University of Technology Sydney. Faculty of Science. With relatively large surface areas and tunable composition and morphology, nanoparticles possess unexpected properties comparing with their bulk counterparts. Controlled synthesis and fine-tuning of their composition, morphology, and surface properties are the fundamental cornerstones in nanoscience and nanotechnology towards optimized overall performance of a variety of nanoparticles discovered today. Lanthanide-doped upconversion nanoparticles (UCNPs) are a new family of luminescent nanomaterials attracting a large amount of research interests, because these materials are capable of converting two or more low-energy photons into one high-energy photon. The controlled synthesis of lanthanide-doped UCNPs with fine-tuning the size, shape and composition becomes the next grand challenge aiming to push their real applications. Also, heterogeneous structured UCNPs have the potential to integrate many different functionalities and positively enhance each property within one nanoscale platform. Direct size/shape/composition-control in the conventional synthesis process, however does remain a challenge via the one-pot wet chemical synthesis route. This thesis thus aims to establish new methods for the fabrication of UCNPs with fine-tuning the size, shape and composition. Then, I use these well-controlled nanocrystals to study the optical property at single nanocrystal level. Further, this knowledge is leveraged to develop a robust engineering protocol for fabrication of monodisperse multifunctional luminescent nanocrystals. The outcomes of this thesis not only include a series of knowledge discovered for controlled growth of lanthanide-doped nanomaterials, but also lead to a range of new applications demonstrated, such as super-resolution nanoscopy imaging, single-particle tracking, and multimodal bioimaging. This thesis begins with a comprehensive review of the size-/shape-dependent properties and the controlled wet-chemical synthesis of nanomaterials, especially for the recently developed UCNPs, forming the introduction Chapter 1. In Chapter 2, I provide the full details of materials and methods employed for seed-mediated growth in this thesis. And the following four chapters summarize the core results from 666 synthesis experiments to produce a range of homogeneous nanocrystals (Chapter 3), heterogeneous core@shell@shell nanocrystals (Chapter 5) and single directional grown barcoded nanorods at arbitrary sizes (Chapter 6), as well as a comprehensive characterization and investigation of crystal growth mechanisms (Chapter 4) that underpins these synthesis techniques. In Chapter 7, I summarize the key achievements presented in this thesis and discusses the potentials of using this developed approach for the fabrication of high-quality on-demand hybrid nanocrystals. Also, I discuss their potential application in new nanotechnologies, such as super-resolution imaging, multimodal bioimaging, optogenetics, nanothermometry, photovoltaics, and laser refrigeration.