J. Phys. Chem. C 2007, 111, 4111-4115
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Improved Luminescence of Lanthanide(III)-Doped Nanophosphors by Linear Aggregation Ling Li,†,‡ Wenge Jiang,† Haihua Pan,† Xurong Xu,† Yinxuan Tang,§ Jiangzhou Ming,§ Zhude Xu,† and Ruikang Tang*,† Department of Chemistry and Center for Biomaterials and Biopathways, Zhejiang UniVersity, Hangzhou, China, School of Chemical and Material Engineering, Southern Yangtze UniVersity, Wuxi, China, and Research Department of Daming Fluorescent Materials Co. Ltd and Daming ProVincial Laboratory of Rare Earth Materials, Hangzhou, China ReceiVed: December 9, 2006; In Final Form: January 22, 2007
The studies and applications of nanophosphors are considerably important, but unfortunately, their luminescence is always rather poor. Here, we report that the oriented aggregation of nanophosphors can significantly enhance their luminescent emissions at the nanoscale. LnPO4 (Ln ) La, Ce, and Tb) is extensively used in luminescent lighting industry, but the total quantum yield of synthesized individual ∼20 nm La0.60Ce0.26Tb0.14PO4 particles is only ∼21%. This value is improved up to ∼80% when five to seven of these nanophosphors are linearly aggregated to form an oriented structure. The luminescence intensities of these linear aggregates of nanophosphors increase correspondingly. It is suggested that one-dimensional nanoaggregation plays an essential role in this luminescence improvement, which might be a useful strategy in the application of nanophosphors.
Introduction Nanometric luminescent materials have attracted great interest for they may serve as the active component in light-emitting devices,1-3 displays,4 optical amplifiers,5 and biological markers6,7 with nanometer dimensions, biological assays,8 and light sources in low-threshold lasers.9 Among these studies, lanthanide(III)-doped nanocrystals have received considerable attention10-20 since they are excellent light-emitting agents.21,22 The bulk materials, normally with sizes of 3-10 µm, have been extensively used in luminescent lighting industry. Although the preparation and characteristics of LnPO4 nanoparticles are of great importance, their quantum yields are always lower than the corresponding bulk materials. This phenomenon might be caused by energy-transfer processes to the surface through adjacent doping ions or be caused by the luminescence quenching of surface-doping ions. So, if the energy-transferring or the luminescence-quenching processes could be suppressed, the quantum yield of the nanoparticles could surely be improved greatly. One effective strategy to improve emission of nanophosphors is to synthesize core-shell structure. The core-shell structure of green-emitting LnPO4 (Ln ) La, Ce, and Tb) nanoparticles with improved quantum yield was first reported by Ko¨mpe et al.10 In this core-shell structure, the core was doped with the luminescent lanthanide ions while the shell was not doped.10,11 Thus, the energy could not be transferred and the nonradiative pathways could be reduced with the structure of an undoped shell around each doped nanoparticle. In current studies of materials, nanoarchitecture via aggregation-based growth is considered as a useful tool to modulate the characteristics of materials. Herein, we displayed that one-dimensional linear aggregation could significantly enhance the luminescent * To whom correspondence should be addressed. Telephone: (86)5718795 3736. Fax: (86)571-8795 3736. E-mail:
[email protected]. † Zhejiang University. ‡ Southern Yangtze University. § Daming Fluorescent Materials Co. Ltd and Provincial Laboratory of Advanced Materials of Zhejiang.
emission of nanophosphors. The value of the total quantum yield could be increased greatly from only 21% up to 80% when five to seven individual La0.60Ce0.26Tb0.14PO4 nanoparticles (∼20 nm) were aggregated linearly. This value of the nanophosphors almost approached the corresponding value of the bulk material. Experimental Section First, appropriate amounts of La2O3 (99.99%), Ce(NO3)3‚ 6H2O (99.99%), and Tb(NO3)3‚5H2O (99.99%) were dissolved in 250 mL 1.0 M HNO3 aqueous solution. (NH4)2HPO4 (A.R.) was then added to the above solution. The final concentrations of the total rare earth ions and phosphate were 3.0 and 10.0 mM, respectively. Since the pH value of the mixture solution was low (105 times higher than that of La(OH)3. Thus, the crystallization of LnPO4 was dominated absolutely in this precipitation system
Luminescence of Lanthanide(III) Nanophosphors
Figure 2. (a) XRD patterns of La0.60Ce0.26Tb0.14PO4 (curve A) and NLs with aggregating period of 1 day (curve B) and 7 days (curve C). (b) Typical EDX result of LnPO4 solids, the normalized ratio of La:Ce: Tb:O was 0.60 ((0.03):0.26 ((0.02):0.14 ((0.01):1.00 ((0.06):4.04 ((0.12), and the unlabeled weak peaks belonged to the substrates. (c) IR pattern of La0.60Ce0.26Tb0.14PO4.
and the formation of Ln(OH)3 could be ignored. In IR pattern (Figure 2c), the bands at 3447 and 1628 cm-1 were due to the remaining water in the samples. The band at 1057 cm-1 was characteristic of the asymmetric stretch vibration of P-O of PO43- group. The bands at 616 and 542 cm-1 were attributed to the bend vibration of O-P-O. The signal of N-H or N-O
J. Phys. Chem. C, Vol. 111, No. 11, 2007 4113 was not detected by IR, which indicated that the byproduct, NH4NO3, was almost eliminated from the La0.60Ce0.26Tb0.14PO4 solids during the washing. All XRD peaks of NP and NLs could be indexed by using LnPO4 (e.g., JCPDS card No.04-0635, LaPO4). The experimental results showed that the crystallographic structures of the NLs were not affected by the aggregating time since all the NLs had almost the same XRD and EDX patterns, for example, the curves of B and C in Figure 2. However, the intensities of peaks for (200) and (102) of NP and NLs were different. For NP, the intensity of (102) peak was slightly greater than that of (200); however, this order was reversed for NLs since the peak of (200) became the strongest one. This result implied that the morphologies and orientations of the nanophosphors should be altered during the aggregation. The study of selective area electron diffraction (SAED) under TEM could also lead to the same conclusion that the oriented linear aggregates were induced. The electron diffractions could be attributed to (100), (200), (102), and (211) of the sample (Figure 1d), which were in agreement with the XRD results. However, the diffraction rings were incomplete and discontinuous and each of them consisted of some symmetrical curves or dots, implying that the particles in NL were not randomly aggregated. It was interesting to note that the orientation of (100)/(200) diffractions in the SAED was parallel to the examined NL (arrows in Figure 1d), indicating a tendency of oriented organization of the crystallized particles in this linear structure. On the basis of these observations, it was demonstrated that NL was formed by a controlled self-assembly of NPs along the a-axis of La0.60Ce0.26Tb0.14PO4. The oriented aggregation of nanoparticles had been observed in other cases,24-26 for example, Giersig et al. reported that Ag nanoparticles could form one-dimensional chain,24 which was similar to the case of La0.60Ce0.26Tb0.14PO4. It was suggested that dipole-dipole attraction and reactivity between nanoparticles could be induced by their polarized crystal faces.24,27,28 Different facets had different polarity27 and reactivity,28 and the aggregation along the direction with the highest polarity or reactivity was always preferred to reduce the energetic factors. Actually, the details of the mechanism of this linear selfaggregation would be studied in the future. Emission spectrums of all the solid samples under UV excitation (λex ) 275 nm) were the same and their characteristics were not influenced by changing the size of particles or by changing the aggregation time of the particles, since they were in agreement with the previously reported bulk materials of LnPO4 (Ln ) La, Ce, and Tb). Two absorption peaks at 255 and 275 nm were for Ce3+ ions. The emission spectra consisted mainly of four groups of signals, corresponding to the f-f transitions of the Tb3+ (450-650 nm) and a weak band (300400 nm) in the UV region as a result of the d-f transitions of Ce3+. The strong emission of Tb3+ was due to transitions between the excited 5D4 state and the 7FJ (J ) 6-3) ground states of Tb3+ ions (Figure 3a).29 This examination was also repeated in their aqueous suspensions. As expected, a similar energy transfer had been observed in the bulk powder materials30 and in their aqueous colloids.12 Luminescence intensities of the solids were measured and compared using two parallel windows. The NP window was used as the control and upon UV excitation at 275 nm; the luminescence intensity of NP monitored at the main terbium line at 547 nm was used as the standard. It was found that the luminescence intensities of NP and NL were significantly different (Figure 3b). The green emissions of NL were always
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Figure 4. Luminescence decay curves of Tb3+ in La0.60Ce0.26Tb0.14PO4 NP (curve A) and NLs with aggregating period of 1 day (curve B) and 7 days (curve C). (λex ) 345 nm, tex ) 5ns, λem ) 547 nm).
Figure 3. (a) Absorption (curve A), excitation (curve B), and emission (curve C) spectra of the La0.60Ce0.26Tb0.14PO4. The spectra of all the samples, NP and NLs, were identical. (b) Relative luminescent intensities of NLs as a function of aggregating time, the sample of NP was used as the standard since its luminescent intensity was considered as 1. They were measured at 547 nm and were excited by 275 nm UV.
2.70 times higher than the corresponding value of NP. When the NLs were initially formed (aggregating time was 1 day), the relative intensity was suddenly promoted to 3.62. With the extending of aging time, their relative intensities were slightly increased and approached a steady value, about 3.77. An aggregating period of 1 month was also applied. However, it seemed that the “evolution” of NL could not further improve the luminescence of LnPO4. Therefore, the initial linear aggregation could be considered as the predominate factor in this luminescence enhancement, in which the NP was linearly aggregated to form NL. This enhancement at the nanoscale was confirmed by the measurements of photoluminescence quantum yields (φ) in colloidal ethanol solutions. By comparing their integrated emission to the emission of a solution of Rhodamine 6G (dye content 99%, Aldrich), which has the same optical density of 0.1 at the excitation wavelength of 275 nm, a reproducible value of φ, 21 ( 2%, was deduced for the individual NP. This low value was in consistent with the previous understanding of the nanosized LnPO4 (Ln ) La, Ce, and Tb) phosphors. In contrast, the total quantum yields of NL were improved to 80 ( 3%, and this value was close to that of the bulk material (the size distribution was 3.8 ( 1.0 µm and the sample was provided by
Daming, China), 90∼94%. Thus, it could be concluded that a simply oriented aggregation could greatly improve the luminescence of nanophosphors, which could make the application of nanophosphors become possible. Figure 4 is the typical luminescence decay curves of Tb3+ in NP and NLs. For all the NLs, the curves are almost the same. They could overlap with each other. However, the luminescence decay curve of NP exhibits a different feature. These decays are clearly not single-exponential in contrast to the singleexponential luminescence decay characteristic. Analysis of these curves showed that they could be fit successfully to biexponential decay function.31,32 The lifetime constants, τ1 and τ2 of NP, NL (1 day), and NL (7 days) were 1.04 and 0.13, 2.42 and 0.36, and 2.93 and 0.50, respectively. Their corresponding relative luminescence intensities were 1.00, 3.70, and 3.77. Normally, the photoluminescence quantum yields were roughly proportional to their lifetimes in the model of single-exponential decay.30 Although this relationship did not satisfy the resulting data estimated from the biexponential function exactly, it could be found that lifetimes, τ1 and τ2, of NLs were always significantly longer than NP. The τ1 and τ2 values of NL were about 2 and 3 times higher than those of NP, respectively. However, the difference between the two NL samples was not so remarkable. The increase of lifetimes implied the inhibition of the nonradiative relaxation.33 In this study, it also indicated that the luminescence quenching and surface defects were reduced by the linear aggregation. These defects acted as the important channels for nonradiative relaxation, which could catch and exhaust the energy of excited electrons. Conclusions We have showed that five to seven individual La0.60Ce0.26Tb0.14PO4 nanoparticles (∼20 nm) could linearly aggregate spontaneously along the a-axis and that this oriented aggregation could improve the luminescence of the nanophosphors greatly. The total quantum yield of the individual nanoparticles was only ∼21%, but their oriented linear aggregates could promote this value to ∼80%. The luminescence intensities and the luminescence lifetimes were also enhanced correspondingly. Here, we suggested that the controlled oriented aggregation might offer an effective and easy strategy for the luminescent enhancements of phosphors at the nanoscale. Acknowledgment. We thank Prof. Hongwei Song and Prof. Ping Lu for their helpful discussions. This work was supported by National Natural Science Foundation of China (20571064),
Luminescence of Lanthanide(III) Nanophosphors Changjiang Scholars Program (R. T.), and Hangzhou Daming Fluorescent Materials Co. Ltd. References and Notes (1) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (2) Dabousi, B. O.; Bawendi, M. G.; Onitsuka, O.; Rubner, M. F. Appl. Phys. Lett. 1995, 66, 1316. (3) Tessler, N.; Medvedev,V.; Kazes, M.; Kan, S. H.; Banin, U. Science 2002, 295, 1506. (4) Justel, T.; Nikol, H.; Ronda, C. Angew. Chem., Int. Ed. 1998, 37, 3085. (5) Harrison, M. T.; Kershaw, S. V.; Burt, M. G.; Rogach, A. L.; Kornowski, A.; Eychmu¨ller, A.; Weller, H. Pure Appl. Chem. 2000, 72, 314. (6) Bruchez, M. P.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013. (7) Dubertret, B.; Skourides, P.; Norris, D. J.; Noireaux, V.; Brivanlou, A. H.; Libchaber, A. Science 2002, 298, 1759. (8) Dahan, M.; Lauwrence, T.; Pinaud, F.; Chemla, D. S.; Alivisatos, A. P.; Sauer, M.; Weiss, S. Opt. Lett. 2001, 26, 825. (9) Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H. J.; Bawendi, M. G. Science 2000, 290, 314. (10) Ko¨mpe, K.; Borchert, H.; Storz, J.; Lobo, A.; Adam, S.; Mo¨ller, T.; Haase, M. Angew. Chem., Int. Ed. 2003, 42, 5513. (11) Stouwdam, J. W.; van Veggel, F. C. J. M. Langmuir 2004, 20, 11763. (12) Riwotzki, K.; Meyssamy, H.; Schnablegger, H.; Kornowski, A.; Haase, M. Angew. Chem., Int. Ed. 2001, 40, 573. (13) Riwotzki, K.; Meyssamy, H.; Kornowski, A.; Haase, M. J. Phys. Chem. B 2000, 104, 2824.
J. Phys. Chem. C, Vol. 111, No. 11, 2007 4115 (14) Hebbink, G. A.; Stouwdam, J. W.; Reinhoudt, D. N.; van Veggel, F. C. J. M. AdV. Mater. 2002, 14, 1147. (15) Yu, M.; Lin, J.; Fu, J.; Zhang, H. J.; Han, Y. C. J. Mater. Chem. 2003, 13, 1413. (16) Buissette, V.; Moreau, M.; Gacoin, T.; Boilot, J.-P.; Chane-Ching, J.-Y.; Mercier, T. L. Chem. Mater. 2004, 16, 3767. (17) Song, H.; Yu, L.; Lu, S.; Wang,T.; Liu, Z.; Yang, L. Appl. Phys. Lett. 2004, 85, 470. (18) Yu, L.; Song, H.; Lu, S.; Liu, Z.; Yang, L.; Kong, X. J. Phys. Chem. B 2004, 108, 16697. (19) Gallini, S.; Jurado, J. R.; Colomer, M. T. Chem. Mater. 2005, 17, 4154. (20) Karmaoui, M.; Ferreira, R. A. S.; Mane, A. T.; Carlos, L. D.; Pinna, N. Chem. Mater. 2006, 18, 4493. (21) Feldmann, C.; Ju¨stel, T.; Ronda, C. R.; Schmidt, P. J. AdV. Funct. Mater. 2003, 13, 511. (22) Maestro, P.; Huguenin, D. J. Alloys Compd. 1995, 225, 520. (23) Tananaev, I. V.; Vasil’eva, V. P. Russ. J. Inorg. Chem. 1963, 8, 555. (24) Giersig, M.; Pastoriza-Santos, I.; Liz-Marza´n, L. M. J. Mater. Chem. 2004, 14, 607. (25) Sahah, P. S.; Sigman, M. B., Jr.; Stowell, C. A.; Lim, K. T.; Johnston, K. P.; Korgel, B. A. AdV. Mater. 2003, 15, 971. (26) Penn, R. L. J. Phys. Chem. B 2004, 108, 12707. (27) Tang, Z.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237. (28) Pacholski, C.; Kornowski, A.; Weller, H. Angew. Chem., Int. Ed. 2002, 41, 1188. (29) Yu, M.; Lin, J.; Fu, J.; Zhang, H. J.; Han, Y. C. J. Mater. Chem. 2003, 13, 1413. (30) Bourcet, J.-C.; Fong, F. K. J. Chem. Phys. 1974, 60, 34. (31) Grinvald, A.; Steinberg, I. Z. Anal. Biochem. 1974, 59, 583. (32) Periasamy, N. Biophys. J. 1988, 54, 961. (33) Nofsinger, J. B.; Ye, T.; Simon, J. D. J. Phys. Chem. B 2001, 105, 2864.
