UC Merced

Semiconductor nanoparticles (NPs) are an emerging type of material with the potential to revolutionize consumer electronics, and photonics. These particles derive their properties from their small size and can be tuned based on their diameter. Their small size however makes controlling and using them hard, and solving this problem is the focus of this thesis. One way to control such small particles is to orient them spatially using self-assembly. There are two main approaches to self-assembly; top-down assembly and bottom-up assembly. These top-down processes can be extremely expensive, time-consuming, and are typically only be prepared on small scales with specific substrates. Bottom-up processes involve taking colloidal nanoparticles that are manipulated into an array or cluster using a host material such as a liquid crystal (LC) or a block co-polymer, etc. These processes are more cost-effective, can be prepared on numerous substrates, and have the potential for large-scale production.

In this research, I use the nematic liquid crystal, an anisotropic fluid, as a host. The orientation of liquid crystal can be changed by applying an electric field, changing temperature, or applying a magnetic field. NPs used in this research are quantum dots (QDs) that fluoresce and can be tracked to map out their position during the assembly process. The liquid crystal’s ordering changes over large length-scales during a phase transition, this alteration of the bulk liquid crystal can be used to assemble the NPs into 3D micron structures. I have developed quantum dots modified with a special mesogenic (liquid-crystal-like) ligand that aids particle dispersion into the liquid crystal host. The mesogenic ligand’s flexible arm structure enhances ligand alignment with the local liquid crystal director, enhancing QDs dispersion in the isotropic and nematic phases.

My work has focused on understanding the mechanism of assembly of NPs in 3D, overcoming current limitations and introducing better approaches. This has revolved around understanding how I can control the size of assembled structures based on their applications. My focus here was to study the effect of two main parameters; concentration of nanoparticles in liquid crystal and the system cooling rate through the isotropic to nematic phase transition. I observed that the cooling rate not only changes the size of the 3D structures, but also that it changes the morphology of these 3D structures to give various hollow assemblies made of closely packed functionalized NPs. As the liquid crystal undergoes the phase transition and nematic domains nucleate and grow, NPs tend to remain in the isotropic regions. A secondary nematic nucleation can then occur inside the shrinking isotopic domains under certain conditions to produce hollow capsules and multi-compartment foam-like materials. The mechanism for this process is explored in this thesis.

As part of this thesis I focused on NP surface modification, which is necessary for the dispersion of NPs in LC as well as a potential tunable mechanism to modulate QD emission spectra (including decreasing Forster Resonance energy transfer (FRET), by varying the spacing between NPs. I modified the surface of NPs using a series of pro-mesogenic ligands to study the effect of NP separation distance on QD emission in drop-cast films. Methods included transmission electron microscopy (TEM), photoluminescence spectroscopy (PL), and small angle x-ray scattering (SAXS).

An important goal of this thesis work was to modify the surface of NPs with mesogenic/ pro-mesogenic ligands. This surface modification makes the particles more soluble in a liquid crystal host. The interaction between ligands also acts to keep the NPs close together and to stabilize the closely packed micron-scale NP structures. As a result of these modifications I demonstrated the ability to control not only the size, but the morphology of NP structures.