Synthesis of rare-earth oxide/titanate for upconversion luminescence with an ultra-high power efficiency

• Main Author: Wu, Mingda
• Other Authors: Dong Zhili
• Format: Theses and Dissertations
• Language: English
• Published: 2019
• Subjects: DRNTU::Engineering::Materials::Nanostructured materials; DRNTU::Engineering::Materials::Functional materials
• Online Access: https://hdl.handle.net/10356/89870; http://hdl.handle.net/10220/47727
• Institution: Nanyang Technological University

In the last decade, lanthanide-doped upconversion(UC) materials have become a shining star and been focused by a growing number of researchers all over the world, due to its unique and fascinating NIR-to-visible light conversion. Effective upconversion materials (e.g. NaYF4:Yb3+,Er3+ and NaYF4:Yb3+,Tm3+) have been demonstrated to be promising in a wide range of bio-medical and luminescent applications. However, such lanthanide-doped fluorides are found structurally unstable in ambient conditions and sensitive to aqueous solutions, leaving shadows in the further development and promotion of upconversion luminescence. Compared with fluorides, their oxide counterparts possess higher binding energies, more stable structures, and better tolerance to aqueous solutions, while being limited by difficulties in synthesizing nanocrystals and achieving effect upconversion luminescence. Hence, it’s of great promising to synthesize oxides-based upconversion materials in nanoscale and with high upconversion efficiency. In the first part, a systematic study in both controlled synthesis and enhanced luminescence of lanthanide-doped gadolinium oxides was performed. To start with, uniform and size-tunable gadolinium hydroxide rods were prepared through surfactant-free hydrothermal synthesis and could further transform into corresponding oxides through calcination treatment. The upconversion luminescence of as-prepared lanthanide-doped gadolinium oxide (Gd2O3:Ln3+) was substantially enhanced through a thermal treatment, being accompanied with the improvement of crystallinity. As a sequence, Gd2O3:Yb3+,Er3+ (10/1 mol%) nanorods with an upconversion efficiency comparable to fluoride counterparts were successfully obtained. To achieve a better understanding of energy transfer and upconversion luminescence, an investigation of host matrix-sensitized upconversion material (erbium-doped ytterbium oxide, denoted as Yb2O3:Er3+) was performed. By using ytterbium oxide powder and erbium chloride solution as precursors, Yb2O3:Er3+ with three-dimensional porous structure were prepared through hydrothermal synthesis followed by calcination treatment. The retention of the three-dimensional porous structure was demonstrated critical to overcoming the quenching effect. Such work proved a potential platform to study the short-term energy transfer between sensitizers and activators. Subsequently, oxide perovskite crystals with tunable cavities, in which doped cations were immobilized, were synthesized and used as host matrices for upconversion luminescence. To make it specific, Na1/2Y1/2TiO3 nanocubes were found promising for flexible lanthanide-doping and tunable upconversion luminescence. Markedly, through a further structural improvement and compositional optimization, effective upconversion luminescence with an ultrahigh power efficiency of 40.7 % was achieved on Na1/2Y1/2TiO3:Yb3+,Er3+ (20/2 mol%) nanocubes. Admittedly, effective upconversion nanoparticles could be potentially utilized in plenty of applications. As a proof of concept, a Gd2O3:Ln3+-based phosphor chip was fabricated to generate a combinational RGB-emission, achieving bright white-light emission consequently. Similarly, Na1/2Y1/2TiO3:Yb3+,Er3+ nanocubes were demonstrated eligible for light emitting and security pattern drawing. Furthermore, the dispersibility, stability, and biocompatibility of rare-earth oxide and titanate (e.g. Gd2O3 and Na1/2Y1/2TiO3) were assessed and demonstrated by cellular imaging. Taken together, rare-earth oxide/titanate nanocrystals were versatile and promising candidates for upconversion applications.