Electric Field Directed Nanowire Assembly: Towards Reconfigurable Materials with Switchable Properties

Ordered particle arrays often amplify the optical and electronic properties of individual particles, making assemblies of nanowires especially valuable as device components for applications including optical cloaking, solar cells, and chemical sensing. Dynamic and reconfigurable control of nanoparticle assemblies are increasingly important for the development of functional materials. Electric field directed assembly is particularly promising because of the inherent ability to control particle placement and to anneal the resulting structure, resulting in fewer defects. This dissertation explores the driving forces that govern electric field directed assembly of metal nanowires and the characterization of the emergent optical properties of ordered nanoparticle assemblies. Metallic nanowires were synthesized through templated electrodeposition and sol-gel silica coating methods. Lithographically patterned electrode geometries were designed and fabricated to assemble nanowires within an alternating current (AC) electric field and optical microscopy was used to investigate the particle assembly behavior. The assembly mechanism of solid gold and partially etched nanowires in an AC field is explored in Chapter 2. We found that dielectrophoretic forces concentrated wires between electrode gaps, with their long axes aligned parallel to the field lines, resulting in particle dense, ordered nanowire lattices. This result was the first 2D lattice of metal particles of its kind. Multiphysics simulations showed that the formation of the lattice was driven by field-induced dipolar interactions and mutual dielectrophoresis between neighboring nanowires. We were able to tune the resulting 2D structures in three distinct ways: (1) frequency-dependent dipolar coupling, which varied lattice periodicity in real iv time, (2) particle striping pattern, which fixed the periodicity for different particle types at a given field condition and, (3) switching the field off/on which converted between lattice and smectic arrangements. The demonstration of a reconfigurable broadband polarizer, designed to exploit the optical anisotropy of the gold nanowires is detailed in Chapter 3. Simulations of the lattice indicated excellent discrimination between polarized light aligned parallel vs. perpendicular to the long axes of the nanowires. A dual electrode design was developed to realize a reconfigurable polarizer in the near-infrared by utilizing two orthogonal interdigitated electrodes to rotate the lattice by 90° in situ. The corresponding optical response was investigated with Fourier transform infrared spectroscopy. In good agreement with theory, the nanowire lattice was transparent to incident light whose polarization was perpendicular to the nanowires within the lattice, but highly reflective for the orthogonal polarization. Unlike polarizers fabricated by top down methods that have static optical properties, these electric field assembled structures can be reorganized after assembly, switching the nanowire orientation by 90° and consequently selecting for a different polarization. The demonstrated broadband nanowire polarizer provides potential for electrically reconfigurable photonic devices such as ultra-compact polarization components, electro-optic switches, and on-chip modulators. Smart windows are an exciting practical application for these nanowire polarizers, where varying levels of transparency and thermal absorption/reflection could be controlled to decrease energy