• Acta Physica Sinica
  • Vol. 69, Issue 3, 034207-1 (2020)
Wei Gao, Bo-Yang Wang, Ze-Yu Sun, Lu Gao, Chen-Xue Zhang, Qing-Yan Han, and Jun Dong*
DOI: 10.7498/aps.69.20191333 Cite this Article
Wei Gao, Bo-Yang Wang, Ze-Yu Sun, Lu Gao, Chen-Xue Zhang, Qing-Yan Han, Jun Dong. Tuning upconversion emissions of Ho3+ through changing excitation conditions [J]. Acta Physica Sinica, 2020, 69(3): 034207-1 Copy Citation Text show less

Abstract

The upconversion (UC) emission properties of rare-earth ions are not only dependent on the host materials, but also relate to the excitation conditions. In this work, taking the Ho3+ ions for example, upconversion emission properties are studied in two NaYF4 and LiYF4 fluoride microcrystals through changing excitation conditions, namely the excitation power and the sample environment. The NaYF4:20%Yb3+/2%Ho3+ and NaYF4:20%Yb3+/2%Ho3+ microcrystal are synthesized by the hydrothermal method. The typical X-ray diffraction patterns of NaYF4:20%Yb3+/2%Ho3+ and LiYF4:20%Yb3+/2%Ho3+ microcrystal indicate that the prepared samples possess pure hexagonal phase NaYF4 structure and the pure tetragonal phase LiYF4 structure with high crystallinity, respectively. Most of NaYF4:20%Yb3+/2%Ho3+ microcrystals show uniform and regular rod shape with diameter and length of approximately 3 μm and 10 μm, respectively. Few rods with a length of approximately 5 μm are also observed. The LiYF4:20%Yb3+/2%Ho3+ microcrystals are all octahedral in shape with a smooth surface, the average size is around 10 μm. The spectral peculiarities of Ho3+ are investigated by using confocal microscopy equipment under near infrared 980 nm excitation. Beautiful patterns with different upconversion emissions of Ho3+ are discovered in single NaYF4 and LiYF4 microcrystal. As the excitation power increases, the upconversion emission of Ho3+ turns from green to pink in single NaYF4 microrods due to the cross-relaxation between Ho3+ and the energy back transfer from Ho3+ to Yb3+. However, in single LiYF4:Ho3+ microcrystal no similar phenomenon is observed. Nevertheless, when the powder of NaYF4 and LiYF4 microcrystals are excited by a 980 nm laser, increasing the power can turn the output colours of Ho3+ all green. Because particles outside the laser radiation are not directly covered by the laser, most of them are excited by the scattered light from the laser, and the actual excitation energy is low compared with at the center position. This result can be proved in the single NaYF4 and LiYF4 microcrystal under low excitation power. Thus, the results indicate that UC emission of rare-earth ions is controlled by changing the excitation condition. Using the new testing methods we can not only observe more interesting spectral phenomena, but also find a new way to further study its luminescence mechanism.
Wei Gao, Bo-Yang Wang, Ze-Yu Sun, Lu Gao, Chen-Xue Zhang, Qing-Yan Han, Jun Dong. Tuning upconversion emissions of Ho3+ through changing excitation conditions [J]. Acta Physica Sinica, 2020, 69(3): 034207-1
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