Controlled synthesis to produce upconversion materials with multicolour luminescence

Significant development has been done in rare earth (RE³⁺) ion-doped upconversion (UC) materials over the past few years, however one challenge remaining lies in the controlled synthesis of UC materials with tunable, wide-gamut, multicolour luminescence and high-throughput production. This thesis focuses on exploring the distribution of RE³⁺ ions, understanding the network of energy transfer systems within interior UC materials, and developing resource- and time-saving methods for fine-tuning UC materials with multicolour luminescence, high performance and wide colour gamuts. Chapter 1 summarizes the motivations for the thesis and background knowledge relevant to the development of multicolour luminescent UC materials, as well as the specific aims of this thesis: controlled synthesis of multicolour luminescent UC materials and distribution study of RE³⁺ ions within them. In Chapter 2, Rietveld refinement of X-ray powder diffraction (XRD) was employed to characterize the distribution of RE³⁺ ions in bulk UC materials. Different RE³⁺ ions produced distinct emission peaks. Therefore, multicolour luminescence of activators, such as Er³⁺, Tm³⁺ and Ho³⁺, was achieved in alkaline indium oxide UC materials. In Chapter 3, synchrotron-based X-ray photoelectron spectroscopy (XPS) measurements were used to investigate the depth-resolved distribution of RE³⁺ within fluoride upconversion nanoparticles (UCNPs). The author proposed a natural Gd³⁺-rich shell in Yb³⁺/Tm³⁺ doped NaGdF₄ UCNPs, which can effectively bridge the gap of energy transfer between sensitizers and activators to realized multicolour luminescence via cation exchange. Chapter 4 reports on a novel direct cation exchange method for UCNPs without removing surface ligands in organic solvent. It avoids the tedious pre-treatment of synthesized UCNPs, and the luminescent intensities using the new method are much stronger than those using conventional cation exchange in water. This facile and rapid cation exchange strategy opens a new path to the synthesis of multicolour-emitting nanoparticles expeditiously with high performance and high-throughput. Chapter 5 further applies the knowledge obtained from Chapter 4. We attempted to develop hybrid heterostructures of UCNPs and lead halide perovskite quantum dots (PQDs), and to produce and fine-tune multicolour luminescence. The cation-exchanged ions were expected to bridge these two kinds of nanomaterials. However, it remains a great challenge. Conclusions and perspectives are given in Chapter 6, which also summarizes the key achievements of the thesis. Controlled synthesis and fine-tuning of the spectral UC emission properties of UCNPs may open a path to more complex applications.