Controlled engineering of rare-earth doped core-shell nanocrystals

Controlled engineering of rare-earth doped core-shell nanocrystals to improve the original physical and chemical properties, and to integrate multiple functionalities in one particle with desirable size and shape, promises a huge potential in enabling new nanotechnologies, such as ultra-sensitive bio-sensing, multi-modal biomedical imaging,targeted delivery and release of drugs, high efficiency hybrid catalyst, green energy harvesting, lightening, and 3D volumetric displays. This thesis focuses on exploring the epitaxial shell growth of nanocrystals as an efficient method for controlled synthesis of rare-earth doped core-shell nanocrystals. I demonstrate that wet-chemical synthesis can be developed as a very promising approach towards the high-yield, low-cost and mass production of a library of new types of heterogeneous nanocrystal in the solution phase. Particularly, I find that much more freedom and flexibility of control shell growth of nanocrystal can be achieved in wet chemical synthesis. This thesis starts with a comprehensive review of controlled growth of nanomaterials with particular focus on the recent development of rare-earth doped (core-shell) nanomaterials.The thesis has two major parallel projects to investigate techniques to enable the unidirectional synthesis with uniform shells (chapter 2) and directional controlled growth of heterogeneous shells (chapters 3 and 4) respectively. Chapter 2 reports a simple technique for homogeneous shell growth by adjusting the amount of oleylamine. I extend this technique to study the upconversion emission stability and reversibility at different pH and temperature conditions. By controlled synthesis of the homogeneous core-shell nanocrystals via adjusting the ratio of oleic acid and oleylamine, I find that the intact shells at controlled thickness are useful in fully protecting the core nanocrystal from quenching by the surface ligands and solvent. I demonstrate the passivation effect by the intact shells will not only enhance the luminescence intensity but also improve the emission stability against temperature and pH variations for biomedical applications. In Chapters 3 and 4, I systemically study the reaction mechanisms of the epitaxial shell growth of rare earth fluoride nanocrystal in wet chemical synthesis. I find that oleateanions (OA-), as the dissociated form of oleic acid molecules (OAH), have variable, dynamic roles in mediating the growth of alkaline rare-earth fluoride (AREF₄) nanocrystals. I demonstrate that the control over the ratio of OA- to OAH can be used to directionally inhibit,promote, or etch the crystallographic facets of the nanoparticles. This control enables selective grafting of shells with complex morphologies grown over nanocrystal cores, thus allowing for access to a diverse library of monodisperse sub-50 nm nanoparticles. With such programmable additive and subtractive engineering, the heterogeneous nanocrystals in 3Dshapes can be designed and scaled from the bottom-up. Our findings may lead to a new classof multifunctional nanomaterials and provide the first bases for developing previously unforeseen applications of nanoparticles with complex programmable shapes and surface properties. The results reported in this thesis suggest that using epitaxial shell growth of nanocrystals via wet chemical route, some other heterogeneous nanocrystals may be synthesized based on semiconductor, noble metal and metal oxide nanocrystals, (e.g. Pd@Pt,Au@Fe₃O₄, Au@CdS, et. al.) The interesting synthesis mechanisms that direct the epitaxia lshell growth of nanocrystals and the combinatory factors that jointly determine the growth direction are the key to unlock a new horizon of nanomaterials science