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Abstract We investigate the effect of bis(imino)pyridine (BIP) ligands in guiding self-assembly of semiconducting CdSe/ZnS quantum dots (QDs) into three-dimensional multi-layered shells with diameters spanning the entire mesoscopic range, from 200 nm to 2 μm. The assembly process is directed by guest–host interactions between the BIP ligands and a thermotropic liquid crystal (LC), with the latter’s phase transition driving the process. Characterization of the shell structures, through scanning electron microscopy and dynamic light scattering, demonstrates that the average shell diameter depends on the BIP structure, and that changing one functional group in the chemical scaffold allows systematic tuning of shell sizes across the entire range. Differential scanning calorimetry confirms a relationship between shell sizes and the thermodynamic perturbation of the BIP molecules to the LC phase transition temperature, allowing analytical modeling of shell assembly energetics. This novel mechanism to controllably tune shell sizes over the entire mesoscale via one standard protocol is a significant development for research on in situ cargo/drug delivery platforms using nano-assembled structures.
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Calamitic ligand design for quantum dot assembly
We report the design and use of calamitic ligands for quantum dot surface modification and nanoparticle assembly. Ligands incorporating a rigid aromatic rod-like core have previously been shown to facilitate the formation of porous nanoparticle-based structures, such as solid-walled capsules and multi-compartment quantum dot foams and networks via liquid crystal phase transition templating—a process in which the host phase is quenched through the isotropic-nematic phase transition. The effect of the calamitic ligand structure on particle dispersion, transport, and subsequent assembly, however, requires further investigation, particularly in the case of anisotropic liquid crystal solvents. In this report, we vary the structure of six new calamitic ligands and characterize quantum dot size and packing into superstructures when modified with each ligand. Dynamic light scattering is used to measure the effective nanoparticle size for each ligand in dilute toluene solution. Transmission electron microscopy reveals nanoparticle distribution in dense drop-cast films for each ligand, and small-angle x-ray scattering is used to measure interparticle separations in the assembled porous structures. Together, these methods provide a full picture of particle packing for each ligand. Notably, our findings demonstrate that while longer, more rigid aromatic cores promote a closer packing structure in drop-cast films (a slow quasi-equilibrium process)—such effects are not evident using a rapid quenching method. This study highlights the fact that when nanoparticles are formed into macroscopic assemblies, both ligand design and the particular method of assembly can contribute significantly to the final packing structure.
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- PAR ID:
- 10609266
- Publisher / Repository:
- American Institute of Physics
- Date Published:
- Journal Name:
- Journal of Applied Physics
- Volume:
- 137
- Issue:
- 24
- ISSN:
- 0021-8979
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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