Interactions and Assemblies of Polymeric Materials and Colloidal Nanocrystals

Author(s): Williams, Teresa Elaine | Advisor(s): Helms, Brett A; Xu, Ting | Abstract: Our need to reduce global energy use is well known and without question, not just froman economic standpoint but also to decrease human impact on climate change. Emergingadvances in this area result from the ability to tailor-make materials and energy-savingdevices using solution–phase chemistry and deposition techniques. Colloidallysynthesized nanocrystals, with their tunable size, shape, and composition, and unusualoptical and electronic properties, are leading candidates in these efforts. Because ofrecent advances in colloidal chemistries, the inventory of monodisperse nanocrystals hasexpanded to now include metals, semiconductors, magnetic materials, and dielectricmaterials. For a variety of applications, an active layer composed of a thin film ofrandomly close-packed nanocrystals is not ideal for optimized device performance; here,the ability to arrange these nano building units into mesoporous (2 nm l d l 50 nm)architectures is highly desirable. Given this, the goal of the work in this dissertation is todetermine and understand the design rules that govern the interactions between ligand-strippednanocrystals and polymeric materials, leading to their hierarchical assembly intocolloidal nanocrystal frameworks. I also include the development of quantitative, andnovel, characterization techniques, and the application of such frameworks in energyefficiency devices such as electrochromic windows.Understanding the local environment of nanocrystal surfaces and their interactionwith surrounding media is vital to their controlled assembly into higher-order structures.Though work has continued in this field for over a decade, researchers have yet toprovide a simple and straightforward procedure to scale across nanoscale materialsystems and applications allowing for synthetic and structural tunability and quantitativecharacterization. In this dissertation, I have synthesized a new class of amphiphilic blockcopolymer architecture-directing agents based upon poly(dimethylacrylamide)-b-poly(styrene) (PDMA-b-PS), which are strategically designed to enhance the interactionbetween the hydrophilic PDMA block and ligand-stripped nanocrystals. As a result,stable assemblies are produced which, following solution deposition and removal of theblock copolymer template, renders a mesoporous framework. Leveraging the use of thissacrificial block copolymer allows for the formation of highly tunable structures, wherecontrol over multiple length scales (e.g., pore size, film thickness) is achieved through thejudicious selection of the two building blocks. I also combine X-ray scattering, electronimaging, and image analysis as novel quantitative analysis techniques for the physicalcharacterization of the frameworks.Last, I demonstrate the applicability of these porous frameworks as platforms forchemical transformation and energy efficiency devices. Examining the active layer in anelectrochromic window, I show a direct comparison between, and improved performancefor, devices built from both randomly close-packed nanocrystals and those arranged inmesoporous framework architectures. I show that the framework also serves as a scaffoldfor in-filling with a second active material, rendering a dual–mode electrochromic device.These results imply that there may exist a broad application space for these techniques inthe development of ordered composite architectures.