Self-assembly is utilized by many natural processes to make large-scale complex structures and materials from much smaller, simpler components. These structures are stable without the continuous influx of energy. It is a “bottom-up” approach that is not restricted by scale and is therefore a very promising approach for the fabrication and manufacturing of both new synthetic materials or mimics of biological materials. This is especially true in the field of nanomaterials. Objects that exist on the nanoscale often have unique and interesting properties that are dependent on their size, which makes them very appealing for a variety of applications. Most of these applications depend on arranging nanoparticles into very specific patterns on a large scale. Currently, many of the fabrication techniques that excel in terms of order and control, such as lithography-based techniques, lack scalability. Self-assembly is one conceivable pathway for achieving a high degree of control in a scalable manufacturing process. In this thesis, I present investigations into two different nanoparticle two-dimensional (2D) self-assembly systems, which advance our understanding of the principles that govern them and provide the groundwork for further exploration into these systems.
In Chapter 1, I discuss the basic principles of self-assembly at the nano-length scale. This discussion includes highlighting the most common forces used to drive self-assembly, and how different components of a nanoparticle system can be used to alter those forces. I also provide additional analysis on the specific challenges and merits of 2D self-assembly as compared to 3D self-assembly. Chapter 2 is a commentary that provides a structure and language for discussing interparticle interactions and how self-assembly systems can carry the information that directs them. The terms valency, directionality, and specificity are used to describe the type and degree of information that is encoded into a system. In Chapter 3, I present an investigation into the role of DNA flexibility during the DNA-mediated 2D self-assembly of gold nanospheres. Introducing an “ambidextrous” design of the sticky end of the DNA strands directing the self-assembly, I was able to deconvolute the particle-particle interactions and the particle-surface interactions in a 2D system. This revealed that the system favored softer, more flexible particle-particle interactions but harder, more energetically stable particle-substrate interactions. I performed additional experimentation and analysis that suggests the preference for hard particle-substrate interactions is most likely a result of having faster kinetics under those circumstances, while the softer particle-particle interaction is more strongly dictated by the ability of softer ligand shells to overcome lattice defects. The final chapter presents a second 2D self-assembly system in which cubes and orthocentric bitetrahedra are co-assembled into two distinct superstructures based on the size and sharpness of both shapes. While this project is still in its infancy, the initial work for determining a standard and the boundary conditions for each of the different elements of the system is present. In this chapter, I also discuss the promising initial results and the future experiments needed to resolve standing questions and to quantify my observations. Self-assembly is a simple and universal principle that governs many of the most complex materials in our lives. Understanding and applying it, however, is anything but simple. The discussions, observations, and results presented in this thesis add to the growing body of knowledge about this ubiquitous process so that it may one day be utilized with accuracy and efficiency in the fabrication of complex materials and structures.