Synthesis of Tetragonal Phase LiYF4: Yb and Tm Microcrystals with Strong UV Upconversion Fluorescence

Abstract

In this work, we synthesized pure tetragonal phase LiYF₄ microcrystals by a hydrothermal method. In the reaction, sodium ions acted as an intermedium species. The long axis of the quasi-octahedron crystals was about 5 to 20 micrometers. Strong UV ¹I₆ → ³H₆ (~291 nm), ¹I₆ → ³F₄ (~346 nm), and ¹D₂ → ³H₆ (~362 nm) and blue emissions ¹D₂ → ³F₄ (~450 nm) and ¹G₄ → ³H₆ (~475 nm) could be observed in our samples. We could see blue luminescence from the microcrystals by our naked eyes in the daylight even if the pump power of the 980 nm laser diode was less than 10 mW. Obvious energy level splitting was also observed, which indicated that Tm³⁺ ions were embedded into the crystal lattice very well. The ¹G₄ energy level split into four subenergy levels owing to the effect of crystal field. The upconversion mechanisms were discussed in detail.

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1. Introduction

Lanthanide (Ln) ion-doped upconversion (UC) nano- and microcrystals can convert the near-infrared excitation into the visible or ultraviolet (UV) emissions. They have attracted much attention in recent years owing to their great fundamental interests and numerous applications in biological labeling and imaging, color display, and optical communications [1–3]. The UC process has been extensively studied in Ln³⁺-doped fluorides due to the lower energy phonons of their fluoride host compared with other oxides, which results in decreased nonradiative relaxation and increased efficiencies of the UC emissions [4]. The most widely studied host material is NaYF₄ because it exhibits intensive UC luminescence under 980 nm excitation when sensitizer ions are Yb³⁺ ions and luminescence center ions are Er³⁺, Tm³⁺, Ho³⁺, or other Ln³⁺. NaYF₄ can be synthesized in different sizes or different phases when using different methods or under different conditions according to the previously published work [5–7]. Recently, Mahalingam et al. reported the synthesis of colloidal tetragonal phase LiYF₄ nanocrystals codoped with Yb³⁺ and Tm³⁺ by a thermal-decomposition method and this kind of UC material exhibits low luminescence threshold [8]. This improved luminescent property might promote the practical applications of UC materials. We considered that LiYF₄ might be a very good host material for its low UC luminescence threshold and it is worth making further research on this material. However, the thermal-decomposition method needs high reaction temperature and precise condition control to synthesize well-crystallized LiYF₄. In this letter, we successfully synthesized pure tetragonal phase LiYF₄ microcrystals by a completely different hydrothermal method. The reaction is safe, moderate, and simple. We did a series of experiments to investigate the reaction mechanisms. Our samples also had a low UC threshold; strong UV and blue emissions could be observed. It is interesting that obvious energy level splitting could be observed in the UC spectra. We also studied its UC mechanisms, which were discussed in detail.

2. Experimental

2.1. Preparation of LiYF₄: Yb³⁺ and Tm³⁺ Microcrystals

In a typical preparation, 0.815 mmol Y (NO₃)₃, 0.18 mmol Yb (NO₃)₃, 0.005 mmol Tm (NO₃)₃, 1 mmol Ethylene Diamine Tetraacetic Acid (EDTA), and 15 ml deionized water were put into plastic beakers (LiF is a covalent compound and hydrolytic, when it dissolves in water the hydrofluoric acid will corrode the glass if we use glass beakers), mixed together, and stirred for 30 minutes. Then fluorine source was added into beakers according to the molar ratio and stirred for 2 hours. The mixture was transferred into autoclaves and heated at 180°C for 16 hours. After the reaction, the autoclaves were cooled at room temperature. Then the microcrystals were collected by centrifugation and washed several times with deionized water. Finally, the microcrystals were dried at 80°C for 8 hours [5–11].

2.2. Characterizations

The X-ray diffraction pattern of our samples was taken by a Rigaku RU-200b X-ray powder diffractometer (XRD) equipped with a nickel-filtered Cu Ka radiator ($\lambda =1.4518$ Å). The size and morphology were characterized by scan electron microscope (Hitachi TM-1000). The UC luminescence spectra were recorded with a Hitachi F-4500 fluorescence spectrophotometer (1.0 nm for spectral resolution (FWHM) and 400 V for PMT voltage) under a 980 nm laser diode excitation. All the measurements were taken at room temperature.

3. Results and Discussion

We synthesized pure tetragonal phase LiYF₄ microcrystals using 1 : 1 molar ratio of LiF and NaF as fluorine source by the hydrothermal method at 180°C for 16 hours. Figure 1(b) shows the measured X-ray diffraction pattern, which matches well with the literature values (JCPDS no. 77-816) (Figure 1(a)). The samples can be indexed to tetragonal phase LiYF₄. If LiF is used as a fluorine source, the product is hard to purify. In this reaction, Na⁺ ions acted as the intermedium. We studied the function of Na⁺ ions in this reaction. So we designed a series of experiments: the total F⁻ ions should be the same and changed the molar ratio of Na⁺ ions and Li⁺ ions; they were 6 : 0, 4 : 2, 3 : 3, and 2 : 4. We named the sample with the molar ratio (Na⁺ : Li⁺) of 6 : 0 as sample 1, that with the molar ratio of 4 : 2 as sample 2, and so on. All the measurements were taken under the same conditions, as shown in Figure 2. Figure 2(a)–2(c) shows XRD patterns of literature data for standard NaYF₄ crystals, LiYF₄ crystals, and LiF crystals, respectively. Figure 2(d) shows the X-ray diffraction pattern of sample 1, indicating that it was cubic phase NaYF₄. The diffraction peaks of sample 2 can be indexed to cubic phase NaYF₄ and tetragonal phase LiYF₄ (Figure 2(e)). The diffraction peaks of sample 3 can be indexed to the pure tetragonal phase LiYF₄ (Figure 2(f)). The diffraction peaks of sample 4 can be matched well with LiYF₄ and LiF, no diffraction peaks of NaYF₄ can be observed (Figure 2(g)). These experimental results indicate: firstly, we cannot synthesize pure tetragonal phase LiYF₄ microcrystals by only using LiF as fluorine source in the system, it can be proved by sample 5; secondly, Na⁺ ions acted as intermedium, when the amount of Na⁺ ions is larger than that of Li⁺ ions, the product is a mixture of LiYF₄ and NaYF₄. When the amount of Na⁺ ions is equal to that of Li⁺ ions, the product is pure tetragonal phase LiYF₄. When the amount of Na⁺ ions is less than that of Li⁺ ions, the product is a mixture of LiYF₄ and LiF. By increasing the molar ratio of Li⁺ ions, more Na⁺ ions will be replaced by Li⁺ ions. The morphology of this crystal is very special, which is different from the previously published work by Mahalingam et al. [8]. Figure 1(c) shows the SEM of corresponding sample. We can see that it is quasi-octahedron, the edges of the long axis grow into faces. The long axis of the crystal is about 5 to 20 micrometers. By changing the reaction time, the morphology did not change, the size changed, as shown in Figure 3.

[figure omitted; refer to PDF] [figure omitted; refer to PDF]

[figures omitted; refer to PDF]

Figure 4 shows the upconversion emission spectrum of LiYF₄: Yb³⁺ and Tm³⁺ microcrystals under the excitation of a 980 nm laser diode. Strong UV and blue emission have been observed in our samples. We could see blue light by our bare eyes in the daylight when the pump power of the 980 nm laser diode was less than 10 mW. Obvious energy level splitting was also observed owing to the effect of crystal field, which indicated that Tm³⁺ ions were embedded into the crystal lattice very well. These emissions come from the following transitions of Tm³⁺ ions: ¹I₆ → ³H₆ (~291 nm), ¹I₆ → ³F₄ (~346 nm), ¹D₂ → ³H₆ (~362 nm), ¹D₂ → ³F₄ (~450 nm), ¹G₄ → ³H₆ (~475 nm), ¹D₂ → ³H₅ (~510 nm), ¹D₂ → ³H₄ (~575 nm), ¹G₄ → ³F₄ (~645 nm), ³F₃ → ³H₆ (~689 nm), and ³H₄ → ³H₆ (~795 nm). The Yb³⁺ ions have the highest absorption coefficient at 980 nm among all Ln³⁺ ions, which is often used as a sensitizer [12, 13]. Figure 5 shows energy transfer process in LiYF₄: Yb³⁺ and Tm³⁺ systems under excitation of a 980 nm laser diode. The Yb³⁺ ions are excited by the pump light and successively transfer to Tm³⁺ to populate ³H₅,(³F₃,³F₂), and ¹G₄. The cross relaxation process of ³F₂ → ³H₆ (Tm³⁺): ³H₄ → ¹D₂ (Tm³⁺) may alternatively play the most important role in populating ¹D₂. On the one hand, Yb³⁺ ions continuously absorb 980 nm photons and transfer the energy to the ¹D₂ levels of Er³⁺ to populate ³P₂ levels of Er³⁺, followed by relax to ¹I₆ levels of Er³⁺ by ³P₂ → ³P₁ → ³P₀ → ¹I₆ nonradiative relaxation process [14, 15]. Then the ¹I₆ → ³H₆ and ¹I₆ → ³F₄ transition gives the UV upconversion emission at 291 and 346 nm. On the other hand, the ¹D₂ level of Er³⁺ radiatively relaxes to the ground state and interstates, which causes 362, 450, 510, and 575 nm emissions. Generally speaking, the population of the states ¹I₆, ¹D₂, ¹G₄, and ³H₄ is five-photon, four-photon, three-photon, and two-photon process, respectively [5, 16].

[figure omitted; refer to PDF] [figure omitted; refer to PDF]

4. Conclusion

We successfully synthesized pure tetragonal phase LiYF₄ microcrystal using 1 : 1 molar ratio of LiF and NaF as fluorine source by hydrothermal method at 180°C. We could not synthesize pure tetragonal phase LiYF₄ microcrystals by only using LiF as fluorine source in this hydrothermal system, Na⁺ ions acted as intermedium. By increasing the molar ratio of Li⁺ ions, more Na⁺ ions would be replaced by Li⁺ ions. By changing the reaction time, the morphology did not change, only the size changed. Strong UV and blue emission could be observed in our samples. Obvious energy level splitting was also observed, which indicated that Tm³⁺ ions were embedded into the crystal lattice very well, owing to the effect of crystal field.

Conflicts of Interest

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This work was supported by (1) the Science and technology development plan project of Jilin Province of China (20180520199JH, 20180520191JH), (2) the Science and technology project of the Education Department of Jilin Province (no. JJKH20180762KJ), (3) the National Natural Science Foundation of China (no. 21701047), and (4) the State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (Grant no. RERU2017013).

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