Hierarchical Nanoparticle Assemblies in Thin Films of Supramolecular Nanocomposites
- Author(s): Kao, Joseph
- Advisor(s): Xu, Ting
- et al.
This dissertation focuses on the co-assembly of nanoparticles (NPs) and block copolymer (BCP)-based supramolecules in thin films for the generation of functional nanocomposites that may find applications in next-generation nanodevices. Nanocomposites, composed of organic and inorganic building blocks, can combine the properties from the parent constituents and generate new properties to meet current and future demands in functional materials. Recent developments in NP synthesis provide a plethora of inorganic building blocks, laying the foundation for constructing hybrid nanocomposites with unlimited possibilities. The properties of nanocomposite materials depend not only on those of individual NPs, but also on their spatial organization at different length scales. Block copolymer-based supramolecules, which microphase separate into various hierarchical nanostructures, have shown their potential for organizing inorganic NPs in bulk. For practical applications and device fabrication, it is requisite to translate the NP assemblies in bulk to thin films, which has yet to be realized due to the complicated energy landscape and assembly kinetics in thin film configurations.
Here, the thermodynamics and kinetics of the NP assemblies in supramolecular thin films were systematically investigated to achieve desirable and, potentially, functional NP assemblies over multiple length scales. In particular, the complicated interplay between the entropic and
enthalpic contribution in the co-assembly was explored by tuning the chain architecture of the supramolecules and the NP loading. The delicate balance among the thermodynamic driving forces was further manipulated by changing NP size, leading to a rich library of multidimensional assemblies of not only a single kind, but also mixtures of NPs in thin films. In addition, the assembly kinetics of the NPs was studied during solvent annealing. By varying the solvent volume fraction and small molecule loading in the film, the mobility, the activation energy barrier for inter-domain diffusion, and the thermodynamic driving force for defect elimination can be precisely tailored to manipulate the kinetic pathway. The results not only enable precise control over the macroscopic morphology of the supramolecular nanocomposite, but also optimize the processing conditions for the fabrication of functional nanocomposites.
Preliminary optical property results demonstrate strong wavelength dependence optical anisotropy in the supramolecular nanocomposites, showing the promise of this new family of materials for light manipulation and information transmission. To manipulate the coupling in the NP arrays over multiple length scales, faceted and lithographically patterned surfaces were employed to direct the macroscopic 2-D alignment as well as the 3-D structure in supramolecular nanocomposites. This work demonstrates that the hierarchically structured supramolecular nanocomposite thin films are fundamentally intriguing as well as technologically relevant. The fundamental knowledge gained lays a solid foundation for future studies on the investigation of structure-property correlations, which may enable the realization of novel functional nanocomposites for a wide range of applications including, but not limited to, nanoscopic energy waveguides, optical coatings, and nanoelectronics.