Tunable Yellow-Red Emission via Energy Transfer in Dy3+, Mn4+: NaLaMgWO6 Phosphors for UV-Excited LED

The Dy 3+ , Mn 4+ : NaLaMgWO 6 phosphors are synthesized successfully using the technique of solid-state reaction at 1150 ℃ . The XRD and SEM-EDS are measured to analyze the phase purity. The concentrations of Dy 3+ , Mn 4+ , Mg 2+ , Na + , La 3+ and W 6+ are measured by ICP. The absorption and excited spectra are presented, which indicate the 390 nm is suitable for the tunable yellow-red light excitation. Using the 390 nm LED as the excitation source, the emission spectra with different doped concentrations ratios of Dy 3+ to Mn 4+ are obtained, which shows good tunable yellow-red emission performance. make some of the electrons of Dy 3+ transit from the 4 F 9/2 (Dy 3+ ) to 4 T 2g (Mn 4+ ) by the cross relaxation. Fortunately, the doped concentration of Dy 3+ can be up to 10at% or more, which can guarantee the emission intensity of 575 nm enough.


Introduction
As well known, the LED technology has been applied in many elds such as display, lighting, medical, health, transportation, etc. And the commercial LED sources can provide almost all wavelengths in the visible bands from 360-650 nm and the power of per-chip is up to Watt level, which indicates the LED can act as the excitation source like Xe-lamp to generate the needed green-yellow-red emission. Namely, one economical and compact white LED solution might be that using LED source to excite phosphor to generate white light.
In the phosphor family, the rare-earth phosphors occupy the vast majority such as YAG:Ce 3+ [1][2], SrAl 12 O 19 :Eu 2+ [3]…Among them, the Dy 3+ has been widely used to generate the blue, yellow and red light, corresponding to the transition of 4 F 9/2 → 6 H 15/2, 6 H 13/2, 6 H 11/2 [4][5] because of its plentiful absorption and emission spectra.. Whereas, the emission intensity of red band of Dy 3+ ions is weak, which cannot contribute to the warm-white LED enough. And so, for the warm white LED application, the Dy 3+ needs the help of other ions.
For the purpose to investigating the micro-structure deep, the line-scanning SEM -EDS was measured, shown in Figure 2 . The line-EDS spectra were operated with the maximum length of 40 μm and maximum voltage of 15 KeV. In Fig2.(a), the total length is 40 μm and the diameter of cell particles is 0.5-1μm. The Fig.2 (b) shows the counts per-second vs scanning energy. From the gure, in general, the sum counts keep nearly uniform with the scanning energy of 0-10 KeV. The distributions of Dy 3+ and Mn 4+ always appear simultaneously, which are mainly distributed in 1-3KeV and 7-8KeV. According to the data, we can conclude that the Dy 3+ and Mn 4+ evenly replace the sites of NaLaMgWO 6 and the micro-structure keeps periodicity and integrity.
The 20 ml aqua regia was added in the samples, and then heated them to 180 0 C to chemical reaction.  The absorption spectrum of the xDy 3+ , yMn 4+ : NaLaMgWO 6 (x=5 at%, y=0.5 at%) phosphors is shown in Fig.3  Using the 390 nm LED as the excitation source, the emission spectra are shown in Fig.4. In Fig.4, Three emission peaks centered at 575 nm ( 4 F 9/2 → 6 H 13/2 (Dy 3+ )), 698 nm and 712 nm ( 2 E g → 4 A 2g, 2 T 1 → 4 A 2g (Mn 4+ )) are observed. With the increasing doped concentration of Dy 3+ , the emission intensity of 575 nm becomes stronger. When the ratio of Dy 3+ to Mn 4+ is up to 5at%:0.5at%=10:1, the peak intensity of 575 nm is equal to that of 698 nm. When the ratio of Dy 3+ to Mn 4+ is 10at%:0.5at%=20:1, the peak intensity of 575 nm is near double to 698 nm (15300:7950=1.92), but the uorescence integral intensity is only half of the 698-712 nm (670825 a.u*nm). And so, we can obtain the tunable yellow-red emission with the concentration ratio of Dy 3+ to Mn 4+ , which contributes to the broad application eld such as display, health and plant growth [18].

The in uence of Dy 3+ concentrations on the Correlated Color Temperature (CCT)
For the purpose of investigating the color temperature of LED, we calculated the total relative spectrum energy by using the following equations, which are stated in CIE 1931 and CIE 1964 [19]. Tristimulus values X (R=700 nm), Y (G=546.1 nm), Z (B=435.8 m) are de ned: The P(λ) is the relative spectrum energy distribution and x, y, z are the three basic colors. The λ is the color light wavelength and the k is the normalized coe cient. And so, the CIE coordination (x, y) can be obtained: x=X/(X+Y=Z); y=Y/(X+Y+Z) According to the emission spectra shown in Fig. 4, we can get the CIE coordination (x, y) with the different ratio of Dy 3+ to Mn 4+ , shown in Table 2. Excitation with 390 nm, the white light performance has been investigated. The white light photos and the Correlated Color Temperature (CCT) also have been calculated. The Fig.5 shows the CIE coordination and uorescence photos of xDy 3+ , 0.5at%Mn 4+ : NaLaMgWO 6 (x=0.5at%-10 at %) phosphors excited with 390 nm LED source. Results indicate that the Correlated Color Temperature (CCT) is in the region of 3000K-4000K, which shows it's the warm white-LED and can be useful for many applications.

Conclusions
In the work, the xDy 3+ , 0.5at%Mn 4+ : NaLaMgWO 6 (x=0.5at%-10 at %) phosphors were successfully synthesized with the solid state reaction method. The XRD pattern is measured, which indicates the pattern is well identical with the standard JCPDS # 88-1761. The line-scanning SEM-EDS is measured to check the homogeneity and result indicates that the doped ions of Dy 3+ and Mn 4+ are evenly distributed in the micro-crystal. The absorption and excitation spectra are presented, which indicates the 390 nm suitable for the excitation source. Excited with 390 nm LED, the emission spectra are measured with different concentrations ratio of Dy 3+ to Mn 4+ . Results indicate that the 575 nm yellow emission can be tunable with different concentrations of Dy 3+ . And then, the 390 nm, 575 nm and 689-712 nm can be mixed to near white light. The CIE coordinate and CCT are also measured. The CCT is in the region of 3000-4000K, which indicates it's the warm white-LED. The uorescence mechanism is also presented, which imply that share poll and cross relaxation exist in the xDy 3+ , 0.5at%Mn 4+ : NaLaMgWO 6 (x=0.5at%-10 at %) phosphors. And the high doped concentration of Dy 3+ can guarantee the emission intensity of 575 nm enough. In this paper, commercial LED 390nm light source is used for excitation, which makes the products easy to be commercialized, while most other products are red by Xe-lamp, which is not conducive to commercial applications.