Core/shell colloidal semiconductor nanocrystals are one of the most active areas of nanotechnology research. CdSe/nCdS core/shell heterostructures show remarkable suppressed fluroscence intermittency at the single-particle level. Although syntheses of thin- shelled core/shells have been reported in the past, reproducible syntheses for thick-shelled core/shells are less understood. Moreover, most of the core/shell reports are on CdSe/nCdS heterostructures, while other core materials -such as Ge- are absent from the literature.

In the first part of this thesis, we present a thorough investigation of thick-shelled nanocrystal synthesis. We successfully grow metal sulfide shells on two different cores: CdSe and Ge. We explored the effect that the concentration of amine, amine type, core size, cadmium precursor concentration, annealing time, injection-rate and surface priming have on the synthesis of core/shell heterostructures. Adopting a similar method that uses surface priming, we are also able to grow epitaxial cadmium sulfide and zinc sulfide shells on Ge cores. The obtained Ge/nMS heterostructures show a large emission enhancement in the near-infrared range. We discuss the optical behavior of thick-shelled CdSe/nCdS nanocrystals at the ensemble and single-particle levels. Through collaboration with analytical research groups, we study the applications of these core/shell nanocrystals in bio-imaging and tracking using total internal reflection microscopy and stimulated emission depletion microscopy.

Beyond spherical or spheroidal nanocrystals, anisotropic nanostructures such as nanorods and tetrapods are of particular interest in photocatalysis and energy harvesting. Controlling the size and morphology of more ophisticated structures such as these remains difficult. Dichalcogenide precursors enable the isolation of metastable nanocrystalline phases with unusual composition and morphology. It remains unclear what factors play a determinant role in controlling the outcome of preparations that utilize these interesting family of precursors.

In the second part of this thesis, by studying a variety of commercially available dichalcogenides and the outcome of their reaction, and with the help of computational calculations, we demonstrate that the formation and degree of anisotropy of different nanocrystaline products can be traced back to the precise molecular structure and reactivity of the precursor used. We expect our results will not only lead to a larger throughput of these materials, but also lead to reliable syntheses of colloidal nanomaterials for customized applications.