoichiometric molar ratio of calcium carbonate (CaCO 3 , 99.95%, Aldrich), silicon dioxide (SiO 2 , 99.5 %, Aldrich), aluminium oxide (Al 2 O 3 , 99.9%, Aldrich), europium oxide (Eu 2 O 3 , 99.99 %, Aldrich).All regents were taken without any further purification and mixture, then powders were grinded and mixed by the planetary ball-milling method.Powders have been sintered at 1100 -1500 o C.Here we have increase temperature from room temperature to given sintered temperature with 4 o C per a minute, then all samples are keep in sintered temperature for 4 hour in carbon atmosphere, respectively.
The sintered phosphors were ground using an alumina mortar.The thermal properties of the precursors were examined by a thermogravimetric-differential thermal analysis (TG-DTA) at a heating rate of 4 °C/min.And the crystal structure of the phosphor was obtained by X-Ray diffractometer (PHILIPS X'Pert-MPD) with CuKα = 1.546 radiation at 40 kV and 30 mA.The Xray diffraction patterns show the intensities of the diffraction peaks as a function of the detecting angle 2θ.The interplaner spacing (d), matching peaks our samples and reference for JCDPS data, is calculated using the software X'pert High Score Plus.Miller indices were then assigned to the reflection peaks in the diffraction patterns by comparing the data obtained from the samples.Both the photoluminescence and lifetime measurements of emission bands of the phosphors were recorded on the luminescence spectrophotometer (Photon Technology International (PTI)) with a Xe-arc lamp (60 W) as an excitation source.Figure 1 shows the TG-DTA measurements of CaAl 2 Si 2 O 8 precursor, which were carried out to monitor the decomposition process in the host lattice.The endothermic peaks and weight loss were observed until 161 o C. The weight loss is originated from the evaporation of water molecules in the precursor samples.In the heat absorbing section between 161 and 640 o C, one can see the weight loss again.It means that the absorbing section arise from the weight loss due to the decomposition of the remaining evaporation of water molecules and organic material.After 660 o C, endothermic behaviour without weight loss is confirmed and it means that remaining materials participate in the synthesis.FE-SEM images in Figure 3(a) -(e) of Ca0.88Al2Si2O8:Eu 2+ 0.12 phosphor represent the particle shape and surface at each sintering temperature.The particles shows increasing irregular shape with increasing the sintering temperature.Also, the average particle and the crystallite sizes are increases depending on sintering temperature.The average crystallite sizes are displayed in Fig 5 .Interestingly, we have observed the significantly increased average crystallite sizes until 1300 o C.After 1300 o C, the average crystallite size is almost independent on sintering temperature.Note that the particle size is increased with temperature.This can be explained by the Ostwald ripening process [20].As shown in Figure 3(f), EDS was used to examine the elemental composition of Ca 0.88 Al 2 Si 2 O 8 :Eu 3+ 0.12 powder sintered at 1300 o C. The EDS spectrum of the Ca 0.88 Al 2 Si 2 O 8 :Eu 3+ 0.12 powder shows the presence of Calcium (Ca), Aluminum (Al), Silicone (Si) and Europium (Eu), but other components are not detected.0.12 at different sintering temperatures.The PLE spectrum monitored by the emission at 440nm exhibited a broad band within the range from 200 to 420 nm, which can be assigned to the transitions from the ground state 8 S 7/2 (4f 7 ) to the crystal-field-split components of the 4f 6 5d 1 configuration.The excitation band represent a triple overlapping peaks cantered at 320, 350 and 380 nm.The 320 nm (t 2g ) and 380 nm (e g ) are due to the result of 5d orbital separation caused by crystal field splitting peaks, and 350nm is excitation by CaAl 2 O 4 [21][22].The Ca 0.88 Al 2 Si 2 O 8 :Eu 3+ 0.12 phosphor displayed blue luminescence excited by near-UV light.The emission spectrum of Ca 0.88 Al 2 Si 2 O 8 :Eu 2+ 0.12 have a broad band from 400 to 670 nm with a maximum wavelength of about 450 nm, which can be ascribed to the electric dipole-allowed 4f 6 5d 1 -8S 7/2 (4f 7 ) transition of the Eu 2+ ions.PL spectra indicate wide emission region extending to green region.It is obtained triple multi emission band consisted with wave length of 440, 460, and 505 nm by Gaussian-fitting method as shown in figure 4(b).The transition from 5D 1 to be ground states can be split by crystal field.Actually, the emission bands are possible due to the allowed character of the 5d → 4f transitions, and also to the existence of 460 nm of alkaline earth sites available for the activator.Although Eu 2+ can occupy into the interstitial site in SiO 2 crystal [23].Therefore, the overlapped 460nm and 505nm are due to different Eu 2+ site in CaAl 2 Si 2 O 8 and this make different environment to Eu 2+ .Also, the other emission peak (440 nm) is caused by CaAl 2 O 4 [24].From the inset of Figure 4(b), one can see the strongest peak in the blue region described by the coupling of 440 nm and 460 nm.In the emission spectrum, a third contribution in the yellow range around 560 nm is clearly visible.Phosphor converted LEDs (pc-LEDs), based on a blue In 1-x Ga x N diode and a yellow phosphor which is kind of Y 3 Al 5 O 12 :Ce 3+ [3][4][5][6], suffered from a low color rendering due to the lack of green and red light in the spectrum and of inherently high correlated color temperatures (CCT).To solve this problem consists by using a cyan phosphor with a small Stokes shift and uses a violet or near UV excited diode in combination with an additional blue phosphor.Therefore, it is completely explained that Ca 0.88 Al 2 Si 2 O 8 :Eu 2+ 0.12 phosphors have good color rendering.where I is a luminescence intensity at time, I   0.12 could be a potential blue color emitting candidate, which can be used to improve the quality of white.

Figure 3 .
Figure 3. XRD patterns of Ca 0.88 Al 2 Si 2 O 8 :Eu 3+ 0.12 phosphors at different sintering temperatures The XRD pattern of Ca0.88Al2Si2O8:Eu 2+ 0.12 phosphor synthesized at 1100 -1500 o C for 4hour under reduce atmosphere are shown in Figure 2. The main peaks of phosphors from XRD pattern corresponding to CaAl 2 Si 2 O 8 (JCDPS 41-1486, triclinic), CaAl 2 O 4 (JCDPS 34-0440, Monoclinic) and SiO 2 (JCDPS 33-1161, Hexagonal) phase.Samples were crystallized into a triclinic phase with a space group of P-1.The calculated lattice constants sintered at 1300 o C were a = 8.1430 Å, b = 12.8789 Å, and c = 14.1828Å , which are very close to the corresponding values (a = 8.1756 Å, b = 12.8720 Å, c = 14.1827Å) in the standard JCPDS 41-1486.We have observed the increasing the intensity of the CaAlO 4 until 1300 o C, then the intensity gradually decreasing, and opt-out silicone oxide in the reaction is confirmed by increasing intensity of SiO 2 marked as triangle body in Fig 2.

Figure 4 .
Figure 4. (a) PLE and (b) PL spectra of Ca 0.88 Al 2 Si 2 O 8 :Eu 2+ 0.12 phosphors as a function of sintering temperature Figure 4 shows the measured photoluminescence (a) excitation (PLE) and (b) emission (PL) spectra of Ca 0.88 Al 2 Si 2 O 8 :Eu 2+0.12 at different sintering temperatures.The PLE spectrum monitored by the emission at 440nm exhibited a broad band within the range from 200 to 420 nm, which can be assigned to the transitions from the ground state 8 S 7/2 (4f 7 ) to the crystal-field-split components of the 4f 6 5d 1 configuration.The excitation band represent a triple overlapping peaks cantered at 320, 350 and 380 nm.The 320 nm (t 2g ) and 380 nm (e g ) are due to the result of 5d orbital separation caused by crystal field splitting peaks, and 350nm is excitation by CaAl 2 O 4[21][22].The Ca 0.88 Al 2 Si 2 O 8 :Eu 3+

Table 1 .
In addition, CIE 1931 color coordinate of Ca 0.88 Al 2 Si 2 O 8 :Eu 2+ 0.12 phosphor was presented in Figure 7.The CIE coordinate of Ca 0.88 Al 2 Si 2 O 8 :Eu 2+ 0.12 phosphor sintered at 1300 o C located at blue region (0.168, 0.144), and CIE coordinate of other samples are almost similar values.The Ca 0.88 Al 2 Si 2 O 8 :Eu 2+ 0.12 phosphors were prepared by a solid state reaction method.The sintering temperature influences the structural properties, which were confirmed by analysing the XRD patterns, SEM images and luminescence properties.XRD patterns indicate that phosphors have mixed phases of both CaAl 2 Si 2 O 8 (JCDPS 41-1486 triclinic) and CaAl 2 O 4 (JCDPS 34-0440 Monoclinic).The Eu 2+ ion was well occupied in monoclinic crystalline structure.Luminescence properties of Ca 0.88 Al 2 Si 2 O 8 :Eu 2+ 0.12 have PL (λ = 440 nm) and PLE (λ = 350 nm) maximum intensity at 1300 o C. PL spectra have emission region extending to green region.Hence, based on the short life time, CIE coordinates and wide blue region of the broad PL spectra, Ca 0.88 Al 2 Si 2 O 8 :Eu 2+