- Email: [email protected]
shell nanostructure with enhanced near infrared (NIR) emission
Materials Letters 63 (2009) 376–378
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Synthesis and photoluminescent properties of α-NaYF4:Nd/α-NaYF4 core/shell nanostructure with enhanced near infrared (NIR) emission Qiang Zhang a,⁎, Qi-Ming Zhang b a b
Department of Chemistry, Fudan University, 220 Handan Road, Shanghai 200433, People's Republic of China Department of Optical Science and Engineering, Fudan University, 220 Handan Road, Shanghai 200433, People's Republic of China
a r t i c l e
i n f o
Article history: Received 17 October 2008 Accepted 22 October 2008 Available online 9 November 2008 Keywords: Core/shell nanostructure Lifetime analysis Optical materials and properties Nanomaterials
a b s t r a c t A simple and efficient method has been established for the synthesis of α-NaYF4:Nd/α-NaYF4 core/shell nanoparticles under hydrothermal conditions. The as-prepared products were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectra and lifetimes. TEM images show that the as-prepared samples have sphere-like shape. The lifetime values of the 4F3/2 energy level of the α-NaYF4:Nd/α-NaYF4 core/shell nanoparticles increase significantly in comparison with those of α-NaYF4:Nd core nanoparticles, with a quantum efficiency of as high as 52%. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Neodymium (Nd3+) doped nanomaterials are of high importance for their applications both in the optical window (spectra range of 600–1100 nm) of cells and tissues and the optical telecommunication window (spectra range of 900–1600 nm) [1–9]. Extensive attention has been recently paid to liquid phase co-precipitation synthesis of lanthanide-doped nanocrystals, which yield nanocrystals that can be well dispersed in unpolar organic solvents or water to form stable colloidal solutions [1,3–9]. Meanwhile, core/shell nanostructures have drawn increasingly attention to further shield the doped core nanoparticles by coating undoped inorganic shell or silica shell, which is especially important for near-infrared (NIR) emitting rare earth ions, such as Nd, Er and Tm, et al., because their low-lying excited states can be readily quenched by surface effects and quenching groups. The lifetime of Tm and Nd doped LaF3 can be largely improved by coating undoped shell of LaF3 and then encapsulated in silica shell or in silica and alumina films by calcinations [6–9]. These results indicates that the key role of the shells is to block energy transfer and improve photoluminescent properties. Monodisperse cubic and hexagonal NaREF4 (RE = Pr to Lu) nanocrystals has been fabricated via decomposition of NaCF3CO2 and RE(CF3CO2)3 in high boiling-point solvents [10]. Water–ethanol–oleic acid mixing system with a facile hydrothermal technology first developed by Li's group has been widely adopted to synthesize monodisperse nanoparticles and nanorods [11– 13]. This strategy is a green method without high-temperature decomposition of rare earth salts. Considering these, it's necessary to
⁎ Corresponding author. Tel.: +86 21 5801 1054. E-mail address: [email protected] (Q. Zhang). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.10.064
modify the hydrothermal method to synthesize core/shell nanostructures. Herein, a facile and simple hydrothermal method for synthesis of α-NaYF4:Nd/α-NaYF4 core/shell nanostructures has been developed. The motivation of this research is to develop an extendable hydrothermal method to obtain core/shell nanostructure with enhanced photoluminescent properties. 2. Experimental section 2.1. Synthesis (i) For synthesis of core α-NaYF4:Nd nanoparticles: Various nanoparticles were prepared by a modified hydrothermal process [11–13]. Sodium oleate (4.0 g), anhydrous ethanol (16 mL) and oleic acid (20 mL) were mixed under agitation at 30 °C. Then 8 mL of mixed solution of Y(NO3)3 (1.96 mmol) and Nd(NO3)3 (0.04 mmol) were dropped. Excessive amount of 10 ml of 1 M NaF (10 mmol) solution were dropped and agitated, then transferred to a 200 mL autoclave, sealed, and hydrothermally treated at 160 °C for 7 hours. The system was cooled to room temperature naturally. (ii) For preparation of oleic acid capped α-NaErF4/α-NaYF4 core/shell nanoparticles: 2.75 mL solution of Y(NO3)3 (0.70 mmol) were dropped into the above system of (i) under agitation. Then the autoclave was sealed again and hydrothermally treated at 160 °C for 1 h. The αNaErF4 and α-NaErF4/α-NaYF4 nanoparticles for XPS measurement: The oleic acid-capped core α-NaErF4 nanoparticles was synthesized under the same conditions as the core α-NaYF4:Nd nanoparticles process. The different shell thickness was controlled by the different concentration of 2.75 mL Y(NO3)3 solution, such as containing (i) 0.5 mmol Y(NO3)3, (ii) 0.7 mmol Y(NO3)3 and (iii) 1.0 mmol Y(NO3)3, with additional 2 mL of 1 M NaF solution for (iii) only.
Q. Zhang, Q.-M. Zhang / Materials Letters 63 (2009) 376–378
Fig. 1. XRD patterns of α-NaYF4:Nd (b), α-NaYF4:Nd/α-NaYF4 (c), α-NaErF4 for XPS measurement(c) and the standard data for α-NaYF4 (a) (JCPDS card 77-2042).
377
Fig. 3. XPS spectra of the α-NaErF4 (A) core and α-NaErF4/α-NaYF4 (B, C, D) core/shell nanoparticles. (B, C and D stand for that 0.5, 0.7 and 1.0 mol Y(NO3)3 was added when the α-NaErF4/α-NaYF4 nanoparticles was synthesized.).
2.2. Characterization Decay curve were obtained using a Nd:YAG (EKSPLA PL2143A) laser coupled with an optical parameter oscillator (OPO) to give 800 nm laser with pulse width of 30 ps. Emission of 1058 nm was monitored by a digital oscilloscope synchronized to the single-pulse excitation. The luminescence lifetime (τ1, τ2) was fitted using a biexponential decay equation in OriginPro 8 software. Transmission electron microscope (TEM) was carried out on a JEOL JEM-2010F. X-ray diffraction (XRD) patterns were measured with a Bruker D8-Advance X-ray diffractometer with Cu-Kα radiation (λ = 0.15418 nm). XPS experiments were carried out on a RBD upgraded PHI-5000C ESCA system (Perkin Elmer) with Mg Kα radiation (hν = 1253.6 eV). 3. Results and discussion 3.1. Structure and morphology of the α-NaYF4:Nd and α-NaYF4:Nd/α-NaYF4 nanostructures As shown in Fig. 1, the powder X-ray diffraction (XRD) patterns indicate that all samples are of pure cubic phase. The diffraction peaks of all samples can be well indexed to the cubic phase (JCPDS card No. 77-2042). The average crystalline sizes of αNaYF4:Nd and α-NaYF4:Nd/α-NaYF4 nanopaticles calculated from the (111), (220) and (311) diffraction peaks using the Scherrer equation are about 14 and 15 nm, respectively. These results are in agreement with the TEM observation which was shown in Fig. 2. TEM images of the core and core/shell nanoparticles showed in Fig. 2A and B clearly reveal that the size is about 15 nm. The difference of the sizes of the core and core/shell nanostructures is very small, consistent with the XRD results. Especially, there are no free standing small nanoparticles and small nanoparticles inlayed on the large ones in TEM image of the core/shell nanoparticles, depicted in Fig. 2B, and this means that homogeneous α-NaYF4 shell growth probably dominates over heterogeneous nucleation, which is unfavorable to coat shell structures [14].
To confirm that the α-NaYF4 nanoparticles have coated on α-NaYF4:Nd nanoparitcles successfully by our new strategy, XPS analysis has been performed. X-ray photoelectron spectroscopy (XPS) is known to probe only the surface of the particles and allows us to obtain information to be obtained from the top few atomic layers of a surface. Because the Er and Y elements with very similar chemical properties and ions radii which can preclude the lattice mismatch during the process of coating an α-NaYF4 shell, we synthesize the α-NaErF4 core (Fig. 1c) and α-NaErF4/α-NaYF4 core/shell nanoparticles for XPS measurement and prove the feasibility of our strategy. Fig. 3 shows the XPS spectra of the α-NaErF4 and α-NaErF4/α-NaYF4 nanoparticles. For the pure α-NaErF4 nanoparticles, the binding energy at 175 eV attributed to the Er 4d peak is measured (see Fig. 3A). After coated with a thin layer of NaYF4, a sharp and strong peak at binding energy of 164 eV attributed to the Y 3d5/2 peak is observed obviously, indicating Y occurs on the surface of the α-NaErF4 nanoparticles. Meanwhile, as increased the thickness of NaYF4 shell, the Er 4d peak at 175 eV gradually collapsed and weakened and the Y 3d5/2 peak at 164 eV gradually increased from Fig. 3A to D, indicating that a shell layer with different thickness is coated onto the α-NaErF4 nanoparticles. Therefore, we can safely conclude that the α-NaErF4/α-NaYF4 core/shell nanoparticles can be obtained by our developed strategy. 3.2. Photoluminescent properties Decay curves for the α-NaYF4:Nd, α-NaYF4:Nd/α-NaYF4 core/shell nanoparticles are depicted in Fig. 4. For all of the Nd3+ doped nanoparticles, decay corresponding to 4 F3/2 energy level is bi-exponential, because the Nd3+ ions doped in the nanoparticles are separated to two kinds: inside the core and near or at the surface [1]. The average lifetime (τ av) of the core/shell nanoparticles is longer than that of the core nanoparticles, increased by 13%. It is worth to note that the fast decay component changes from 43 µs (37%) to 97 µs (37%), almost increasing by 100%. The lifetime analysis gives further evidence for the proposed core/shell structure. With the obtained fluorescence decay time, the quantum yield of the 4F3/2–4I11/2 transition can be estimated from the ratio of the fluorescence decay time (τav) to radiative or “natural” decay time (τr). However, α-NaYF4:Nd is a metastable phase and there is no published
Fig. 2. TEM images of the as-prepared core α-NaYF4:Nd (A) and core/shell α-NaYF4:Nd/α-NaYF4 (B) samples.
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Q. Zhang, Q.-M. Zhang / Materials Letters 63 (2009) 376–378
XPS measurement. Moreover, the lifetime value analysis indicates that the luminescence quantum efficiency of the α-NaYF4:Nd/α-NaYF4 core/shell nanoparticles increased significantly in comparison of that of the free α-NaYF4:Nd nanoparticles. Further work will focus to oxidize the hydrophobic nanoparticles to water-dispersible ones with the Lemieux-von Rudloff reagent [13]. We expect that this new strategy to synthesize core/shell nanostructure could be extended to other nanomaterials of lanthanide fluorides, such as NaYF4:Yb, Er/Tm nanoparticles or nanorods. Acknowledgements The authors thank Shanghai Leading Academic Discipline Project (B108) for financial support. Also, we thank Prof. Chunhui Huang and Prof. Fuyou Li for helpful discussions. Fig. 4. Decay curves for the α-NaYF4:Nd core (A), α-NaYF4:Nd/α-NaYF4 core/shell (B). The dried powder samples were excited at 800 nm, and emission was monitored at 1058 nm at room temperature. τav = (τ21α1 + τ22α2) / (τ1α1 + τ2α2) [9]. The units of τ1, τ2 and τav is microsecond and αi indicates the relative percentages of the different lifetime components.
results of calculated radiative or “natural” decay time for α-NaYF4:Nd. To roughly estimate the quantum yield of the 4F3/2–4I11/2 emission, we are bold to select the calculated radiative decay time of 842 µs of LaF3:Nd following the well known Judd– Ofelt procedure and a quantum efficiency of 52% is obtained for the core/shell nanoparticles [15].
4. Conclusion In summary, we demonstrated a facile hydrothermal method for the preparation of the NIR emitting α-NaYF4:Nd/α-NaYF4 core/shell nanoparticles. The proposed core/shell structure was confirmed by the
References [1] Stouwdam JW, van Veggel FCJM. Nano Lett 2002;2:733–7. [2] Dekker R, Klunder DJW, Borreman A, Diemeer MBJ, Wörhoff K, Driessen A. Appl Phys Lett 2004;85:6104–6. [3] Yu XF, Chen LD, Li M, Xie MY, Zhou L, Li Y. Adv Mater 2008;20, doi:10.1002/ adma.200801224. [4] König K. J Microsc 2000;200:83–104. [5] Wang F, Zhang Y, Fan XP, Wang MQ. J Mater Chem 2006;16:1031–4. [6] Sudarsan V, Sivakumar S, van Veggel FCJM, Raudsepp M. Chem Mater 2005;17:4736–42. [7] Diamente PR, Raudsepp M, van Veggel FCJM. Adv Funct Mater 2007;17:363–8. [8] Stouwdam JW, van Veggel FCJM. Langmuir 2004;20:11763–71. [9] Stouwdam JW, Hebbink GA, Huskens J, van Veggel FCJM. Chem Mater 2003;15:4604–16. [10] Mai HX, Zhang YW, Si R, Yan ZG, Sun LD, You LP. J Am Chem Soc 2006;128:6426–36. [11] Wang LY, Li YD. Chem Mater 2007;19:727–34. [12] Wang X, Zhuang J, Peng Q, Li YD. Nature 2005;437:121–4. [13] Chen ZG, Chen HL, Hu H, Yu MX, Li FY, Zhang Q. J Am Chem Soc 2008;130:3023–9. [14] Fang YP, Xu AW, Dong WF. Small 2005;1:967–71. [15] Kumar GA, Chen CW, Ballato J, Riman RE. Chem Mater 2007;19:1523–8.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Synthesis and photoluminescent properties of α-NaYF4:Nd/α-NaYF4 core/shell nanostructure with enhanced near infrared (NIR) emission Qiang Zhang a,⁎, Qi-Ming Zhang b a b
Department of Chemistry, Fudan University, 220 Handan Road, Shanghai 200433, People's Republic of China Department of Optical Science and Engineering, Fudan University, 220 Handan Road, Shanghai 200433, People's Republic of China
a r t i c l e
i n f o
Article history: Received 17 October 2008 Accepted 22 October 2008 Available online 9 November 2008 Keywords: Core/shell nanostructure Lifetime analysis Optical materials and properties Nanomaterials
a b s t r a c t A simple and efficient method has been established for the synthesis of α-NaYF4:Nd/α-NaYF4 core/shell nanoparticles under hydrothermal conditions. The as-prepared products were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectra and lifetimes. TEM images show that the as-prepared samples have sphere-like shape. The lifetime values of the 4F3/2 energy level of the α-NaYF4:Nd/α-NaYF4 core/shell nanoparticles increase significantly in comparison with those of α-NaYF4:Nd core nanoparticles, with a quantum efficiency of as high as 52%. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Neodymium (Nd3+) doped nanomaterials are of high importance for their applications both in the optical window (spectra range of 600–1100 nm) of cells and tissues and the optical telecommunication window (spectra range of 900–1600 nm) [1–9]. Extensive attention has been recently paid to liquid phase co-precipitation synthesis of lanthanide-doped nanocrystals, which yield nanocrystals that can be well dispersed in unpolar organic solvents or water to form stable colloidal solutions [1,3–9]. Meanwhile, core/shell nanostructures have drawn increasingly attention to further shield the doped core nanoparticles by coating undoped inorganic shell or silica shell, which is especially important for near-infrared (NIR) emitting rare earth ions, such as Nd, Er and Tm, et al., because their low-lying excited states can be readily quenched by surface effects and quenching groups. The lifetime of Tm and Nd doped LaF3 can be largely improved by coating undoped shell of LaF3 and then encapsulated in silica shell or in silica and alumina films by calcinations [6–9]. These results indicates that the key role of the shells is to block energy transfer and improve photoluminescent properties. Monodisperse cubic and hexagonal NaREF4 (RE = Pr to Lu) nanocrystals has been fabricated via decomposition of NaCF3CO2 and RE(CF3CO2)3 in high boiling-point solvents [10]. Water–ethanol–oleic acid mixing system with a facile hydrothermal technology first developed by Li's group has been widely adopted to synthesize monodisperse nanoparticles and nanorods [11– 13]. This strategy is a green method without high-temperature decomposition of rare earth salts. Considering these, it's necessary to
⁎ Corresponding author. Tel.: +86 21 5801 1054. E-mail address: [email protected] (Q. Zhang). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.10.064
modify the hydrothermal method to synthesize core/shell nanostructures. Herein, a facile and simple hydrothermal method for synthesis of α-NaYF4:Nd/α-NaYF4 core/shell nanostructures has been developed. The motivation of this research is to develop an extendable hydrothermal method to obtain core/shell nanostructure with enhanced photoluminescent properties. 2. Experimental section 2.1. Synthesis (i) For synthesis of core α-NaYF4:Nd nanoparticles: Various nanoparticles were prepared by a modified hydrothermal process [11–13]. Sodium oleate (4.0 g), anhydrous ethanol (16 mL) and oleic acid (20 mL) were mixed under agitation at 30 °C. Then 8 mL of mixed solution of Y(NO3)3 (1.96 mmol) and Nd(NO3)3 (0.04 mmol) were dropped. Excessive amount of 10 ml of 1 M NaF (10 mmol) solution were dropped and agitated, then transferred to a 200 mL autoclave, sealed, and hydrothermally treated at 160 °C for 7 hours. The system was cooled to room temperature naturally. (ii) For preparation of oleic acid capped α-NaErF4/α-NaYF4 core/shell nanoparticles: 2.75 mL solution of Y(NO3)3 (0.70 mmol) were dropped into the above system of (i) under agitation. Then the autoclave was sealed again and hydrothermally treated at 160 °C for 1 h. The αNaErF4 and α-NaErF4/α-NaYF4 nanoparticles for XPS measurement: The oleic acid-capped core α-NaErF4 nanoparticles was synthesized under the same conditions as the core α-NaYF4:Nd nanoparticles process. The different shell thickness was controlled by the different concentration of 2.75 mL Y(NO3)3 solution, such as containing (i) 0.5 mmol Y(NO3)3, (ii) 0.7 mmol Y(NO3)3 and (iii) 1.0 mmol Y(NO3)3, with additional 2 mL of 1 M NaF solution for (iii) only.
Q. Zhang, Q.-M. Zhang / Materials Letters 63 (2009) 376–378
Fig. 1. XRD patterns of α-NaYF4:Nd (b), α-NaYF4:Nd/α-NaYF4 (c), α-NaErF4 for XPS measurement(c) and the standard data for α-NaYF4 (a) (JCPDS card 77-2042).
377
Fig. 3. XPS spectra of the α-NaErF4 (A) core and α-NaErF4/α-NaYF4 (B, C, D) core/shell nanoparticles. (B, C and D stand for that 0.5, 0.7 and 1.0 mol Y(NO3)3 was added when the α-NaErF4/α-NaYF4 nanoparticles was synthesized.).
2.2. Characterization Decay curve were obtained using a Nd:YAG (EKSPLA PL2143A) laser coupled with an optical parameter oscillator (OPO) to give 800 nm laser with pulse width of 30 ps. Emission of 1058 nm was monitored by a digital oscilloscope synchronized to the single-pulse excitation. The luminescence lifetime (τ1, τ2) was fitted using a biexponential decay equation in OriginPro 8 software. Transmission electron microscope (TEM) was carried out on a JEOL JEM-2010F. X-ray diffraction (XRD) patterns were measured with a Bruker D8-Advance X-ray diffractometer with Cu-Kα radiation (λ = 0.15418 nm). XPS experiments were carried out on a RBD upgraded PHI-5000C ESCA system (Perkin Elmer) with Mg Kα radiation (hν = 1253.6 eV). 3. Results and discussion 3.1. Structure and morphology of the α-NaYF4:Nd and α-NaYF4:Nd/α-NaYF4 nanostructures As shown in Fig. 1, the powder X-ray diffraction (XRD) patterns indicate that all samples are of pure cubic phase. The diffraction peaks of all samples can be well indexed to the cubic phase (JCPDS card No. 77-2042). The average crystalline sizes of αNaYF4:Nd and α-NaYF4:Nd/α-NaYF4 nanopaticles calculated from the (111), (220) and (311) diffraction peaks using the Scherrer equation are about 14 and 15 nm, respectively. These results are in agreement with the TEM observation which was shown in Fig. 2. TEM images of the core and core/shell nanoparticles showed in Fig. 2A and B clearly reveal that the size is about 15 nm. The difference of the sizes of the core and core/shell nanostructures is very small, consistent with the XRD results. Especially, there are no free standing small nanoparticles and small nanoparticles inlayed on the large ones in TEM image of the core/shell nanoparticles, depicted in Fig. 2B, and this means that homogeneous α-NaYF4 shell growth probably dominates over heterogeneous nucleation, which is unfavorable to coat shell structures [14].
To confirm that the α-NaYF4 nanoparticles have coated on α-NaYF4:Nd nanoparitcles successfully by our new strategy, XPS analysis has been performed. X-ray photoelectron spectroscopy (XPS) is known to probe only the surface of the particles and allows us to obtain information to be obtained from the top few atomic layers of a surface. Because the Er and Y elements with very similar chemical properties and ions radii which can preclude the lattice mismatch during the process of coating an α-NaYF4 shell, we synthesize the α-NaErF4 core (Fig. 1c) and α-NaErF4/α-NaYF4 core/shell nanoparticles for XPS measurement and prove the feasibility of our strategy. Fig. 3 shows the XPS spectra of the α-NaErF4 and α-NaErF4/α-NaYF4 nanoparticles. For the pure α-NaErF4 nanoparticles, the binding energy at 175 eV attributed to the Er 4d peak is measured (see Fig. 3A). After coated with a thin layer of NaYF4, a sharp and strong peak at binding energy of 164 eV attributed to the Y 3d5/2 peak is observed obviously, indicating Y occurs on the surface of the α-NaErF4 nanoparticles. Meanwhile, as increased the thickness of NaYF4 shell, the Er 4d peak at 175 eV gradually collapsed and weakened and the Y 3d5/2 peak at 164 eV gradually increased from Fig. 3A to D, indicating that a shell layer with different thickness is coated onto the α-NaErF4 nanoparticles. Therefore, we can safely conclude that the α-NaErF4/α-NaYF4 core/shell nanoparticles can be obtained by our developed strategy. 3.2. Photoluminescent properties Decay curves for the α-NaYF4:Nd, α-NaYF4:Nd/α-NaYF4 core/shell nanoparticles are depicted in Fig. 4. For all of the Nd3+ doped nanoparticles, decay corresponding to 4 F3/2 energy level is bi-exponential, because the Nd3+ ions doped in the nanoparticles are separated to two kinds: inside the core and near or at the surface [1]. The average lifetime (τ av) of the core/shell nanoparticles is longer than that of the core nanoparticles, increased by 13%. It is worth to note that the fast decay component changes from 43 µs (37%) to 97 µs (37%), almost increasing by 100%. The lifetime analysis gives further evidence for the proposed core/shell structure. With the obtained fluorescence decay time, the quantum yield of the 4F3/2–4I11/2 transition can be estimated from the ratio of the fluorescence decay time (τav) to radiative or “natural” decay time (τr). However, α-NaYF4:Nd is a metastable phase and there is no published
Fig. 2. TEM images of the as-prepared core α-NaYF4:Nd (A) and core/shell α-NaYF4:Nd/α-NaYF4 (B) samples.
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Q. Zhang, Q.-M. Zhang / Materials Letters 63 (2009) 376–378
XPS measurement. Moreover, the lifetime value analysis indicates that the luminescence quantum efficiency of the α-NaYF4:Nd/α-NaYF4 core/shell nanoparticles increased significantly in comparison of that of the free α-NaYF4:Nd nanoparticles. Further work will focus to oxidize the hydrophobic nanoparticles to water-dispersible ones with the Lemieux-von Rudloff reagent [13]. We expect that this new strategy to synthesize core/shell nanostructure could be extended to other nanomaterials of lanthanide fluorides, such as NaYF4:Yb, Er/Tm nanoparticles or nanorods. Acknowledgements The authors thank Shanghai Leading Academic Discipline Project (B108) for financial support. Also, we thank Prof. Chunhui Huang and Prof. Fuyou Li for helpful discussions. Fig. 4. Decay curves for the α-NaYF4:Nd core (A), α-NaYF4:Nd/α-NaYF4 core/shell (B). The dried powder samples were excited at 800 nm, and emission was monitored at 1058 nm at room temperature. τav = (τ21α1 + τ22α2) / (τ1α1 + τ2α2) [9]. The units of τ1, τ2 and τav is microsecond and αi indicates the relative percentages of the different lifetime components.
results of calculated radiative or “natural” decay time for α-NaYF4:Nd. To roughly estimate the quantum yield of the 4F3/2–4I11/2 emission, we are bold to select the calculated radiative decay time of 842 µs of LaF3:Nd following the well known Judd– Ofelt procedure and a quantum efficiency of 52% is obtained for the core/shell nanoparticles [15].
4. Conclusion In summary, we demonstrated a facile hydrothermal method for the preparation of the NIR emitting α-NaYF4:Nd/α-NaYF4 core/shell nanoparticles. The proposed core/shell structure was confirmed by the
References [1] Stouwdam JW, van Veggel FCJM. Nano Lett 2002;2:733–7. [2] Dekker R, Klunder DJW, Borreman A, Diemeer MBJ, Wörhoff K, Driessen A. Appl Phys Lett 2004;85:6104–6. [3] Yu XF, Chen LD, Li M, Xie MY, Zhou L, Li Y. Adv Mater 2008;20, doi:10.1002/ adma.200801224. [4] König K. J Microsc 2000;200:83–104. [5] Wang F, Zhang Y, Fan XP, Wang MQ. J Mater Chem 2006;16:1031–4. [6] Sudarsan V, Sivakumar S, van Veggel FCJM, Raudsepp M. Chem Mater 2005;17:4736–42. [7] Diamente PR, Raudsepp M, van Veggel FCJM. Adv Funct Mater 2007;17:363–8. [8] Stouwdam JW, van Veggel FCJM. Langmuir 2004;20:11763–71. [9] Stouwdam JW, Hebbink GA, Huskens J, van Veggel FCJM. Chem Mater 2003;15:4604–16. [10] Mai HX, Zhang YW, Si R, Yan ZG, Sun LD, You LP. J Am Chem Soc 2006;128:6426–36. [11] Wang LY, Li YD. Chem Mater 2007;19:727–34. [12] Wang X, Zhuang J, Peng Q, Li YD. Nature 2005;437:121–4. [13] Chen ZG, Chen HL, Hu H, Yu MX, Li FY, Zhang Q. J Am Chem Soc 2008;130:3023–9. [14] Fang YP, Xu AW, Dong WF. Small 2005;1:967–71. [15] Kumar GA, Chen CW, Ballato J, Riman RE. Chem Mater 2007;19:1523–8.