Multifarious methods for enhancement of luminescence in rare earth ion-doped upconversion nanocrystals

Upconversion nanocrystals (UCNCs) have become a sought-after delivery system for advanced biomedical and photonics applications since they have the unique ability to convert low energy infrared photons into high-energy visible and ultraviolet photons. In this work, the author has developed a number of approaches to the fabrication of UCNCs that are small in size and so are of much use in the field of biology and medicine. Since a long-pursued goal for UCNCs is the further enhancement of their upconversion efficiency and the generation of a stronger emission, the author also describes new procedures for the fabrication of particles that possess enhanced efficiency in their upconversion luminescence. To achieve the two aims, the author first investigated the nature of the interior crystal quality of these particles and how to enhance it. Experiments that formed part of this work demonstrated that smaller and more efficient UCNCs can be formed with improved interior crystal quality. In parallel to this line of investigation, the author also demonstrated the importance of controlling the NaYF4 transit stage from cubic phase to hexagonal phase. Following this, an efficient method for synthesizing sub-10nm UCNCs, and then fabricating a novel sandwich structure, was developed. The sandwich structure nanocrystal can realize size-tunable and doped maximum sensitizer ions. The author also found that the particles, when enclosed by an inert shell, were able to achieve minimal surface quenching as well. Finally, the author shows that a photon-avalanche-like effect plays the key role in bringing down the intensity requirement for stimulated emission depletion with high Tm3+ ions doped nanocrystals s. The sensitizer ions present as a radial gradient distribute from the core to the surface. This kind of UCNCs offer efficient optical switching with saturation intensity at low power and provides a large contrast to conventional fluorescent probes used for super resolution stimulated emission depletion (STED) nanoscopy. The three major results are presented in four publications. The author presents a summary and prospect chapter at the end as well. This thesis provided several good ways to solve the key problems of size-tunable and luminescence enhancement for UCNCs applications. It has profound implications for the upconversion optical property exploration and realizes further bioimaging and biomedical applications.