Eu3+-activated CdY4MoO16 nanoparticles were synthesized via the sol-gel method. The phase formations were confirmed by the structural refinements. The photoluminescence properties such as the excitation and emission spectra, optimal doping level, internal absolute quantum efficiency (QE), decay lifetimes and the thermal stability, were measured. The charge transfer band (CTB) has a dependence on the Eu3+-content, showing an obvious red-shift with the increase of doping levels. Especially, CTB could reach a longer wavelength than the reported Eu3+-doped molydates. Moreover, the phosphor has some priorities such as high quantum efficiency, high doping levels and good thermal stability, etc. The excellent luminescence of Eu3+-activated CdY4MoO16 was discussed on its structural characteristics such as the cubic fluorite-like crystalline phase, framework constructed by Mo-O polyhedral groups, and the positive charge deficiency in the Eu3+-occupied cation sites of (Cd0.5, Y0.5)2.5 in the lattices.
© 2016 Optical Society of America
CorrectionsRuijin Yu, Aiping Fan, Maosen Yuan, Tianbao Li, Qin Tu, Jinyi Wang, and Vincent Rotello, "Eu3+-activated CdY4Mo3O16 nanoparticles with narrow red-emission and broad excitation in near-UV wavelength region: publisher’s note," Opt. Mater. Express 6, 3469-3469 (2016)
3 October 2016: Corrections were made to the author listing and the funding section.
The luminescence of Eu3+ is characterized by sharp peaks from electronic transitions of 5D0 to 7FJ (J = 0-6) states. The usual emission at 610–615 nm is the best choice for red fluorescent light source. Eu3+-doped inorganics have been widely developed as efficient red-emitting phosphors [1,2]. It is well known that the oscillation strength of Eu3+ in UV-near UV region is very low (<10−6) due to f–f forbidden transitions. So direct absorption of excitation wavelength in visible region by Eu3+ will be weak. It is important to choose an appropriate host matrix, in which the energy can transfer from host to Eu3+ ions. The best situation is that excitation from charge transfer state (CTS) lies in near-UV (360–420 nm) and blue range (430–475 nm). Molybdates have been confirmed to be one of the best candidates, which can efficiently absorb UV-near light through CTS in MoOx groups and easily transfer energy to Eu3+ for red emission.
Eu3+-doped CdY4Mo3O16 was chosen to develop a new red-emitting phosphor. The dominated motivate is that this molybdate matches for the suggested criterion: CdY4Mo3O16 has a cubic molybdate structure with the space group Pn-3n . The structure is a derivative of a fluorite-type, in which some anions are displaced to provide the tetrahedral environments for Mo6+. Under excitation with UV light, broad and intense CT absorption in near UV or blue wavelength region are expected.
To preperate phosphors, some methods have been applied such as solid state reaction, single crystal growth, coprecipitation, sol–gel method and so on. The sol-gel synthesis is considered one of the most practical methods due to its simplicity, ultrafine powders at low temperature without complex processes, better chemical homogeneity of the product, reproducibility, low cost etc. In this work, Eu3+-activated CdY4Mo3O16 were synthesized via the sol-gel method. The crystal formations were confirmed by structural refinements. The luminescence were investigated for the excitation and emission spectra, absolute quantum efficiency (QE), thermal stability etc. The phosphor has a possible application for W-LEDs by taking into its luminescence characteristics.
CdY4-4xEu4xMo3O16 (x = 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6) nanophosphors were prepared by the sol-gel method. The starting materials were stoichiometric Cd(NO3)2•4H2O, Y2O3, Eu2O3, (NH4)6MO7O24·4H2O. Firstly, Cd(NO3)2•4H2O, (NH4)6MO7O24·4H2O were dissolved in aqueous solution (about 80 °C) by adjusting the pH at about 7.0. Secondly, Y2O3, Eu2O3, were completely dissolved in HNO3 solutions. Thirdly, the ethylene diamine tetraacetic acid (EDTA) was slowly added into the nitride solutions. The polymeric resin was obtained by adding the ethylene glycol and citric acid under the strong stirring. The gel could be obtained after heating the solution at 100 °C for 2-3 h. Finally CdY4-4xEu4xMo3O16 (x = 0.05-0.6) compounds were synthesized by heating the dry precursor gels at 800 °C for 5 h.
The XRD pattern was measured on a Rigaku D/Max diffractometer at 40 kV, 30 mA with Bragg–Brentano geometry using Cu Kα radiation (λ = 1.5405 Å). The SEM photos were finished on a JEOL, JSM-6360 LA instrument. The photoluminescence excitation (PLE) and emission (PL) spectra were conducted on the luminescence spectrometer (Perkin-Elmer LS-50B) equipped with Monk-Gillieson type monochromators. The xenon discharge lamp was used as an excitation source in the PL measurements. The internal QEs were measured by Edinburgh Instruments FS-920 spectrometer that was equipped with an Edinburgh instruments integrating sphere. The monochromator is connected with CCD sensor and a computer by light guides. QE values were calculated by quantum yield measurement software.
3. Results and discussions
3.1 The Phase Formation and surface characteristics
All the Eu3+-doped CdY4Mo3O16 have similar XRD patterns, which can be well indexed to the cubic structure. As an example, the structural refinement was completed by the GSAS program. Figure 1 is the experimental and calculated XRD profiles of 50mol % Eu3+-doped CdY4Mo3O16. The phosphors well crystallize in a cubic space group Pn3n (222) with Z = 4. The unit parameters are a = 11.0391 Å, α = 90°, and V = 1345.24 Å3. The dependence of lattice parameter a and cell volume V on the Eu3+ doping level is shown in Fig. 1(b). It could be observed that the structural parameters get larger with the increase of Eu3+-doping. In CdY4Mo3O16, Eu3+ ions prefer to occupy Y3+ sites. The the lattices expansion was induced by the substitution of smaller ions Y3+ (0.9 Å, CN = 6) by Eu3+ (0.947 Å, CN = 6) ions. The cell parameters have a linear dependence on the Eu3+-doping (x = 0.05-0.6). This relationship is in agreement with the Vegard's law, which empirically concludes that unit cell parameters of a host can linearly vary with the composition for a continuous substitutional solid solution .
The structure sketch map (Fig. 2(a)) of CdY4Mo3O16 was drawn by the Diamond Crystal and Molecular Structure Visualization software using the refined structural data. The structure is a derivative of a fluorite-type structure, in which some anions are displaced to provide some tetrahedral environment for the Mo6+ ions in the lattices. The Mo6+ cation is located in a 12(d) site; while parts of the Y3+ ions are in pure 12(e) sites and the other Y3+ ions are statistically arranged with Cd2+ in the 8(c) sites. While the O2- ions stay in the 48(i) sites in this fluorite-type sublattice.12 There are two kinds of Y3+ ions in the deformed cubic coordination (Fig. 2 b). The Y(1) cations fully occupy 48i sites, while Y(2) ions are statistically distributed with Cd2+ in the 2a sites in the lattices.
The morphological characteristics were investigated by SEM images. Figures 3(a) and 3(b) are the typical SEM photos of Eu3+-doped CdY4Mo3O16 phosphor. The crystalline morphology presents many ball-like particles. The average size estimated by the SEM micrograph is about 100 nm as shown in Fig. 3(b). This SEM characteristic was confirmed by the TEM photo in Fig. 3(c). The HRTEM image of CdY4Mo3O16 particles is shown in Fig. 3(d). In the HRTEM image, the clear lattice fringes were observed indicating the high crystallinity of the as-prepared sample. The distance of 0.273 nm between the adjacent fringes is agree well with the distance of the (004) reﬂection plane of the lattices. In addition, the observed SAED pattern (Fig. 3€) exhibited the cubic symmetric diffraction style matched to CdY4Mo3O16.
3.2 The optical absorption and the electronic calculation
Figure 4(a) shows the excitation of CdY4-4xEu4xMo3O16, which consists of a broad band and some absorption lines. The first origin of the band (CT1 and CT2) is CT transitions from O2− ions to Mo6+ ions , i.e., interband energy from Mo6+4d to O2-2p. The second possible origin is an inter-valence charge transfer (IVCT) states , i.e., the electron transitions from 4f states of Eu3+ ions to Mo6+ ions. The third contribution is the electronic transferring from O2− to the neighbor Eu3+. This band was not observed on the excitation spectrum because it could overlap with the CTB of O2−-Mo6-. Usually in a Eu3+-doped molydate, CTB is mainly attributed to O2−→Mo6+ and O2−→Eu3+ transitions . The excitation band CT1 at 265 nm can be assigned to CT of Eu3+-O2−. While the band CT2 is from O2−→Mo6+. The excitation presents the intra-configurational 4f–4f transitions of Eu3+ 330 to 500 nm: 394 nm (7F0→5L6), 464 nm (7F0→5D2). It can be concluded that this excitation can matches with the emission of a near-UV or blue LED chip. This is critical for phosphor-converted W-LEDs.
To discuss the Eu3+ doping effects, Ecut-off was introduced, i.e., wavelength position at the intersection point between the x-axle and the tangent line of excitation edge. The Eu3+ lowly doped sample CdY4-4xEu4xMo3O16 (x = 0.1) has the Ecut-off value of around 420 nm. However, the Ecut-off value of the heavily doped phosphor CdY4-4xEu4xMo3O16 (x = 0.5) has a longer wavelength of 457 nm. The relationship of Ecut-off values and the Eu3+ doping concentrations is shown in Fig. 4(b). The results indicate that the excitation edge of Ecut-off has great red-shift (to the longer wavelength side) extending to blue wavelength on the increase of Eu3+-doping. There is also strong excitaion from the 7F0→5D2 transitions locating in the blue wavelength region. This is greatly valuable for a phosphor-converted W-LEDs because of the excellent match with the near-UV or blue LED chip.
According to the references, the CTB excitation position of Eu3+-doped molybdates or tungstates usually locate in the wavelength of 230-350 nm, which is limited for a pc-WLED. However, the maximum CTB excitation wavelength in CdY4-4xEu4xMo3O16 (x = 0.5) is measured to be 457 nm (Fig. 4). As far as we know, this is longer than any one phosphor in the reported Eu3+-doped molybdates . This is due to the special structural characteristic of CdY4Mo3O16, i.e., the cubic framework lattices, providing the longer CTB wavelength.
Another emission characteristic is that the f-f transitions of Eu3+ show dominated intensity, which is stronger than the CTB. It has been reported that Eu3+ ions could present efficient intraionic (f-f) transitions and weak interionic transitions (Eu3+ and O2-) when it is subject to a site with a positive effective charge . Following this model, the excitation in CdY4Mo3O16:Eu3+ can be explained in the specialty of the microstructure. In CdY4Mo3O16, there are disordered cation sites (Cd0.5,Y0.5)2.5 polyhedra in the lattices . When Eu3+ ions are doped in the lattices, the activators with positive charge will be created leading to the low intensity of CT bands and stronger f-f transitions of Eu3+. Similar results have been reported in the other Eu3+-activated red-emitting phosphors such as Ca9R(PO4)7 (R = Al, Lu):Eu3+ .
3.3 The photoluminescence
Figure 5 shows the luminescence of CdY4-4xEu4xMo3O16 (x = 0.05, 0.5). The spectra present the electronic transitions the excited states 5D0 to the ground states 7FJ (J = 0, 1, 2, 3, 4) in the 4f6 configuration. The upper energy transitions from Eu3+ ions such as 5D1,2→7FJ (J = 0, 1, 2, 3, 4), were not observed in all the samples. The dominated emission is the electric dipole transition 5D0→7F2 at 611 nm due to the fact that the Eu3+ ions in CdY4Mo3O16 are subject to the non-inversion centers in the lattice. The value of (5D0→7F2)/(5D0→7F1) ratio can measure the site symmetry. A higher value of (5D0→7F2)/(5D0→7F1) will be created in a lower crystal field symmetry around Eu3+ ions. The value of (5D0→7F2)/(5D0→7F1) in CdY4-4xEu4xMo3O16 (x = 0.5) is 3.35. This could bring to a pure color for a red-emitting phosphor. The CIE (Commission Internationale de l’Eclairage) color coordinates of CdY4-4xEu4xMo3O16 (x = 0.5) are calculated from the emission spectrum to be x = 0.665 y = 329. The CIE coordinates are closer to the NTSC (National Television System Committee) standards (x = 0.670, y = 0.330) than the commercial red-emitting Y2O2S:Eu3+ (x = 0.630, y = 0.350) .
The dependence of the luminescence of CdY4-4xEu4xMo3O16 (x = 0.05-0.6) on the Eu3+-doping is displayed inset of Fig. 5. The emission intensity gets stronger with the increases of Eu3+-content. However, the luminescence becomes a turnover when the doping level reach at is 50.0 mol %. The luminescence begins to quench when the doping is above 50.0 mol %.
3.4 The luminescence dynamics, stability and efficiency
The representative luminescence spectra at several temperatures are displayed in Fig. 6(a). It can be seen that the luminescence keeps stable from 20 to 125 °C. And it decreases when the temperature is above 150 °C. The luminescence intensity at 150 °C is about 80% of the initial value at 20 °C. Importantly, the emission peaks of the Eu3+ ions in CdY4Mo3O16 don’t change with the increase of temperature. This indicates that the luminescence color didn’t change with the increase of temperature.
The luminescence lifetimes are calculated as a function of temperature and displayed in Fig. 6 (b). The decay time drops at higher temperature from 300 K, presenting a typical temperature quenching behavior. The data is fitted to equation as:
The experimental QEs were measured and are listed in Table 1. The maximum QE measured for CdY4-4xEu4xMo3O16 (x = 0.5) is 67.5% (λex = 395 nm). Althrough this QE value is still lower than some reported phosphors such as CaLa1.98−y(MoO4)4:0.02Dy3+, 0.03Eu3+ (80%, λex = 353 nm), , NaCsGaSi2O7:RE3+ (RE = 1%Ce, 10%Eu, 25%Tb) (76.72%, λex = 351 nm) , etc, however, the value is higher than that of the commercial red-emitting Y2O2S:Eu3+ (QE = 35%, λex = 317 nm, λem = 611 nm) . Moreover, this value is also higher than some red-emitting molybdates such as Gd6MoO12:0.25Eu3+ nano-powder (62%, λex = 395 nm) , SrY0.7Eu1.4(MoO4)4 (48%, λex = 395 nm) , etc. The results show that CdY4-4xEu4xMo3O16 (x = 0.05-0.6) could be a potential candidate for lighting and display.
In summary, CdY4-4xEu4xMo3O16 (x = 0.05-0.6) prepared by sol-gel method crystallize in ball-like particles with average size of 100 nm. This phosphor displays broad CT transitions from O2− to Mo6+, which have a great red-shift with increase of Eu3+-doping. In CdY4-4xEu4xMo3O16 (x = 0.5), the maximum CTB can reach to 457nm, which is longer than any one phosphor in the reported Eu3+-doped molybdates. This is due to the structural characteristic of CdY4Mo3O16, i.e., the cubic framework lattices provide the longer CTB wavelength. Another characteristic is that f-f excitation show stronger intensity than the CTB band. This is due to the fact that the Eu3+ ions in (Cd0.5, Y0.5)2.5 sites will bring positive effective charges in the lattices. The optimal Eu3+ doping in CdY4-4xEu4xMo3O16 is 50 mol%. The maximum QE for CdY4-4xEu4xMo3O16 (x = 0.5) is 67.5%. Moreover, the red-emitting phosphor has a high thermal stability with the activation energy of 0.412 eV. The results have demonstrated the potentiality of the phosphor for near-UV/blue GaN-based white LEDs.
National Natural Science Foundation of China (21201141); The Chinese Universities Scientific Fund (QN2011119, QN2452015424); The Young Faculty Study Abroad Program of Northwest A&F University; University of Massachusetts Research Trust Fund.
References and links
1. X. Y. Chen, Z. J. Zhang, F. F. Xu, S. Q. Shi, and J. T. Zhao, “Color-Tunable Carbon Dots/Y2WO6:Eu3+ Embedded Composite Bulk,” Opt. Mater. Express 6(2), 374–380 (2016). [CrossRef]
2. P. S. Dutta and A. Khanna, “Eu3+ Activated Molybdate and Tungstate Based Red Phosphors with Charge Transfer Band in Blue Region,” ECS J. Solid State Sci. Technol. 2(2), R3153–R3167 (2012). [CrossRef]
3. J. B. Bourdet, R. Chevalier, J. P. Fournier, R. Kohlmuller, and J. Omaly, “A Structural Study of Cadmium Yttrium Molybdate CdY4Mo3O16,” Acta Crystallogr. B 38(9), 2371–2374 (1982). [CrossRef]
5. P. Dorenbos, A. H. Krumpel, E. Kolk, P. Boutinaud, M. Bettinelli, and E. Cavalli, “Lanthanide Level Location in Transition Metal Complex Compounds,” Opt. Mater. 32(12), 1681–1685 (2010). [CrossRef]
7. Y. L. Yang, X. M. Li, W. L. Feng, W. J. Yang, W. L. Li, and C. Y. Tao, “Effect of Surfactants on Morphology and Luminescent Properties of CaMoO4:Eu3+ Red Phosphors,” J. Alloys Compd. 509(3), 845–848 (2011). [CrossRef]
8. F. Du, Y. Nakai, T. Tsuboi, Y. Huang, and H. J. Seo, “Luminescence Properties and Site Occupations of Eu3+ Ions Doped in Double phosphates Ca9R(PO4)7 (R = Al, Lu),” J. Mater. Chem. 21(12), 4669–4678 (2011). [CrossRef]
9. C. H. Huang, T. W. Kuo, and T. M. Chen, “Thermally stable green Ba(3)Y(PO(4))3:Ce(3+),Tb(3+) and red Ca(3)Y(AlO)(3)(BO(3))4:Eu(3+) phosphors for white-light fluorescent lamps,” Opt. Express 19(S1), A1–A6 (2011). [CrossRef] [PubMed]
10. L. Han, G. Liu, X. Dong, J. Wang, and W. Yu, “Single-Phase and Warm White-light-emitting Phosphors CaLa2−x−y(MoO4)4:xDy3+, yEu3+: Synthesis, Luminescence and Energy Transfer,” J. Lumin. 178, 61–67 (2016). [CrossRef]
11. K. Y. Yeh and W. R. Liu, “Luminescence Properties of NaCaGaSi2O7:RE, Li+ (RE = Ce3+, Eu3+ or Tb3+) Phosphors for UV Excitable White Light Emitting Diodes,” Mater. Res. Bull. 80, 127–134 (2016). [CrossRef]
12. C. H. Huang, T. W. Kuo, and T. M. Chen, “Thermally stable green Ba(3)Y(PO(4))3:Ce(3+),Tb(3+) and red Ca(3)Y(AlO)(3)(BO(3))4:Eu(3+) phosphors for white-light fluorescent lamps,” Opt. Express 19(S1), A1–A6 (2011). [CrossRef] [PubMed]
13. L. Qin, D. Wei, Y. Huang, S. I. Kim, Y. M. Yu, and H. J. Seo, “Efficient and Thermally Stable Red Luminescence from Nano-Sized Phosphor of Gd6MoO12:Eu3+,” J. Nanopart. Res. 15(9), 1940–1948 (2013). [CrossRef]
14. L. Qin, Y. Huang, T. Tsuboi, and H. J. Seo, “The Red-Emitting Phosphors of Eu3+-Activated MR2(MoO4)4 (M=Ba, Sr, Ca; R = La3+, Gd3+, Y3+) for Light Emitting Diodes,” Mater. Res. Bull. 47(12), 4498–4502 (2012). [CrossRef]