Warm-white luminescence of Dy 3+ and Sm 3+ co-doped NaSrPO 4 phosphors through energy transfer between Dy 3+ and Sm 3+ ions

: Series of white-light-emitting NaSrPO 4 :Dy 3+ phosphors were synthesized via a sol-gel method. Small amounts of Sm 3+ ions were co-doped into the phosphors to enhance their luminescence. The effects of Dy 3+ /Sm 3+ co-doping concentrations and synthesis temperature on the phosphors’ structures and luminescence properties were investigated. The results of X-ray diffraction confirmed that the characteristic diffraction peaks of the phosphors were in accord with those of the standard NaSrPO 4 structure. Energy dispersive spectroscopy indicated that the NaSrPO 4 :Dy 3+ samples were prepared with stoichiometric ratios of elements. Under ultraviolet light excitation of 348 nm, the NaSrPO 4 :Dy 3+ phosphors emitted a white light which was composed of two emission peaks at 484 nm (blue) and 577 nm (yellow) corresponding to the magnetic dipole transition 4 F 9/2 → 6 H 15/2 and electronic dipole transition 4 F 9/2 → 6 H 13/2 of Dy 3+ ions, respectively. Based on Sm 3+ co-doping, a characteristic red emission at 602 nm arose and a warm-white light containing a lower CCT value was obtained. Besides, the luminescence lifetime of Dy 3+ decreased while that of Sm 3+ increased as dosages of Sm 3+ were added, indicating efficient energy transfer from Dy 3+ to Sm 3+ happened. Therefore, NaSrPO 4 :Dy 3+ , Sm 3+ phosphors are promising candidates for application in warm-white light-emitting diodes.


Introduction
As environmental pollution and the energy crisis have become increasingly serious, white-light-emitting diodes (w-LEDs) are being increasingly utilized in illuminating systems and display applications [1][2][3][4], by virtue of their environmental friendliness, energy savings, and high energy efficiency [5][6][7]. Typically, widespread w-LEDs are composed of a blue InGaN chip and a yellow Y3Al5O12:Ce 3+ (YAG:Ce) phosphor [8][9][10][11]. There are shortcomings to this combination, just as highly correlated color temperature (CCT > 5000 K) and low color rendering index (CRI < 80), since lack of a red emission component [12][13][14][15]. Additionally, some researchers have made novel w-LEDs with an ultraviolet (UV) LED chip and tri-color (RGB) phosphors to realize white light. However, this method has several disadvantages related to color ratio control, color reabsorption, and luminous efficiency [16,17]. Therefore, a novel single-component phosphor that produces excellent white light emission under UVlight excitation is urgently needed [18,19].
However, for the shortage of a red ingredient, the white emission of these Dy 3+ -doped phosphors have the drawbacks of low CRI and high CCT values. This is similar to using the combination of an InGaN chip with a YAG:Ce phosphor. In order to ameliorate these defects, red-emitting center Sm 3+ ions can be introduced into Dy 3+ doped phosphors as they have a forceful emission peak relatively at 602nm ( 4 G5/2→ 6 H7/2) that can improve the luminescence properties (CRI and CCT) of the white emission of Dy 3+ [28]. By adjusting the ratio of the Dy 3+ to Sm 3+ concentration, excellent white light emissions, especially warm white, can be achieved. Phosphates are extensively utilized as phosphor hosts for their low synthesis temperatures and good chemical and thermal stability, including Ca2Sr(PO4)2:Dy 3+ [29], LiCaPO4:Ce [30], and YPO4:Dy 3+ \Eu 3+ \Tb 3+ [31]. Wet chemical methods can provide low synthesis temperatures, good crystal structures, and excellent luminescence properties. However, there has been little research on its structure or the preparation of rare-earth-doped NaSrPO4 by wet chemical methods [32][33][34], especially in Dy 3+doped NaSrPO4 white-emitting phosphors [35].
In this paper, NaSrPO4:Dy 3+ , Sm 3+ phosphors were prepared using a sol-gel method and white light was expected to be obtained from their blue and yellow emissions. Sm 3+ co-doping would adequately ameliorate the CRI and CCT values of white light emission. The impacts of Dy 3+ /Sm 3+ co-doping concentration and synthesis temperature on the structure and white light emission of NaSrPO4:Dy 3+ samples were investigated. The results provide a reference for research into other white-lightemitting phosphors.

Experimental
All NaSrPO4:Dy 3+ , Sm 3+ specimens were synthesized through a sol-gel method. The 4 starting materials CH3COONa, Sr(NO3)2, NH4H2PO4, C6H8O7, Dy2O3, and Sm2O3 were highly purified. The doping concentrations of Dy 3+ were controlled within 0.5-5 mol%, while that of Sm 3+ were regulated as 1-3 mol% (relative to the molar amount of Sr 2+ ). All raw materials were weighed out with stoichiometric ratios. First, a certain amount of Dy2O3 and Sm2O3 were dissolved in nitric acid to produce a nitrate solution, while suitable CH3COONa, Sr(NO3)2, and NH4H2PO4 were added to a conical flask containing distilled water. Afterward, an appropriate amount of citric acid was drizzled into the above solution to form a stable system. The system was stirred fully by vigorous magnetic stirring at 70 °C for 2.5 h to form a transparent sol and heated by an air circulation oven at 80 °C for 36 h until a swelled yellow porous gel was formed. Ultimately, the sample was moved to a furnace and heated at 800-1200 °C for 3 h. The as-prepared phosphors were obtained when cooled naturally to room temperature (RT).
The phase structure and purity of these as-prepared specimens were evaluated utilizing X-ray diffraction (XRD) on an X-ray diffractometer (DX-2700, Aolong

Results and discussion
The XRD patterns of NaSrPO4:Dy 3+ powders heated at 800-1200 °C are represented in Fig. 1. All diffraction peaks of these specimens present a good match with the standard phase of NaSrPO4 (PDF#33-1282) [32] and can be indexed as a monoclinic crystal with lattice parameters of a = 20.414 Å, b = 5.429 Å, and c = 17.426 Å. This proves that NaSrPO4:Dy 3+ samples with NaSrPO4 structure are successfully prepared and no obvious impurity phases corresponding to Dy 3+ appear. In addition, a low Dy 3+ -ion doping concentration does not lead to significant phase changes in the NaSrPO4 structure in view of the close ionic radii of Sr 2+ (r = 0.118 nm) and Dy 3+ (r = 0.091 nm). Table 1 displays the corresponding lattice parameters and volumes of these samples. Apparently, increasing the thermal energy enhances diffusion and migration among diverse ions, which causes a slight decrease in cell volume. This can be interpreted as resulting from an increasing number of Dy 3+ ions entering the host structure to form a luminescence center [36].
The SEM images and corresponding EDS results of NaSrPO4:Dy 3+ samples prepared at 800 °C and 1000 °C are shown in Fig. 2. The samples prepared at 800 ℃ and 1000 °C (Figs. 2(a) and 2(b), respectively) exhibit homogeneous size distributions with particle diameters of about 2 μm and 4 μm at 800 °C and 1000 °C, respectively.
Meanwhile, the particle sizes increase rapidly with increases in synthesis temperature.
As the synthesis temperature increases, the particle size and degree of bonding are affected to some extent. For these grains acquire an increasing thermal energy, there is a tighter bonding among particles, and the particle size of the phosphor prepared at 1000 °C is larger than that prepared at 800 °C. Moreover, the EDS spectrum shown in Fig. 2(c) provides a distribution of the elemental content at a marker point. The elemental weight percentage of the NaSrPO4:2%Dy 3+ sample prepared at 1000 °C and 6 stoichiometric atom ratio are listed and compared. It can be clearly observed that the sample contained the elements O, Na, P, Sr, and Dy. Compared with the elemental weight percentage of the stoichiometric ratio obtained by theoretical calculation, the as-prepared phosphor has good correspondence to the standard composition, and only a small amount of Na is evaporated at high temperature. There are no significant shifts of the emission peaks, although the luminescence intensities of NaSrPO4:Dy 3+ increases with synthesis temperature up to 1000 °C and later declines. The synthesis temperature produces two concrete roles on its interior structure, which affects the luminescence property of the specimens [14]: On one hand, the crystallization of NaSrPO4 and diffusion of the Dy 3+ ions in the structure are favored by high temperatures; on the other hand, heating at high temperatures leads to the formation of large crystal grains. At synthesis temperatures of 800-1000 °C, the first aspect acts a critical section in improving the luminescence performance, while the second affects the luminescence obtained above 1000 °C.
As Dy 3+ ions are the luminescence center, the Dy 3+ doping concentration significantly influences the relevant luminescence intensities of these as-prepared powders. The XRD mappings of NaSrPO4:xDy 3+ samples with particular Dy 3+ amounts are represented in Fig. 4. With increased Dy 3+ doping concentrations, the diffraction mappings of all as-prepared powders exhibit much consistency in the standard NaSrPO4 structure without impurity peaks, indicating that Dy 3+ ions do not affect the host structure. 7 By zooming the patterns in the 2θ angle range of 30°-34°, an enlarged image of the main diffraction peaks is obtained ( Fig. 4(b)). As smaller Dy 3+ (0.091 nm) ions successfully enter the larger Sr 2+ (r = 0.118 nm) sites in the monoclinic crystal lattice of NaSrPO4 , the diffraction peaks offset mildly to the larger 2θ angle positions.
Moreover, the unit-cell volume of Dy 3+ -doped NaSrPO4 is slightly lower than that of the original host, as shown in Table 2.
The UV-Vis absorption spectrogram of NaSrPO4:Dy 3+ is shown in Fig. 5 Therefore, it is suggested that ideal white-light will hopefully be manufactured via regulating the blue-yellow ratio and color coordinates. 9 To further explore the concentration quenching phenomenon of luminescence among adjacent Dy 3+ ions, the critical radius distance (Rc) can be counted as [37]: where Rc denotes the critical distance, V is the unit cell volume, N represents the cation sites from a unit cell in NaSrPO4 host, and xc refers the critical concentration of Dy 3+  Accordingly, There are two types of energy transfer: one is exchange interaction, another is multipole-multipole interaction. Based on the calculation of Rc (> 5 Å), the conclusion of multipole-multipole interaction mechanism plays main role in energy transfer of Dy 3+ ions can be offered. To further analyze the mechanism, the following equation can be used according to Dexter's theory [38]: where k and β indicate two constants in a NaSrPO4 host; and I and x represent the luminescence intensity of NaSrPO4:xDy 3+ (x = 2%, 3%, 4%, 5%, 6%) and molar fraction of the specific Dy 3+ concentration, respectively. Q shows the form of interaction among dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole interactions, correspond to Q = 6, 8, and 10, respectively. Provided β(x) Q/3 is much larger than 1, the above equation can be equivalently converted to the following [39]: Hence, the dominating cause of concentration quenching of NaSrPO4:xDy 3+ can be 10 considered to involve dipole-dipole interactions.
The above consequences are verified that white light is promising to be achieved through uniting the two characteristic blue and yellow emissions of NaSrPO4:Dy 3+ phosphors. Moreover, Sm 3+ ions should be co-doped in the phosphors to improve luminescence, especially the CCT value.
The XRD patterns of NaSrPO4:2%Dy 3+ , ySm 3+ (y = 1%, 2%, 3%) phosphors are shown in Fig. 8. Like Fig. 4 structure, by virtue of the tiny difference in ion radius, the relevant cell volumes and lattice parameters of the as-prepared specimens decrease and distort slightly. In addition, this phenomenon shows that Dy 3+ and Sm 3+ ions enter the host structure without altering its structure.
The emission spectra (λex = 348 nm) and corresponding energy transfer efficiency of NaSrPO4:2%Dy 3+ , ySm 3+ phosphors with specified Sm 3+ co-doping concentrations are put forward in Fig. 9. Apart from the two original peaks of Dy 3+ ions located at 484 nm and 577 nm, a new emission peak appears at 602 nm ( Fig. 9(a)), which stems from the characteristic 4 G5/2→ 6 H7/2 transition from Sm 3+ . The results show that as the co-doping concentration of Sm 3+ increases gradually, the luminescence intensities of the Sm 3+ characteristic peak at 602 nm increase, while the luminescence intensities of the Dy 3+ characteristic emission peaks decrease. This phenomenon is interpreted as 11 the result of transmission in energy from Dy 3+ to Sm 3+ .
To describe the energy transfer (Dy 3+ →Sm 3+ ) and their level structure more clearly, a schematic plot of energy transfer is presented in Fig. 10. Under UV-light irradiation, the electrons in the 4f electronic shell of Dy 3+ in the ground-state orbitals 6 H15/2 absorb light and leap into higher energy levels ( 6 P7/2, 6 P5/2, 4 I15/2, etc.). Radiative transition occurs among high energy levels and results in the electrons falling into the formula [40]: where A1 and A2 are fitting constants; while τ1 and τ2 signify the lifetimes of exponential sections; I0 and I(t) represent the luminescence intensities at times 0 and t, respectively. Conforming to the above parameters, the decay times for different concentrations of Sm 3+ co-doped NaSrPO4:2%Dy 3+ phosphors can be assessed through the following functional equation [41]: CCT = -437n 3 + 3601n 2 -6861n + 5514.31 (7) where n = ( − )/( − ); and and both equal 0.33; and equal 0.1858.
Although the NaSrPO4:Dy 3+ phosphor with the 2% doping concentration has a higher luminescence intensity, its CCT value is as high as 5695 K, which can be considered a "cold" white light (blueish). Fig.12  phosphor has great potential for use in commercial applications requiring warm white light.

Conclusion
In summary, NaSrPO4:Dy 3+ , Sm 3+ phosphors were synthesized through a sol-gel method and the white-light emissions from Dy 3+ and Sm 3+ were surveyed in detail.
The PL spectra indicate that NaSrPO4:Dy 3+  phosphors are strong candidates for application to w-LEDs.

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.