Structure design and applications of upconversion nanoparticles

Lanthanide (Ln³⁺) doped upconversion nanoparticles (UCNPs) have displayed attractive potentiality in numerous fields such as solar cells, biology, display, anti-counterfeiting due to their unique luminescence property of converting near-infrared (NIR) photons to ultraviolet (UV) or visible (VIS) photons. To further promote the practical applications of UCNPs, enhancing the UC efficiency is therefore regarded as one of the most important goals in the UCNPs-related fields. In this aspect a thorough comprehension of the UC process is of great necessity. With traditional analysis methods based on the rate equations it is difficult to obtain a distinct microscopic picture on the role of excitation migration in UC process since they primarily focus on the interactions just between sensitizer and activator. This thesis describes the exploration of UC dynamic processes by quantitatively considering the influence of energy migration from sensitizer to sensitizer. By integrating Monte Carlo simulations with spectroscopic studies and nanostructure designs, we have unraveled the significant role of the excitation migration process in UC. On the basis of this improved picture, we have designed “dopant ions spatial separation” (DISS) structures to precisely tune the rise and decay behaviors of the UC emission over a broad time range. We also designed novel structures, i.e., NaErF₄@NaYF₄ and its derivatives to realize a relatively efficient UC with advanced properties. The intensive UC luminescence of Er³⁺-rich nanosystem breaks the concentration quenching limit and provides a new possibility for realizing highly efficient UC as well as more promising applications.