The successful growth of colloidal lead halide perovskite nanocrystals has generated tremendous interest in the community, due to the unique properties and the promise PQDs offer for use in applications involving light-emitting devices and solar cell technology. However, tangible progress in probing their fundamental properties and/or their integration into optoelectronic devices has been hampered by issues of colloidal and photophysical instability. In this dissertation, we aim to understand the surface chemistry of all inorganic perovskite nanocrystals, exploring several different strategies to address the stability issues. We successfully developed sets of polymer ligands exhibiting strong binding affinity towards perovskite NC surface and endow them both enhanced stability and optical properties. In Chapter 1, we provide a summary of basic physical and chemical properties behind semiconducting quantum dots (QDs). A though introduction of metal halide perovskite is also provided, which include their crystal structures, growth method, shape control, ion exchange and phase transformation features. At the end, we summarize all the common strategies that have been reported to enhance the stability of perovskite NCs. In Chapter 2, we introduce a promising surface coating strategy relying on a polyzwitterion polymer, where high-affinity binding onto the QDs is driven by multicoordinating electrostatic interactions with the ion-rich surfaces of CsPbBr₃ PQDs. The polymer ligands were synthesized by installing a stoichiometric mixture of amine-modified sulfobetaine anchors and solubilizing motifs on poly(isobutylene-alt-maleic anhydride), PIMA, via nucleophilic addition reaction. We find that this coating approach imparts enhanced colloidal and photophysical stability to the nanocrystals over a broad range of solvent conditions and in powder form. This approach also allows easy phase transfer of the PQDs from nonpolar media to an array of solutions with varying polarities and properties. Additionally, the stabilization strategy preserves the photophysical and structural characteristics of the nanocrystals over a period extending to 1.5 years under certain conditions. In Chapter 3, we detail the synthesis of a new family of multi-coordinating, bromidebased polysalt ligands and test their ability to stabilize CsPbBr₃ nanocrystals in polar solutions. The ligands present multiple salt groups involving quaternary cations, namely ammonium and imidazolium as anchors for coordination onto PQD surfaces, along with several alkyl chains with varying chain length to promote solubilization in various conditions. The ligands provide a few key benefits including the ability to repair damaged surface sites, allow rapid ligand exchange and phase transfer, and preserve the crystalline structure and morphology of the nanocrystals. The polysalt-coated PQDs exhibit near unity PLQY and significantly enhanced colloidal stability in ethanol and methanol. In Chapter 4, we report the surface ligand-engineering strategy to enhance the photoluminescence quantum yields and stability of perovskite NPLs using a PbBr₂-complexed polysalt ligands. The polysalt ligand contains either ammonium bromide or imidazolium bromide salt as anchoring group and alkyl chain with different length as solubilizing blocks. The presence of the quaternary ammonium salt could facilitate the dissolution of PbBr₂ in organic solvents, forming polysalt-[PbBrₓ] complexes. The L-PbBr₂ complex could simultaneously eliminate the surface defects on the pristine NPLs as well as improve their stability to maintain color purity and anisotropic morphology. This strategy enables us to obtain pure blue-emitting NPLs with PLQY up to 80% at ~460 nm and with significantly enhanced stability to resist long-term storage, UV irradiation and dilution. In Chapter 5, we provide a summary on the work presented in this dissertation focusing on the surface functionalization of perovskite NCs. At the end we discuss the future direction based on expanding the structure of the multi-coordinating polymer ligands and their applications on other perovskite NCs.
