Composition tuning the upconversion emission in NaYF₄:Yb/Tm hexaplate nanocrystals

Abstract

Single crystal hexagonal NaYF₄:Yb/Tm nanocrystals have been synthesized with uniform size, morphology and controlled chemical composition. Spectroscopic studies show that these nanocrystals exhibit strong energy upconversion emission when excited with a 980 nm diode laser, with two primary emission peaks centered around 452 nm and 476 nm. Importantly, the overall and relative emission intensity at these wavelengths can be readily tuned by controlling the concentration of the trivalent rare earth element dopants at the beginning of the synthesis which has been confirmed by EDX for the first time. Through systematic studies, the optimum rare earth ion doping concentration can be determined for the strongest emission intensity at the selected peak(s). Confocal microscopy studies show that the upconversion emission from individual NCs can be readily visualized. These studies demonstrate a rational approach for fine tuning the upconversion properties in rare-earth doped nanostructures and can broadly impact areas ranging from energy harvesting, energy conversion to biomedical imaging and therapeutics.

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When a medium emits fluorescence as a result of being excited with an incident light, the wavelength of the fluorescence is usually longer than that of the excitation light which is called down-conversion.¹ However, under some special circumstances, one can observe upconversion fluorescence, in which the wavelength of the emitted light is shorter than that of the excitation light.^2,3 This unique property makes upconversion materials attractive for a wide range of applications.^4–7 In particular, the fact that they can be excited with infrared light makes them attractive candidates as biological probes for deeper imaging with lower noise, in contrast to traditional organic dyes and quantum dots.^8–11 Recently, considerable efforts have been devoted to the synthesis of energy upconversion nanoparticles.^12,13

Trivalent rare earth element (RE³⁺) doped materials are among the most promising materials for energy upconversion because they have multiple long-lived excited states and can exhibit highly efficient energy transfer processes that are responsible for upconversion emission under certain conditions.^2,3,14–16 Here, we focus our efforts on Yb–Tm doped systems that can emit violet (452 nm) or blue (476 nm) light. The ability to achieve this shorter wavelength emission is of significance for activating certain chemical or biological processes. For example, the 476 nm light can be used to excite channelrhodopsins, which are a subfamily of opsin proteins that function as light-gated ion channels in neural cells.¹⁷ Therefore, it is important that the emission wavelength can be tuned by introducing different dopants or doping ratios. By varying the exact doping concentration during the chemical synthesis process, we show that the overall and relative emission intensity of the selected spectral peaks can be readily tuned.

NaYF₄ can adopt either a cubic or hexagonal structure.^12,15,18 It is well known that the hexagonal structure is a more efficient matrix for upconversion phosphors.^19–21 However, most synthetic approaches result in the cubic phase NaYF₄/NaYbF₄.^22–24 Here we discuss the synthesis of NaYF₄ hexaplate nanocrystals (NCs) through a solution chemical approach at high temperature.

The synthesis was carried out with standard oxygen-free procedures as reported before.^25,26 All chemicals were purchased from Sigma-Aldrich and used without further purification. In a typical procedure, a certain amount of yttrium(III) oxide (Y₂O₃, 99.99%), ytterbium(III) oxide (Yb₂O₃, 99.9%) and thulium(III) oxide (Tm₂O₃, 99.9%) (1.25 mmol in all with the relative ratio depending on the exact doping concentration) were dissolved in 5 mL trifluoroacetic acid (TFA, 99%) in a 100 mL three-necked flask. The slurry was then heated to 80 °C with vigorous magnetic stirring under vacuum for 30 minutes to remove water and excessive TFA. Next, 2.5 mmol sodium trifluoroacetate (NaCOOCF₃, 98%) was added, along with 7.5 mL of oleic acid (OA, 90%) and 7.5 mL of 1-octadecene (ODE, 90%) at 100 °C. Afterwards, the solution was heated to 330 °C with a rate of 30 °C min⁻¹ and maintained at 330 °C for 60 minutes to obtain the NCs.

The microstructures, morphologies and compositions of the NaYF₄:Yb/Tm NCs were characterized by a JEOL 6700 FEG scanning electron microscope (SEM) with an energy-dispersive X-ray spectroscopy (EDX). The microstructures and lattice images of NaYF₄:Yb/Tm NCs were observed by an FEI CM120 transmission electron microscope (TEM) and an FEI Titan high-resolution transmission electron microscope (HRTEM). Room temperature photoluminescence spectra were collected on a Spec-10® system from Princeton Instruments including a liquid nitrogen cooled CCD camera (Spectra Pro® 2300i, Model: 7509-0001). Confocal photoluminescence images were taken on a Leica TCS-SP2 AOBS inverted Confocal and Multiphoton Microscope (Mannheim, Germany) equipped with a Spectra-Physics MaiTai picosecond pulsed infrared laser (Mountain View, CA) set at 980 nm for infrared excitation.

The SEM and TEM images show that the NaYb_0.99Tm_0.01F₄ NCs have a hexagonal plate structure with a nearly mono-dispersed size (Fig. 1a and b). The diagonal length of the hexagonal plate is about 180 nm with a thickness of 50 nm. The well-defined hexagonal morphology of the NCs suggests that they are single crystals. Indeed, the HRTEM and SAED images confirmed that the as-prepared NCs are single crystals with a hexagonal phase. The lattice spacing of 5.13 Å in HRTEM image corresponds to (100) lattice planes (Fig. 1c and inset).

Fig. 1 (a) SEM, (b)TEM, (c and inset) HRTEM and SAED images of NaYbF₄:Tm NPs with uniform hexagonal shape (the scale bars correspond to 400 nm, 200 nm and 5 nm, respectively).

Although the upconversion emission peak positions usually cannot be changed for a given upconversion material, the emission efficiency at different wavelengths can be greatly affected by different doping concentrations. To this end, we have carried out systematic studies to control the rare earth element doping concentrations in these nanocrystals. Importantly, our studies show that the doping concentration in the final NC product is directly correlated to the initial reactant ratio. For example, we have prepared a series of NaY_0.8−xYb_0.2Tm_xF₄ and NaYb_1−xTm_xF₄ NCs with variable Tm³⁺ ion concentrations. The plot of the Tm³⁺ concentration in the NCs vs. the starting reactant ratio shows a clear linear relation with a slope close to one (Fig. 2). These studies demonstrate that all the rare earth elements can be effectively incorporated into the final NCs with our synthetic conditions. Therefore, the RE³⁺ doping concentration can be readily controlled by varying the initial reactant ratio. The size, morphology and dispersity of the NCs of variable compositions remained similar to what have been shown in Fig. 1.

Fig. 2 Diagram of final Tm³⁺ concentration versus original Tm³⁺ concentration in (a) NaY_0.8−xYb_0.2Tm_xF₄ NCs and (b) NaYb_1−xTm_xF₄ NCs.

In RE³⁺ doped materials, the energy transfer process between adjacent ions is the primary reason responsible for the upconversion emission. Therefore, a close match between different transition levels of the rare earth ions can critically impact the probability of the energy transfer process and the efficiency of the upconversion process. In the Yb–Tm co-doped materials, the infrared radiation can be efficiently converted into blue emission by a three-photon process or violet emission by a four-photon upconversion process due to the existence of multiple energy resonances in the system (Fig. 3). One energy transfer step from Yb³⁺ ion to Tm³⁺ ion populates the ³H₅ level of Tm³⁺ from ³H₆. The ³H₅ decays rapidly to the ³F₄ level. The second energy transfer step raises the Tm³⁺ ion from ³F₄ to ³F₂ that quickly decays to ³H₄. Subsequently, the third transfer step raises the Tm³⁺ ion from ³H₄ to ¹G₄ that can yield a blue emission upon radiative relaxation back to ³H₆. A fourth energy transfer step from Yb³⁺ ion to Tm³⁺ ion may also take place to populate the Tm³⁺ ion from ¹G₄ to ¹D₂, which is however usually less efficient due to the relative large energy mismatch. An alternative way to populate ¹D₂ is through cross relaxation between adjacent Tm³⁺ ions with three possible routes: ³F₂ → ³H₆ and ³H₄ → ¹D₂; ¹G₄ → ³F₄ and ³H₄ → ¹D₂; and ¹G₄ → ¹D₂ and ³H₄ → ³F₄.²⁷ The relaxation of ¹D₂ to ³F₄ leads to a violet emission.

Fig. 3 Schematic illustration of the energy levels involved in the upconversion process of Tm³⁺ with Yb³⁺ as the promoter for NaYF₄:Yb,Tm NCs.

The upconversion emission of the NCs was characterized in chloroform solution with a fixed amount of NCs (typically 1 wt%) of variable Tm³⁺ concentration under 980 nm diode laser excitation with a total power of 38 mW.

Fig. 4a shows the emission spectra of NaY_0.8−xYb_0.2Tm_xF₄nanoparticles at different doping concentrations of Tm³⁺ ions. The two strongest peaks occur at 452 nm (violet) and 476 nm (blue), corresponding to relaxation of ¹D₂ → ³F₄ and ¹G₄ → ³H₆, respectively. The intensity increases slightly when the Tm³⁺ concentration is increased from 0.05 to 0.2 molar fraction and then decreases with a further increase of Tm³⁺ concentration. It is reasonable to assume that if the Tm³⁺ ion concentration is too low, there will not be enough Tm³⁺ ion to be populated or excited.^2,3 At this point, the emission intensity increases with increasing number of Tm³⁺ ions in the system. On the other hand, a further increase in the Tm³⁺ concentration beyond a certain threshold can lead to a decrease in the emission intensity. This is partly because a cross-relaxation mechanism exists between Tm³⁺ ions that can lead to a self-quenching effect: the increase of the Tm³⁺ concentration leads to a decrease in the distance between adjacent Tm³⁺ ions and an increase in the probability of cross-relaxation that reduces radiative relaxation, and thus suppresses the upconversion emission in high Tm³⁺ ion concentrated samples. At the same time, for a given overall excitation energy, the excitation received by individual Tm ions decreases with increasing Tm doping concentrations, which could also contribute to the weakened Tm emission.^28–30 Plotting the upconversion emission intensity normalized by the number of Tm³⁺ ions as a function of Tm³⁺ concentration, a monotonic decrease in intensity could be observed (Fig. 4c), suggesting that the cross-relaxation between neighboring Tm³⁺ ions exists in all samples including the very low-dope samples.

Fig. 4 (a) Upconversion emission spectra of NaY_0.8−xYb_0.2Tm_xF₄ NCs with different Tm³⁺ concentrations, x = 0.0005, 0.002, 0.02, 0.1, for (1), (2), (3), (4) respectively. (b) Digital camera pictures ofNaY_0.7995Yb_0.2Tm_0.0005F₄, NaY_0.798Yb_0.2Tm_0.002F₄ and NaY_0.78Yb_0.2Tm_0.02F₄ (from left to right) samples under 980 nm excitation. (c) Upconversion emission intensity (normalized by Tm³⁺ ion concentration) versus doping concentration of Tm³⁺ in NaY_0.8−xYb_0.2Tm_xF₄ NCs. (d) Diagram of violet to blue emission (f_v/b) ratio versusTm³⁺ doping concentration in NaY_0.8−xYb_0.2Tm_xF₄ NCs.

Fig. 4a also shows that the preferred emission wavelength changes from blue to violet with increasing the Tm³⁺ concentration. The digital camera images of NCs fluorescence clearly show different colors (from blue to violet) for the samples with increasing Tm³⁺ ion doping concentration (Fig. 4b). Plotting the ratio of violet vs. blue emission (f_v/b) shows a monotonic increase with the increasing Tm³⁺ ion concentration (Fig. 4d). This colour change as a function of Tm³⁺ ion concentration may be attributed to possible cross-relaxation processes: ¹G₄ → ³F₄ and ³H₄ → ¹D₂; and ¹G₄ → ¹D₂ and ³H₄ → ³F₄. These cross-relaxation processes decrease the ¹G₄ population and increase the ¹D₂ population. Therefore, with increasing Tm³⁺ ion concentration and increasing the probability of cross-relaxation, one would expect to see relatively stronger violet emission originated from ¹D₂ state and weaker blue emission originated from ¹G₄ state, resulting in an increase in violet/blue ratio (f_v/b) with the increase of Tm³⁺ concentration.

Additionally, we have further explored a simpler NaYbF₄:Tm system, using NaYbF₄ as the matrix and Tm as the dopant, and investigated the emission intensity vs. doping concentration. Fig. 5a shows the emission spectra of a series of NaYbF₄:Tm NCs with variable Tm³⁺ concentrations. Interestingly, only one dominant emission peak at 452 nm can be seen in this system. A maximum violet over blue emission ratio of 5.5 is observed. Removing Y³⁺ ion away from the system increases the Yb³⁺ ion concentration, which makes it possible that cross-relaxation between Tm³⁺ ions to Yb³⁺ ions can also take place, reducing the population of the ¹G₄ state in a more significant way than the ¹D₂ state,³¹ and thus significantly suppressing 476 nm emission from ¹G₄ state, leaving the 452 nm (violet) emission from the ¹D₂ state the dominant peak here.

Fig. 5 (a) Upconversion emission spectra of NaYb_1−xTm_xF₄ NCs with different Tm³⁺ concentrations, x = 0.002, 0.005, 0.01, 0.02, 0.05, 0.1, for (1), (2), (3), (4), (5), (6) respectively. (b) Tm³⁺ normalized upconversion emission intensity versus doping concentration of Tm³⁺ in NaYb_1−xTm_xF₄.

The spectra in Fig. 5a also show that the best intensity is observed in the sample with a composition of NaYb_0.99Tm_0.01F₄; an additional decrease or increase in the Tm³⁺ concentration will result in a decrease in overall upconversion intensity. Normalizing the emission intensity by the number of Tm³⁺ ions, a monotonic decrease of the emission intensity vs. doping concentration could be easily observed (Fig. 5b), which is consistent with the result shown in Fig. 4c, suggesting that the cross-relaxation between adjacent Tm³⁺ ions is primarily responsible for the self-quenching and the decrease of emission intensity at high Tm³⁺ concentration.

Lastly, we have also used confocal microscopy to investigate the upconversion emission from individual NCs. To this end, the NCs were first spin-coated onto a glass slide. The reflection image of the NCs on glass slide shows well separated NCs or NC clusters (Fig. 6a). SEM image of a similar sample prepared on silicon wafer further shows the NCs are either single NCs or a few NC clusters (inset, Fig. 6a). Importantly, the confocal photoluminescence image (Fig. 6b) clearly shows strong emission from the NCs when excited with the 980 nm laser, with each bright spot in the image corresponding to emission from one or a few NCs. These studies clearly show that upconversion emission from the NCs can be readily visualized by a confocal microscope, which is of significance for exploring them as fluorescence tags for biomedical imaging.

Fig. 6 (a) Reflection image of NCs on glass substrate. (Inset) SEM image of a similar sample prepared on silicon wafer. (b) Confocal photoluminescence image of the upconversion nanocrystals shown in (a). (Scale bar: 25 µm for (a) and (b), and 5µm for the inset).

In summary, we successfully prepared NaYF₄:Yb/Tm hexagonal plate NCs with controlled composition by varying the doping concentrations when synthesizing the NCs. Photoluminescence studies show strong upconversion emission in blue and violet region from the NCs when excited with 980 nm light. Importantly, the overall emission intensity and relative ratio of these two emission colours can be readily tuned by varying the exact RE³⁺ doping concentration. These studies demonstrate a rational approach to tune the upconversion emission from rare earth element doped nanomaterials, and can broadly impact fields ranging from energy harvesting and energy conversion to biomedical imaging and therapeutics.

Acknowledgements

X.D. acknowledges support by the NIH Director's New Innovator Award Program, part of the NIH Roadmap for Medical Research, through grant number 1DP2OD004342-01. We acknowledge Electron Imaging Center for Nanomachines (EICN) at UCLA for the support of TEM. Confocal laser scanning microscopy was performed at the CNSI Advanced Light Microscopy/Spectroscopy Shared Resource Facility at UCLA, supported with funding from NIH-NCRR shared resources grant (CJX1-443835-WS-29646) and NSF Major Research Instrumentation grant (CHE-0722519). We acknowledge Dr Matthew Schibler for technical support on the confocal miscroscope. Y.H. acknowledges partial support from the Chinese American Faculty Association (CAFA) Faculty Development award.

Notes and references

1. G. Blasse and B. C. Grabmaier, Luminescent Materials, Springer, Berlin, 1994.
2. F. Auzel, Chem. Rev., 2004, 104, 139.
3. F. Wang and X. Liu, Chem. Soc. Rev., 2009, 38, 976.
4. S. F. Lim, R. Riehn, W. S. Ryu, N. Khanarian, C. K. Tung, D. Tank and R. H. Austin, Nano Lett., 2006, 6, 169.
5. D. K. Chatterjee, A. J. Rufaihah and Y. Zhang, Biomaterials, 2008, 29, 937.
6. D. K. Chatterjee, L. S. Fong and Y. Zhang, Adv. Drug Delivery Rev., 2008, 60, 1627–1637.
7. L. Y. Wang, R. X. Yan, Z. Y. Huo, L. Wang, J. H. Zeng, J. Bao, X. Wang, Q. Peng and Y. D. Li, Angew. Chem., Int. Ed., 2005, 44, 6054.
8. F. Wang, W. B. Tan, Y. Zhang, X. P. Fan and M. Q. Wang, Nanotechnology, 2006, 17, R1.
9. C. A. Mirkin, R. L. Letsinger, R. C. Mucic and J. J. Storhoff, Nature, 1996, 382, 607.
10. J. L. Seifert, R. E. Connor, S. A. Kushon, M. Wang and A. Armitage, J. Am. Chem. Soc., 1999, 121, 2987.
11. W. C. W. Chan and S. M. Nie, Science., 1998, 281, 2016.
12. H. X. Mai, Y. W. Zhang, R. Si, Z. G. Yan, L. D. Sun, L. P. You and C. H. Yan, J. Am. Chem. Soc., 2006, 128, 6426.
13. O. Ehlert, R. Thomann, M. Darbandi and T. Nann, ACS Nano, 2008, 2, 120.
14. D. R. Gamelin and H. U. Gudel, Top. Curr. Chem., 2001, 214, 1.
15. H. X. Mai, Y. W. Zhang, L. D. Sun and C. H. Yan, J. Phys. Chem. C, 2007, 111, 13730.
16. H. X. Mai, Y. W. Zhang, L. D. Sun and C. H. Yan, J. Phys. Chem. C, 2007, 111, 13721.
17. G. Nagel, T. Szellas, W. Huhn, S. Kateriya, N. Adeishvili, P. Berthold, D. Ollig, P. Hegemann and E. Bamberg, Proc. Natl. Acad. Sci. U. S. A., 2003, 100, 13940.
18. L. Y. Wang and Y. D. Li, Chem. Mater., 2007, 19, 727.
19. L. F. Liang, H. Wu, H. L. Hu, M. M. Wu and Q. Su, J. Alloys Compd., 2004, 368, 94.
20. A. Aebischer, M. Hostettler, J. Hause, K. Kramer, T. Weber, H. U. Gudel and H. B. Buergi, Angew. Chem., Int. Ed., 2006, 45, 2802.
21. K. W. Kramer, D. Biner, G. Frei, H. U. Gudel, M. P. Hehlen and S. R. Luthi, Chem. Mater., 2004, 16, 1244.
22. S. Heer, K. Kompe, H. U. Gudel and M. Haase, Adv. Mater., 2004, 16, 2102.
23. J. C. Boyer, F. Vetrone, L. A. Cuccia and J. A. Capobianco, J. Am. Chem. Soc., 2006, 128, 7444.
24. Z. Q. Li and Y. Zhang, Angew. Chem., Int. Ed., 2006, 45, 7732.
25. H. Zhang, Y. Li, I. A. Ivanov, Y. Qu, Y. Huang and X. Duan, Angew. Chem., Int. Ed., 2010, 16, 2865.
26. H. Zhang, D. Xu, Y. Huang and X. Duan, Chem. Commun., 2011, 47, 979.
27. G. Wang, W. Qin, L. Wang, G. Wei, P. Zhu and R. Kim, Opt. Express, 2008, 16, 11907.
28. F. Wang and X. G. Liu, J. Am. Chem. Soc., 2008, 130, 5642.
29. F. Wang, Y. Han, C. S. Lim, Y. Lu, J. Wang, J. Xu, H. Chen, C. Zhang, M. Hong and X. Liu, Nature, 2010, 463, 1061.
30. A. Yin, Y. Zhang, L. Sun and C. Yan, Nanoscale, 2010, 2, 953.
31. W. Ryba-Romanowski, S. Golab, G. Dominiak-Dzik, M. Zelechower and J. Gabrys-Pisarska, J. Alloys Compd., 2001, 325, 215.

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