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In this work, rare earth (RE) ions tri-doped YV1-xPxO4: RE3+ (RE = Tm, Dy, Eu) inverse opal photonic crystals (IOPCs) were fabricated by the PMMA template method, which demonstrated efficient white light emissions under ultraviolet excitation. It is significant to observe that the chrominance of the white light could be largely modulated by the photonic stop band of the IOPCs. And more, the photoluminescence quantum yield in the IOPCs was largely improved over the grinded reference (REF) because the undesired energy transfer (ET) process was effectively restrained.
© 2013 Optical Society of America
1. Introduction
Since the pioneering works by Yablonovitch [1] and John [2] in 1987, photonic crystals (PCs) have attracted considerable interests. Possessing spatial periodicity in their dielectric constants on the length scale of the optical wavelength, PCs behave with respect to electromagnetic waves like atomic crystals do with respect to electrons. As an electronic band gap is created by the periodic arrangement of atoms in a semiconductor, the periodic electromagnetic modulation created by the PCs can yield a photonic stop band (PSB), a band of frequency for which light propagation in the PCs is forbidden. PCs have demonstrated a wide variety of applications, such as optical waveguides with sharp bands, optical circuits, optical signal modulators and etc [3–8]. Among various applications, controlling the spontaneous emission of excited atoms and molecules is attracting particular interests. Up to now, the modification of spontaneous emission by embedding luminescent species in the three-dimensional PCs, including organic dyes, semiconductors and RE ions has been widely studied and various phenomena have been observed, such as the modulation of PCs on spontaneous emission rate and ET process, Lamb shift and so on [9–16].
On the other hand, the searching for efficient white light phosphors is of great significance in the field of lighting and display [17]. White light can be traditionally generated by mixing blue, green, and red phosphors together. However, in a multiphase system, the luminescent efficiency is unavoidably affected due to the reabsorption of blue light by red and green phosphors [18, 19]. It is expected that more efficient white emission could be realized by a single phase phosphor with co-excited activators and some efforts have been contributed [20].
In this work, we demonstrate an efficient white light emission in Tm3+, Dy3+ and Eu3+ tri-doped YV1-xPxO4 IOPCs under the excitation of ultraviolet (UV) light. It is known that YVO4 is one of the most famous phosphors under UV excitation due to its outstanding physical and chemical stability, high thermal conductivity, large absorption cross-section and effective ET from VO43+ to RE ions [21]. Through the suitable introduction of P element in the YVO4 host, the cross ET among VO43- groups can be suppressed, thus the ET from VO43- groups to RE ions becomes more effective [22]. In fact, RE ions triply doped method has been already used to realize white light emission [23]. It is expected that in the YV1-xPxO4 IOPCs, by the modulation of IOPC structure, the unexpected cross ET among different RE ions could be further suppressed and the luminescent quantum efficiency could be improved. In this experiment, it is significant to realize efficient white light emissions in the tri-doped three-dimensional IOPCs and to observe considerably improved luminescent efficiency.
2. Experiments
All the YV1-xPxO4 IOPCs were prepared by the PMMA latex sphere template technique, similar to [13, 16]. PMMA template method has some advantages such as low cost of Methyl methacrylate (MMA), simple synthesis process and highly uniformed and dispersed of Polymethyl methacrylate (PMMA) spheres. YVO4 can be prepared by sol-gel method and the precursor solution can uniformly infiltrates into the space of PMMA sphere template. Thus, through PMMA template method, controllable thickness YVO4 IOPCs with different PSBs can be prepared easily. The PSBs of YV1-xPxO4 IOPCs were controlled by diameters of the PMMA latex spheres (The PSBs of the YV1-xPxO4 IOPCs measured in the normal direction located at 380, 478, 516, 570 and 615 nm and they were named as, PC1-PC5, respectively, in Fig. 3 and Fig. 4. The IOPCs used in Fig. 5 and Fig. 6 owned the PSBs at 615nm. All the IOPCs in this work were controlled with the same thickness). The REF samples were prepared by grinding the corresponding IOPCs to destroy the regular 3D structure. The experimental conditions for structure characterization and optical measurement have been described in detail in [16].
3. Results and discussions
Firstly, the structure of all the IOPCs and REF samples were characterized and compared. Figs. 1(a) and 1(b) show the X-ray diffraction (XRD) patterns of the IOPCs and REF samples, respectively, in contrast to the standard card JCPDS 17-0341 for tetragonal YVO4. In Fig. 1(a), the broad band ranging of 15-40 degree comes from the diffraction of the glass substrate. The diffraction peaks at 2θ≈25° are due to the (200) reflection of YV1-xPxO4. No impurity peaks appear, implying that all YV1-xPxO4 samples are in pure tetragonal phase. The insert of Figs. 1(a) and 1(b) show that the diffraction peaks present a gradually shift towards higher 2θ values with the increasing of P concentration, owning to the decrease in unit cell parameters [24]. As a typical case, the SEM and TEM images of PC5 are shown in Figs. 1(c) and 1(d), respectively. The SEM image shows that the PC5 sample yields a long-range ordered hexagonal arrangement of IOPC, and the center-to-center distance of the IOPC is 338 nm. The TEM image shows that the wall thickness of the IOPC is about 10 nm, which consists of a large amount of small nanoparticles (NPs). The structure characteristics of the other IOPCs are similar to PC5 except the change of IOPC center to center distance. Figure 2 shows the EDS spectra of the YV0.95P0.05O4: Dy3+ IOPC. It shows that Y, V, P, and O elements all exist in the sample and this is the direct experimental evidence to prove the P element in the YVO4 host.
Fig. 1 The XRD patterns of YVxP1-xO4:Dy3+ PCs (a) and REF samples (b) with different x values (x = 0, 0.03, 0.05, 0.07 and 0.1), and insets of (a) and (b) show the closeup of (200) reflection of the IOPCs and REF samples, respectively. (c) The SEM image of PC5. (d) The TEM image of PC5.
Figure 3 shows the transmittance spectra with different PSBs at normal (θ = 0°) and the corresponding steady-state emission spectra in the Y0.986V0.95P0.05O4: Tm3+0.008Dy3+0.004Eu3+0.002 IOPCs . It can be seen that the PC2 displays a PSB centering at 478 nm, corresponding to the 1G4-3H6 transition of Tm3+ and the 4F9/2-6H15/2 transition of Dy3+. PC3 demonstrates a PSB at 516 nm, which is nearby the 1G4-3H6/4F9/2-6H15/2 transition. The PC4 and PC5 display PSBs centering at 570 nm and 615 nm, overlapping with the 4F9/2-6H13/2 transition of Dy3+ and the 5D0-7F2 transition of Eu3+, respectively. From the steady-state emission spectra, when the emission wavelength locates on the center of PSBs, a significant inhibition of luminescent intensity can be observed in contrast to PC1, which displays a PSB centering at 380nm and is far away from all the emissions. The inhibition of light emission is an universal phenomenon for luminescent species embedded in PCs [12, 16]. Suppression of the emission can be understood as being due to the reduction in the number of opticalmodes available for photonpropagation at frequencies within the PSB [25]. It can be seen from Figs. 3(b) and 3(d), when the emission wavelength locates on the edge of PSBs, a weak enhancement of luminescent intensity is distinguished, owing to the improvement in the number of opticalmodes on the edge of PSBs [15, 26]. It should be highlighted that besides the transition of RE ions, the broad band emission ranging of 400-450nm originating from vanadate groups can be also observed, which cannot be observed in YVO4: Dy3+ IOPCs [16]. This is the direct evidence that the ET process from vanadate groups to defects is suppressed by the doping of P element.
Fig. 3 The transmittance of PC2 (a), PC3 (b), PC4 (c), PC5 (d), and the steady-state emission spectra (ex = 280nm) of PC2 (a), PC3 (b), PC4 (c), PC5 (d) in contrast with that of PC1 (the black line in (a)-(d)).
Figure 4 shows the CIE chromaticity coordinates diagram and correlated color temperature (CCT) (inset of Fig. 4) of Y0.986V0.95P0.05O4: Tm3+0.008Dy3+0.004Eu3+0.002 IOPCs with different PSBs. As is shown in the inset of Fig. 4, when the PSB locates at 478 nm, the blue emission is suppressed and a warm white light (CCT = 4192K) can be obtained. When the PSB locates at 570 nm, yellow emission is suppressed and a cold white light (CCT = 6966K) can be realized. Overall, the conversion between warm white and cold white light can be accurately modulated through the PSB effect of IOPCs. Thus, it is expected that IOPC-based white-light sources could be widely used in art exhibition and scene arrangement.
Fig. 4 The CIE chromaticity coordinates diagram of Y0.986V0.95P0.05O4: Tm3+0.008Dy3+0.004Eu3+0.002 IOPCs with different PSBs. Inset is the detail information.
Our previous studies indicated that in weakly modulated PCs, the radiative decay time constants of RE ions were independent of PSBs. However, due to the modulation of effective refractive index, the radiative decay time constants of RE ions in PCs were prolonged in contrast to the REF samples, which was dependent of the refractive index and the filling factor of the PCs [12, 16].
The study on luminescent dynamics of the IOPCs in this work further supports our previous conclusion. In the Tm3+, Dy3+, and Eu3+ tri-doped phosphors, cross ETs may occur among different ions, such as from Tm3+ to Dy3+, which usually induce the quenching of PL. In order to deduce the cross ET rates from Tm3+ to Dy3+ in different samples, the decay time constants of 1G4-3H6 transitions for Tm3+ in Y0.992-xV0.95P0.05O4:Tm3+0.008Dy3+x (x = 0, 0.002, 0.004 and 0.006) IOPCs (PSBs located in 615nm and with the same thickness) and REF (grinding the corresponding IOPCs) samples were measured. All the decay curves can be well fitted by the ET model and the fitting result shows that dipole-dipole interaction dominates the ET process [27]. As shown in Fig. 5, it can be seen that the decay time constants of Tm3+ decrease gradually with the increasing doping concentration of Dy3+, implying the existence of ET from Tm3+ to Dy3+. When x = 0, the lifetime of 1G4-3H6 for Tm3+ in PCs was prolonged 1.65 times in comparison to that in REF samples, while that of 5D0-7FJ for Eu3+ and of 4F9/2-6HJ for Dy3+ was prolonged as large as 2.5 times [12, 16]. Generally, the spontaneous emission rate (SER) is the sum of the radiative transition rate and the nonradiative relaxation rate. In the former case, the lifetime for 1G4 depends on both the radiative rate and the nonradiative relaxation rate, thus the variation of the lifetime is very complex. In the latter case, the lifetime is dominated by the radiative rate of 5D0-7FJ or 4F9/2-6HJ due to large energy gap from 5D0, 4F9/2 to the nearest down level and the negligible nonradiative transition rate from 5D0, 4F9/2.
Fig. 5 The decay time constant of Tm3+ in YV0.95P0.05O4:Tm3+Dy3+x (x = 0, 0.002, 0.004, 0.006) IOPCs (green dots) and REF samples (red dots). Inset: the ET efficiency of Tm3+ to Dy3+ in YV0.95P0.05O4:Tm3+Dy3+x (x = 0, 0.002, 0.004, 0.006) IOPCs (green bar) and REF (red bar).
Based on the luminescent dynamics of Tm3+, the ET efficiency from Tm3+ to Dy3+ can be roughly estimated by:
where is the ET efficiency, τTm1 is the lifetime constant of Tm3+ with Dy3+ doping, and τTm2 is the lifetime constant of Tm3+ without Dy3+ doping. The inset of Fig. 5 shows the deduced dependence of ET efficiency from Tm3+ to Dy3+ in the IOPCs and REF samples. As is shown in the inset, the ET efficiency of Tm3+ to Dy3+ in the REF samples increases rapidly with the increasing doping concentration of Dy3+, but the ET efficiency in the IOPCs increases more slowly. This is because, in IOPCs, a high surface-to-volume ratio can effectively increase the chance that photons come into the air and the periodic dielectric cavity acts as a local resonance mode for the emission propagation. Photons can couple to the local resonance mode and scatter out of the structure [28]. Thus, more energy of excited Tm3+ could be converted to light in IOPCs, and the unexpected nonradiative process of Tm3+ and the ET process from Tm3+ to Dy3+ could be suppressed. Note that the white emission and ET between Tm3+ and Dy3+ ions were reported in [29], which tuned the chrominance and luminescent intensity through the doping concentration of Tm3+ and Dy3+. In this work, IOPC structure was used and the chrominance could be largely modulated between warm white and cold white light by the PSB of the IOPCs. Besides, the ET between Tm3+ and Dy3+ could be suppressed by using IOPC structure and the luminescent quantum efficiency could be improved.The PL quantum yield (QY) of YV1-xPxO4:Dy3+ IOPCs (PSBs located at 615nm) and REF (grinding the corresponding IOPCs with the same thickness) samples were measured with a fluorescence spectrophotometer equipped with a BaSO4 integrated sphere similar to [30] and the result as shown in Fig. 6. It can be observed that the PL quantum yield first increases with the increasing of x value (x = 0-0.05) both in the IOPCs and REF samples, and then decreases with the further increase of x value (x = 0.05-0.1). As mentioned above, luminescent quenching will inevitably happen due to the ET from VO43- groups to defect states because of the existence of long-term resonant ET among VO43- groups. The replacement of a part of VO43- by PO43- can prevent this ET process, leading to the initial increase of QY. With the further increase of PO43-, the ET process can be suppressed not only from VO43- groups to defects, but also from VO43- groups to the RE ions, resulting in the decrease of QY.
Fig. 6 The PL quantum yield of YV1-xPxO4:Dy3+ (x = 0, 0.03, 0.05, 0.07, 0.1) PCs (red bar) and REF samples (blue bar). Inset: the decay time constant of Dy3+ in YV1-xPXO4:Dy3+ (x = 0, 0.03, 0.05, 0.07, 0.1) PCs (red line) and REF samples (black line).
In order to prove this viewpoint, the decay time constants of Dy3+ in YV1-xPxO4:Dy3+ (x = 0, 0.03, 0.05, 0.07, and 0.1) IOPCs and REF samples were measured. As shown in the inset of Fig. 6, Whether in IOPCs or REF samples, the decay time constants of Dy3+ rarely change with P concentration. This result implies that the change of QY with P concentration is mainly due to ET process from VO43- groups to Dy3+ as well as the other RE ions. In Fig. 6, it is exciting to observe that the PL quantum yield of IOPCs was considerably improved than that of REF samples. Because in the inverse opals, the long-range ET among VO43- groups should be restrained largely due to the thin thickness of each YV1-xPxO4 layer and the existence of a long periodic air cavity between the two layers.
4. Conclusions
In conclusion, tri-doped YV1-xPxO4: RE3+ (RE = Tm, Dy, Eu) IOPCs were fabricated by the PMMA template method. As x = 0.05, both YV1-xPxO4: RE3+ IOPCs and REF samples demonstrated a maximum PL quantum yield. Using the IOPC structure, the unexpected ET process was effectively restrained and the PL quantum yield was considerably increased. Meanwhile, the color conversion between warm white and cold white light was modulated by the PSB effect of PCs. Overall, YV1-xPxO4:RE3+ (RE = Tm, Dy, Eu) IOPCs have a remarkable application prospect in the field of white-light source.
Acknowledgments
This work was supported by National Talent Youth Science Foundation of China (Grant no. 60925018), the National Natural Science Foundation of China (Grant no. 61204015, 51002062, 11174111, 61177042, and 81201738), funding from the State Key Laboratory of Bioelectronics of Southeast University.
References and links
1. E. Yablonovitch, “Inhibited spontaneous emission in solid-state physics and electronics,” Phys. Rev. Lett. 58(20), 2059–2062 (1987). [CrossRef] [PubMed]
2. S. John, “Strong localization of photons in certain disordered dielectric superlattices,” Phys. Rev. Lett. 58(23), 2486–2489 (1987). [CrossRef] [PubMed]
3. A. Majumdar, J. Kim, J. Vuckovic, and F. Wang, “Electrical control of silicon photonic crystal cavity by graphene,” Nano Lett. 13(2), 515–518 (2013). [CrossRef] [PubMed]
4. N. Matsuda, H. Takesue, K. Shimizu, Y. Tokura, E. Kuramochi, and M. Notomi, “Slow light enhanced correlated photon pair generation in photonic-crystal coupled-resonator optical waveguides,” Opt. Express 21(7), 8596–8604 (2013). [CrossRef] [PubMed]
5. F. Raineri, T. J. Karle, V. Roppo, P. Monnier, and R. Raj, “Time-domain mapping of nonlinear pulse propagation in photonic-crystal slow-light waveguides,” Phys. Rev. A 87(4), 041802 (2013). [CrossRef]
6. V. Liu, D. A. B. Miller, and S. H. Fan, “Ultra-compact photonic crystal waveguide spatial mode converter and its connection to the optical diode effect,” Opt. Express 20(27), 28388–28397 (2012). [CrossRef] [PubMed]
7. A. Hosseini, X. C. Xu, H. Subbaraman, C. Y. Lin, S. Rahimi, and R. T. Chen, “Large optical spectral range dispersion engineered silicon-based photonic crystal waveguide modulator,” Opt. Express 20(11), 12318–12325 (2012). [CrossRef] [PubMed]
8. C. Xiong, C. Monat, A. S. Clark, C. Grillet, G. D. Marshall, M. J. Steel, J. Li, L. O’Faolain, T. F. Krauss, J. G. Rarity, and B. J. Eggleton, “Slow-light enhanced correlated photon pair generation in a silicon photonic crystal waveguide,” Opt. Lett. 36(17), 3413–3415 (2011). [CrossRef] [PubMed]
9. K. Yoshino, S. B. Lee, S. Tatsuhara, Y. Kawagishi, M. Ozaki, and A. A. Zakhidov, “Observation of inhibited spontaneous emission and stimulated emission of rhodamine 6G in polymer replica of synthetic opal,” Appl. Phys. Lett. 73(24), 3506–3508 (1998). [CrossRef]
10. P. Lodahl, A. Floris Van Driel, I. S. Nikolaev, A. Irman, K. Overgaag, D. L. Vanmaekelbergh, and W. L. Vos, “Controlling the dynamics of spontaneous emission from quantum dots by photonic crystals,” Nature 430(7000), 654–657 (2004). [CrossRef] [PubMed]
11. A. Rodenas, G. Zhou, D. Jaque, and M. Gu, “Rare-earth spontaneous emission control in three-dimensional lithium niobate photonic crystals,” Adv. Mater. 21(34), 3526–3530 (2009). [CrossRef]
12. Y. S. Zhu, W. Xu, H. Z. Zhang, W. Wang, L. Tong, S. Xu, Z. P. Sun, and H. W. Song, “Highly modified spontaneous emissions in YVO4:Eu3+ inverse opal and refractive index sensing application,” Appl. Phys. Lett. 100(8), 081104 (2012). [CrossRef]
13. Y. S. Zhu, Z. P. Sun, Z. Yin, H. W. Song, W. Xu, Y. F. Wang, L. G. Zhang, and H. Z. Zhang, “Self-assembly, highly modified spontaneous emission and energy transfer properties of LaPO4:Ce3+, Tb3+ inverse opals,” Dalton Trans. 42(22), 8049–8057 (2013). [CrossRef] [PubMed]
14. Q. Liu, H. W. Song, W. Wang, X. Bai, Y. Wang, B. Dong, L. Xu, and W. Han, “Observation of Lamb shift and modified spontaneous emission dynamics in the YBO3:Eu3+ inverse opal,” Opt. Lett. 35(17), 2898–2900 (2010). [CrossRef] [PubMed]
15. C. Blum, A. P. Mosk, I. S. Nikolaev, V. Subramaniam, and W. L. Vos, “Color control of natural fluorescent proteins by photonic crystals,” Small 4(4), 492–496 (2008). [CrossRef] [PubMed]
16. Y. S. Zhu, W. Xu, H. Z. Zhang, W. Wang, S. Xu, and H. W. Song, “Inhibited long-scale energy transfer in dysprosium doped yttrium vanadate inverse opal,” J. Phys. Chem. C 116(3), 2297–2302 (2012). [CrossRef]
17. A. S. Osvaldo, A. C. Simone, and R. I. J. Renata, “A new procedure to obtain Eu3+ doped oxide and oxosalt phosphors,” Alloys Compd. 303–304, 316–319 (2000).
18. Y. H. Won, H. S. Jang, W. B. Im, D. Y. Jeon, and J. S. Lee, “Tunable full-color-emitting La0.827Al11.9O19.09: Eu2+,Mn2+ phosphor for application to warm white-light-emitting diodes,” Appl. Phys. Lett. 89(23), 231909 (2006). [CrossRef]
19. D. Gao, H. Zheng, X. Zhang, W. Gao, Y. Tian, J. Li, and M. Cui, “Luminescence enhancement and quenching by codopant ions in lanthanide doped fluoride nanocrystals,” Nanotechnology 22(17), 175702 (2011). [CrossRef] [PubMed]
20. C. H. Huang and T. M. Chen, “A novel single-composition trichromatic white-light Ca3Y(GaO)3(BO3)4:Ce3+,Mn2+,Tb3+ phosphor for UV-light emitting diodes,” J. Phys. Chem. C 115(5), 2349–2355 (2011). [CrossRef]
21. R. J. Wiglusz, A. Bednarkiewicz, and W. Strek, “Role of the sintering temperature and doping level in the structural and spectral properties of Eu-doped nanocrystalline YVO4.,” Inorg. Chem. 51(2), 1180–1186 (2012). [CrossRef] [PubMed]
22. M. Yu, J. Lin, Y. H. Zhou, M. L. Pang, X. M. Han, and S. B. Wang, “Luminescence properties of RP1−xVxO4: A (R=Y, Gd, La; A=Sm3+, Er3+x=0, 0.5, 1) thin films prepared by Pechini sol–gel process,” Thin Solid Films 444(1-2), 245–253 (2003). [CrossRef]
23. C. Lorbeer and A. V. Mudring, “White-light-emitting single phosphors via triply doped LaF3 nanoparticles,” J. Phys. Chem. C 117(23), 12229–12238 (2013). [CrossRef]
24. C. H. Lu and R. Jagannathan, “Cerium-ion-doped yttrium aluminum garnet nanophosphors prepared through sol-gel pyrolysis for luminescent lighting,” Appl. Phys. Lett. 80(19), 3608–3610 (2002). [CrossRef]
25. G. von Freymann, V. Kitaev, B. V. Lotsch, and G. A. Ozin, “Bottom-up assembly of photonic crystals,” Chem. Soc. Rev. 42(7), 2528–2554 (2013). [CrossRef] [PubMed]
26. S. Fan, P. R. Villeneuve, J. D. Joannopoulos, and E. F. Schubert, “High extraction efficiency of spontaneous emission from slabs of photonic crystals,” Phys. Rev. Lett. 78(17), 3294–3297 (1997). [CrossRef]
27. L. P. Xie, H. W. Song, Y. Wang, W. Xu, X. Bai, and B. Dong, “Influence of concentration effect and Au coating on photoluminescence properties of YVO4:Eu3+ NPs colloids,” J. Phys. Chem. C 114(21), 9975–9980 (2010). [CrossRef]
28. H. Li, J. X. Wang, H. Lin, L. Xu, W. Xu, R. M. Wang, Y. L. Song, and D. B. Zhu, “Amplification of fluorescent contrast by photonic crystals in optical storage,” Adv. Mater. 22(11), 1237–1241 (2010). [CrossRef] [PubMed]
29. X. Wang, X. H. Yan, Y. Y. Bu, J. Zhen, and Y. Xuan, “Fabrication, photoluminescence, and potential application in white light emitting diode of Dy3+-Tm3+ doped transparent glass ceramics containing GdSr2F7 nanocrystals,” Appl. Phys., A Mater. Sci. Process. 112(2), 317–322 (2013). [CrossRef]
30. J. C. de Mello, H. F. Wittmann, and R. H. Friend, “An improved experimental determination of external photoluminescence quantum efficiency,” Adv. Mater. 9(3), 230–232 (1997). [CrossRef]
