The XRD patterns of CYA: Mn4+ x (x =0, 0.001, 0.005, 0.05) are shown in Figure 1. The results indicate that all the peaks of Mn4+ ion doped CYA can be indexed to a pure CaYAlO4 (JCPDS 81–0742). The dopants have no obvious influence on the crystalline structure of the host. The CaYAlO4 has tetragonal system with a space group of I4/mmm (139) and a = 3.6750(5) Å c = 12.011(2) Å c/a = 3.2683 V = 162.22(4) Å3, Z = 2 [20]. There are two types of cation sites in CaYAlO4. The Ca2+ and Y3+ ions are distributed in the nine-coordinated sites and the Al3+ ions occupy the six-coordinate site. It is reported that the effective ionic radius of Ca2+ ion (CN = 9), Y3+ ion (CN = 9), Al3+ ion (CN = 6) and Mn4+ ion (CN = 6) are 1.18 Å, 1.075 Å, 0.535 Å and 0.53 Å, respectively [21]. It is obvious that the ionic radius of Mn4+ is close to Al3+ and smaller than Ca2+ or Y3+, suggesting that Mn4+ ions prefer to occupy Al3+ site in the present host.
Figure 2 shows the powder DRS of CYA: Mn4+ x (x = 0.001, 0.005, 0.01, 0.03, 0.05). It is clearly observed that the phosphors of CYA: Mn4+ 0.001 shows a platform of high reflection in the wavelength range of 580–1200 nm and then starts to decrease dramatically from 580 to 200 nm. As the increasing of Mn4+ concentration, two broad absorption bands appears at 200–425 nm and 425–580 nm, which is derived from the 4A2 → 4 T1 and 4 T2 transition of Mn4+, respectively.
The PLE and PL spectra of CYA: Mn4+ 0.001 are showed in Figure 3. The PLE spectrum contains two broad bands at 250–420 nm and 420-550 nm, which can be attributed to 4A2 → 4 T1 and 4 T2 transition of Mn4+, respectively. The PL spectra under the excitation at 335 nm, 370 nm and 460 nm exhibit a narrow band between 660 nm and 770 nm with a sharp peak at 710 nm, which is due to 4E → 4A2 transition of Mn4+. It is to say this phosphor can be effectively excited by UV or blue LED chip and emits red light.
In order to further optimize the red emission of Mn4+ ion, the concentration dependent emission intensity of CYA: Mn4+ x (x = 0.001, 0.005, 0.01, 0.03, 0.05) is studied. It can be seen in Figure 4 that the emission intensity of Mn4+ ion at 710 nm initially increase, then reaches a maximum at x = 0.005 and decrease due to concentration quenching. It is interesting that the chromaticity coordinates of CYA: Mn4+ x are almostly the same with the change of Mn4+ ion dopt content. The inset of Figure 4 gives the chromaticity coordinates of CYA: Mn4+ 0.005 under 370 nm excitation. The color purity of the point in spectrum locus is 100%. So the color purity of phosphor CYA: Mn4+ is near 100%.
The QE of phosphor CaYAlO4: Mn4+ was recorded using an integrating sphere attached to the FSP920. QE is defined as the ratio of the number of emitted photons (I
em) to the number of absorbed photons (I
abs), and can be calculated by the following equation [22].
(1)
where E
R, E
S are the spectra of the excitation light without and with the sample in the integrating sphere, respectively, and L
S is the luminescence emission spectrum of the sample in the integrating sphere. The QE of the CYA: Mn4+ 0.005 was measured and calculated to be about 26% and 28% under 335 nm and 460 nm excitation, respectively.
The luminous efficiency of the radiation (LER) is an important parameter which shows how bright the radiation is perceived by the average human eye. Figure 5 shows the nonalized spectral eye sensitivity cures for photopic vision and spectrum of phosphor CYA: Mn4+ 0.005. It can be caculated from the emission sepctrum as: [23].
(2)
Where V (λ) and I (λ) are eye sensitity cure and phosphor emission spectrum respectively. The LER of the CYA: Mn4+ 0.005 is 3 lum/w which indicates the phosphor is too red for general lighting . However, it may be a promising phosphor for other artificial lighting applications, such as in plant photomorphogenesis [24].
For LEDs application, the thermal stability of phosphor is one of the important factors. Figure 6 shows the PL spectral (λex = 370 nm) of CYA: Mn4+ 0.005 at the temperature range of 300–460 K. It illustrates that the position and shape of the emission spectra do not change with increasing temperature. The temperature-dependence of the integrated emission intensity for CYA: Mn4+ 0.005 is presented in the inset of Figure 6. It is clearly observed that the integrated emission intensity of CYA: Mn4+ 0.005 decreases as the temperature increases from 300 K to 460 K. The integrated emission intensity at 100 and 150°C remain about 70% and 50% when compared to room temperature. The above results mean this phoshor has a good thermal stability and is a candidate for pc-LEDs.
Figure 7 shows the decay curves of Mn4+ 4E → 4A2 emission excited by 335 nm. The decay behavior can be expressed asfollows: [25].
where I and I0 are emission intensity, A is constant, t is time and, τ is decay time for exponential component. For x = 0.001, 0.005, 0.01 all samples show a nearly single exponential decay behavior like CYA: Mn4+ 0.001 (the inset of Figure 7) and the life time is estimated to be 1.590 ms, 1.444 ms and 1.306 ms, respectively. When the Mn4+ concentration is further increased, the decay curves decrease more rapidly and become nonexponential. Such a fast decline of Mn4+ 4E is due to the interaction or energy migration between Mn4+ ions. The same phenomenon was found in the decay curves of Mn4+ emission excited by 460 nm.