Development of Core-shell Upconversion Nanoparticles for Ultraviolet Lasing


Student thesis: Doctoral Thesis

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Awarding Institution
Award date26 Apr 2017


Photon upconversion generally results from a series of successive electronic transitions within complex energy levels of lanthanide ions that are embedded in the lattice of a crystalline solid. Current study on upconversion is mainly focused on core-shell nanoparticles comprising spatially confined dopant ions. By doping upconverting lanthanide ions in the interior of a core-shell nanoparticle, the upconversion emission can be substantially enhanced and the optical integrity of the nanoparticles can be largely preserved. Core-shell nanoparticles comprising optically active shells are also frequently employed to impart multiple functionalities to upconversion nanoparticles. Intriguingly, core-shell design raises the possibility of constructing novel upconversion nanoparticles by exploiting the energy exchange interactions across the core-shell interface.

The thesis begins with recent advances in the development of core-shell upconversion nanoparticles with particular emphasis on the emerging strategies to regulating interplay of dopant interactions through core-shell nanostructural engineering that lead to unprecedented upconversion properties. The improved control over photon energy conversion would open up new opportunities for technological application.

In chapter 4, we report a size effect on upconversion nanoparticles that can be harnessed to enhance multiphoton upconversion. Our mechanistic investigations suggest that the phenomenon stems from spatial confinement of energy migration in nanosized frameworks. We show that confining energy migration constitutes a general and versatile strategy to manipulating multiphoton upconversion, demonstrating an efficient five-photon upconversion emission of Tm3+ in a stoichiometric Yb lattice without suffering from concentration quenching.

In chapter 5, we develop a heterogeneous core-shell-shell nanostructure of NaYbF4:Gd/Tm@NaGdF4@CaF2:Ce that enables efficient photon upconversion in Ce3+ ions through a Gd-mediated energy migration process. The design takes advantage of high covalency of CaF2 host to reduce the energy of 4f-5d transition in Ce3+. Meanwhile, CaF2 is isostructural with NaGdF4 and can form a continuous crystalline lattice with the core layer. As a result, Ce3+ ions are able to capture the migrating energy stored in the Gd3+ sublattice at the heterogeneous core/shell interface.

In chapter 6, we realize room-temperature ultraviolet lasing based on upconversion nanoparticles. We used NaYF4@NaYbF4:Gd/Tm@NaYF4 nanoparticles as a gain medium to realize deep-ultraviolet lasing emission at around 311 nm through the formation of whispering gallery modes in a microcavity under a five-pulse excitation scheme. Lasing behavior of NaYbF4:Gd/Tm@NaGdF4@ CaF2:Ce core-shell-shell nanoparticles under near-infrared excitation were also investigated, suggesting that the nanoparticles can be used as gain media supporting a wide tunable range of lasing emission from 310-361 nm. Our findings thus highlight the viability of realizing diode-pumped lasing in ultraviolet regime for various practical applications.

    Research areas

  • Lanthanide-doped, Upconversion Luminescence, Core–shell, Nanoparticles, Energy migration, Ultraviolet, Lasing