- Email: [email protected]
shell nanoparticles with enhanced near infrared (NIR) emission
Chemical Physics Letters 494 (2010) 60–63
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Luminescent properties of Nd3+-doped LaF3 core/shell nanoparticles with enhanced near infrared (NIR) emission Xiaoxia Cui a,b, Jiangbo She a,b, Chao Gao a, Kai Cui a,b, Chaoqi Hou a,b, Wei Wei a,c,*, Bo Peng a,c,** a
State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Science (CAS), Xi’an, Shaanxi 710119, PR China Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China c Institute of Advanced Materials, Nanjing University of Posts and Telecommunications, Nanjing 210003, PR China b
a r t i c l e
i n f o
Article history: Received 25 March 2010 In final form 24 May 2010 Available online 27 May 2010
a b s t r a c t A kind of Nd3+-doped LaF3 nanoparticles with core/shell structure were synthesized via a co-precipitation method. The possible formation mechanisms for the core/shell architectures were presented. The X-ray diffraction (XRD) patterns indicated that the obtained nanoparticles exhibit hexagonal structure. In comparison with LaF3:Nd core nanoparticles, the fluorescence intensity and lifetime were increased by 140% and 150% respectively in LaF3:Nd/LaF3 core/shell nanoparticles. The obvious features are mainly attributed to the formation of LaF3 shell which effectively protected the Nd3+ ions from surrounding influence. The results show that rare-earth-doped nanoparticles with core/shell structure have potential applications in biological labeling and light-emitting devices. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Neodymium ions (Nd3+) doped nano-materials have a variety of applications in the optical cells window and the optical telecommunication window due to their near infrared emissions [1–3]. Recently, rare-earth (RE) fluorides have become the research focus because of their very low vibrational energies; this advantage will minimize the quenching effects of the excited states of the rareearth ions [4]. However, the near infrared (NIR) emitting rare-earth ions such as Nd3+ and Er3+ can be quenched efficiently when they are incorporated into nanoparticles [5]. It may be explained that a large surface-to-volume ratio of nanoparticles cause a number of dopant ions to reside on the nanoparticles surface, which form the surface defects and hamper the optical efficiency. On top of that, the surface ions would adsorb some groups such as O–H and C–H which also noticeably quench the emissions of luminescence ions [6,7]. Similar effect has been observed in the LaF3:Nd nanoparticles reported in our previously work [8], although the nanoparticles surface were modified by oleic acid, the O–H groups were not eliminated completely. In order to reduce these influence, the passivated inorganic shells including NaYF4, SiO2, LaPO4 and
* Corresponding author at: State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Science (CAS), Xi’an, Shaanxi 710119, PR China. Fax: +86 25 83535502. ** Corresponding author at: State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Science (CAS), Xi’an, Shaanxi 710119, PR China. Fax: +86 25 83535502. E-mail addresses: [email protected] (W. Wei), [email protected] (B. Peng). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.05.070
LaF3 [9–12] have been introduced to shield the lanthanide ion. These core/shell nanoparticles, in which the core is doped with the lanthanide ion and the shell is not doped, could confine the excited state well away from the surface and in this way reduce surface quenching. An appropriate shell should possess two features to improve the emission properties: (1) low phonon energy and environmental stability to minimize the nonradiative quench [13]; (2) similar crystalline structure between the shell and core to decrease lattice mismatch [14]. LaF3 as doping host meets these requirements due to its low vibration energies and superior optical transparency in the 0.2–1.1 lm region. Rare-earth fluoride (REF3) core/shell nanocrystals have been successfully prepared by the thermolysis of organometallic precursors [15] and hydrothermal synthesis [16]. However, these methods usually require high temperature and high pressure, which may lead to the formation of the no-doped nanoparticles and the relatively large diameter, limiting their applications in biological labeling. Water–ethanol–ammonium di-n-octadecyldithioposhate (ADDP) mixing system with a facile precipitation technology was first developed by Ballato’s group to synthesize core/shell nanoparticles [17]. This strategy is a green method without high temperature decomposition of rare-earth salts. In this work, we modified the method to synthesize a kind of LaF3:Nd/LaF3 core/shell nanoparticles. This architecture contributed to significant enhancement of fluorescence intensity and lifetime, which implied that the LaF3 shell with low vibration energy has protected the emission of Nd3+ centers. It provides a useful way to improve luminescent properties of Nd3+-doped LaF3 nanoparticles.
X. Cui et al. / Chemical Physics Letters 494 (2010) 60–63
2. Experimental The LaF3:Nd3+/LaF3 core/shell nanoparticles were synthesized as follows [17]. NH4F (8 mmol) and oleic acid (OA) surfactant (0.6 mmol) were firstly dissolved in 70 mL ethanol/water (1:1), and then the solution was heated to 75 °C. Aqueous solution (4.0 mL) with La(NO3)36H2O (3.6 mmol) and Nd(NO3)36H2O (0.4 mmol) were added dropwise into the NH4F solution to form the core particles. After 30 min, LaF3 shell was grown by the alternating addition in 10 parts of a 2.5 mL aqueous NH4F solution (4.0 mmol) and a 4.0 mL aqueous Ln(NO3)3 solution (2.0 mmol). The result solution was stirred at 65 °C for 2 h and allowed cooling to room temperature. The products were centrifugated and washed by ethanol and water, the resultant powder was dried over P2O5 for 2 days under vacuum. The morphologies of the samples were characterized by JEOL JEM-3010 transmission electron microscope (TEM) under a working voltage of 300 kV. The X-ray diffraction (XRD) was measured on a Rigaku Dmax-2400 X-ray powder diffractometer with graphite monochromatized Cu Ka radiation (k = 0.15405 nm). The XPS spectra were obtained by Axis Ultra Spectrometer (Kratos, UK) using monochromatic Al Ka radiation (150 W, 15 KV, and 1486 eV), and the vacuum in the spectrometer is 10 9 Torr. Photoluminescence emission spectra were recorded on a Zolix Omini-k 300 spectrophotometer pumped by a laser diode at 800 nm. Luminescence decay times were measured by modulating the laser with a mechanical chopper, and the signal was collected and analyzed by a 300 MHz Tektronix oscilloscope (Model 3032B).
3. Results and discussion
61
reaction system alternatively, the nanoparticles surface exhibits excessive La3+ or F ions in turn, which bond additive counter ions via the electrostatic attraction. Therefore, the additive ions were grafted on the core surface to form shell layer instead of individual nanoparticles. 3.2. Morphology, structure and surface composition Fig. 2 shows the TEM images of core and core/shell nanoparticles. The average diameters of these nanoparticles are 9.6 and 11.1 nm respectively. The thickness of the shells is about 1.5 nm. No small free-standing nanoparticles are inlayed on the large ones in TEM images of the core/shell nanoparticles, indicating that all of the added monomer had grown on the existing core nanoparticles and no new nucleation had occurred. Because the core and the shell have similar electron density and lattice parameters, they cannot be clearly distinguished [18]. Fig. 3 shows the XRD patterns of LaF3:Nd/LaF3 core/shell and LaF3:Nd nanoparticles. All of the diffraction peaks were indexed to the hexagonal LaF3 crystal structure (JCPDS: 32–0483). No other peaks were observed in the patterns, revealing that the products were single. By means of Debye–Scherer equation, the average sizes of core and core/shell nanoparticles were estimated to be about 9.2 and 11.3 nm, respectively. These results were in good agreement with the TEM image. Compared with the core nanoparticles, the peak intensities of core/shell nanoparticles increased obviously, which demonstrate that the formation of LaF3 shell reduced the surface defects successfully. In order to confirm that the LaF3:Nd core nanoparticles have been coated by LaF3 shell successfully; the surface compositions
3.1. Formation mechanism of core/shell nanoparticles Fig. 1 is a scheme diagram of the formation mechanism of the LaF3:Nd/LaF3 core/shell nanoparticles. The modified method is to grow the shell layer around the core by the alternate addition of small portions of the shell reagents. At the beginning of the core formation, the surfactant and excessive amounts of F ions were introduced. When the rareearth ions were added dropwise into the solution, LnF3 nuclei was generated by the chemical reaction between Ln3+ and F . Meanwhile, the OA acted as surfactant kept the core nanoparticles growing gradually. The excessive F ions were absorbed on the core surface, then the nanoparticles exhibit negative charge surface. That makes the coordination of positive charge La3+ ions possible. At the second step, the reaction temperature was adjusted to 65 °C to slow the reaction rate, avoiding additional nucleation to form un-doped nanoparticles. When the shell solution containing La3+ ions was added, the La3+ interacted with the F ions gradually. Accordingly, the excessive La3+ ions were located at the nanoparticles surface, leading to a positive charge surface. When the F ions were added subsequently, there is competition between the OA molecules and fluoride ions. The Ln3+ ions first react with F ions instead of OA molecules on account of its smaller radius, Thus, when the solutions containing La3+ and F ions were added into
Fig. 2. TEM images of the as-prepared core LaF3:Nd (a) and LaF3:Nd/LaF3 core/shell samples (b).
Fig. 1. A scheme diagram of the formation mechanism for the LaF3:Nd/LaF3 core/ shell nanoparticles.
Fig. 3. XRD patterns of standard data for LaF3 (a) (PDF card No. 32-0483), LaF3:Nd core nanoparticles (b) and LaF3:Nd/LaF3 core/shell nanoparticles (c).
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X. Cui et al. / Chemical Physics Letters 494 (2010) 60–63
of the samples were investigated by XPS analysis (as shown in Fig. 4). For LaF3:Nd nanoparticles, the obvious peaks of visible Nd 3d (binding energy = 1000 eV), La 3d (binding energy = 836 eV) and F 1s (binding energy = 635 eV) were observed. For LaF3:Nd/ LaF3 core/shell structure, the sharp and enhanced peaks of La 3d and F 1s are observed obviously, and the intensity of peak of Nd 3d was weakened. This is consistent with the proposed structure where the LaF3 shell encapsulated the LaF3:Nd core. The oxygen and carbon detected in the XPS measurement may come from the atmosphere. Furthermore, low amounts of oxygen and carbon are frequently observed in the XPS spectra.
3.3. Photoluminescent properties Fig. 5 shows the emission spectra of LaF3:Nd/LaF3 core/shell and LaF3:Nd nanoparticles under excitation at 800 nm. The emission peaks at 880, 1053, and 1330 nm correspond to the transitions from 4F3/2 to 4I9/2, 4I11/2 and 4I13/2, respectively. The 4F3/2 ? 4I11/2 is a typical magnetic dipole transition, generally investigated and applied in near-IR. For LaF3:Nd/LaF3 core/shell nanoparticles, the shapes of the emission patterns are similar to that of LaF3:Nd nanoparticles, while the emission intensity was increased by about 140% at 1053 nm. The ratio is much higher than the previous reported [19].
Fig. 6. Decay curves of the 4F3/2 ? 4I11/2 transition for LaF3:Nd core and LaF3:Nd/ LaF3 core/shell nanoparticles (Nd10 mol%). The samples were excited at 800 nm and emission was monitored at 1053 nm at room temperature.
Decay curves for the core and core/shell nanoparticles are shown in Fig. 6. The fluorescence lifetime (s) was measured to be 56 ls for LaF3:Nd core and 140 ls for LaF3:Nd/LaF3 core/shell nanoparticles by fitting the decay curves with double exponential function. Compared with the core nanoparticles, the fluorescence lifetime of the core/shell nanoparticles was increased by 150%. The increase ratio is much bigger than that of literature [9]. This remarkable enhancement can be explained that when the shell-ion solution was added into the core solution, there is competition between the organic molecules (OA) and F ions. The OA has little chance to bond with Ln3+ ions adsorbed on the core surface, avoiding the quenching of –CH3, –COO groups. After the shell layer formed, the coordination of OA to the lanthanide ions on the shell surface increased the distance between neighbouring nanoparticles, reducing the nonradiative pathways and suppressing the quenching in the energy transfer process. Besides, in the proposed core/shell structure, the light-emitting center ions (Nd3+) located on the core surface own fewer dangling bonds, thereby reducing the surface defects. Amounts of nonradiative centers existing on the core surface are reduced and the emitting centers were less sensitive to surface quencher. The enhanced fluorescence intensity and lifetime indicate that the strategy is an effective way to improve luminescent properties of LaF3:Ln3+ nanoparticles.
Fig. 4. XPS spectra of the LaF3:Nd core (A) and LaF3:Nd/LaF3 core/shell (B) nanoparticles.
4. Conclusions In summary, we have successfully synthesized LaF3:Nd/LaF3 core/shell nanoparticles via a co-precipitation method. The fluorescence intensity and lifetime were increased by 140% and 150% respectively compared to LaF3:Nd nanoparticles. It can be ascribed to the formation of shell layer which provides a physical barrier between the optically active core and the surrounding medium, reducing the nonradiative pathways. Moreover, the shell further provides an efficient passivation of the surface trap states, and gives rise to a strongly enhanced fluorescence intensity and lifetime. The advantage of this facile method appears to provide an effective way to improve the luminescence properties for other rare-earth fluoride compounds. Acknowledgments Fig. 5. Emission spectra of LaF3:Nd/LaF3 core/shell and LaF3:Nd (Nd10 mol%) nanoparticles. The peaks at 880, 1053, and 1330 nm correspond to the transitions from 4F3/2 to 4I9/2, 4I11/2 and 4I13/2, respectively.
This work was financially supported by the National Natural Science Foundation of China (No. 60977023) and one Hundred Talents Programs of the Chinese Academy of Sciences.
X. Cui et al. / Chemical Physics Letters 494 (2010) 60–63
References [1] J.W. Stouwdam, F.C.J.M. van Veggel, Nano Lett. 2 (2002) 733. [2] R. Dekker, D.J.W. Klunder, A. Borreman, M.B.J. Diemeer, K. Wörhoff, A. Driessen, Appl. Phys. Lett. 85 (2004) 6104. [3] G. Sinha, A. Patra, Chem. Phys. Lett. 473 (2009) 151. [4] C.M. Bender, J.M. Burlitch, Chem. Mater. 12 (2000) 1969. [5] J.W. Stouwdam, G.A. Hebbink, J. Huskens, F.C.J.M. van Veggel, Chem. Mater. 15 (2003) 4604. [6] R.C. Hole, W.DeW. Harrocks Jr., Inorg. Chim. Acta 171 (1990) 193. [7] S.T. Frey, C.A. Chang, K.L. Pounds, W.DeW. Harrocks Jr., Inorg. Chem. 33 (1994) 2882. [8] X.X. Cui, J.B. She, K. Cui, C. Gao, C.Q. Hou, W. Wei, B. Peng, Chem. Phys. Lett. 489 (2010) 191. [9] Q. Zhang, Q.M. Zhang, Mater. Lett. 63 (2009) 376. [10] X.F. Yu, L.D. Chen, M. Li, M.Y. Xie, L. Zhou, Y. Li, Q.Q. Wang, Adv. Mater. 20 (2008) 1.
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[11] K. KÖmpe, H. Borchert, J. Storz, A. Lobo, S. Adam, T. MÜller, M. Haase, Angew. Chem. Int. Ed. 42 (2003) 5513. [12] S.H. Bo, J. Hu, Z. Chen, Q. Wang, G.M. Xu, X.H. Liu, Z. Zhen, Appl. Phys. B 97 (2009) 665. [13] O.V. Kudryavteseva, L.S. Garashina, K.K. Rivkina, B.P. Sobolev, J. Sov. Phys. Crystallogr. 18 (1974) 531. [14] X.B. Chen, Y.B. Lou, A.C. Samia, C. Burda, Nano Lett. 3 (2003) 799. [15] X. Sun, Y.W. Zhang, Y.P. Du, Z.G. Yan, R. Si, L.P. You, C.H. Yan, Chem. Eur. J. 13 (2007) 2320. [16] C.X. Li, X.M. Liu, P.P. Yang, C.M. Zhang, H.Z. Lian, J. Lin, J. Phys. Chem. C 112 (2008) 2904. [17] J.R. DiMaio, B. Kokuoz, J. Ballato, Opt. Express 14 (2006) 11412. [18] Z.L. Wang et al., Chem. Mater. 18 (2006) 2030. [19] M.Y. Xie, L. Yu, H. He, X.F. Yu, J. Solid State Chem. 182 (2009) 597.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Luminescent properties of Nd3+-doped LaF3 core/shell nanoparticles with enhanced near infrared (NIR) emission Xiaoxia Cui a,b, Jiangbo She a,b, Chao Gao a, Kai Cui a,b, Chaoqi Hou a,b, Wei Wei a,c,*, Bo Peng a,c,** a
State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Science (CAS), Xi’an, Shaanxi 710119, PR China Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China c Institute of Advanced Materials, Nanjing University of Posts and Telecommunications, Nanjing 210003, PR China b
a r t i c l e
i n f o
Article history: Received 25 March 2010 In final form 24 May 2010 Available online 27 May 2010
a b s t r a c t A kind of Nd3+-doped LaF3 nanoparticles with core/shell structure were synthesized via a co-precipitation method. The possible formation mechanisms for the core/shell architectures were presented. The X-ray diffraction (XRD) patterns indicated that the obtained nanoparticles exhibit hexagonal structure. In comparison with LaF3:Nd core nanoparticles, the fluorescence intensity and lifetime were increased by 140% and 150% respectively in LaF3:Nd/LaF3 core/shell nanoparticles. The obvious features are mainly attributed to the formation of LaF3 shell which effectively protected the Nd3+ ions from surrounding influence. The results show that rare-earth-doped nanoparticles with core/shell structure have potential applications in biological labeling and light-emitting devices. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Neodymium ions (Nd3+) doped nano-materials have a variety of applications in the optical cells window and the optical telecommunication window due to their near infrared emissions [1–3]. Recently, rare-earth (RE) fluorides have become the research focus because of their very low vibrational energies; this advantage will minimize the quenching effects of the excited states of the rareearth ions [4]. However, the near infrared (NIR) emitting rare-earth ions such as Nd3+ and Er3+ can be quenched efficiently when they are incorporated into nanoparticles [5]. It may be explained that a large surface-to-volume ratio of nanoparticles cause a number of dopant ions to reside on the nanoparticles surface, which form the surface defects and hamper the optical efficiency. On top of that, the surface ions would adsorb some groups such as O–H and C–H which also noticeably quench the emissions of luminescence ions [6,7]. Similar effect has been observed in the LaF3:Nd nanoparticles reported in our previously work [8], although the nanoparticles surface were modified by oleic acid, the O–H groups were not eliminated completely. In order to reduce these influence, the passivated inorganic shells including NaYF4, SiO2, LaPO4 and
* Corresponding author at: State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Science (CAS), Xi’an, Shaanxi 710119, PR China. Fax: +86 25 83535502. ** Corresponding author at: State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Science (CAS), Xi’an, Shaanxi 710119, PR China. Fax: +86 25 83535502. E-mail addresses: [email protected] (W. Wei), [email protected] (B. Peng). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.05.070
LaF3 [9–12] have been introduced to shield the lanthanide ion. These core/shell nanoparticles, in which the core is doped with the lanthanide ion and the shell is not doped, could confine the excited state well away from the surface and in this way reduce surface quenching. An appropriate shell should possess two features to improve the emission properties: (1) low phonon energy and environmental stability to minimize the nonradiative quench [13]; (2) similar crystalline structure between the shell and core to decrease lattice mismatch [14]. LaF3 as doping host meets these requirements due to its low vibration energies and superior optical transparency in the 0.2–1.1 lm region. Rare-earth fluoride (REF3) core/shell nanocrystals have been successfully prepared by the thermolysis of organometallic precursors [15] and hydrothermal synthesis [16]. However, these methods usually require high temperature and high pressure, which may lead to the formation of the no-doped nanoparticles and the relatively large diameter, limiting their applications in biological labeling. Water–ethanol–ammonium di-n-octadecyldithioposhate (ADDP) mixing system with a facile precipitation technology was first developed by Ballato’s group to synthesize core/shell nanoparticles [17]. This strategy is a green method without high temperature decomposition of rare-earth salts. In this work, we modified the method to synthesize a kind of LaF3:Nd/LaF3 core/shell nanoparticles. This architecture contributed to significant enhancement of fluorescence intensity and lifetime, which implied that the LaF3 shell with low vibration energy has protected the emission of Nd3+ centers. It provides a useful way to improve luminescent properties of Nd3+-doped LaF3 nanoparticles.
X. Cui et al. / Chemical Physics Letters 494 (2010) 60–63
2. Experimental The LaF3:Nd3+/LaF3 core/shell nanoparticles were synthesized as follows [17]. NH4F (8 mmol) and oleic acid (OA) surfactant (0.6 mmol) were firstly dissolved in 70 mL ethanol/water (1:1), and then the solution was heated to 75 °C. Aqueous solution (4.0 mL) with La(NO3)36H2O (3.6 mmol) and Nd(NO3)36H2O (0.4 mmol) were added dropwise into the NH4F solution to form the core particles. After 30 min, LaF3 shell was grown by the alternating addition in 10 parts of a 2.5 mL aqueous NH4F solution (4.0 mmol) and a 4.0 mL aqueous Ln(NO3)3 solution (2.0 mmol). The result solution was stirred at 65 °C for 2 h and allowed cooling to room temperature. The products were centrifugated and washed by ethanol and water, the resultant powder was dried over P2O5 for 2 days under vacuum. The morphologies of the samples were characterized by JEOL JEM-3010 transmission electron microscope (TEM) under a working voltage of 300 kV. The X-ray diffraction (XRD) was measured on a Rigaku Dmax-2400 X-ray powder diffractometer with graphite monochromatized Cu Ka radiation (k = 0.15405 nm). The XPS spectra were obtained by Axis Ultra Spectrometer (Kratos, UK) using monochromatic Al Ka radiation (150 W, 15 KV, and 1486 eV), and the vacuum in the spectrometer is 10 9 Torr. Photoluminescence emission spectra were recorded on a Zolix Omini-k 300 spectrophotometer pumped by a laser diode at 800 nm. Luminescence decay times were measured by modulating the laser with a mechanical chopper, and the signal was collected and analyzed by a 300 MHz Tektronix oscilloscope (Model 3032B).
3. Results and discussion
61
reaction system alternatively, the nanoparticles surface exhibits excessive La3+ or F ions in turn, which bond additive counter ions via the electrostatic attraction. Therefore, the additive ions were grafted on the core surface to form shell layer instead of individual nanoparticles. 3.2. Morphology, structure and surface composition Fig. 2 shows the TEM images of core and core/shell nanoparticles. The average diameters of these nanoparticles are 9.6 and 11.1 nm respectively. The thickness of the shells is about 1.5 nm. No small free-standing nanoparticles are inlayed on the large ones in TEM images of the core/shell nanoparticles, indicating that all of the added monomer had grown on the existing core nanoparticles and no new nucleation had occurred. Because the core and the shell have similar electron density and lattice parameters, they cannot be clearly distinguished [18]. Fig. 3 shows the XRD patterns of LaF3:Nd/LaF3 core/shell and LaF3:Nd nanoparticles. All of the diffraction peaks were indexed to the hexagonal LaF3 crystal structure (JCPDS: 32–0483). No other peaks were observed in the patterns, revealing that the products were single. By means of Debye–Scherer equation, the average sizes of core and core/shell nanoparticles were estimated to be about 9.2 and 11.3 nm, respectively. These results were in good agreement with the TEM image. Compared with the core nanoparticles, the peak intensities of core/shell nanoparticles increased obviously, which demonstrate that the formation of LaF3 shell reduced the surface defects successfully. In order to confirm that the LaF3:Nd core nanoparticles have been coated by LaF3 shell successfully; the surface compositions
3.1. Formation mechanism of core/shell nanoparticles Fig. 1 is a scheme diagram of the formation mechanism of the LaF3:Nd/LaF3 core/shell nanoparticles. The modified method is to grow the shell layer around the core by the alternate addition of small portions of the shell reagents. At the beginning of the core formation, the surfactant and excessive amounts of F ions were introduced. When the rareearth ions were added dropwise into the solution, LnF3 nuclei was generated by the chemical reaction between Ln3+ and F . Meanwhile, the OA acted as surfactant kept the core nanoparticles growing gradually. The excessive F ions were absorbed on the core surface, then the nanoparticles exhibit negative charge surface. That makes the coordination of positive charge La3+ ions possible. At the second step, the reaction temperature was adjusted to 65 °C to slow the reaction rate, avoiding additional nucleation to form un-doped nanoparticles. When the shell solution containing La3+ ions was added, the La3+ interacted with the F ions gradually. Accordingly, the excessive La3+ ions were located at the nanoparticles surface, leading to a positive charge surface. When the F ions were added subsequently, there is competition between the OA molecules and fluoride ions. The Ln3+ ions first react with F ions instead of OA molecules on account of its smaller radius, Thus, when the solutions containing La3+ and F ions were added into
Fig. 2. TEM images of the as-prepared core LaF3:Nd (a) and LaF3:Nd/LaF3 core/shell samples (b).
Fig. 1. A scheme diagram of the formation mechanism for the LaF3:Nd/LaF3 core/ shell nanoparticles.
Fig. 3. XRD patterns of standard data for LaF3 (a) (PDF card No. 32-0483), LaF3:Nd core nanoparticles (b) and LaF3:Nd/LaF3 core/shell nanoparticles (c).
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X. Cui et al. / Chemical Physics Letters 494 (2010) 60–63
of the samples were investigated by XPS analysis (as shown in Fig. 4). For LaF3:Nd nanoparticles, the obvious peaks of visible Nd 3d (binding energy = 1000 eV), La 3d (binding energy = 836 eV) and F 1s (binding energy = 635 eV) were observed. For LaF3:Nd/ LaF3 core/shell structure, the sharp and enhanced peaks of La 3d and F 1s are observed obviously, and the intensity of peak of Nd 3d was weakened. This is consistent with the proposed structure where the LaF3 shell encapsulated the LaF3:Nd core. The oxygen and carbon detected in the XPS measurement may come from the atmosphere. Furthermore, low amounts of oxygen and carbon are frequently observed in the XPS spectra.
3.3. Photoluminescent properties Fig. 5 shows the emission spectra of LaF3:Nd/LaF3 core/shell and LaF3:Nd nanoparticles under excitation at 800 nm. The emission peaks at 880, 1053, and 1330 nm correspond to the transitions from 4F3/2 to 4I9/2, 4I11/2 and 4I13/2, respectively. The 4F3/2 ? 4I11/2 is a typical magnetic dipole transition, generally investigated and applied in near-IR. For LaF3:Nd/LaF3 core/shell nanoparticles, the shapes of the emission patterns are similar to that of LaF3:Nd nanoparticles, while the emission intensity was increased by about 140% at 1053 nm. The ratio is much higher than the previous reported [19].
Fig. 6. Decay curves of the 4F3/2 ? 4I11/2 transition for LaF3:Nd core and LaF3:Nd/ LaF3 core/shell nanoparticles (Nd10 mol%). The samples were excited at 800 nm and emission was monitored at 1053 nm at room temperature.
Decay curves for the core and core/shell nanoparticles are shown in Fig. 6. The fluorescence lifetime (s) was measured to be 56 ls for LaF3:Nd core and 140 ls for LaF3:Nd/LaF3 core/shell nanoparticles by fitting the decay curves with double exponential function. Compared with the core nanoparticles, the fluorescence lifetime of the core/shell nanoparticles was increased by 150%. The increase ratio is much bigger than that of literature [9]. This remarkable enhancement can be explained that when the shell-ion solution was added into the core solution, there is competition between the organic molecules (OA) and F ions. The OA has little chance to bond with Ln3+ ions adsorbed on the core surface, avoiding the quenching of –CH3, –COO groups. After the shell layer formed, the coordination of OA to the lanthanide ions on the shell surface increased the distance between neighbouring nanoparticles, reducing the nonradiative pathways and suppressing the quenching in the energy transfer process. Besides, in the proposed core/shell structure, the light-emitting center ions (Nd3+) located on the core surface own fewer dangling bonds, thereby reducing the surface defects. Amounts of nonradiative centers existing on the core surface are reduced and the emitting centers were less sensitive to surface quencher. The enhanced fluorescence intensity and lifetime indicate that the strategy is an effective way to improve luminescent properties of LaF3:Ln3+ nanoparticles.
Fig. 4. XPS spectra of the LaF3:Nd core (A) and LaF3:Nd/LaF3 core/shell (B) nanoparticles.
4. Conclusions In summary, we have successfully synthesized LaF3:Nd/LaF3 core/shell nanoparticles via a co-precipitation method. The fluorescence intensity and lifetime were increased by 140% and 150% respectively compared to LaF3:Nd nanoparticles. It can be ascribed to the formation of shell layer which provides a physical barrier between the optically active core and the surrounding medium, reducing the nonradiative pathways. Moreover, the shell further provides an efficient passivation of the surface trap states, and gives rise to a strongly enhanced fluorescence intensity and lifetime. The advantage of this facile method appears to provide an effective way to improve the luminescence properties for other rare-earth fluoride compounds. Acknowledgments Fig. 5. Emission spectra of LaF3:Nd/LaF3 core/shell and LaF3:Nd (Nd10 mol%) nanoparticles. The peaks at 880, 1053, and 1330 nm correspond to the transitions from 4F3/2 to 4I9/2, 4I11/2 and 4I13/2, respectively.
This work was financially supported by the National Natural Science Foundation of China (No. 60977023) and one Hundred Talents Programs of the Chinese Academy of Sciences.
X. Cui et al. / Chemical Physics Letters 494 (2010) 60–63
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