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Title: Design and fabrication of lanthanide-doped upconversion particles for optical applications
Authors: Wang, Ting
Degree: Ph.D.
Issue Date: 2019
Abstract: Upconversion (UC) emission via lanthanide (Ln3+) ions is a nonlinear optical process that can convert near-infrared (NIR) pump light to visible output. A wide emission wavelength from ultraviolet (UV) to infrared regime, which can be obtained from the Ln3+-doped UC materials, is a necessary condition to realize broadband-tunable lasers. However, the relatively low UC emission efficiency leads to the high excitation threshold of Ln3+-doped UC lasers. If the lasing threshold can be further reduced, the wide bandwidth emission characteristics of UC lasers may find enormous applications such as all-optical on-chip information processing, biomedical imaging, and optogenetics. Therefore, in this thesis, we propose using Ln3+-doped UC particles, including the micro- and nano-particles, to obtain multi-wavelength lasing emission based on improving UC emission efficiency. In chapter 3, Ln3+ ions (Yb3+, Er3+, and Tm3+) doped β-NaYF4 hexagonal microrods are proposed to support UC lasing emission under 980 nm ns-pulsed laser excitation. The single β-NaYF4 hexagonal microrod, which hexagonal shape supports whispering gallery modes (WGMs), obtains multi-wavelength lasing emission under 980 nm ns-pulsed laser excitation. Initially, UC lasing emission intensities at red/green/blue (RGB) areas are optimized by modulating the Ln3+ concentration and then further controlling the β-NaYF4 hexagonal microrods radius to decrease the lasing threshold. In this chapter, the RGB and white lasing emissions are successfully produced. In chapter 4, we further improve the single β-NaYF4 hexagonal microrods UC lasing intensity by introducing surface plasmonic effect. Here, Ag film is utilized to introduce the plasmonic effect and the experimental results show that the spontaneous UC emission intensity of the single β-NaYF4 hexagonal microrods can be increased by more than 10 times and the values of the UC lasing threshold can be decreased by 50%. Besides, we note that the UC improvement is due to the optical coupling between the WGMs and the surface plasmonic resonance modes. UC characteristics of Ln3+-doped nanoparticles on the lasing enhancement are investigated in chapter 5. Firstly, we find visually that the improvement of UC photoluminescence is due to the suppression of surface-related deactivations of the nanoparticles. This can be done by using aberration corrected high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) to directly observe the surface conditions of KLu2F7:Yb3+, Er3+ bare core UC nanoparticles in atomic scale. Due to the very thin and uniform thickness of the KLu2F7 nanoparticles, we observe unambiguously that the removal of surface defects by using the thermal annealing method can enhance UC photoluminescence. The realization of dominant green lasing emission under 980 nm ns-pulsed laser excitation further confirm the high crystallinity of the KLu2F7 nanoparticles. Although the use of thermal annealing can strongly suppress the surface defects of Ln3+-doped UC nanoparticles during their growth process, the corresponding mechanism of surface defects restoration process is still unknown. Understanding the detail growth process of the UC nanoparticles can guide us to improve the UC efficiency of Ln3+-doped nanoparticles. In chapter 6, we observe the growth process of Yb3+, Er3+ co-doped LuF3 nanoparticles by using in situ transmission electron microscopy (TEM) imaging technique to deduce the corresponding surface defects restoration mechanism. Here, we verify that the enhancement of LuF3 nanoparticles UC efficiency can be realized by crystallization of the surface clusters of the LuF3:Yb3+, Er3+ nanoparticles. This is because the surface clusters act as defect centers which decrease the UC efficiency of the nanoparticles. Hence, it is shown that the enhancement of UC intensity can be attributed to the removal of surface defects after using the thermal annealing method during the nanoparticles growth process.
Subjects: Hong Kong Polytechnic University -- Dissertations
Pages: xxv, 131 pages : color illustrations
Appears in Collections:Thesis

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