Patterning and Characterization of Reconfigurable Nanoparticle Assemblies

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The assembly of nanoscale building blocks into larger ensembles with well-defined architecture has the potential to create entirely new classes of designer photonic and plasmonic metamaterials with unique properties not found in nature. Electromagnetic metasurfaces, or 2D metamaterials, operating at optical wavelengths are of particular interest due to ease of integration with current fabrication processes for electronic and optical devices. The fabrication of metasurfaces with nanometer-scale meta-atoms has largely been limited to top-down lithographic processes. However, directed assembly approaches offer an alternative route utilizing colloidal nanoparticles as the meta-atoms, deterministically positioned by a combination of top-down lithography and bottom-up self-assembly. Nanoparticle metasurfaces have the potential for higher performance due to reduced scattering at grain boundaries by utilizing single-crystal nanoparticles. In addition, nanoparticles can be synthesized with complex 3D shapes and compositional engineering (Janus particles, core-shell structures, etc.), providing a large parameter space for metasurface design. Moreover, directed assembly provides the ability to manipulate the short-range forces that drive particle assembly to achieve reconfigurable, stimuli-responsive architectures. In tandem with new patterning and assembly strategies for reconfigurable directed assembly, the development of new tools to analyze the assembly process in situ is critical for constructing a detailed and mechanistic understanding of these systems.', 'While structural tunability has been demonstrated in metamaterials at longer wavelengths, most of these approaches fail at nanometer-length scales due to the increased surface forces and difficulty in addressing the individual meta-atoms. Similarly, self-assembly strategies can produce a range of structures with short- and long-range order, but the degree of architectural control and ability to register these assemblies for device applications is limited. In this work, the top-down lithographic control provided by electron beam lithography (EBL) is combined with a bottom-up assembly strategy using DNA hybridization. DNA-functionalized substrates are modified by grayscale EBL patterning (DNA-EBL) to achieve local control over the surface density of the DNA to modulate interactions with complementary DNA-grafted colloidal gold nanoparticles. By this approach, the thermodynamics and kinetics of nanoparticle binding can be manipulated to achieve nanoparticle configurations that change as a function of temperature. First, the DNA-EBL process is described and characterized, demonstrating patterning of DNA density with high spatial resolution to direct nanoparticle assembly at the single-particle length scale. Second, the DNA-EBL approach is utilized to drive temperature-dependent size-selective nanoparticle assembly from a bimodal suspension of spherical gold nanoparticles. Third, this patterning strategy is extended to drive temperature-dependent ordering, alignment and positioning of both gold nanorods and spherical gold nanoparticles in 2D arrays. Finally, the ability to image these processes in situ is explored by the development of a liquid-cell platform based on microelectromechanical systems (MEMS) technology compatible with scanning electron microscopy (SEM), transmission electron microscopy (TEM) and correlative techniques.

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  • 10/08/2019
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