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Electric Field Directed Nanowire Assembly: Towards Reconfigurable Materials with Switchable Properties
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Boehm, Sarah Jane
Doctor of Philosophy
Date of Defense:
October 31, 2016
Christine D. Keating, Dissertation Advisor
Christine D. Keating , Committee Chair
Philip C. Bevilacqua, Committee Member
Paul S. Cremer, Committee Member
Douglas H. Wener, Outside Member
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 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 consumption. A different type of hybrid top-down/bottom-up approach from those detailed above is presented in Chapter 4. Here, the effect of patterned topography on nanowire assemblies in electric fields was studied. Particle-particle, particle-field and particle-substrate interactions were investigated using silica-coated gold nanowires assembled in AC electric fields along with complementary multiphysics simulations. Solid gold nanowires were assembled in aqueous solution, between top and bottom electrodes, where the bottom electrode was patterned with cylindrical dielectric posts. Dielectrophoretic forces were manipulated through frequency and voltage variation, organizing nanowires parallel to the field lines, i.e., standing perpendicular to the substrate surface. We investigated the effects of patterned feature diameter and spacing as well as the effects of particle striping pattern on the assembly behavior. Due to the substrate topography altering the electric field gradient profile, particle placement and organization could be controlled. Varying the applied frequency enabled reconfiguration of the assembly between nanowire organization around post perimeters and in clusters between neighboring posts. The observed clustering behavior of the nanowires in the field was studied in detail. The assembly of partially etched nanowires with small gold segments in AC field is studied in Chapter 5. We observed a positive to negative dielectrophoretic transition for striped nanowires with small gold segments as a function of applied frequency. This observation was unexpected due to the high polarizability of gold, which typically prevents metal particles from experiencing negative dielectrophoresis (DEP). Very few examples of negative DEP assembly of metal particles have been demonstrated. We designed striping patterns to elucidate the particle parameters that dictate the assembly behavior. Multiphysics simulations were employed to map out a larger phase space of particle types that do and do not assemble via the aforementioned behavior. Overall conclusions and future directions are presented in Chapter 6. The vertical assembly of nanowires in shallow microwells is described and potential applications are offered. In order to build materials with switchable modality, the ability to form and rapidly reconfigure assemblies of functional particles at predetermined locations is essential. It should be possible to transfer the knowledge gained from this dissertation to other systems where reconfigurable particle assemblies are desired for switchable material functionality.
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