Open Access
Add to BibSonomyAbstract
The self-activated yellow-emitting phosphors of vanadates Ca5M4(VO4)6 (M = Mg, Zn) were synthesized via the solid-state reaction route. The formation of single phase compound with garnet structure was verified through X-ray diffraction (XRD) studies. The excitation and emission spectra and the thermal quenching of luminescence intensities were measured. The different luminescence properties of Ca5Mg4(VO4)6 and Ca5Zn4(VO4)6 phosphors were presented, e.g., the spectra shift, the luminescence lifetimes, the absolute quantum efficiency, the color coordinates and the Stokes shift. This deference was discussed on the base of the relationship between the micro-structure and the charge transfer transitions in [VO4]3- groups in the lattices. Ca5Mg4(VO4)6 could be suggested to be a potential yellow-emitting phosphor for the application on near-UV excited white LEDs.
©2012 Optical Society of America
1. Introduction
Vanadates have many applications in the field of optical laser, electrochemistry, biology materials, and catalysis, etc [1–5]. Especially as a kind of efficient luminescence material, vanadates have been widely investigated in the past years [6–10]. For example, in recent years these materials have been applied to various types of white light emitting diodes (W-LEDs) and flat-panel displays (FPDs) due to their better chromaticity.
Vanadate group, [VO4]3−, where the central metal ion is coordinated by four oxygen ions in a tetrahedral (Td) symmetry, is known to be an efficient luminescent center [6,8]. The potential use of vanadates in phosphor application can be understood as due to their long-wavelength excitation and the excellent chemical stabilities. Usually vanadates have broad and intense charge transfer (CT) absorption bands in the near-UV region and are therefore capable of efficiently capturing the emission over a large range of wavelengths. Meanwhile numbers of vanadates can show a quite broadband emission from 400 to more than 700 nm [6]. It has been possible to tune the AlGaN-based LEDs that are developed to operate at the short wavelength UV region [11]. When excited by 350–400 nm light from emission of near-UV LED chips, these materials have the capability to convert the ultraviolet emission into white light [10,12]. Bayer [13] firstly described garnets containing vanadium, e.g., {NaCa2}[Mg2](V3)O12 where {} represents a dodecahedral site, [] an octahedral site and () a tetrahedral site. However, the detailed luminescence properties such as an absolute quantum efficiencies (QE) and luminescent color properties of Ca5M4(VO4)6 (M = Mg, Zn) vanadium oxide phosphors have not been reported.
In the present work, the yellow-emitting phosphors Ca5M4(VO4)6 (M = Mg, Zn) with garnet structure were synthesized via the solid-state reaction. The phase formation, photoluminescence excitation (PLE) and luminescence (PL) spectra, the luminescence decay, the QE values, and the thermal quenching were investigated. The different luminescence properties between Ca5Mg4(VO4)6 (called CMV, hereafter) and Ca5Zn4(VO4)6 (CZV) have been discussed on the base of the crystal structure and the luminescence spectra.
2. Experimental
Polycrystalline samples Ca5M4(VO4)6 (M = Mg, Zn) were synthesized using a conventional solid-state reaction. The starting material was a stoichiometric mixture of reagent grade ammonium vanadate NH4VO3, 4(MgCO3)·Mg(OH)2·5H2O (magnesium carbonate basic pentahydrate), ZnO and CaCO3. Firstly, the stoichiometric mixture was slowly heated up to 350 °C in 5 h and kept at this temperature for 3-5 h. The obtained powder was mixed again and then heated up to 700 °C for 5 h in air. After that, the sample was thoroughly mixed and heated at 750–850 °C for 6-10 h in air.
The XRD patterns were collected on a Rigaku D/Max diffractometer operating at 40 kV, 30 mA with Bragg–Brentano geometry using Cu Kα radiation (λ = 1.5405 Å) and analyzed by using Jade-5.0 software program. The photoluminescence excitation spectra and luminescence spectra were recorded on a Perkin-Elmer LS-50B luminescence spectrometer with Monk–Gillieson type monochromators and a xenon discharge lamp used as excitation source. Quantum efficiency was measured by an Absolute Photoluminescence Quantum Yield Measurement System (C9920-02, Hamamatsu) at room temperature. The excitation was done by changing excitation wavelength of light from 150 W Xe-lamp.
3. Results
3.1 Phase formation
Figure 1 shows the X-ray powder diffraction patterns of CMV and CZV, which well match with the standard PDF2 cards No.34-0014 (Ca5Mg4(VO4)6) and No.53-1164 (Ca5Zn4(VO4)6), respectively, selected in the International Centre for Diffraction Data (ICDD) database. No impurity lines were observed. All peaks are well indexed to the cubic garnet structure with space group of Ia-3d (230). A schematic of the crystal structure of Ca5Mg4(VO4)6 is presented in Fig. 2 . In this vanadate garnet structure, Ca2+ ions are located in eightfold dodecahedral sites (i.e. a distorted cube with D2 symmetry). The Mg2+ and Zn2+ ions are in six fold octahedral sites. The metal ion V5+ (in isolated [VO4]3−) completely occupies the fourfold Td site [13].
The cell parameters are fitted to be a = 12.411 Å, Vol = 1910.70 Å3 for CMV, and a = 12.462 Å, Vol = 1935.37 Å3 for CZV. The unit cell of CZV is much bigger than that of CMV. This can be confirmed by the low degree-shift of XRD pattern in CZV as shown in inset in Fig. 1, indicating an expansion of CZV lattices. This could be due to the larger atomic radii of Zn2+ (0.74 Å, coordination number CN = 6) than the Mg2+ (0.72 Å, CN = 6).
3.2 The excitation and luminescence spectra
Figure 3 is the PL and PLE spectra for CMV and CZV samples. Figure 4 is the schematic model to display the excitation and emission processes in VO4 tetrahedron [6,14]. The molecular orbitals of V5+ ion with Td symmetry are expressed as a ground 1A1 state and excited 1T1, 1T2, 3T1, and 3T2 states [8]. The absorption bands for Ex1 and Ex2 in the PLE spectra are corresponding to the spin-allowed transitions from the ground state 1A1 to the excited states 1T2 and 1T1 levels, respectively [6,15].
Fig. 3 the normalized PL and PLE for Ca5Mg4(VO4)6 and Ca5Zn4(VO4)6 samples. The two dashed lines are the fitted the emission spectrum of Ca5Mg4(VO4)6 by two Gaussian components named as Em1 and Em2.
Fig. 4 The excitation and emission processes in VO4 tetrahedron with Td symmetry in Ca5Mg4(VO4)6 and Ca5Zn4(VO4)6.
The two samples show broad excitation spectra between 250 and 400 nm with two peaks at 280 nm (Ex1:1A1 → 1T2) and 365 nm (Ex2:1A1 → 1T1) ascribed to charge transfer transitions in [VO4]3− group. This indicates that the phosphor can well match with the light of UV-LED chips (360–400 nm), which is essential for improving the efficiency of W-LEDs. The nearly the same excitation spectrum is in agreement with the conclusion suggested by Ronde et al [8] that the position of the excitation bands is only slightly influenced by the vanadate host.
The origin of the luminescence in vanadate based systems has been well documented and attributed to the ligand–metal charge transfer (CT) bands (2p orbital of oxygen ion→3d orbital of vanadium ion) localized within the tetrahedrally coordinated [VO4]3− group [12]. The two samples show yellow emission band between 450 and 750 nm due to CT emission transition in [VO4]3− (Fig. 3). The CMV sample shows a broad luminescence spectrum with the maximum position at 530 nm. Compared with CMV (530 nm), the emission spectrum of CZV shows a red-shift with the maximum wavelength at 550 nm. The CIE (Commission International de l'Eclairage 1931) coordinate of CMV and CZV were calculated to be (x = 0.382, y = 0.499) and (x = 0.425, y = 0.511), respectively.
The broad emission band of CMV has a peak at 530 nm and extends from 420 nm to 700 nm. Therefore, taking into account that its emission spectrum is similar to the solar spectrum regarding the peak wavelength and broad band in the visible region, this material is useful for lighting when it is pumped with UV-LED.
According to the Gaussian components by fitting the emission spectrum (only showing the fitted CMV in Fig. 3), the emission band of each sample consists of double peaks: Em1 (520 nm for CMV and 540 nm for CZV) and Em2 (590 nm for CMV and 610 nm for CZV). This indicates that the energy separation between 3T1 and 3T2 is about 2200 cm−1 for CMV and 2100 cm−1 for CZV. The detailed energy levels are shown in Fig. 4. The Stokes shift of CMV and CZV were estimated to be around 8530 and 9220 cm−1, respectively.
3.3 The quantum efficiency and luminescence decay
One of the most important properties of a phosphor is the QE for its application. The QE values of CMV and CZV samples are compared with the well-know yellow-emitting phosphors YAG:Ce3+ in Table 1 . The QE values of CMV and CZV measured (excitation 320 nm) at room temperature are 41.6%, and 15.9%, respectively. The QEs for CMV and CZV are lower than the commercial yellow-emitting YAG:Ce3+, but are far better than the sulfide-based yellow-emitting phosphor.
However, the luminescence efficiencies of the YAG:Ce3+ are strongly depended on the particle size and the redox reaction of cerium [16]. The QE usually depends on the synthesis conditions. The higher quantum yields can be obtained by further improving the synthesis conditions to reduce the number of defects and impurities. However, CMV phosphor has some advantages, for example, low sintering temperature (below 850 °C), environment-friendly (sintering in air), no rare earth ions doping, low cost, etc. Additionally, the non-rare-earth-based phosphors have been suggested to be excellent candidates for replacing rare earth ions doped materials in the preparation of LEDs [17].
Figure 5 shows emission decay curves of CMV (530 nm) and CZV (550 nm). The non-exponential decay curves can be fitted to the effective lifetime defined as the following equation:
where I(t) represents the luminescence intensity at a time t after the cutoff of the excitation light. The decay curves for CMV (530 nm) and CZV (550 nm) exhibit different lifetimes of 2.151 and 1.714 μs, respectively.Fig. 5 The luminescence decay curves of Ca5Mg4(VO4)6 and Ca5Zn4(VO4)6 samples under the excitation of the third harmonic 355 nm of a pulsed Nd:YAG laser.
3.4 The dependence of luminescence on temperature
In general, the temperature dependence of phosphors is important because it has great influence on the light output and color rendering index. The luminescence thermal quenching effects of CMV (530 nm) and CZV (550 nm) were evaluated by measuring the temperature-dependent emission. Figure 6 shows the representative thermal quenching effects on the luminescence of CMV. The emission spectra have not obvious shift with changing the temperatures. The CZV sample has the same situation. This is benefit for the color stability of phosphors at high temperature.
Fig. 6 Temperature-dependent luminescence of CMZ. Inset shows the luminescence intensity of thermal quenching normalized to the value at 25 °C.
Inset in Fig. 6 is the luminescence intensity of thermal quenching normalized to the value at 25 °C. The thermal quenching temperature, T0.5, defined as the temperature at which the emission intensity is 50% of its original value, is about 115 °C for CMV, while T0.5 = 85 °C for CZV.
4. Discussions
It can be found that the luminescence properties of two samples are very different, for example, CIE (x,y), QE, the luminescence lifetimes, and Stokes shift as listed in the Table 1. They have the same crystal structure, so the micro-structure in the lattice would be different, especially the structural difference in the VO4 tetrahedron because the emission is from the charge transfer transitions in [VO4]3− group [6,15].
As seen in the XRD studies in Fig. 1, the lattice of CZV shows some expansion because the bigger ionic radius of Zn2+ ions. The average V–O bond distance (1.753 Å) in CZV is larger than that in CMV (1.727 Å) [13]. Ronde et al [19] have confirmed that the energy of the [VO4]3− charge-transfer transition is depended on the V-O distance. With increasing the V-O distance the transition energies △E (t1- 2e) decrease. Consequently, the maximum emission position for the CZV is red-shifted by about 30 nm compared to that for CMV. This variation is not from a changing ratio of the emission intensity for the Em1 and Em2 but from peak shifts of both emissions, indicating that the difference in the structural distortion of the VO4 tetrahedra leads to a large change of the energy levels for both of the ground and excited states (see Fig. 4). This change brings the very different luminescence properties in CMV and CZV phosphors, e.g., the quantum efficiency, the CIE color coordinates, Stokes shifts, the luminescence lifetimes, and the thermal stabilities, as listed in Table 1.
Usually, the thermal stability can be related to the stiffness of a host lattice. The vanadate host-lattice dependence of the quenching temperature of the emission has been investigated in terms of a configurational-coordinate mode by Ronde, and G. Blasse [8]. It can be understood that the higher value of T0.5 in the case of CMV implies that △r (i.e. the difference between the equilibrium distances of the parabola for the ground and the excited state in the single configurational coordinate model) must be relatively small upon optical excitation. This can be related to the crystal structure: the stiffness of the garnet lattice will be relatively high as a consequence of the fact that the Mg2+ ions in octahedra are small (0.72 Å) than that of Zn2+ (0.74 Å). The rigid structure of the surroundings of VO4 group could hinder the V-O bond to expand after optical excitation (compared with the loose CZV sample). This implies a high thermal quenching temperature. For example, in isostructural structure of NaCaVO4 and LiCdVO4, the Ca2+ and Cd2+ have the similar ionic radius. The substitution of lithium by the bigger sodium leads to great luminescence thermal quenching in NaCaVO4 [8]. This is due to the relaxation of the stiffness of the lattice and thus for the lowering of the quenching temperature in Na composition.
Usually larger Stokes shift leads to thermal quenching at lower temperature. This is understood using Fig. 7 , where a schematic configuration coordinate diagram plotted against V-O distance R is presented for the excited state Eexc responsible for emission and the ground state Eg of VO4. ΔE is the activation energy, while Rg and Rexc are the equilibrium distances of the ground and excited states, respectively. Figure 7 also illustrates the radiative transition kr and non-radiative (i.e., thermal phonon-assisted) transition knr from the ground state to the excited state. When the separation between Rexc and Rg becomes large, the Stokes shift becomes large. At the same time the activation energy ΔE becomes small, resulting in the thermal quenching at lower temperature. The shorter V-O distance of VO4 for CMV than for CZV leads to shorter Rexc-Rg distance for CMV as mentioned above. Therefore we understand the reason why CZV with larger Stokes shift than CMV shows thermal quenching at lower temperature.
Fig. 7 A schematic configuration coordinate diagram for the excited state level Eexc and the ground state Eg in VO4, together with the electronic transitions.
The observed difference of luminescence QE values (41.6 and 15.9% for CMV and CZV, respectively) is also explained by the difference of V-O distance. The shorter V-O distance gives rise to larger distortion of the [VO4]3− tetrahedron, leading to lowering the symmetry of the [VO4]3− tetrahedron from the Td symmetry. Consequently, (1) the parity-forbidden transition in the d-d transition of V ion becomes partially parity-allowed and (2) spin-forbidden transition in the 3T2 (3T1) - 1A1 transition becomes partially allowed, by the mixing of the 3T2 (3T1) states with high-energy excited singlet states. As a result the radiative transition probability becomes higher for CMV than for CZV. Therefore, taking into account that the non-radiative thermal transition knr from the 3T2 (3T1) state to the ground state is smaller in CMV than CZV as mentioned above, the luminescent QE is expected to become higher for CMV than for CZV because of less-dissipation of the excited energy from the 3T2 (3T1) states. In this way we can understand the observed difference of QE between CMV and CZV.
In addition, Nakajima et al [18] have widely investigated the correlation between QEs and structural properties of vanadates AVO3 (A: Li, Na, K, Rb, and Cs), M2V2O7 (M: Mg, Ca, Sr, Ba, and Zn), and M3V2O8 (M: Mg, Ca, Sr, Ba, and Zn) with chained, dimerized, and isolated VO4 tetrahedra, respectively. They concluded that the luminescence QE of the vanadates with VO4 tetrahedra was strongly enhanced by the strong interaction between V ions and the weak interaction between V and A(M) ions in the crystal structures. As listed in Table 1, the QEs of vanadates reported by Nakajima et al [18] have great changes due to the different structural properties. In vanadates containing of the VO4 chains. CsVO3 has maximum QE of 87.0%; all the vanadates containing VO4 dimers have low QEs. Similar to the structures of Ca5Mg4(VO4)6 and Ca5Zn4(VO4)6, M3V2O8 (M: Mg, Ca, Sr, Ba, and Zn) consist of isolated VO4 tetrahedra. Except for Zn3V2O8, the other M3V2O8 (M: Mg, Ca, Sr, and Ba) have much lower QEs than those of Ca5Mg4(VO4)6 and Ca5Zn4(VO4)6.
It can be found that the shortest distance between the close V ions in Ca5Mg4(VO4)6 and Ca5Zn4(VO4)6 are 3.8062 Å and 3.8129 Å, respectively. It can be expected that the interaction between V ions in Ca5Mg4(VO4)6 could be stronger than that in Ca5Zn4(VO4)6 lattice. Consequently, the luminescent QE of Ca5Mg4(VO4)6 could be higher than that of Ca5Zn4(VO4)6 phosphor according to the suggestions proposed by Nakajima et al [18].
5. Conclusions
The self-activated yellow-emitting phosphors Ca5M4(VO4)6 (M = Mg, Zn), were synthesized by solid state reactions. The phosphors show broad excitation band from 200 to 430 nm associated with CT bands of [VO4]3-, which can be effectively excited by UV chips (360–400 nm) for the potential applications in the W-LEDs. In the Ca5Mg4(VO4)6 sample, the broadband emission spectra from 450 to 750 nm derived from the CT transition in the VO4 tetrahedra were observed with the maximum emission at 530 nm. Ca5Zn4(VO4)6 shifts the luminescence spectra to long wavelength with the maximum at 550 nm. The luminescence QE values of Ca5Mg4(VO4)6 and Ca5Zn4(VO4)6 are 41.6% and 15.9%, respectively. The thermal quenching temperature T0.5 for Ca5Mg4(VO4)6 and Ca5Zn4(VO4)6 are 115 and 85 °C, respectively. The higher QE and thermal stability of Ca5Mg4(VO4)6 than Ca5Zn4(VO4)6 are explained by the distortion of VO4 tetrahedron in Ca5Mg4(VO4)6, which leads to smaller Stokes shift, higher radiative transition and smaller non-radiative thermal transition from the emitting excited state to the ground state in Ca5Mg4(VO4)6 than in Ca5Zn4(VO4)6. The present results indicate that the novel yellow-emitting phosphor Ca5Mg4(VO4)6 is possible to develop a suitable phosphor for the application on near-UV excited white LEDs.
Acknowledgments
This work was supported by Mid-career Researcher Program through National Research Foundation of Korea (NRF) grant funded by the Ministry of Education, Science and Technology (MEST) (Project No. 2009-0078682), and the Industrial Strategic technology development program (Project No: 10037416, Establishment of infrastructure for LED-marine convergence technology support and technology development for commercialization) funded by the Ministry of Knowledge Economy (MKE, Korea), and by A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
References and links
1. F. C. Hawthorne and C. Calvo, “The crystal chemistry of the M+VO3 (M+= Li, Na, K, NH4, Tl, Rb, and Cs) pyroxenes,” J. Solid State Chem. 22(2), 157–170 (1977). [CrossRef]
2. M. Koichi, K. Miyamoto, S. Ujita, T. Saito, H. Ito, and T. Omatsu, “Dual-frequency picosecond optical parametric generator pumped by a Nd-doped vanadate bounce laser,” Opt. Express 19(19), 18523–18528 (2011). [CrossRef] [PubMed]
3. J. Azkargorta, M. Bettinelli, I. Iparraguirre, S. Garcia-Revilla, R. Balda, and J. Fernández, “Random lasing in Nd:LuVO4 crystal powder,” Opt. Express 19(20), 19591–19599 (2011). [CrossRef] [PubMed]
4. Y. J. Huang, Y. P. Huang, H. C. Liang, K. W. Su, Y. F. Chen, and K. F. Huang, “Comparative study between conventional and diffusion-bonded Nd-doped vanadate crystals in the passively mode-locked operation,” Opt. Express 18(9), 9518–9524 (2010). [CrossRef] [PubMed]
5. G. M. Thomas and M. J. Damzen, “Passively Q-switched Nd:YVO4 laser with greater than 11 W average power,” Opt. Express 19(5), 4577–4582 (2011). [CrossRef] [PubMed]
6. T. Nakajima, M. Isobe, T. Tsuchiya, Y. Ueda, and T. Kumagai, “A revisit of photoluminescence property for vanadate oxides AVO3 (A:K, Rb and Cs) and M3V2O8 (M:Mg and Zn),” J. Lumin. 129(12), 1598–1601 (2009). [CrossRef]
7. K. C. Park and S. I. Mho, “Photoluminescence properties of Ba3V2O8,Ba3(1-x)Eu2xV2O8 and Ba2Y2/3V2O8:Eu3+,” J. Lumin. 122–123, 95–98 (2007). [CrossRef]
8. H. Ronde and G. Blasse, “The nature of the electronic transitions of the vanadate group,” J. Inorg. Nucl. Chem. 40(2), 215–219 (1978). [CrossRef]
9. A. R. Dhobale, M. Mohapatra, V. Natarajan, and S. V. Godbole, “Synthesis and photoluminescence investigations of the white light emitting phosphor, vanadate garnet, Ca2NaMg2V3O12 co-doped with Dy and Sm,” J. Lumin. 132(2), 293–298 (2012). [CrossRef]
10. G. Gundiah, Y. Shimomura, N. Kijima, and A. K. Cheetham, “Novel red phosphors based on vanadate garnets for solid state lighting applications,” Chem. Phys. Lett. 455(4–6), 279–283 (2008).
11. T. Nishida, T. Ban, and N. Kobayashi, “High-color-rendering light sources consisting of a 350 nm ultraviolet light-emitting diode and three-basal-color phosphors,” Appl. Phys. Lett. 82(22), 3817–3819 (2003). [CrossRef]
12. 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]
13. G. Bayer, “Vanadates A3B2V3O12 with garnet structure,” J. Am. Ceram. Soc. 48(11), 600 (1965). [CrossRef]
14. T. Ziegler, A. Rauk, and E. J. Baerends, “On the calculation of multiplet energies by the hartree-fock-slater method,” Theor. Chim. Acta 43(3), 261–271 (1977). [CrossRef]
15. S. Benmokhtar, A. El Jazouli, J. P. Chaminade, P. Gravereau, F. Guillen, and D. de Waal, “Synthesis, crystal structure and optical properties of BiMgVO5,” J. Solid State Chem. 177(11), 4175–4182 (2004). [CrossRef]
16. D. Haranath, H. Chander, P. Sharma, and S. Singh, “Enhanced luminescence of Y3Al5O12:Ce3+ nanophosphor for white light-emitting diodes,” Appl. Phys. Lett. 89, 173118 (2006).
17. C. C. Lin and R. S. Liu, “Advances in phosphors for light-emitting diodes,” J. Phys. Chem. Lett. 2(11), 1268–1277 (2011). [CrossRef]
18. T. Nakajima, M. Isobe, T. Tsuchiya, Y. Ueda, and T. Manabe, “Correlation between luminescence quantum efficiency and structural properties of vanadate phosphors with chained, dimerized, and isolated VO4 tetrahedra,” J. Phys. Chem. C 114(11), 5160–5167 (2010). [CrossRef]
19. H. Ronde and J. G. Snijder, “The position of the VO3−4 charge-transfer transition as a function of the V-O distance,” Chem. Phys. Lett. 50(2), 282–283 (1977). [CrossRef]
