Directed assembly of magnetic and semiconducting nanoparticles with tunable and synergistic functionality

The ability to fabricate new materials using nanomaterials as building blocks, and with meta functionalities, is one of the most intriguing possibilities in the area of materials design and synthesis. Semiconducting quantum dots (QDs) and magnetic nanoparticles (MNPs) are co-dispersed in a liquid crystalline (LC) matrix and directed to form self-similar assemblies by leveraging the host's thermotropic phase transition. These co-assemblies, comprising 6 nm CdSe/ZnS QDs and 5-20 nm Fe₃O₄ MNPs, bridge nano- to micron length scales, and can be modulated in situ by applied magnetic fields <250 mT, resulting in an enhancement of QD photoluminescence (PL). This effect is reversible in co-assemblies with 5 and 10 nm MNPs but demonstrates hysteresis in those with 20 nm MNPs. Transmission electron microscopy (TEM) and energy dispersive spectroscopy reveal that at the nanoscale, while the QDs are densely packed into the center of the co-assemblies, the MNPs are relatively uniformly dispersed through the cluster volume. Using Lorentz TEM, it is observed that MNPs suspended in LC rotate to align with the applied field, which is attributed to be the cause of the observed PL increase at the micro-scale. This study highlights the critical role of correlating multiscale spectroscopy and microscopy characterization in order to clarify how interactions at the nanoscale manifest in microscale functionality.