Nanocrystals with precisely tuned dimensions, spatial compositions, and surface chemistry offer unique properties that may not be realized otherwise. This dissertation takes on two distinct preparative approaches to craft such nanocrystals and explore their unique optical and optoelectronic properties. The first approach capitalizes on star-like copolymers with distinct polymer blocks as nanoreactors to create nanostructured materials with precisely tuned dimensions and enhanced stability that cannot be achieved via conventional ligand-assisted methods. I synthesized PS-capped gold nanoparticles (PS-capped Au NPs), PS-capped silver nanoparticles (PS-capped Ag NPs) and PS-capped all-inorganic perovskite quantum dots (PS-capped CsPbX3 QD) with tailored dimensions that are intimately and permanently tethered with polymers by employing rationally designed star-like poly(acrylic acid)-block-polystyrene (PAA-b-PS) as nanoreactors. By synthesizing the copolymer blocks with low polydispersity via atom transfer radical polymerization (ATRP), I accurately controlled the size of PS-capped Au NPs, PS-capped Ag NPs, and PS-capped CsPbX3 QDs, thus achieving strict control over light-harvesting (for Au NPs, Ag NPs and CsPbX3 QDs) and emission (for CsPbX3 QDs) at desired wavelengths in the visible region. Moreover, by manipulating the length of permanently tethered polymers, I improved their colloidal stability (for Au NPs, Ag NPs and CsPbX3 QDs) as well as water stability (for CsPbX3 QDs). It is important to note that each PS-capped CsPbX3 QD, for the first time, carries a layer of protective hydrophobic PS chains that can be readily regulated to any desired length during the ATRP of styrene monomers, thus allowing strikingly improved water and colloidal stabilities. The second approach utilizes cation-exchange of well-defined inorganic QDs as nanotemplates to yield new QDs that maintain the same anionic framework of the original inorganic nanotemplates. I demonstrated the precision synthesis of composition gradient PbSe/PbSe1-ySy/PbS QDs by capitalizing on cation-exchange of pre-synthesized CdSe/Cd1-xZnxSe1-ySy/ZnS QDs as nanotemplates. The obtained PbSe/PbSe1-ySy/PbS QDs had PL that range between 1700 nm to 2300 nm (SWIR region) that was precisely tunable by controlling the thickness of PbS shell, which can be easily tailored by the cation-exchange time. Moreover, I showed that the initial CdSe/Cd1-xZnxSe1-ySy/ZnS QD dimensions can be utilized to accurately control the optical properties of PbSe/PbSe1-ySy/PbS QDs in the IR region. It is worth noting that the PbSe/PbSe1-ySy/PbS QDs have excellent colloidal stability (> 6 months) as well as oxidative stability (> 50 days).
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