Tuning upconversion emissions of Ho³⁺ through changing excitation conditions

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 Ho 3+ ions for example, upconversion emission properties are studied in two NaYF 4 and LiYF 4 fluoride microcrystals through changing excitation conditions, namely the excitation power and the sample environment. The NaYF 4 :20%Yb 3+ /2%Ho 3+ and NaYF 4 :20%Yb 3+ /2%Ho 3+ microcrystal are synthesized by the hydrothermal method. The typical X-ray diffraction patterns of NaYF 4 :20%Yb 3+ /2%Ho 3+ and LiYF 4 :20%Yb 3+ /2%Ho 3+ microcrystal indicate that the prepared samples possess pure hexagonal phase NaYF 4 structure and the pure tetragonal phase LiYF 4 structure with high crystallinity, respectively. Most of NaYF 4 :20%Yb 3+ /2%Ho 3+ 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 LiYF 4 :20%Yb 3+ /2%Ho 3+ microcrystals are all octahedral in shape with a smooth surface, the average size is around 10 μm. The spectral peculiarities of Ho 3+ are investigated by using confocal microscopy equipment under near infrared 980 nm excitation. Beautiful patterns with different upconversion emissions of Ho 3+ are discovered in single NaYF 4 and LiYF 4 microcrystal. As the excitation power increases, the upconversion emission of Ho 3+ turns from green to pink in single NaYF 4 microrods due to the cross-relaxation between Ho 3+ and the energy back transfer from Ho 3+ to Yb 3+ . However, in single LiYF 4 :Ho 3+ microcrystal no similar phenomenon is observed. Nevertheless, when the powder of NaYF 4 and LiYF 4 microcrystals are excited by a 980 nm laser, increasing the power can turn the output colours of Ho 3+ 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 NaYF 4 and LiYF 4 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.