Single-phase white-emitting and tunable color phosphor Na3Sc2(PO4)3:Eu2+,Dy3+: Synthesis, luminescence and energy transfer☆
Graphical abstract
A series of Eu2+/Dy3+ single doped and co-doped Na3Sc2(PO4)3 phosphors was synthesized by the high-temperature solid-state method. Under the excitation of 370 nm, the Na3Sc2(PO4)3:Eu2+,Dy3+ phosphor can emit white light whose spectrum is composed of a broad emission band centered at 460 nm and another three peaks at 483, 577, and 672 nm, respectively. There is ET from Eu2+ to Dy3+ ion in Na3Sc2(PO4)3:Eu2+,Dy3+ phosphor. With the increase of Dy3+ ion concentration, the color coordinates of Na3Sc2(PO4)3:0.01Eu2+,xDy3+ phosphors change from blue to white, then to yellow. The CIE coordinates of the packaged LED by using the white phosphor NSPO:0.01Eu2+,0.08Dy3+ on a near-UV InGaN chip (λ = 370 nm) were calculated to be (x = 0.3782, y = 0.3760) in the white region.
Introduction
With the progress of science and technology and the process of industrialization, LED has become a new-generation material of lighting technology, which has the advantages of energy-saving, high luminous efficiency, accurate light distribution, stability, reliability, long service life, and high cost-effectiveness.1, 2, 3 It has been widely used in urban road lighting, airports, docks, urban lighting projects, tourist attractions, etc. The mainstream LED technology uses blue chips to excite the four phosphors of red, green, blue, and yellow to form white light to achieve a high rendering index.4 However, there is a serious self-absorption phenomena after the phosphors of multiple colors are mixed, and the temperature tolerance performances of different phosphors are quite different, which causes the color drift after lighting for a long time.5,6 Therefore, more and more people study the use of a single-phase phosphor to produce white light.7, 8, 9, 10, 11
Dy3+ ion is a good candidate as a white light generating activator. When doped into a suitable host, Dy3+ ion could give two characteristic emissions in blue and yellow regions due to the magnetic dipole transition 4F9/2 → 6H15/2 and the dipole transition 4F9/2 → 6H13/2 transitions.12, 13, 14, 15 White light can be obtained from the combination of blue and yellow light in an appropriate ratio from a single Dy3+ ion. The intensity ratio of the yellow to blue emission (Y/B) has considerable dependency on the concentration of Dy3+ ion and host composition, since the 4F9/2 → 6H13/2 is a hypersensitive transition and has a stronger intensity dependent on the host than the 4F9/2 → 6H15/2 transition.16,17 In most cases, the mixed light from Dy3+ ion only presents near white light or yellow light. Moreover, the Dy3+ ion activated phosphors usually suffer from their low luminous efficiency due to 4f–4f forbidden transitions of Dy3+ ion. To improve the luminescence properties of the Dy3+ ion activated phosphors, another luminescent ion can be adopted as a sensitizer. For example, in Ce3+ ion and Dy3+ ion co-doped KBaY(BO3)2 and Ca3(P1–xBxO4)2 phosphors, white emission with considerable luminescence intensity have been achieved owing to the effective Ce3+→Dy3+ energy transfer (ET).18,19 Similar to Ce3+ ion, Eu2+ ion is also a widely used rare earth luminescent ion. Due to the parity-allowed electric dipole d-f emission of Eu2+ ion, the broadband emitting rare-earth Eu2+ ion is an important activator for luminescent materials, which has been studied extensively.20, 21, 22 Numerous Eu2+ ion-doped luminescent materials have been widely used in people's life, such as Sr2Si5N8:Eu2+, Sr3SiO5:Eu2+, SrAl2O4:Eu2+,Dy3+, and Sr4Al14O25:Eu2+,Dy3+ phosphors.23, 24, 25, 26 According to theory of resonant energy transfer (ET), Eu2+ ion is a good sensitizer for Dy3+ ion, and the energy transfer between Eu2+ and Dy3+ could occur when overlap happens between the absorption of Dy3+ ion and the emission band of the Eu2+ ion.27,28 However, there are few reports on the energy transfer from Eu2+ ion to Dy3+ ion, which might be due to the limit of the distance between Eu2+ ion and Dy3+ ion and the low luminescent efficiency of Dy3+ in the phosphors.29 In fact, in most of the Eu2+ ion and Dy3+ ion co-doped luminescent materials, Dy3+ ion only acts as the trap center instead of the luminescence centers to promote the long-lasting phosphoresce of Eu2+ ion.25,26
Phosphate is an important type of host material for rare earth ions activated phosphors. The luminescent materials based on phosphates such as Ca5(PO4)3Cl:Eu2+, Ca6BaP4O17:Eu2+ and K2BaCa(PO4)2:Eu2+, have the advantages of high luminous efficiency, low sintering temperature, extensive luminescence property, and good stability.30, 31, 32 Na3Sc2(PO4)3 crystallizing trigonally in the space group was firstly reported by Tkachev et al., in 1984.33 In 2016, Wang et al. reported the Eu2+ doped Na3Sc2(PO4)3 phosphor as a novel blue-emitting phosphor for near-UV LEDs.34 Recently, Kim et al. found that the thermal stability of NSPO:Eu2+ is excellent. The phosphor exhibited the characteristic of zero thermal quenching even up to 200 °C.35 Given the high luminescent efficiency and excellent thermal quenching resistance of Na3Sc2(PO4)3:Eu2+, as well as the near white light emission of Na3Sc2(PO4)3:Dy3+,36 we synthesized the compound Na3Sc2(PO4)3:Eu2+,Dy3+ aiming to enhance the luminescence of Dy3+ ion by energy transfer from Eu2+ ion and obtain a single-phase white-emitting phosphor.
In this article, we reported the ET and luminescence properties of the Eu2+ ion and Dy3+ ion co-doped Na3Sc2(PO4)3 phosphors. Due to the Eu2+→Dy3+ ET, under near-UV excitation at 370 nm, Na3Sc2(PO4)3:0.01Eu2+,xDy3+ phosphors can generate color-tunable emissions from blue to white, then to yellow by changing the Eu2+/Dy3+ concentration ratio. The results showed that the Na3Sc2(PO4)3:Eu2+,Dy3+ phosphor exhibits excellent luminescence thermal stability and can be used as a potential single-substrate white LED luminescent material.
Section snippets
Material and synthesis
The Eu2+/Dy3+ single doped Na3Sc2(PO4)3 samples (NSPO:0.01Eu2+ and NSPO:0.04Dy3+) and co-doped samples NSPO:0.01Eu2+,xDy3+ (x = 0.02, 0.04, 0.06, 0.08 and 0.10) samples were synthesized by the traditional high-temperature solid-state reaction method. Na2CO3 (99.0%), Sc2O3 (99.99%), NH4H2PO4 (99.0%), Dy2O3 (99.99%) and Eu2O3 (99.99%) were used as starting materials. The raw materials were calculated according to the stoichiometric ratio, accurately weighed, and finely ground. The mixtures were
Phase and structural analysis
Fig. 1(a) shows the XRD patterns of Eu2+/Dy3+ single doped samples NSPO:Eu2+, NSPO:Dy3+, and the Eu2+, Dy3+ co-doped samples NSPO:0.01Eu2+,xDy3+ (x = 0.02, 0.04, 0.06, 0.08 and 0.10). The results show that all the XRD patterns obtained are consistent with the crystal structure of Na3Sc2(PO4)3 (JCPDS Card No. 79-0479), and no other impurity peaks are detected, which means that the main phase in the phosphors is Na3Sc2(PO4)3 and the incorporation of Eu2+ or Dy3+ ion does not cause a significant
Conclusions
In summary, a series of color-tunable NSPO:Eu2+,Dy3+ phosphors was successfully synthesized. Under the excitation at 370 nm, the NSPO:Eu2+,Dy3+ phosphor has four emission peaks of 460, 483, 577, and 672 nm. There was ET from Eu2+ to Dy3+ ion in NSPO:Eu2+,Dy3+ phosphor due to the good overlap between the PL spectra of NSPO:Eu2+ and the PLE spectra of NSPO:Dy3+, which was further confirmed by the fluorescence lifetime decrease of Eu2+ emission with the increase of Dy3+ content. According to the
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Foundation item: Project supported by the National Key Research and Development Program of China (2019YFA0709101), the Science and Technology Cooperation Program between Jilin Province and Chinese Academy of Sciences (2019SYHZ0008), and R & D Projects in Key Areas of Guangdong Province (2020B0101010001).