Abstract: Nano-sized Y2Mo4O15:Eu3+ was synthesized by the pechini method. The crystalline phase was confirmed by the structural refinements. The photoluminescence (PL) excitation and emission spectra, and decay lifetimes were investigated. The phosphors can be efficiently excited by near-ultraviolet light and exhibit a red luminescence around 616 nm from the forced electric dipole transition 5D0→7F2 of Eu3+ ions. The thermal stability was investigated from the temperature-dependent luminescence lifetimes and intensities. The Eu3+ ions were confirmed to distribute in one kind of crystallographic site with a high “ordered state” in this lattices. The structure provides long distances between Eu3+ ions limiting the luminescence energy transfer or diffusions. Crystal structure of Y2Mo4O15 is beneficial to the luminescence of activators such as Eu3+ ions. The absolute luminescence internal quantum efficiency (QE), CIE color coordinates and thermal activation energy (∆E) were reported.
© 2015 Optical Society of America
Rare earth ions (RE) activated compounds have broad absorption and present emission from UV-visible to IR wavelength range duo to transitions of 4f electrons . It has been widely investigated for luminescence or display. Among RE activators, Eu3+ is one of the most popular ions for its efficient 5D0 to 7FJ (J = 0-6) transitions in red wavelength region . Eu3+-doped materials are attractive as red-emitting phosphors for lighting and display such as white light-emitting diodes (W-LEDs). Of various hosts, molybdate oxides are attractive hosts owing to energy transfer from Mo-O complex to RE ions . Molybdate phosphors have been intensively investigated for solid-state lighting , for example, CaMoO4:Eu3+ , Gd2MoO6:Eu3+ , La2Mo2O9:Eu3+ , Gd2(MoO4)3:Eu3+ , NaYEu(MoO4)2:Eu3+ .
Molybdates with a formula of Ln2Mo4O15 (Ln = lanthanides and yttrium), two times of MIIIMo2O7.5, form a large family of compounds with interesting physical properties depending on crystal structures . The structure of Ln2Mo4O15 can be divided into three types, type I: La2Mo4O15 (monoclinic space group P21/n) ; type II: Ln2Mo4O15 (Ln = Ce-Tb) (triclinic space group P-1) ; and type III: Ln2Mo4O15 (Ln = Dy–Lu and Y) (monoclinic space group P21/c) . The well-known and intensively investigated properties of Ln2Mo4O15 are anomalous negative thermal expansion (NTE) over a temperature range in 30-500 °C , for example, Y2Mo4O15 shows NTE property (25-100 °C) with a TEC of −1.3 × 10−5 K−1 . Laufer et al  reported the structural and luminescence properties of Eu3+-doped Y2[MoO4]2[Mo2O7] with the dominated 5D0→7F2 transitions.
In this work, a red-emitting nano-phosphor of Eu3+-doped Y2Mo4O15 was synthesized by the Pechini method. The structure was investigated by powder X-ray diffraction (XRD) measurement and structure refinements, SEM and TEM measurements. The photoluminescence excitation and emission together with the decay curves were measured; The luminescence properties related to application such as the absolute internal quantum efficiency (QE) and thermal stability were reported.
Y2Mo4O15:Eu3+ was prepared by the Pechini method. The raw materials are (NH4)6Mo7O24·4H2O, Eu2O3 and Y2O3 (99.9%). Firstly, Eu2O3 and Y2O3 were dissolved with nitric acid; the solution was complexed by citric acid with two times molar weight of Ln3+. The solution was neutralized by ammonium hydroxide (28% wt.). Secondly, (NH4)6Mo7O24·4H2O was dissolved (60-70 °C) by adjusting the pH to 7.0 and complexing them with citric acid in two times molar weight of molybdate. Metal complexation in both cases was promoted by heat treatment at 100 °C for 2 h. Thirdly, the as-prepared two solutions were mixed together with adding aqueous polyvinylalcohol (PVA) to adjust the viscoelastic, which was stirred for 3 h to obtain a homogeneous viscous solution for the spin-coating on clean glasses. The precursor film can be obtained by natural withering of the coated glasses. Finally, the film was annealed to the desired temperature (800 °C) producing Y2Mo4O15:Eu3+ with the heating rate of 5 °C /min. Note that, we cannot obtain the pure crystal phase when the doping level is above x = 0.4.
XRD was carried on a Rigaku D/Max diffractometer at 40 kV, 30 mA with Bragg-Brentano geometry using CuKα radiation (λ = 1.5405 Å). The photoluminescence (PL) spectra were recorded on a Perkin-Elmer LS-50B with Monk-Gillieson type monochromators. For the measurements of luminescence decay, the samples were excited by a pulsed Nd:YAG laser at 355 nm (Spectron Laser System SL802G). The luminescence was dispersed by the 75 cm monochromator (ActonResearch Corp. Pro-750) and multiplied by the PMT (Hamamatsu R928). The data were displayed and recorded with the LeCloy 9301 digital storage oscilloscope. PL quantum efficiency (QE), i.e., internal QE, was measured by a standard Edinburgh Instruments FLS-920 spectrometer equipped with an Edinburgh instruments integrating sphere. The excitation wavelength was selected from the output of the xenon lamp by a monochromator, which is connected with CCD sensor and a computer by light guides. QE value was calculated by the quantum yield measurement software.
3.1 Phase formation and SEM
The structural refinement of Y2Mo4O15:Eu3+ was finished by the GSAS program of Rietveld method shown in Fig. 1(a).The sample grows in pure monoclinic phase of P21/c (14). The structure sketch map of Y2Mo4O15 is shown in Fig. 1(b), which was modeled using the Diamond Crystal and Molecular Structure Visualization software on the results of the refinements. The framework is constructed by the MoO4 groups. O8 atom lying at a center of symmetry bridges two Mo(2)O4 tetrahedra to form a pyromolybdate Mo(2)2O7 groups. Each Y(Eu)3+ is coordinated with seven oxygen to constitute singly capped trigonal prism with the bond distances of 2.2514(3) to 2.3224(2) Å. Mo(1) and Mo(2) centers are almost tetrahedron with Mo-O bond lengths from 1.689(3) to 1.878(2) Å. Mo(2)2O7 polyhedron and two Mo(1)O4 tetrahedra joined together by sharing O14 atoms to form an entire Mo4O15 group.
The typical scanning electron microscopy (SEM) image of Y2−2xEu2xMo4O15 (x = 0.4) are shown in Fig. 2(a).The particles are obviously agglomerated. Figure 2(c) is a representative transmission electron microscopy (TEM) image. The nano-powders consist of roughly spherical. The synthesized particles are smaller than 100 nm. The TEM image confirms that the Y2−2xEu2xMo4O15 nanoparticles are well crystallized with a single-phase structure. The minimum and maximum diameters are 9 nm and 90 nm, respectively. The statistical average size of the particles by the micrograph is about 35 nm.
3.2 The luminescence and excitation spectra
Figure 3(a) presents the photoluminescence of Y2-2xEu2xMo4O15 (x = 0.05, 0.2, 0.3, 0.4). The emission lines are the characteristic transitions from 5D0→7FJ (J = 0, 1, 2, 3, 4) of Eu3+ ions. The 5D0→7F2 transitions show the dominated intensities. This is in agreement with the Laufer et al’s reports . The value of (5D0→7F2)/(5D0→7F1) for Y2-2xEu2xMo4O15 (x = 0.4) is 3.51, which indicates low ligand symmetry and high bond covalency. This larger ratio is favorable to improve the color purity of the red phosphor .
Figure 3(b) shows the excitation spectra obtained by monitoring the 5D0→7F2 emission (616 nm). It consists of a broad band at 310 nm and some lines in 370-550 nm. The broad excitation bands with an asymmetry shape can be decomposed by two Gaussian components. A band at 275 nm is identified as the charge transfer (CT) transition from O2− to Eu3+. The band centered at 330 nm is attributed to CT from O-orbitals to Mo-4d orbitals. The sharp lines are characteristic 4f–4f transitions indicating that Y2Mo4O15:Eu3+ can be efficiently excited by UV-LED (350–400 nm) and blue-LED chips (450 nm). The presence of the excitation bands due to MoO4 groups in Y2Mo4O15 (Fig. 3(b)) gives the fact that there is an energy transfer from the host to Eu3+. The Eu3+ could be activated through two channels, i.e., to sensitize MoO4 groups in the lattices and then transferring the energy to Eu3+ ions, or to sensitize the Eu3+ ions directly via the f-f transitions. Usually, the excitations due to MoO4 groups are spin and parity allowed, and therefore they usually are more efficient for the luminescence of Eu3+ ions.
3.3 Absolute PL quantum efficiency
Absolute PL QE is one of important parameters for a phosphor’s application. Inset Fig. 3(a) shows the relationship between the QEs and the Eu3+-doping (x) of Y2-2xEu2xMo4O15 (x = 0.05-0.4). The efficiency increases with the enhancement of Eu3+ doping and the reach at a maximum at x = 0.4. Consequently the optimal QE in Y2-2xEu2xMo4O15 (x = 0.4) nano-powders is decided to be 63% (λex = 395 nm). This value is higher than that of commercial red-emitting Y2O2S:Eu3+ (QE = 35%, λex = 317 nm, λem = 611 nm) .
3.4 Thermal stability of Eu3+-luminescence
The thermal stability on temperature is one of important technological parameters for applications in W-LEDs. Usually W-LED packages could tolerate high temperatures (>100 °C) and light fluxes (>10 W/cm2) in comparison with traditional fluorescent lighting . The thermal quenching was also investigated by the temperature-dependent emission as shown in Fig. 4(a).The integrated intensities decrease with increasing the temperature to 150 °C with the intensity 80% of the initial value (20°C). Generally, the quenching at high temperature is due to energy migration and transfer to nonradiative traps within the lattice . To confirm the influence of temperature on the energy transfer, the decay curves were measured shown Fig. 4(b). It is obvious that the decay of the 5D0 level at 15 and 350 K have a similar exponential profile. The lifetime keeps nearly the same value of 0.69 ms at 15-350 K indicating that the temperatures have no obvious influence on Eu3+. The lifetime starts to drop at 350 K by a decrease of the emission intensity. The luminescence lifetimes of the Eu3+ ions are calculated as a function of temperature and displayed in Fig. 4(b). The thermal activation energy (∆E) for the thermal quenching was determined by the equation as follows:17].
One of the motivations in this work is based on the fact that MoO4 groups exhibit intensive absorption of near-UV light due to CT transition from O2- to Mo6+ and therewith efficient energy transfer from MoO4 to Eu3+ could happen in the hosts . In this process, there are energy migration and quenching processes among MoO4, which compete with the energy transfer process and consequently reduce the luminescent efficiency. Anyway, it is well-known that a vital influence on the luminescence efficiency is structure in Eu3+-activated hosts, for example, the microstructures of Eu3+ has great influences on the luminescence.
The maximum QE 63% of Y2-2xEu2xMo4O15 (x = 0.4) indicates that this is an efficient red-emitting phosphor. It has been confirmed that in highly charged hosts MOx such as M = V, Mo, W, etc. the excitation energy can be transferred from MOx groups to Eu3+ ions via either an exchange or super-exchange mechanism. The efficiency of the energy transfer depends on the M-O-Eu bond angle . Usually, a 180° angle would maximize wave-function overlap and enhances the CT efficiency from MOx to Eu3+. For example, Blasse et al. concluded that energy transfer VO43-→ Eu3+ is not efficient in Eu3+-doped vanadate garnets due to the V–O–Eu bond angle is 90 or 120°, which reduce the wave-function overlap and the exchange energy transfer efficiency. As shown in Fig. 1 (b), the Mo–O–Eu bond angle is 165°, which would enhance the CT efficiency MoO4→Eu3+ in Y2Mo4O15.
It is a common phenomenon that a heavy Eu3+-doping leads to a “concentration quenching” of the red luminescence due to a possible energy migration among Eu3+ ions. In un-doped Y2Mo4O15 structure, the minimum distance between the Y3+ ions is 5.5670 Å. By an approximation, the shortest distances between En3+ ions in 40% mol Eu3+-doped Y2Mo4O15 is at least 5.5670 × 2 = 11.134 Å. It is far longer that the reported values in some Eu3+-concentrated phosphors such as Eu3BWO9 (4.2117 Å) . This indicates that Y2Mo4O15 host lattices can provide a long distance between Eu3+ ions even at high concentration. This can limit the luminescence quenching by energy transfer of Eu3+ ions hampering the energy migration to the killer centers.
Figure 5 presents the excitation spectra of Y2-2xEu2xMo4O15 (x = 0.4) detected by monitoring 5D0→7F2 luminescence of 616 nm. The sharp emission line with 7F0→5D0 transition at 579.1 nm (17,268 cm−1). One F0→5D0 transition together with the three-fold splitting of 7F1 level and five-fold splitting of 7F2 level indicate that there is only one crystallographic Eu3+ site with symmetry Cs in this lattice. The Eu3+ ions have low-ðeld sites with small ΔE (7F1) of 148 cm−1 in Y2Mo4O15 indicating that Eu-O bonding is more covalent. Usually in Eu3+-doped compound disordered distribution of Eu3+ ions is responsible for the thermal quenching. In Y2Mo4O15 the Eu3+ ions are distributed in the host with a high “ordered state” in only one crystallographic. The excitation energy can have a quick diffusion and give an emission from Eu3+ ions. This could be one reason for the high thermal stability for the Eu3+ in Y2Mo4O15.
Nano-sized red-emitting phosphor of Eu3+-doped Y2Mo4O15 has been synthesized by pechini method with the average size of 35 nm. The crystal phase and structure were performed by XRD structural refinements. The sample keeps monoclinic P21/c crystalline phase below the doping level 40 mol% of the Eu3+ substitution for Y3+ ions in the lattices. Eu3+-doped Y2Mo4O15 show effective excitation in near-UV and blue light regions. The phosphor shows the strong emission from the 5D0→7F2 transitions of Eu3+ ions (616 nm) with CIE coordinates (x = 0.662, y = 0.334). Under the excitation of near UV light, Y2Mo4O15:Eu3+ presents maximum absolute internal QE value of 67% and a high thermal activation energy ∆E of 0.395 eV. The luminescence properties are ascribed to the special structure characteristic such as big Mo–O–Eu bond angle, the long distances between Eu3+ ions, only one crystallographic site with a high “ordered state” distributed in the lattices.
This work was supported by the National Natural Science Foundations of China (Grant No. 21201141), the Chinese Universities Scientific Fund (Grant No. QN2011119), and the Open Foundation of Key Laboratory of Photoelectric Technology and Functional Materials (Culture Base) in Shaanxi Province (ZS12020). Y2Mo4O15:Eu3+ phosphor is supplied by the Display and Lighting Phosphor Bank at Pukyong National University.
References and links
1. C. C. Lin and R. S. Liu, “Advances in phosphors for light-emitting diodes,” J. Phys. Chem. Lett. 2(11), 1268–1277 (2011). [CrossRef]
2. X. Y. Chen, W. Zhao, R. E. Cook, and G. K. Liu, “Anomalous luminescence dynamics of Eu3+ in BaFCl microcrystals,” Phys. Rev. B 70(20), 205122 (2004). [CrossRef]
3. 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. and Tech. 2(2), R3153–R3167 (2013). [CrossRef]
4. H. Li, H. K. Yang, B. K. Moon, B. C. Choi, J. H. Jeong, K. Jang, H. S. Lee, and S. S. Yi, “Crystal structure, electronic structure, and optical and photoluminescence properties of Eu(III) ion-doped Lu6Mo(W)O12.,” Inorg. Chem. 50(24), 12522–12530 (2011). [CrossRef] [PubMed]
5. 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. Alloy. Comp. 509(3), 845–848 (2011). [CrossRef]
6. Z. Y. Hou, H. Z. Lian, M. L. Zhang, L. L. Wang, M. F. Lü, C. M. Zhang, and J. Lin, “Preparation and luminescence properties of Gd2MoO6:Eu3+ nanofibers and nanobelts by electrospinning,” J. Electrochem. Soc. 156(8), J209–J214 (2009). [CrossRef]
7. X. Li, Z. P. Yang, L. Guan, C. Liu, and P. L. Li, “Luminescent properties of Eu3+-doped La2Mo2O9 red phosphor by the flux method,” J. Cryst. Growth 310(12), 3117–3120 (2008). [CrossRef]
8. J. Wan, L. H. Cheng, J. S. Sun, H. Y. Zhong, X. P. Li, W. L. Lu, Y. Tian, H. Lin, and B. J. Chen, “Energy transfer and colorimetric properties of Eu3+/Dy3+ co-doped Gd2(MoO4)3 phosphors,” J. Alloy. Comp. 496(1–2), 331–334 (2010). [CrossRef]
9. G. D. Fallon and B. M. Gatehouse, “The crystal structure of a complex cerium(III) molybdate containing a dimolybdate chain, Ce2(MoO4)2(Mo2O7),” J. Solid State Chem. 44(2), 156–161 (1982). [CrossRef]
10. F. Dubois, F. Goutenoire, Y. Laligant, E. Suard, and P. Lacorre, “Ab-Initio determination of La2Mo4O15 crystal structure from x-rays and neutron powder diffraction,” J. Solid State Chem. 159(1), 228–233 (2001). [CrossRef]
11. L. Sebastian, J. Manjanna, A. M. Umarji, and J. Gopalakrishnan, “Anomalous thermal expansion behaviour of Ln2Mo4O15(Ln=Y, Dy, Ho, Tm),” Mater. Sci. Eng. B 103(3), 289–296 (2003). [CrossRef]
12. H. Wang, J. Peng, X. J. Li, Y. M. Hao, D. F. Chen, and Z. Hu, “Structural and thermal expansion properties of solid solution Y2−xSmxMo4O15 (x=0.0–0.8),” Physica B 388(1–2), 278–284 (2007).
13. S. Laufer, S. Strobel, T. Schleid, J. Cybinska, A. V. Mudring, and I. Hartenbach, “Yttrium(III) oxomolybdates(VI) as potential host materials for luminescence applications: an investigation of Eu3+-doped Y2[MoO4]3 and Y2[MoO4]2[Mo2O7],” New J. Chem. 37(7), 1919–1926 (2013). [CrossRef]
14. Y. L. Huang, Y. Nakai, T. Tsuboi, and H. J. Seo, “The new red-emitting phosphor of oxyfluoride Ca2RF4PO4:Eu3+ (R=Gd, Y) for solid state lighting applications,” Opt. Express 19(7), 6303–6311 (2011). [CrossRef] [PubMed]
15. A. A. Setlur, H. A. Comanzo, A. M. Srivastava, and W. W. Beers, “Spectroscopic evaluation of a white light phosphor for UV-LEDs-Ca2NaMg2V3O12:Eu3+,” J. Electrochem. Soc. 152(12), H205–H208 (2005). [CrossRef]
16. R. Zhu, Y. L. Huang, and H. J. Seo, “A red-emitting phosphor of Eu-based borotungstate Eu3BWO9 for white light-emitting diodes,” J. Electrochem. Soc. 157(12), H1116–H1120 (2010). [CrossRef]
17. Y. C. Fang, S. Y. Chu, P. C. Kao, Y. M. Chuang, and Z. L. Zeng, “Energy transfer and thermal quenching behaviors of CaLa2(MoO4)4:Sm3+, Eu3+ red phosphors,” J. Electrochem. Soc. 158(2), J1–J5 (2011). [CrossRef]