Heavy-metal-free colloidal nanocrystal heterostructures – synthetic chemistry and growth mechanisms
Building heterostructures of colloidal nanocrystals by bringing different components into contact can improve existing or impart novel properties, which enables applications as light emitters in already-commercialized and near-future products. However, the majority of the research has been based on compositions with heavy metals (e.g., cadmium or lead) posing toxicity concerns to the consumers. Therefore, a better understanding of the synthetic chemistry and the growth mechanisms for heavy-metal-free nanocrystal heterostructures is critical before their commercialization potential becomes reality. Here, we have developed novel synthetic strategies to achieve the anisotropic growth of colloidal nanocrystals and the introduction of multiple components with tunability of their positions in the heterostructures, as well as to improve the photoluminescence quantum yield (PL QY) of nanocrystals. We first examine how different Cu precursors in the synthesis of colloidal copper sulfide (Cu2-xS) nanocrystals affect the resulting shape and phase. Decreasing aspect ratio in 1D nanorods (eventually transitioning to 2D nanodisks) observed is consistent with the expected effects of decreasing Cu precursor reactivity. Nanorods are predominantly chalcocite at the early stages of growth but a phase transition to djurleite occurs accompanied by a change in tip faceting upon further growth. In contrast, nanodisks appear in the djurleite phase early on and remain so upon continued growth. Secondly, the high copper vacancy density and the high cation mobility in the Cu2-xS nanorods favor cation exchange, and the remaining Cu2-xS after partial cation exchange can serve as the catalyst for the subsequent solution-liquid-solid (SLS)-like growth. The interplay between cation exchange and SLS-like growth leads to tapered rod-rod, body/tail, or barbell-like Cu2-xS/ZnS heterostructures, which can be controlled by the Zn precursor and ligand choice. Finally, using a similar procedure, spontaneous multiple segment formation has been achieved in Cu2-xS/CuGaS2 heterostructured nanorods for the first time. Large strain due to lattice mismatch (-7.4% in the axial direction and -10.6% in the other direction) is responsible, because such segmentation is absent in Cu2-xS/CuInS2 heterostructured nanorods with a smaller mismatch (-4.3% in the axial direction and -6.5% in the other direction). More importantly, ligands with a large steric hindrance are critical to the multiple segment formation, which pack less densely on the surface and enables nucleation in the middle of the nanorods. In contrast, linear ligands provide better surface protection and the new components tend to form at the tips of the nanorods where the ligand packing density is relatively low. In addition to Cu2-xS-based anisotropic heterostructures, we have also explored how to enhance the PL QY for heavy-metal-free color-center (manganese)-doped nanocrystals and ZnTe-based core/shell nanocrystals with spherical shapes. Annealing or growth at high temperatures for an extended period of time is considered detrimental for the synthesis of high-quality Mn-doped II-VI semiconductor nanocrystals, which can lead to the broadening of size distribution and, more importantly, to the loss of dopants. However, ripening can be beneficial to doping in a simple heat-up approach, where high dopant concentrations can be achieved. Smaller nanocrystals in a reaction batch, on average, exhibit higher undesirable band edge PL and lower desirable dopant PL. The optimization of dopant loss and the removal of such smaller undesirable nanocrystals through Ostwald ripening along with surface exchange/passivation to remove Mn clustering lead to high Mn PL QYs (45 to 55 %) for ZnSxSe1-x, ZnS, CdS, and CdSxSe1-x host nanocrystals. Similar with the synthetic optimization for doped nanocrystals, surface passivation by a high-quality ZnSe shell and a cation-rich surface by Zn-cation treatment enable a high PL QY (~40%) for ZnTe-based nanocrystals. The highly reactive selenium powder provides better synthetic control of the ZnSe shell. In addition, highly reductive ligand (diphenylphosphine) can further increase the PL QY presumably by filling surface electron trap states.
Colloidal nanocrystals, Heterostructures, Heavy-metal-free, Synthesis, Growth mechanisms, Precursor chemistry, Cation exchange, Solution-liquid-solid-like growth, Catalytic growth, Shape control, Phase change, 1D nanorods, 2D nanodisks, Djurleite, Chalcocite, Copper sulfides, Zinc sulfide, Copper gallium sulfide, Copper indium sulfide, Spontaneous multiple segment formation, Tailed nanorods, Crystal facet selectivity, Preferential surface adsorption, Steric hindrance, Precursor reactivity, Strain, Doping semiconductor nanocrystals, Ostwald ripening, Dopant loss, Dopant clustering, Size dependence, Surface modification, Surface passivation, Zinc telluride, Zinc selenide shell, Selenium powder, Shell thickness, Photoluminescence quantum yield, Photoluminescence lifetime, Organic ligands, Photoluminescence quenching
University of Illinois - Urbana-Champaign
Networked Digital Library of Theses and Dissertations
University of Illinois at Urbana-Champaign