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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 15 — Jul. 16, 2012
  • pp: 17107–17118
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Bright dual-mode green emission from selective set of dopant ions in β-Na(Y,Gd)F4:Yb,Er/β-NaGdF4:Ce,Tb core/shell nanocrystals

Ho Seong Jang, Kyoungja Woo, and Kipil Lim  »View Author Affiliations


Optics Express, Vol. 20, Issue 15, pp. 17107-17118 (2012)
http://dx.doi.org/10.1364/OE.20.017107


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Abstract

Bright dual-mode green-emitting core/shell nanoparticles (NPs) were synthesized by doping selective set of lanthanide ions. Up-conversion (UC) green-emitting β-NaY0.2Gd0.6F4:Yb0.18,Er0.02 NPs (8.3 nm) were used as core material. Bright down-conversion (DC) green-emitting β-NaGd0.8F4:Ce0.15,Tb0.05 NPs showed ca. 31 times higher photoluminescence (PL) intensity than β-NaGdF4:Tb NPs and they were served as shell material with their excellent PL properties. The UC/DC core/shell NPs showed bright green light under excitations of 980 nm near infrared (NIR) light and 254 nm ultraviolet (UV) light, respectively. The UC/DC core/shell NPs showed ca. 11 times higher UC PL intensity than core UCNPs. Consequently, the core/shell NPs doped with selective set of lanthanide ions showed bright dual-mode green emission under excitations of NIR light and UV light, indicating that they are promising for application to optical imaging.

© 2012 OSA

1. Introduction

2. Experimental

2.1 Synthesis

Synthesis of β-NaY0.2Gd0.6F4:Yb0.18,Er0.02 (β-NYGF:Yb,Er) UCNPs: YCl3.6H2O (99.99%), GdCl3.6H2O (99%), YbCl3.6H2O (99.9%), ErCl3.6H2O (99.9%), NaOH (99.99%), and NH4F (99.99%) were purchased from Aldrich and they were used as received. Oleic acid (OA, technical grade) and 1-octadecene (ODE, technical grade) were also purchased from Aldrich. The Gd-substituted β-NaYF4:Yb,Er UCNPs were synthesized by slightly modifying the method reported by Zhang et al. [28

28. Z. Li, Y. Zhang, and S. Jiang, “Multicolor core/shell-structured upconversion fluorescent nanoparticles,” Adv. Mater. (Deerfield Beach Fla.) 20(24), 4765–4769 (2008). [CrossRef]

]. YCl3.6H2O (0.2 mmol), GdCl3.6H2O (0.6 mmol), YbCl3.6H2O (0.18 mmol), ErCl3.6H2O (0.02 mmol) were mixed with 6 ml of OA and 15 ml of ODE. The mixed solution was heated to 150 °C under vacuum to form transparent yellow solution. After cooling to 50 °C, 10 ml of methanol (MeOH) solution containing 2.5 mmol of NaOH and 4 mmol of NH4F was slowly added to the mixed solution. The resulting solution was stirred for 40 min. After removing MeOH, the solution was heated to 300 °C at the rate of ~12 °C/min and kept for 1 h 30 min under Ar protection. To precipitate the synthesized UCNPs, ethanol (EtOH) was added and the UCNPs were separated by centrifugation (10000 rpm, 10 min). After washing with EtOH and MeOH, final product was dispersed in hexane.

Syntheses of β-NaGd0.95F4:Tb0.05 (β-NaGdF4:Tb) and β-NaGd0.8F4:Ce0.15,Tb0.05 (β-NaGdF4:Ce,Tb) NPs: CeCl3.7H2O (99.999%) and TbCl3.6H2O (99.9%) were purchased from Aldrich and sodium oleate (>97%) was obtained from TCI. First, lanthanide oleate (Ln(oleate)3) complexes were formed by adapting the method for synthesis of iron-oleate complex [29

29. J. Park, K. J. An, Y. Hwang, J.-G. Park, H.-J. Noh, J.-Y. Kim, J.-H. Park, N.-M. Hwang, and T. Hyeon, “Ultra-large-scale syntheses of monodisperse nanocrystals,” Nat. Mater. 3(12), 891–895 (2004). [CrossRef] [PubMed]

]. For synthesis of β-NaGdF4:Tb NPs, GdCl3.6H2O (0.95 mmol), TbCl3.6H2O (0.05 mmol), and sodium oleate (3.1 mmol) were added to a mixed solvent of distilled water (3 ml), EtOH (3.5 ml), and hexane (7 ml). In case of synthesis of β-NaGdF4:Ce,Tb NPs, GdCl3.6H2O (0.80 mmol), CeCl3.7H2O (0.15 mmol), and TbCl3.6H2O (0.05 mmol) were used. The solution was heated to 70 °C and maintained for 4 hours. After completing the reaction, upper organic layer was separated and washed three times with 15 ml of distilled water. Hexane was evaporated and then Ln(oleate)3 (Ln = Gd, Tb for β-NaGdF4:Tb and Ln = Gd, Ce, Tb for β-NaGdF4:Ce,Tb), 6 ml of OA, and 15 ml of ODE were loaded into the three neck flask. The mixed solution was heated to 150 °C under vacuum. After cooling to 50 °C, 10 ml of MeOH solution containing NaOH (2.5 mmol) and NH4F (4 mmol) was added to the flask. The mixed solution was stirred for 40 min. Heat-treatment and washing processes were the same as the case of the synthesis of β-NYGF:Yb,Er and the synthesized NPs were dispersed in hexane.

Syntheses of β-NYGF:Yb,Er/β-NaGdF4 core/shell UCNPs and β-NYGF:Yb,Er/β-NaGdF4:Ce,Tb UC/DC core/shell NPs: One mmol of Gd(oleate)3 for β-NaGdF4 inert shell and 1 mmol of Ln(oleate)3 for β-NaGdF4:Ce,Tb luminescent shell (Ln = 80% Gd, 15% Ce, 5% Tb) were prepared as described above using sodium oleate (3.1 mmol). Obtained Ln(oleate)3 (Ln = Gd for β-NaGdF4 shell and Ln = Gd, Ce, Tb for β-NaGdF4:Ce,Tb shell) were mixed with OA (6 ml) and ODE (15 ml). The solutions were heated to 150 °C under vacuum and cooled to 80 °C followed by adding the solution of β-NYGF:Yb,Er (1 mmol) in 10 ml of hexane. When the hexane was removed, the solutions were cooled to 50 °C and 10 ml of MeOH solution containing NaOH (2.5 mmol) and NH4F (4 mmol) was added to the flask. The resulting solutions were vigorously stirred for 40 min. Heat-treatment and washing processes were the same as the case of the synthesis of the UCNP core. Finally, synthesized core/shell UCNPs and UC/DC core/shell NPs were dispersed in hexane, separately.

2.2 Characterization

To verify the crystal structures, x-ray diffraction (XRD) patterns were obtained using an x-ray diffractometer (X’pert PRO, PANalytical, Netherland) with Cu Kα radiation (λ = 1.5406 Å). XRD patterns were obtained in the 2θ range of 10-60° since strong Si substrate peak appears at about 65°. Photoluminescence (PL) spectra were collected for 1.5 wt% solution samples by using a Hitachi F-7000 spectrophotometer. For the UC PL measurement, cw NIR diode laser (λ = 980 nm, 1 W, Dragon laser, China) was coupled with the Hitachi F-7000 spectrophotometer. The UCNP solutions were placed in a 10 mm quartz cuvette (Hellma, QS cell). The UC visible light was detected at 90° with respect to the incident NIR light. Transmission electron microscopy (TEM) images were obtained using Tecnai F20 G2 transmission electron microscope (FEI Company) operating at 200 kV.

3. Results and discussion

An ultrasmall β phase NaYF4:Yb,Er core was synthesized by substituting Y with Gd. This is because Gd substitution lowers the energy barrier for the transition from α phase to β phase and makes it possible to form small β-NaYF4 particles without reducing reaction temperature and time which may cause low crystallinity and form α phase [30

30. F. Wang, Y. Han, C. S. Lim, Y. Lu, J. Wang, J. Xu, H. Chen, C. Zhang, M. Hong, and X. G. Liu, “Simultaneous phase and size control of upconversion nanocrystals through lanthanide doping,” Nature 463(7284), 1061–1065 (2010). [CrossRef] [PubMed]

]. In our experimental condition, the smallest β-NaYF4:Yb,Er UCNPs were obtained when 60 mol% of Gd was substituted. That is, β-NaY0.2Gd0.6F4:Yb0.18,Er0.02 (β-NYGF:Yb,Er) showed ultrasmall size of 8.3 ± 1.1 nm (average size ± standard deviation) which is much smaller size than β phase NaY0.8F4:Yb0.18,Er0.02 (β-NaYF4:Yb,Er, 21.3 ± 1.0 nm) as shown in TEM images of Fig. 1
Fig. 1 (a) TEM images (Insets show HR-TEM images and inset scale bars indicate 5 nm.), (b) XRD patterns, and (c) PL spectra of (i) β-NaYF4:Yb,Er and (ii) β-NYGF:Yb,Er UCNPs.
. Particle sizes were determined by measuring two hundred nanoparticles in TEM images. A high resolution-TEM (HR-TEM) image of Fig. 1(a-ii) inset demonstrates high crystallinity and single crystalline nature of the β-NYGF:Yb,Er UCNPs. The HR-TEM image shows highly clear lattice fringes and distance between them is well matched with interplanar d-spacing of literature value of β-NaYF4. In addition, XRD peaks of Fig. 1(b) are well matched with reference peaks (JCPDS 28-1192) and that confirms the formation of β phase. Due to Gd substitution, diffraction peaks of β-NYGF:Yb,Er shifted to smaller angle compared with those of β-NaYF4:Yb,Er and width of XRD peaks of β-NYGF:Yb,Er was broadened due to its smaller size.

As previously reported by Wang et al., particle size of β-NaYF4:Yb,Er was decreased with increasing Gd concentration in the host lattice. However, when all Y sites were substituted by Gd, the size (9.2 ± 1.5 nm) of β-NaGdF4:Yb,Er was larger than that of β-NYGF:Yb,Er (8.3 ± 1.1 nm), which is attributed to different growth mechanism of NaGdF4 from NaYF4 [18

18. H.-X. Mai, Y.-W. Zhang, R. Si, Z.-G. Yan, L.-D. Sun, L.-P. You, and C.-H. Yan, “High-quality sodium rare-earth fluoride nanocrystals: controlled synthesis and optical properties,” J. Am. Chem. Soc. 128(19), 6426–6436 (2006). [CrossRef] [PubMed]

]. Figure 1(c) shows UC PL spectra of β-NaYF4:Yb,Er and β-NYGF:Yb,Er UCNPs under excitation of 980 nm NIR light. Strong emission peak in green spectral region is attributed to electronic transition of 4S3/24I15/2 of Er3+ ions. Other main emission peaks are observed at around 522 and 656 nm due to electronic transitions from 2H11/2 and 4F9/2 states to 4I15/2 state of Er3+ ions, respectively. Although sub-10 nm UCNPs were obtained via Gd substitution, UC PL intensity of Gd-substituted β-NaYF4:Yb,Er was significantly decreased as shown in Fig. 1(c). Because all prepared UCNPs have hexagonal structure and high crystallinity, weak PL intensity of β-NYGF:Yb,Er is ascribed to highly increased ratio of surface area to volume resulted from decrease of particle size.

Growth of epitaxial shell on the core nanoparticle is well known as one of the best methods for enhancing PL intensity of the luminescent nanoparticles [25

25. F. Wang, J. Wang, and X. Liu, “Direct evidence of a surface quenching effect on size-dependent luminescence of upconversion nanoparticles,” Angew. Chem. Int. Ed. Engl. 49(41), 7456–7460 (2010). [CrossRef] [PubMed]

,31

31. F. Vetrone, R. Naccache, V. Mahalingam, C. G. Morgan, and J. A. Capobianco, “The active-core/active-shell approach: a strategy to enhance the upconversion luminescence in lanthanide-doped nanoparticles,” Adv. Funct. Mater. 19(18), 2924–2929 (2009). [CrossRef]

]. However, size of β-NaYF4:Yb,Er-based core/shell structure is larger than 20 nm [32

32. H. Guo, Z. Q. Li, H. S. Qian, Y. Hu, and I. N. Muhammad, “Seed-mediated synthesis of NaYF4:Yb, Er/NaGdF4 nanocrystals with improved upconversion fluorescence and MR relaxivity,” Nanotechnology 21(12), 125602 (2010). [CrossRef] [PubMed]

]. In this study, we used ultrasmall β-NYGF:Yb,Er as core material and β phase NaGdF4 (β-NaGdF4) shell was overgrown on the core particle. The β-NaGdF4 is promising shell material because lattice mismatch between β-NaGdF4 and β-NYGF:Yb,Er is believed to be smaller than that between β-NaYF4 and β-NYGF:Yb,Er. Additionally, β-NaGdF4 shell can serve as a terrific contrast agent for magnetic resonance imaging (MRI) because the presence of Gd3+ ions in the shell heightens the probability that Gd3+ ions physically contact with water protons, which efficiently shortens the relaxation time and enhances T1 contrast [33

33. K. A. Abel, J.-C. Boyer, and F. C. J. M. van Veggel, “Hard proof of the NaYF4/NaGdF4 nanocrystal core/shell structure,” J. Am. Chem. Soc. 131(41), 14644–14645 (2009). [CrossRef] [PubMed]

]. It is obvious that size of core/shell UCNPs (12.8 ± 1.1 nm) shown in Fig. 2(a)
Fig. 2 (a) TEM image of β-NYGF:Yb,Er/β-NaGdF4 core/shell UCNPs (Inset shows HR-TEM image.), (b) PL spectra, and (c) digital camera image showing luminescence of (i) β-NYGF:Yb,Er core and (ii) β-NYGF:Yb,Er/β-NaGdF4 core/shell. Inset of (b) shows magnified PL spectra in low intensity range.
is larger than that of β-NYGF:Yb,Er, and it is the first evidence for the formation of β-NaGdF4 shell on the core NPs. Distinct lattice fringes without lattice mismatch between core and shell in Fig. 2(a) inset indicate that highly crystalline β-NaGdF4 shell was epitaxially grown on the β-NYGF:Yb,Er core.

Figure 2(b) shows UC PL spectra of β-NYGF:Yb,Er core and β-NYGF:Yb,Er/β-NaGdF4 core/shell UCNPs, respectively. PL intensity of β-NYGF:Yb,Er was considerably enhanced by the formation of β-NaGdF4 shell. In addition to major emission peaks ascribed to 2H11/24I15/2 (522 nm), 4S3/24I15/2 (542 nm), and 4F9/24I15/2 (656 nm) transitions, blue emission peak attributed to the electronic transition from 2H9/2 to 4I15/2 (408 nm) of Er3+ ions was observed from the core/shell UCNPs although its intensity was weak (The wavelengths stated in the brackets indicate peak wavelengths.). Because UC blue emission by NIR excitation requires more excitation steps compared with UC green and red emissions, luminescence quenching by surface defects is more probable for UC blue emission [25

25. F. Wang, J. Wang, and X. Liu, “Direct evidence of a surface quenching effect on size-dependent luminescence of upconversion nanoparticles,” Angew. Chem. Int. Ed. Engl. 49(41), 7456–7460 (2010). [CrossRef] [PubMed]

]. For this reason, no blue emission peak was observed in β-NYGF:Yb,Er core NPs. However, β-NaGdF4 shell enormously suppressed surface quenching and emission peak at about 408 nm was observed in β-NYGF:Yb,Er/β-NaGdF4 UCNPs. As a result of β-NaGdF4 shell formation, one can see conspicuous difference of the brightness between core and core/shell UCNPs with the naked eye as shown in Fig. 2(c).

Although formation of β-NaGdF4 leads to enhanced UC luminescence, inert shell is not enough for multifunctionality of the UCNPs which already contain Gd in the core. As shown in Fig. 3
Fig. 3 Schematic illustration showing dual-mode up-conversion and down-conversion luminescence from core/shell structure doped with selective set of lanthanide ions.
, growing luminescent shell through doping lanthanide ions on the UCNP core can be one of the simplest and the most efficient methods to give UC-DC duality to the UCNPs with an enhanced UC PL. Because β-NYGF:Yb,Er core NPs show green emission peaking at 542 nm, if β-NaGdF4 shell is doped with Tb3+ ions which are known as a green-emitting activator [27

27. G. Blasse and B. C. Grabmaier, Luminescent Materials (Springer, Berlin, 1994).

], the core/shell NPs can emit green light under the excitation of UV light as well as NIR light. In this case, highly luminescent material is required because activator ions in the shell are close to the surface. Thus, before forming a luminescent shell on the UCNP core, β-NaGdF4:Tb and β-NaGdF4:Ce,Tb NPs were synthesized and characterized to investigate the luminescent properties of Tb3+ in the β-NaGdF4 host.

Figure 4(a)
Fig. 4 (a) TEM image (Inset shows HR-TEM image and inset scale bar indicates 5 nm.), (b) XRD pattern, and (c) PL spectrum of β-NaGdF4:Tb.
shows TEM images of β-NaGdF4:Tb NPs, indicating that highly monodisperse hexagonal β-NaGdF4:Tb NPs were synthesized. Formation of β phase was confirmed by XRD pattern as shown in Fig. 4(b). XRD peak positions are well matched with peak positions of bulk β-NaGdF4 (JCPDS 27-0699). The widths of diffraction peaks were strongly broadened due to the small size of β-NaGdF4:Tb. The HR-TEM image in the Fig. 4(a) inset exhibits clear lattice fringes, indicating that the synthesized β-NaGdF4:Tb NPs have high crystallinity, and a distance between the lattice fringes are well coincided with an interplanar d-spacing of planes of β-NaGdF4. When the β-NaGdF4:Tb NPs were excited by 254 nm UV light, they showed several emission peaks through the electronic transitions from 5D4 to 7FJ (J = 3, 4, 5, 6). However, emission from the β-NaGdF4:Tb was weak and ratio of signal to noise is low as shown in Fig. 4(c). PL intensity of the β-NaGdF4:Tb was very low and comparable to background signal. The weak PL intensity of the β-NaGdF4:Tb can be attributed to extremely small size (4.4 ± 0.3 nm) and/or inefficient excitation by 254 nm UV light. The difference between the excited 6I7/2 state and the ground 8S7/2 state of Gd3+ ions corresponds with the energy of UV light of 273 nm. Thus, the β-NaGdF4:Tb was excited by 273 nm UV light and intensity of green emission peak due to 5D47F5 transition was increased, indicating that excited energy was transferred from Gd3+ ions to Tb3+ ions. This result indicates that weak green emission from β-NaGdF4:Tb is attributed to inefficient excitation by 254 nm UV light and weak green intensity can be enhanced by changing wavelength of excitation source. However, background intensity was also increased with enhancement of green emission under 273 nm excitation and the β-NaGdF4:Tb solution did not emit pure green light. Therefore, we concluded that the β-NaGdF4:Tb was not appropriate to play a role of luminescent shell.

To improve green luminescence, Ce3+ and Tb3+ ions were co-doped into β-NaGdF4 NPs. When β-NaGdF4 was co-doped with Ce3+ and Tb3+ ions (it is noted as β-NaGdF4:Ce,Tb), particle size (4.0 ± 0.4 nm) of β-NaGdF4:Ce,Tb was slightly smaller than that of β-NaGdF4:Tb (4.4 ± 0.3 nm) as shown in Fig. 5(a)
Fig. 5 (a) TEM and HR-TEM images (inset scale bar = 5 nm) and (b) XRD pattern of β-NaGdF4:Ce,Tb, (c) PL spectra of β-NaGdF4:Tb and β-NaGdF4:Ce,Tb. Inset shows photographs of β-NaGdF4:Ce,Tb solution under room light (left) and 254 nm UV light (right) [i: 5D37F6, ii: 5D37F5, iii: 5D37F4, iv: 5D47F6, v: 5D47F5, vi: 5D47F4, vii: 5D47F3, viii: 5D47F2, ix: 5D47F1, and x: 5D47F0].
. Cerium doping did not influence the crystal structure and β-NaGdF4:Ce,Tb NPs have still high crystallinity (See Figs. 5(a) inset and 5(b).). Although particle size of β-NaGdF4:Ce,Tb is very small (< 5 nm), it showed strong green emission under 254 nm UV light and we can see bright green light from the β-NaGdF4:Ce,Tb solution as shown in Fig. 5(c) inset. Figure 5(c) shows PL spectra of β-NaGdF4:Tb and β-NaGdF4:Ce,Tb, and PL intensity of β-NaGdF4:Ce,Tb was remarkably increased compared with β-NaGdF4:Tb under the same excitation condition (λex = 254 nm). In addition, β-NaGdF4:Ce,Tb NPs clearly showed characteristic emission peaks attributed to the electronic transitions from 5D3 to 7FJ (J = 4, 5, 6) and from 5D4 to 7FJ (J = 0, 1, 2, 3, 4, 5, 6) of Tb3+ ions due to strong PL intensity (Also see energy level diagram of Fig. 7.). The intense green light was generated from ultrasmall β-NaGdF4:Ce,Tb NPs due to the efficient energy transfer from Ce3+ ions to Tb3+ ions via Gd3+ ions in the lattice. Although Ce3+-to-Tb3+ energy transfer can be confirmed by shortened decay time of Ce3+ [34

34. 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]

], huge increase of PL intensity of Tb3+ emission on the addition of Ce is sufficient to justify the energy transfer from Ce3+ ions to Tb3+ ions without showing decay time change of Ce3+.

It is worthy of note that Ce3+-sensitization is much more efficient for achieving strong green emission than energy transfer from Gd3+ to Tb3+ of β-NaGdF4:Tb under irradiation of 273 nm UV light. When the β-NaGdF4:Ce,Tb NPs were excited by 273 nm UV light, the PL intensity was lower than that of the β-NaGdF4:Ce,Tb NPs under the excitation of 254 nm UV light as shown in Fig. 6
Fig. 6 PL spectra of the β-NaGdF4:Ce,Tb under excitations of UV light (black line: λex = 254 nm and red line: λex = 273 nm).
. This result means that green light emitted from Tb3+ ions via energy transfer from Gd3+ to Tb3+ is less bright than that from Tb3+ ions via energy transfer from Ce3+ to Tb3+ through Gd3+ intermediates. The reason why green emission via energy transfer of Gd3+ → Tb3+ under UV light (λex = 273 nm) is weaker than that via energy transfer of Ce3+ → Gd3+ → Tb3+ is sharp absorption peak due to the transition of 8S7/26I7/2 in Gd3+ and nearly monochromatic excitation light source (i.e. narrow band width). In this case, the probability that Gd3+ can be excited by the UV light is not large. On the other hand, absorption band due to 4f → 5d transition in Ce3+ is broad and Ce3+ ions can be efficiently excited by the monochromatic UV light [27

27. G. Blasse and B. C. Grabmaier, Luminescent Materials (Springer, Berlin, 1994).

]. In addition, the transition strength of 4f → 5d transition in Ce3+ is stronger than that of 4f → 4f transition in Gd3+ because 4f → 5d transition is parity allowed transition. Hence, initially, Ce3+ excitation under 254 nm UV light is more efficient than Gd3+ excitation under 273 nm UV light, which indicates that Ce3+-sensitization is more effective to achieve intense DC green emission. Consequently, as shown in Fig. 5(c) inset, β-NaGdF4:Ce,Tb showed bright green light via efficient energy transfer of Ce3+ → Gd3+ → Tb3+ and intense UC and DC green luminescence can be realized when the β-NYGF:Yb,Er core is overcoated with the β-NaGdF4:Ce,Tb shell.

Figure 7(a)
Fig. 7 (a) TEM and HR-TEM images (inset scale bar = 5 nm) of β-NYGF:Yb,Er/β-NaGdF4:Ce,Tb UC/DC core/shell NPs, (b) PL spectra of β-NYGF:Yb,Er core UCNPs and β-NYGF:Yb,Er/β-NaGdF4:Ce,Tb UC/DC core/shell NPs [Inset shows photograph of UC/DC core/shell NPs under (i) room light, (ii) 254 nm UV lamp (4 W), and (iii) 980 nm NIR diode laser (0.4 W).], and (c) energy level diagram showing UC and DC emission from UC/DC core/shell NPs.
shows TEM and HR-TEM images of UC/DC core/shell NPs. Due to the shell formation, particle size of the UC/DC core/shell NPs is larger than that of the β-NYGF:Yb,Er core UCNPs. In the HR-TEM image of Fig. 7(a) inset, lattice mismatch was not found and a boundary between the core and the shell was not distinguishable, indicating that β-NaGdF4:Ce,Tb shell was epitaxially grown on the β-NYGF:Yb,Er core like the case of β-NaGdF4 inert shell growth. Lattice spacing obtained from the highly clear HR-TEM image of Fig. 7(a) inset verifies that the core/shell NPs have hexagonal β phase with high order of crystallinity. Due to the formation of DC luminescent shell, β-NYGF:Yb,Er/β-NaGdF4:Ce,Tb UC/DC NP solution homogeneously emits bright green light under a hand held UV lamp (λex = 254 nm, 4 W) and an NIR laser pointer (λex = 980 nm, 0.4 W) as shown in Fig. 7(b) inset. Figure 7(b) depicts PL spectra of β-NYGF:Yb,Er core UCNPs and β-NYGF:Yb,Er/β-NaGdF4:Ce,Tb core/shell NPs under the excitations of UV (254 nm) and NIR (980 nm) lights. As expected, luminescent shell did not only emit intense green light but it also magnificently enhanced UC PL intensity of the β-NYGF:Yb,Er. Characteristic emission peaks were observed from Er3+ and Tb3+ ions, respectively. Before the formation of the shell, β-NYGF:Yb,Er showed no emission under the irradiation of UV light (Fig. 7(b) lower panel). Compared with PL intensity of β-NaGdF4:Ce,Tb core NPs, DC PL intensity of UC/DC core/shell NPs decreased by 27.5%, whereas UC PL intensity of β-NYGF:Yb,Er/β-NaGdF4:Ce,Tb was significantly increased by ca. 11 times compared with the β-NYGF:Yb,Er core UCNPs. Weakened DC green light emitted from the UC/DC core/shell NPs can be explained as follows: First of all, sensitizer Ce3+ and activator Tb3+ ions are present closer to the surface of UC/DC NPs than in the case of core DC NPs and thus, surface quenching effect becomes larger. Second, energy transfer from Ce3+ to Gd3+ in core lattice occurs as illustrated in Fig. 7(c). Thus, energy loss is generated in the process of Ce3+ → Tb3+ energy transfer. Nevertheless, β-NYGF:Yb,Er/β-NaGdF4:Ce,Tb NPs still show bright green emission as shown in the photograph of Fig. 7(b) inset.

4. Conclusions

Bright dual-mode (UC and DC) green-emitting UC/DC core/shell NPs were successfully synthesized via selective set of dopant ions. Highly crystalline ultrasmall β-NYGF:Yb,Er NPs with size of 8.3 nm were synthesized and their weak PL intensity was significantly enhanced by forming β-NaGdF4 shell. The Ce3+ and Tb3+ co-doped β-NaGdF4 NPs showed ca. 31 times higher PL intensity than Tb3+ doped β-NaGdF4 NPs under excitation of 254 nm UV light despite ultrasmall size (4.0 nm), and β-NaGdF4:Ce,Tb was served as luminescent shell material. The β-NYGF:Yb,Er/β-NaGdF4:Ce,Tb UC/DC core/shell NPs showed bright UC and DC green light under the excitations of NIR and UV light, respectively. These results indicate that β-NYGF:Yb,Er/β-NaGdF4:Ce,Tb UC/DC core/shell NPs are useful to be applied to an optical imaging.

Acknowledgments

This work was supported by the Future key technology program funded by the Korea Institute of Science and Technology and the industrial strategic technology development program (No. 10035274, Quantum dot phosphorus converted LED Module) funded by the Ministry of Knowledge Economy (MKE, Korea). We thank Prof. van Veggel (University of Victoria) for helpful discussion on core upconversion nanoparticle synthesis.

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H. S. Jang, H. Y. Kim, Y.-S. Kim, H. M. Lee, and D. Y. Jeon, “Yellow-emitting γ-Ca2SiO4:Ce3+,Li+ phosphor for solid-sate lighting: luminescent properties, electronic structure, and white light-emitting diode application,” Opt. Express 20(3), 2761–2771 (2012). [CrossRef] [PubMed]

4.

H. S. Jang, H. Yang, S. W. Kim, J. Y. Han, S.-G. Lee, and D. Y. Jeon, “White light-emitting diodes with excellent color rendering based on organically capped CdSe quantum dots and Sr3SiO5:Ce3+,Li+ phosphors,” Adv. Mater. (Deerfield Beach Fla.) 20(14), 2696–2702 (2008). [CrossRef]

5.

D. K. Chatterjee, A. J. Rufaihah, and Y. Zhang, “Upconversion fluorescence imaging of cells and small animals using lanthanide doped nanocrystals,” Biomaterials 29(7), 937–943 (2008). [CrossRef] [PubMed]

6.

D. T. Tu, L. Q. Liu, Q. Ju, Y. S. Liu, H. M. Zhu, R. F. Li, and X. Y. Chen, “Time-resolved FRET biosensor based on amine-functionalized lanthanide-doped NaYF4 nanocrystals,” Angew. Chem. Int. Ed. Engl. 50(28), 6306–6310 (2011). [CrossRef] [PubMed]

7.

A. Shalav, B. S. Richards, T. Trupke, K. W. Krämer, and H. U. Güdel, “Application of NaYF4:Er3+ up-converting phosphors for enhanced near-infrared silicon solar cell response,” Appl. Phys. Lett. 86(1), 013505 (2005). [CrossRef]

8.

H. Q. Wang, M. Batentschuk, A. Osvet, L. Pinna, and C. J. Brabec, “Rare-earth ion doped up-conversion materials for photovoltaic applications,” Adv. Mater. (Deerfield Beach Fla.) 23(22-23), 2675–2680 (2011). [CrossRef] [PubMed]

9.

H. S. Choi, W. Liu, F. Liu, K. Nasr, P. Misra, M. G. Bawendi, and J. V. Frangioni, “Design considerations for tumour-targeted nanoparticles,” Nat. Nanotechnol. 5(1), 42–47 (2010). [CrossRef] [PubMed]

10.

G. Chen, T. Y. Ohulchanskyy, R. Kumar, H. Ågren, and P. N. Prasad, “Ultrasmall monodisperse NaYF4:Yb3+/Tm3+ nanocrystals with enhanced near-infrared to near-infrared upconversion photoluminescence,” ACS Nano 4(6), 3163–3168 (2010). [CrossRef] [PubMed]

11.

D. K. Chatterjee, M. K. Gnanasammandhan, and Y. Zhang, “Small upconverting fluorescent nanoparticles for biomedical applications,” Small 6(24), 2781–2795 (2010). [CrossRef] [PubMed]

12.

L. Cheng, K. Yang, S. Zhang, M. Shao, S. Lee, and Z. Liu, “Highly-sensitive multiplexed in vivo imaging using PEGylated upconversion nanoparticles,” Nano Res. 3(10), 722–732 (2010). [CrossRef]

13.

M. Haase, S. Heer, K. Kömpe, and H. U. Güdel, “Highly efficient multicolour upconversion emission in transparent colloids of lanthanide-doped NaYF4 nanocrystals,” Adv. Mater. (Deerfield Beach Fla.) 16(23–24), 2102–2105 (2004). [CrossRef]

14.

F. Wang and X. Liu, “Upconversion multicolor fine-tuning: visible to near-infrared emission from lanthanide-doped NaYF4 nanoparticles,” J. Am. Chem. Soc. 130(17), 5642–5643 (2008). [CrossRef] [PubMed]

15.

S. Wu, G. Han, D. J. Milliron, S. Aloni, V. Altoe, D. V. Talapin, B. E. Cohen, and P. J. Schuck, “Non-blinking and photostable upconverted luminescence from single lanthanide-doped nanocrystals,” Proc. Natl. Acad. Sci. U.S.A. 106(27), 10917–10921 (2009). [CrossRef] [PubMed]

16.

F. Auzel, “Upconversion and anti-Stokes processes with f and d ions in solids,” Chem. Rev. 104(1), 139–174 (2004). [CrossRef] [PubMed]

17.

G. S. Yi and G. M. Chow, “Synthesis of hexagonal-phase NaYF4:Yb,Er and NaYF4:Yb,Tm nanocrystals with efficient up-conversion fluorescence,” Adv. Funct. Mater. 16(18), 2324–2329 (2006). [CrossRef]

18.

H.-X. Mai, Y.-W. Zhang, R. Si, Z.-G. Yan, L.-D. Sun, L.-P. You, and C.-H. Yan, “High-quality sodium rare-earth fluoride nanocrystals: controlled synthesis and optical properties,” J. Am. Chem. Soc. 128(19), 6426–6436 (2006). [CrossRef] [PubMed]

19.

V. Mahalingam, F. Vetrone, R. Naccache, A. Speghini, and J. A. Capobianco, “Colloidal Tm3+/Yb3+-doped LiYF4 nanocrystals: multiple luminescence spanning the UV to NIR regions via low-energy excitation,” Adv. Mater. (Deerfield Beach Fla.) 21(40), 4025–4028 (2009). [CrossRef]

20.

H. Schäfer, P. Ptacek, O. Zerzouf, and M. Haase, “Synthesis and optical properties of KYF4/Yb, Er nanocrystals, and their surface modification with undoped KYF4,” Adv. Funct. Mater. 18(19), 2913–2918 (2008). [CrossRef]

21.

J.-C. Boyer, F. Vetrone, L. A. Cuccia, and J. A. Capobianco, “Synthesis of colloidal upconverting NaYF4 nanocrystals doped with Er3+, Yb3+ and Tm3+, Yb3+ via thermal decomposition of lanthanide trifluoroacetate precursors,” J. Am. Chem. Soc. 128(23), 7444–7445 (2006). [CrossRef] [PubMed]

22.

J.-C. Boyer, L. A. Cuccia, and J. A. Capobianco, “Synthesis of colloidal upconverting NaYF4:Er3+/Yb3+ and Tm3+/Yb3+ monodisperse nanocrystals,” Nano Lett. 7(3), 847–852 (2007). [CrossRef] [PubMed]

23.

P. Li, Q. Peng, and Y. D. Li, “Dual-mode luminescent colloidal spheres from monodisperse rare-earth fluoride nanocrystals,” Adv. Mater. (Deerfield Beach Fla.) 21(19), 1945–1948 (2009). [CrossRef]

24.

Y. Liu, D. Tu, H. Zhu, R. Li, W. Luo, and X. Chen, “A strategy to achieve efficient dual-mode luminescence of Eu3+ in lanthanides doped multifunctional NaGdF4 nanocrystals,” Adv. Mater. (Deerfield Beach Fla.) 22(30), 3266–3271 (2010). [CrossRef]

25.

F. Wang, J. Wang, and X. Liu, “Direct evidence of a surface quenching effect on size-dependent luminescence of upconversion nanoparticles,” Angew. Chem. Int. Ed. Engl. 49(41), 7456–7460 (2010). [CrossRef] [PubMed]

26.

F. Wang, R. Deng, J. Wang, Q. Wang, Y. Han, H. Zhu, X. Chen, and X. Liu, “Tuning upconversion through energy migration in core-shell nanoparticles,” Nat. Mater. 10(12), 968–973 (2011). [CrossRef] [PubMed]

27.

G. Blasse and B. C. Grabmaier, Luminescent Materials (Springer, Berlin, 1994).

28.

Z. Li, Y. Zhang, and S. Jiang, “Multicolor core/shell-structured upconversion fluorescent nanoparticles,” Adv. Mater. (Deerfield Beach Fla.) 20(24), 4765–4769 (2008). [CrossRef]

29.

J. Park, K. J. An, Y. Hwang, J.-G. Park, H.-J. Noh, J.-Y. Kim, J.-H. Park, N.-M. Hwang, and T. Hyeon, “Ultra-large-scale syntheses of monodisperse nanocrystals,” Nat. Mater. 3(12), 891–895 (2004). [CrossRef] [PubMed]

30.

F. Wang, Y. Han, C. S. Lim, Y. Lu, J. Wang, J. Xu, H. Chen, C. Zhang, M. Hong, and X. G. Liu, “Simultaneous phase and size control of upconversion nanocrystals through lanthanide doping,” Nature 463(7284), 1061–1065 (2010). [CrossRef] [PubMed]

31.

F. Vetrone, R. Naccache, V. Mahalingam, C. G. Morgan, and J. A. Capobianco, “The active-core/active-shell approach: a strategy to enhance the upconversion luminescence in lanthanide-doped nanoparticles,” Adv. Funct. Mater. 19(18), 2924–2929 (2009). [CrossRef]

32.

H. Guo, Z. Q. Li, H. S. Qian, Y. Hu, and I. N. Muhammad, “Seed-mediated synthesis of NaYF4:Yb, Er/NaGdF4 nanocrystals with improved upconversion fluorescence and MR relaxivity,” Nanotechnology 21(12), 125602 (2010). [CrossRef] [PubMed]

33.

K. A. Abel, J.-C. Boyer, and F. C. J. M. van Veggel, “Hard proof of the NaYF4/NaGdF4 nanocrystal core/shell structure,” J. Am. Chem. Soc. 131(41), 14644–14645 (2009). [CrossRef] [PubMed]

34.

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]

OCIS Codes
(160.2540) Materials : Fluorescent and luminescent materials
(160.5690) Materials : Rare-earth-doped materials
(250.5230) Optoelectronics : Photoluminescence

ToC Category:
Materials

History
Original Manuscript: April 30, 2012
Revised Manuscript: June 10, 2012
Manuscript Accepted: June 14, 2012
Published: July 12, 2012

Citation
Ho Seong Jang, Kyoungja Woo, and Kipil Lim, "Bright dual-mode green emission from selective set of dopant ions in β-Na(Y,Gd)F4:Yb,Er/β-NaGdF4:Ce,Tb core/shell nanocrystals," Opt. Express 20, 17107-17118 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-15-17107


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References

  1. C. Feldmann, T. Jüstel, C. R. Ronda, and P. J. Schmidt, “Inorganic luminescent materials: 100 years of research and application,” Adv. Funct. Mater.13(7), 511–516 (2003). [CrossRef]
  2. H. Bechtel, T. Jüstel, H. Gläser, and D. U. Wiechert, “Phosphors for plasma-display panels: demands and achieved performance,” J. Soc. Inf. Disp.10, 63–67 (2002).
  3. H. S. Jang, H. Y. Kim, Y.-S. Kim, H. M. Lee, and D. Y. Jeon, “Yellow-emitting γ-Ca2SiO4:Ce3+,Li+ phosphor for solid-sate lighting: luminescent properties, electronic structure, and white light-emitting diode application,” Opt. Express20(3), 2761–2771 (2012). [CrossRef] [PubMed]
  4. H. S. Jang, H. Yang, S. W. Kim, J. Y. Han, S.-G. Lee, and D. Y. Jeon, “White light-emitting diodes with excellent color rendering based on organically capped CdSe quantum dots and Sr3SiO5:Ce3+,Li+ phosphors,” Adv. Mater. (Deerfield Beach Fla.)20(14), 2696–2702 (2008). [CrossRef]
  5. D. K. Chatterjee, A. J. Rufaihah, and Y. Zhang, “Upconversion fluorescence imaging of cells and small animals using lanthanide doped nanocrystals,” Biomaterials29(7), 937–943 (2008). [CrossRef] [PubMed]
  6. D. T. Tu, L. Q. Liu, Q. Ju, Y. S. Liu, H. M. Zhu, R. F. Li, and X. Y. Chen, “Time-resolved FRET biosensor based on amine-functionalized lanthanide-doped NaYF4 nanocrystals,” Angew. Chem. Int. Ed. Engl.50(28), 6306–6310 (2011). [CrossRef] [PubMed]
  7. A. Shalav, B. S. Richards, T. Trupke, K. W. Krämer, and H. U. Güdel, “Application of NaYF4:Er3+ up-converting phosphors for enhanced near-infrared silicon solar cell response,” Appl. Phys. Lett.86(1), 013505 (2005). [CrossRef]
  8. H. Q. Wang, M. Batentschuk, A. Osvet, L. Pinna, and C. J. Brabec, “Rare-earth ion doped up-conversion materials for photovoltaic applications,” Adv. Mater. (Deerfield Beach Fla.)23(22-23), 2675–2680 (2011). [CrossRef] [PubMed]
  9. H. S. Choi, W. Liu, F. Liu, K. Nasr, P. Misra, M. G. Bawendi, and J. V. Frangioni, “Design considerations for tumour-targeted nanoparticles,” Nat. Nanotechnol.5(1), 42–47 (2010). [CrossRef] [PubMed]
  10. G. Chen, T. Y. Ohulchanskyy, R. Kumar, H. Ågren, and P. N. Prasad, “Ultrasmall monodisperse NaYF4:Yb3+/Tm3+ nanocrystals with enhanced near-infrared to near-infrared upconversion photoluminescence,” ACS Nano4(6), 3163–3168 (2010). [CrossRef] [PubMed]
  11. D. K. Chatterjee, M. K. Gnanasammandhan, and Y. Zhang, “Small upconverting fluorescent nanoparticles for biomedical applications,” Small6(24), 2781–2795 (2010). [CrossRef] [PubMed]
  12. L. Cheng, K. Yang, S. Zhang, M. Shao, S. Lee, and Z. Liu, “Highly-sensitive multiplexed in vivo imaging using PEGylated upconversion nanoparticles,” Nano Res.3(10), 722–732 (2010). [CrossRef]
  13. M. Haase, S. Heer, K. Kömpe, and H. U. Güdel, “Highly efficient multicolour upconversion emission in transparent colloids of lanthanide-doped NaYF4 nanocrystals,” Adv. Mater. (Deerfield Beach Fla.)16(23–24), 2102–2105 (2004). [CrossRef]
  14. F. Wang and X. Liu, “Upconversion multicolor fine-tuning: visible to near-infrared emission from lanthanide-doped NaYF4 nanoparticles,” J. Am. Chem. Soc.130(17), 5642–5643 (2008). [CrossRef] [PubMed]
  15. S. Wu, G. Han, D. J. Milliron, S. Aloni, V. Altoe, D. V. Talapin, B. E. Cohen, and P. J. Schuck, “Non-blinking and photostable upconverted luminescence from single lanthanide-doped nanocrystals,” Proc. Natl. Acad. Sci. U.S.A.106(27), 10917–10921 (2009). [CrossRef] [PubMed]
  16. F. Auzel, “Upconversion and anti-Stokes processes with f and d ions in solids,” Chem. Rev.104(1), 139–174 (2004). [CrossRef] [PubMed]
  17. G. S. Yi and G. M. Chow, “Synthesis of hexagonal-phase NaYF4:Yb,Er and NaYF4:Yb,Tm nanocrystals with efficient up-conversion fluorescence,” Adv. Funct. Mater.16(18), 2324–2329 (2006). [CrossRef]
  18. H.-X. Mai, Y.-W. Zhang, R. Si, Z.-G. Yan, L.-D. Sun, L.-P. You, and C.-H. Yan, “High-quality sodium rare-earth fluoride nanocrystals: controlled synthesis and optical properties,” J. Am. Chem. Soc.128(19), 6426–6436 (2006). [CrossRef] [PubMed]
  19. V. Mahalingam, F. Vetrone, R. Naccache, A. Speghini, and J. A. Capobianco, “Colloidal Tm3+/Yb3+-doped LiYF4 nanocrystals: multiple luminescence spanning the UV to NIR regions via low-energy excitation,” Adv. Mater. (Deerfield Beach Fla.)21(40), 4025–4028 (2009). [CrossRef]
  20. H. Schäfer, P. Ptacek, O. Zerzouf, and M. Haase, “Synthesis and optical properties of KYF4/Yb, Er nanocrystals, and their surface modification with undoped KYF4,” Adv. Funct. Mater.18(19), 2913–2918 (2008). [CrossRef]
  21. J.-C. Boyer, F. Vetrone, L. A. Cuccia, and J. A. Capobianco, “Synthesis of colloidal upconverting NaYF4 nanocrystals doped with Er3+, Yb3+ and Tm3+, Yb3+ via thermal decomposition of lanthanide trifluoroacetate precursors,” J. Am. Chem. Soc.128(23), 7444–7445 (2006). [CrossRef] [PubMed]
  22. J.-C. Boyer, L. A. Cuccia, and J. A. Capobianco, “Synthesis of colloidal upconverting NaYF4:Er3+/Yb3+ and Tm3+/Yb3+ monodisperse nanocrystals,” Nano Lett.7(3), 847–852 (2007). [CrossRef] [PubMed]
  23. P. Li, Q. Peng, and Y. D. Li, “Dual-mode luminescent colloidal spheres from monodisperse rare-earth fluoride nanocrystals,” Adv. Mater. (Deerfield Beach Fla.)21(19), 1945–1948 (2009). [CrossRef]
  24. Y. Liu, D. Tu, H. Zhu, R. Li, W. Luo, and X. Chen, “A strategy to achieve efficient dual-mode luminescence of Eu3+ in lanthanides doped multifunctional NaGdF4 nanocrystals,” Adv. Mater. (Deerfield Beach Fla.)22(30), 3266–3271 (2010). [CrossRef]
  25. F. Wang, J. Wang, and X. Liu, “Direct evidence of a surface quenching effect on size-dependent luminescence of upconversion nanoparticles,” Angew. Chem. Int. Ed. Engl.49(41), 7456–7460 (2010). [CrossRef] [PubMed]
  26. F. Wang, R. Deng, J. Wang, Q. Wang, Y. Han, H. Zhu, X. Chen, and X. Liu, “Tuning upconversion through energy migration in core-shell nanoparticles,” Nat. Mater.10(12), 968–973 (2011). [CrossRef] [PubMed]
  27. G. Blasse and B. C. Grabmaier, Luminescent Materials (Springer, Berlin, 1994).
  28. Z. Li, Y. Zhang, and S. Jiang, “Multicolor core/shell-structured upconversion fluorescent nanoparticles,” Adv. Mater. (Deerfield Beach Fla.)20(24), 4765–4769 (2008). [CrossRef]
  29. J. Park, K. J. An, Y. Hwang, J.-G. Park, H.-J. Noh, J.-Y. Kim, J.-H. Park, N.-M. Hwang, and T. Hyeon, “Ultra-large-scale syntheses of monodisperse nanocrystals,” Nat. Mater.3(12), 891–895 (2004). [CrossRef] [PubMed]
  30. F. Wang, Y. Han, C. S. Lim, Y. Lu, J. Wang, J. Xu, H. Chen, C. Zhang, M. Hong, and X. G. Liu, “Simultaneous phase and size control of upconversion nanocrystals through lanthanide doping,” Nature463(7284), 1061–1065 (2010). [CrossRef] [PubMed]
  31. F. Vetrone, R. Naccache, V. Mahalingam, C. G. Morgan, and J. A. Capobianco, “The active-core/active-shell approach: a strategy to enhance the upconversion luminescence in lanthanide-doped nanoparticles,” Adv. Funct. Mater.19(18), 2924–2929 (2009). [CrossRef]
  32. H. Guo, Z. Q. Li, H. S. Qian, Y. Hu, and I. N. Muhammad, “Seed-mediated synthesis of NaYF4:Yb, Er/NaGdF4 nanocrystals with improved upconversion fluorescence and MR relaxivity,” Nanotechnology21(12), 125602 (2010). [CrossRef] [PubMed]
  33. K. A. Abel, J.-C. Boyer, and F. C. J. M. van Veggel, “Hard proof of the NaYF4/NaGdF4 nanocrystal core/shell structure,” J. Am. Chem. Soc.131(41), 14644–14645 (2009). [CrossRef] [PubMed]
  34. 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]

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