Enhanced upconversion luminescence and tuned red-to-green emission ratio of LiGdF₄ nanocrystals via Ca²⁺ doping

Abstract

LiGdF₄ nanoparticles (NPs) with different Ca²⁺ contents were synthesized by a thermal decomposition method. The as-prepared NPs were able to emit green and strong red light when excited by a 980 nm laser through a two-photon process and they were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and photoluminescence (PL) spectroscopy. Rietveld refinement was performed to explore the structure and phase composition of the product. It was found that Ca²⁺ ions were vital to the successful synthesis of LiGdF₄ NPs and an appropriate Ca²⁺ content (ca. 19%) yielded the product with highest purity. The purity of the obtained product was highly correlated with the PL intensity and red/green light intensity ratio.

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Introduction

In recent years, lanthanide (Ln)-doped upconversion nano-particles (UCNPs) have attracted considerable attention due to their wide applications in electronic devices, optical devices, biomedical devices, and solar cells.^1–6 Compared with traditional fluorescence imaging agents, namely quantum dots and organic dyes, Ln-doped UCNPs possess unique advantages including improved tissue penetration depth, low radiation damage, and weak auto-fluorescence.⁷ The novel performances of UCNPs come from their ability to absorb near-infrared (NIR) irradiation and to emit ultraviolet or visible light.^8–14

However, human tissues strongly absorb the light whose wavelength is below 600 nm, a spectral range contains the light emitted by most UCNPs. Such absorption therefore limits the application UCNPs as in vivo imaging luminescent probes.^15–17 One way to make an improvement is to enlarge the wavelength of the emission from UCNPs since human tissues have weaker adsorption in terms of the light whose wavelength falls into the NIR region (700–1100 nm) and red region (600–700 nm).^18,19 Such spectral range were referred as “optical window” in many publications and tuning both the excitation and emission peaks of the UCNPs into the “optical window” is important for deep tissue imaging in vivo.^20,21

On the other hand, among the various types of UCNPs, tetragonal LiGdF₄ is an outstanding host for its high visible quantum efficiency.²² However, the syntheses of LiGdF₄ via wet chemical methods was difficult^23–26 and an orthorhombic GdF₃ always formed as impurity prior to LiGdF₄.²⁷ In order to synthesis pure LiGdF4 nano crystal, doping was applied. For instance, Ho Seong Jang et al. successfully synthesized tetragonal phase Li(Gd,Y)F₄:Yb,Er nanocrystals by doping Y³⁺ ions.²⁶ The obtained UCNPs based on LiGdF₄ showed stronger green up-converting (UC) luminescence in comparison with β-NaYF₄ host. In addition, doping is not only a convenient method to alter the phase of the product, but also an important method to adjust the UC luminescence of UCNPs. The impact of Ca²⁺, Sr²⁺ ions introduced to UCNPs had been studied and it was found these ions were capable of breaking the symmetry of the crystal field around the rare-earth ions, leading to the enhancement of PL intensity.^28–31

Herein, UCNPs with strong red UC luminescence were synthesized to meet the criteria for tissue imaging. Specifically, LiGdF₄ UCNPs doped with Ca²⁺, Yb³⁺, Er³⁺, and Tm³⁺ ions were obtained by thermal decomposition method. The as-prepared NPs were characterized by X-ray diffractometer (XRD), high-resolution transmission electron microscopy (HRTEM), and photoluminescence (PL) spectrometer. Rietveld refinement was performed to explore the structure and phase fraction of the product. The obtained product was a mixture of LiGdF₄, orthorhombic GdF₃ and fluorite-type Gd_0.67F₂. The last two phases were impurities. They significantly decreased the UC emission intensity and the R/G emission ratio of LiGdF₄ UCNPs. It was found that Ca²⁺ ions stabilized the crystal structure of LiGdF₄ and a proper proportion of Ca²⁺ ions significantly decreased the amount of the impurity, therefore enhanced the PL performance of the product.

Experimental

Materials

Gadolinium(III) oxide (Gd₂O₃, 99%), ytterbium(III) oxide (Yb₂O₃, 99%), erbium(III) oxide (Er₂O₃, 99%), thulium(III) oxide (Tm₂O₃, 99%), 1-octadecene (ODE, 90%), oleic acid (OA, 90%), and trifuoroacetic acid (CF₃COOH, 99.0%) were purchases from Aladdin Incorporation. Lithium hydroxide (LiOH, 90%), calcium hydroxide (Ca(OH)₂, 95%), cyclohexane (C₆H₁₂, 97%) were purchases from Sinopharm Chemical Reagent Incorporation. Silicon (SRM™ 640e) was obtained from National Institute of Standards and Technology (NIST). All chemicals were of analytical grade and were used without further purification. Lanthanide trifluoroacetates ((CF₃COO)₃Ln) were prepared by dissolving the respective lanthanide oxides in trifuoroacetic acid (CF₃COOH) into a 50 mL three-neck flask on a heating mantle. Lithium trifluoroacetate (CF₃COOLi), calcium trifluoro-acetate ((CF₃COO)₂Ca) were prepared by dissolving the respective lithium hydroxide and calcium hydroxide in tri-fluoroacetic acid (CF₃COOH) into a 50 mL three-neck flask on a heating mantle.

Synthesis of Ca²⁺-doped LiGdF₄:20Yb³⁺/4Er³⁺/2Tm³⁺ NPs

CF₃COOLi (1 mmol), (CF₃COO)₃Gd (0.72 mmol), (CF₃COO)₃Yb (0.19 mmol), (CF₃COO)₃Tm (0.02 mmol), (CF₃COO)₃Er (0.04 mmol) and (CF₃COO)₂Ca (x = 0, 3.8, 11.4, 19.0, 26.6, 34.2, 45.6, 57.0) were added to a 50 mL flask containing OA (6 mL) and ODE (4 mL). Another 8 mL of OA was prepared in another flask. Both flasks were heated at 150 °C for 30 min under vacuum to remove water. The OA was then heated to a certain temperature 320 °C at a rate of 20 K min⁻¹ under dry nitrogen atmosphere. The rare-earth trifluoroacetate solution at 150 °C was then injected into the OA solution after 5 minutes. After the injection, the mixture was heated to 330 °C under nitrogen atmosphere for 1 h. The solution was then cooled down to room temperature and cyclohexane was added to precipitated the NPs. The precipitate was collected by centrifugation (10 000 rpm, 5 min) and washed alternately with cyclohexane and ethanol. The obtained NPs were then dried in air at 60 °C for 4 h.

Characterization

The X-ray powder diffraction patterns were measured by a high resolution diffractometer equipped with CuKα radiation (Bruker Discover D8, λ = 1.5418 Å). Quantitative phase analysis of the XRD result was performed based on Rietveld refinement.³² The Rietveld refinement was performed with the assistance of GSASII (latest build).³³ The Silicon (SRM™ 640e) was used to obtain the instrumental broadening profile. The morphology and microstructure of as-synthesized UCNPs were observed using high resolution transmission electron microscope (HRTEM, JEM-2100F STEM/EDS, JEOL Corp, Japan). The UC emission spectra were recorded using a fluorescence spectrometer (970CRT, Shanghai Sanco, China) equipped with an adjustable laser diode (980 nm, Hi-Tech Optoelectronics Co., Ltd.) as the excitation sources.

Results and discussion

Fig. 1 shows the X-ray diffraction (XRD) patterns of the obtained LiGdF₄:xCa²⁺/20Yb³⁺/4Er³⁺/2Tm³⁺ UCNPs (S0, x = 0; S1, x = 11.4; S2, x = 19.0; S3, x = 26.6; S4, x = 34.2; S5, x = 57.0). The diffraction peaks of S0 was weak, therefore indicates that well crystallized tetragonal LiGdF₄ (JCPDS card no. 27-1236) could only be obtained with the assistance of Ca²⁺. It is also noticeable that a small peak appeared at about 28° for sample S1–S5, and such peak was considered belonging to the impurity phase. In the XRD patterns of sample S0 and S1, a tiny peak was observed at about 25° and it was also regarded as impurity.

Fig. 1 The XRD patterns of the as obtained UCNPs. The molar fraction of Ca²⁺ in sample S0, S1, S2, S3, S4, and S5 were 0%, 11.4%, 19.0%, 26.6%, 34.2%, and 57.0%, respectively. The arrows in the figure indicates the presence of the impurity phase.

The result of the quantitative phase analysis of the UCNPs was shown in Fig. 2. The impurity phases were identified to be an orthorhombic GdF₃, which was easy to form according previous study,²⁷ and fluorite-type Gd_0.67F₂ in which all original Ca atoms were replaced by Gd atoms.³⁴ More details on the Rietveld refinement and the corresponding fitting results were listed in the ESI (Fig. S1–S6, Tables S1–S6†). It is clear that all samples had impurities and sample S2 had the largest percentage of LiGdF₄. Specifically, sample S0 had by far the least amount of LiGdF₄ and this suggests that doping Ca²⁺ could not only increase the crystallinity of the product, but also increase the weight percentage of LiGdF₄.

Fig. 2 The result of the quantitative phase analysis based on Rietveld refinement. The red bar indicates the phase fraction of LiGdF₄. The blue bar and yellow bar indicate the phase fraction of fluorite-type Gd_0.67F₂ and orthorhombic GdF₃, respectively.

Sample S2, whose Ca²⁺ fraction was 19%, had the minimum amount of impurity phase, containing only 7.7 wt% Gd_0.67F₂. Sample S1 and S3 had similar phase compositions and there was GdF₃ in these two samples. The phase fractions of Gd_0.67F₂ in sample S4, and S5 were significantly higher than that of sample S1 to S3. This might relate to the excessive Ca²⁺ ions added into the reaction system.

Fig. 3 shows the TEM images of sample S2. As shown in Fig. 3a, LiGdF₄ NPs with uniform octahedron shapes were observed and their average size measured from the image was ca. 150 nm. Small amount of fluorite-type Gd_0.67F₂ NPs were also observed with much smaller sizes. The HRTEM image of the LiGdF₄ was shown in Fig. 3b and the fringes corresponding to (101) plans of LiGdF₄ were observed. Fig. 3c illustrates the HRTEM image of Gd_0.67F₂ in sample S2 and the insert shows the (111) planes of Gd_0.67F₂. Sample S1, S3, S4, S5 had similar morphologies as shown in the ESI (Fig. S7†).

Fig. 3 TEM and HRTEM images of sample S2. (a) TEM image of sample S2; (b) HRTEM of sample S2, the fringes could be ascribed to the (101) planes of LiGdF₄; (c) HRTEM image of sample S2. The insert illustrates part of this image whose region is outlined with a white square. The fringes could be ascribed to the (111) planes of fluorite-type Gd_0.67F₂.

To study the influence of Ca²⁺ concentration on UC emission property, sample S1–S5 were characterized by a PL spectrometer under the excitation of 980 nm laser diode. Fig. 4a shows the emission spectra of sample S1 to S5. It is clear that the spectra of these UCNPs had similar shapes but different intensities. The order of the samples according to the emission intensity was S2 > S3 > S1 > S4 > S5, being roughly identical with the order of the phase fraction of LiGdF₄. Fig. 4b illustrates the deconvolution of the PL spectrum of sample S2. The spectrum was fitted with Gaussian functions. The non-linear fit was performed with the assistance of software Fityk (1.3.0).³⁵ Six peaks appeared in the spectrum, including two main red peaks (663 and 677 nm, ⁴F_9/2 → ²I_15/2 of Er³⁺), two shoulder red peaks (692 and 711 nm, ³F₃ → ³H₆ of Tm³⁺) and two green peaks (535 nm, ²H_11/2 → ²I_15/2 of Er³⁺; 558 nm, ⁴S_3/2 → ²I_15/2 of Er³⁺).^17,26,27,36

Fig. 4 Photoluminescence spectra of sample S1–S5 under the exciting of a 980 nm laser diode. (a) The emission spectra of sample S1–S5; (b) deconvolution of the emission spectrum of sample S2, the labels in the graph indicate the positions of the corresponding Gaussian peaks.

To further investigate effect of purity of the UCNPs on the UC emitting intensity, the correlation of PL emission intensity and the weight percentage of LiGdF₄ of different samples were studied. Fig. 5 illustrates the relationships between intensity of PL emission peaks and the weight percentage of LiGdF₄ from sample S1 to S5. The intensities were obtained by integrating corresponding peaks in the PL spectra and the weight percentages of LiGdF₄ were obtained from the Rietveld refinement (Fig. 2). It is clear that all of the PL features, namely the intensity of red peaks (600 to 750 nm), the intensity of green peaks (500 to 600 nm) and the intensity ratio between red and green peaks, had linear relationships with the weight percentage of LiGdF₄. This indicates that higher purity of LiGdF₄ UCNPs yields higher UC luminescence intensity and a larger red component in the emitted light.

Fig. 5 Relationships between intensities of PL emission peaks and the weight percentage of LiGdF₄ from sample S1 to S5. (a) Relationship between the integration intensity of red emission peaks (650 to 750 nm) and the LiGdF₄ weight percentage; (b) relationship between the integration intensity of green emission peaks (500 to 600 nm) and the LiGdF₄ weight percentage; (c) relationship between the red/green intensity ratio and the LiGdF₄ weight percentage. The five scatters came from the five samples (S1 to S5), with the sample name labels nearby. The fitting parameters and equations are available in the ESI (Table S7†).

In order to determine the number of photons responsible for the UC emission, the luminescence intensities were measured as a function of the pump power for sample S2. For the unsaturated UC process, the number of photons required to populate the upper emitting state can be described by the following relation:^37,38

I_up ∝ I_NIRⁿ

where I_up is the fluorescent intensity, I_NIR is the pump laser intensity, and n is the number of pump photons required. Fig. 6 shows the pump power dependence of the UC emissions in sample S2. Fig. 6a illustrates the PL spectra of the UCNPs excited by 980 nm laser with different power. Fig. 6b shows the log–log plot of the UC emission intensity (red, 650–750 nm; green, 500–600 nm) versus pumping power. The slopes, which reveals the n values, were 1.83 and 2.12 for red and green emissions, respectively. These results indicated that the red and green emissions were two-photon processes in this sample. The values of n were both varied from the required photon numbers (n = 2) to populate the corresponding levels, which can be attributed to saturation effects as well as the local thermal effects induced by the laser exposure.³⁵

Fig. 6 Pump power dependence of the UC emissions in sample S2. (a) The spectra of sample S2 excited by 980 nm laser with different power; (b) the log–log plots of UC emission intensity versus pumping density. The fitting parameters and equations are available in the ESI (Table S7†).

Conclusions

LiGdF₄ NPs doped with Ca²⁺, Yb³⁺, Er³⁺ and Tm³⁺ were synthesized by thermal decomposition method. The obtained NPs were able to emit green and strong red light when excited by 980 nm laser through a two-photon process. It was found that Ca²⁺ ions were required to synthesis the LiGdF₄ NPs and a proper amount of Ca²⁺ (ca. 19%) yielded the product with highest purity. The purity of the obtained product was highly correlated with both the PL intensity and the red/green light intensity ratio. The strong red UC emission of the obtained NPs implies they could be applied for tissue imaging.

Acknowledgements

This work was financially supported by National College Students' innovation and entrepreneurship training of China (20151049701024).

Notes and references

1. J.-C. Boyer, L. A. Cuccia and J. A. Capobianco, Nano Lett., 2007, 7, 847–852.
2. D. K. Chatterjee, A. J. Rufaihah and Y. Zhang, Biomaterials, 2008, 29, 937–943.
3. G. Chen, J. Shen, T. Y. Ohulchanskyy, N. J. Patel, A. Kutikov, Z. Li, J. Song, R. K. Pandey, H. Ågren, P. N. Prasad and G. Han, ACS Nano, 2012, 6, 8280–8287.
4. G. Chen, H. Qiu, P. N. Prasad and X. Chen, Chem. Rev., 2014, 114, 5161–5214.
5. W. Zheng, P. Huang, D. Tu, E. Ma, H. Zhu and X. Chen, Chem. Soc. Rev., 2015, 44, 1379–1415.
6. M. He, X. Pang, X. Liu, B. Jiang, Y. He, H. Snaith and Z. Lin, Angew. Chem., Int. Ed. Engl., 2016, 55, 4280–4284.
7. L. Cheng, C. Wang and Z. Liu, Nanoscale, 2013, 5, 23–37.
8. G. Dantelle, M. Mortier, G. Patriarche and D. Vivien, J. Solid State Chem., 2006, 179, 1995–2003.
9. N.-N. Dong, M. Pedroni, F. Piccinelli, G. Conti, A. Sbarbati, J. E. Ramírez-Hernández, L. M. Maestro, M. C. Iglesias-de la Cruz, F. Sanz-Rodriguez, A. Juarranz, F. Chen, F. Vetrone, J. a. Capobianco, J. G. Solé, M. Bettinelli, D. Jaque and A. Speghini, ACS Nano, 2011, 5, 8665–8671.
10. Y.-P. Du, Y.-W. Zhang, L.-D. Sun and C.-H. Yan, Dalton Trans., 2009, 8574–8581.
11. Q. Huang, J. Yu, E. Ma and K. Lin, J. Phys. Chem. C, 2010, 114, 4719–4724.
12. K. Konig, J. Microsc., 2000, 200, 83–104.
13. L. Lei, D. Chen, P. Huang, J. Xu, R. Zhang and Y. Wang, Nanoscale, 2013, 5, 11298–11305.
14. L. Lei, D. Chen, J. Xu, R. Zhang and Y. Wang, Chem.–Asian J., 2014, 9, 728–733.
15. C. Li, J. Liu, S. Alonso, F. Li and Y. Zhang, Nanoscale, 2012, 4, 6065–6071.
16. M. Misiak, A. Bednarkiewicz and W. Stręk, J. Lumin., 2016, 169 Part B, 717–721.
17. D. Yang, G. Li, X. Kang, Z. Cheng, P. Ma, C. Peng, H. Lian, C. Li and J. Lin, Nanoscale, 2012, 4, 3450–3459.
18. N. Niu, P. Yang, F. He, X. Zhang, S. Gai, C. Li and J. Lin, J. Mater. Chem., 2012, 22, 10889.
19. G. Tian, Z. Gu, L. Zhou, W. Yin, X. Liu, L. Yan, S. Jin, W. Ren, G. Xing, S. Li and Y. Zhao, Adv. Mater., 2012, 24, 1226–1231.
20. F. Vetrone, J. C. Boyer, J. A. Capobianco, A. Speghini and M. Bettinelli, J. Appl. Phys., 2004, 96, 661.
21. C. Wang, L. Cheng and Z. Liu, Biomaterials, 2011, 32, 1110–1120.
22. F. Wang and X. Liu, Chem. Soc. Rev., 2009, 38, 976.
23. F. Wang and X. Liu, J. Am. Chem. Soc., 2008, 130, 5642–5643.
24. J. Wang, F. Wang, C. Wang, Z. Liu and X. Liu, Angew. Chem., Int. Ed., 2011, 50, 10369–10372.
25. R. T. Wegh, H. Donker, K. D. Oskam and A. Meijerink, Science, 1999, 283, 663–666.
26. H. Na, J. S. Jeong, H. J. Chang, H. Y. Kim, K. Woo, K. Lim, K. A. Mkhoyan and H. S. Jang, Nanoscale, 2014, 6, 7461–7468.
27. M. Yang, Y. Sui, S. Wang, X. Wang, Y. Sheng, Z. Zhang, T. Lü and W. Liu, Chem. Phys. Lett., 2010, 492, 40–43.
28. W. Yin, L. Zhao, L. Zhou, Z. Gu, X. Liu, G. Tian, S. Jin, L. Yan, W. Ren, G. Xing and Y. Zhao, Chemistry, 2012, 18, 9239–9245.
29. X.-F. Yu, L.-D. Chen, M. Li, M.-Y. Xie, L. Zhou, Y. Li and Q.-Q. Wang, Adv. Mater., 2008, 20, 4118–4123.
30. J.-H. Zeng, J. Su, Z. Li, R.-X. Yan and Y.-D. Li, Adv. Mater., 2005, 17, 2119–2123.
31. S. Zeng, G. Ren, C. Xu and Q. Yang, CrystEngComm, 2011, 13, 4276–4281.
32. D. L. Bish and S. a. Howard, J. Appl. Crystallogr., 1988, 21, 86–91.
33. B. H. Toby and R. B. Von Dreele, J. Appl. Crystallogr., 2013, 46, 544–549.
34. D. Steele, P. E. Childs and B. E. F. Fender, J. Phys. C: Solid State Phys., 1972, 5, 2677.
35. S. Zeng, G. Ren and Q. Yang, J. Mater. Chem., 2010, 20, 2152.
36. M. Wojdyr, J. Appl. Crystallogr., 2010, 43, 1126–1128.
37. D. Zhou, P. Zhou, D. Liu, W. Xu, Y. Zhu, S. Xu, Q. Dai and H. Song, Opt. Lett., 2014, 39, 4619–4622.
38. J. Zhou, Z. Liu and F. Li, Chem. Soc. Rev., 2012, 41, 1323–1349.

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Footnotes

† Electronic supplementary information (ESI) available: Rietveld refinement results and some TEM images of the product. See DOI: 10.1039/c6ra13441f

‡ These authors contribute equally to this work.

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