Synthesis of Zinc Chalcogenide Nanocrystals as Hosts for Optically Active Defects

Abstract

Doped zinc chalcogenide nanocrystals represent a promising materials platform for quantum information science, offering wide bandgaps, low nuclear spin environments, and the potential to host optically addressable defect states such as neutral shallow donors and lanthanide 4f emitters. This thesis describes the synthesis and characterization of doped ZnO and ZnS nanocrystals as candidate hosts for such states, pursued through three synthetic thrusts of increasing complexity. The first thrust targeted In³⁺-doped ZnO nanorods through alkaline hydrolysis of zinc acetate in ethanol. Despite systematic variation of temperature, precursor concentration, base identity, and addition rate, the synthesis consistently produced oblate wurtzite ZnO nanocrystals rather than the desired anisotropic morphology, attributed to the absence of selective facet-capping agents in the reaction system. The second thrust employed pre-formed Ti₈Yb₂O₁₂(PhCOO)₁₆ heterometallic clusters as seeds for ZnO shell growth, targeting the Yb³⁺ ²F₅/₂ → ²F₇/₂ transition at ~980 nm. Synthesis in both dimethylacetamide and dimethyl sulfoxide produced ZnO-containing products confirmed by XRD and near-band-edge photoluminescence, but no Yb³⁺ emission at 980 nm was observed, indicating unsuccessful energy transfer from the ZnO host to the dopant. DMSO was found to reduce trap emission relative to DMA, attributed to its stronger coordinating character. The third thrust pursued In³⁺-doped ZnS nanorods by solvothermal synthesis using ethylenediamine as a structure-directing agent. Water-based synthesis successfully produced wurtzite ZnS with ICP-OES-confirmed indium incorporation across concentrations of 0.05–10 mol%, and systematic bandgap narrowing with increasing dopant concentration was observed. All samples were dominated by broad defect-mediated emission attributed to surface hydroxylation from aqueous synthesis conditions. A subsequent anhydrous, air-free synthesis using dry DMF as co-solvent yielded a product with dramatically improved optical properties, exhibiting a sharp emission at 377 nm with FWHM of 101–103 meV. Unexpectedly, XRD characterization identified the product as the ZnS·ethylenediamine hybrid phase (ZnS·en) rather than wurtzite ZnS. HRTEM imaging revealed a hierarchical assembly mechanism in which ~2 nm ZnS·en nuclei bridge laterally into larger plate-like structures via bidentate ethylenediamine linkers. A systematic amine functionality study comparing no amine, monoamine (oleylamine), and diamine (ethylenediamine) provided direct experimental evidence that bidentate coordination geometry, not amine functionality alone, is the necessary condition for hybrid phase formation. Oleylamine-passivated samples exhibited improved emission intensity relative to the amine-free control without any change in crystal structure, consistent with surface passivation of zinc dangling bonds. Solvothermal treatment of the ZnS·en hybrid in DMF at 200°C produced a product consistent with wurtzite ZnS by XRD, demonstrating a viable two-step solution-phase conversion route. Together, these results establish the synthetic conditions governing phase selectivity and optical quality in ethylenediamine-directed ZnS synthesis, and identify anhydrous hybrid-to-wurtzite conversion as a promising avenue for producing phase-pure wurtzite ZnS nanocrystals suitable for future quantum optical characterization.