Synthesis and Characterization of Core-shell Upconversion Nanoparticles Containing Anisotropic Interfacial Strain

包含各向異性界面應變的核殼型上轉換納米顆粒的合成及表徵

Student thesis: Doctoral Thesis

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Award date24 Jul 2020

Abstract

Core–shell upconversion nanoparticles typically consist of rare earth/alkaline earth fluoride cores and epitaxial shell layers. The core nanoparticles can accommodate a variety of lanthanide dopant ions, which dominate the upconversion process. The rationally designed shell layers can substantially enhance or alter the upconversion emissions of the core. By integrating different shell layers on performed core nanoparticles, the lanthanide energy transfer process can be tuned by doping ions, shell morphology, and interfacial structure.

Misfit strain is a fundamental factor in core–shell epitaxy growth. Although a diversity of core–shell upconversion nanoparticles have been synthesized, the precise effect of misfit strains on shell growth is still not clear. The understanding and utilization of strain effect are generally constrained by limited control over the magnitude and distribution of misfit strains in a core–shell system. Consequently, deliberate and accurate tuning of core–shell nanoparticles by strain management remains unfulfilled research.

This thesis begins with the basic mechanism of misfit strain in the core–shell nanoparticles and the recent study in the development of core–shell nanoparticles with particular emphasis on the emerging strategies for strain quantification, strain management, and tuning optical properties by strain effects in core–shell nanoparticles.

In Chapter 4, we investigate the epitaxial growth of hexagonal phase NaLnF4 (Ln = lanthanide) core–shell nanoparticles controlled by anisotropic interfacial strains. The role of anisotropic misfit strain in dictating epitaxial habit is elucidated by studying morphology and structure of a series of NaLnF4 epitaxial layers on the same type of NaYF4:Yb/Er (18/2%) core nanoparticles. The strain distribution for dumbbell-shaped NaYF4:Yb/Er@NaEuF4 is revealed by high‐resolution transmission electron microscopy. The strain energy and associated chemical potential can be utilized to fine-tune the shell morphology. The resultant NaYF4:Yb/Er@NaGdF4 nanoparticles show sensitive change of upconversion emissions as the function of temperature.

In Chapter 5, we investigate the effect of calcium doping on epitaxial growth of NaYF4:Yb/Er@NaGdF4:Ca core–shell nanoparticles. The calcium doping effectively prompts shell growth and enhances the shell coverage on the core, which can be attributed to strain relaxation in the shell layer. The strain relaxation is identified by analyzing the peak profile of X-ray diffraction patterns of the core–shell nanoparticles and supported by X-ray diffraction simulation. Specifically, the lattice tilting is observed at the core–shell interface, which plays an important role in strain relaxation. The resultant NaYF4:Yb/Er@NaGdF4:Ca(40%) nanoparticles show enhanced upconversion emissions due to surface passivation provided by the fully coated shell.

In Chapter 6, the epitaxial growth of NaGdF4 and NaNdF4 shells on NaYbF4 nanoparticles is studied. To prompt the shell growth under large lattice misfit, the effects of core particle size and shape as well as reaction temperature was investigated. The NaYbF4 nanorods with large size enables the Stranski–Krastanov growth mode. The NaGdF4 shell with enhanced shell coverage is preferable for upconversion and magnetic resonance imaging applications. With NaGdF4 as a transition layer, further growth of NaNdF4 shells expands the range of excitation wavelengths.

    Research areas

  • Epitaxial growth, Upconversion, Core–shell nanoparticle, Lanthanide, Misfit strain, Strain relaxation