UC Merced

The interactions of semiconducting quantum dots (QDs) and Fe3O4 magnetic nanoparticles (MNPs) co-assembled together with thermotropic liquid crystalline (LC) materials are studied in this work. LC materials allow for bottom-up organization of nanoparticles via thermotropic phase transition. Using nanomaterials as building blocks to create new materials with meta functionalities is of great interest in the materials design and synthesis space. The first study is about dispersing QDs and MNPs together within isotropic LC and co-assembling them into clusters driven by the LC’s thermotropic phase transition. The co-assemblies are micron-size clusters that can be modulated in situ by applied magnetic fields <250 mT that results in the enhancement of QD photoluminescence (PL). The enhancement is reversible in clusters that have MNPs smaller than 10 nm. Using transmission electron microscopy (TEM) and energy dispersive spectroscopy, nanoscale placement of QDs are shown to be aggregated in the center, and MNPs are dispersed throughout the cluster volume. MNPs in LC were observed to be able to rotate and align along the direction of the applied magnetic field with Lorentz TEM. This rotation and alignment are attributed to the cause for PL enhancement. By correlating multiscale spectroscopy and microscopy characterization, this part highlights how the interactions in the nanoscale can manifest in the microscale. This co-assembly of QDs and MNPs is a reversible magnetic field sensor where the emission intensity is tuned by an applied magnetic field. Second, this work covers research done on the addition of charge transfer capabilities to CdSe/ZnS (CZ) QDs and the self-assembly of shells with CZ QDs with LC thermotropic phase transition. Concepts such as surface functionalization and biomaterial strategies for controlled delivery in applications such as drug delivery and tissue engineering are discussed. Surface functionalized NPs with bis(imino)pyridine (BIP) are studied. BIP molecules are a class of molecules that are normally used in base metal charge transfer on the molecular scale. BIPs behavior in the nano and micro scale were unexplored and here we made some interesting discoveries. From crystallography and TEM, we observed that the BIP molecules alter NP interdot separation in QD films. Using ultrafast spectroscopy we were able to determine that due to this interdot separation, the optical and electronic properties were altered. With three different types of BIPs, improved energy and charge transfer were observed. The study also found that the BIP ligands were able to stabilize the QD emission quicker than the native octadecylamine ligands. Third, is a continuation of the second, utilizing the BIP ligands allowing for the controllable size distribution of shell formation when self-assembled using thermotropic LC. Over the years, the Ghosh group and collaborators have developed self-assembled microstructures composed of semiconducting QDs and AuNPs that were functionalized with custom-designed ligands. In this work, I have focused on a new group of ligands, the BIP ligands, which are much smaller and could improve stability, optical properties, biocompatibility, and possibly allow cargo release via electrochemical stimulation. Additionally, it offers a controllable size tunability of the assembled structures based on changing the alkyl group on the BIP molecule. With this size tunability, an optimal nanostructure assembly may be utilized in cargo encapsulation systems. The main goal of this research combines increasing the functionality of QDs or nanoparticles in general, and utilizing surface modification with customized ligands to make them electro-responsive. The use of these ligands are currently good at being photogenerated hole acceptors, provide non-toxic properties to NPs, and the utility of allowing NPs to assemble to shells from LC thermotropic phase transition. The combination of these features allows the use of these materials in charge transfer devices, drug delivery, and tissue engineering platforms.