From single particle polarizability to asembling and imaging hierarchical materials

• Main Author: Cao, Wenhan
• Other Authors: Brown, Keith A.
• Language: en_US
• Published: 2020
• Subjects: Nanoscience; Dielectrophoresis; Field driven assembly; Hierarchical structure; Polarizability; Scanning probe microscopy; Soft matter
• Online Access: https://hdl.handle.net/2144/41479

High performance natural materials typically employ highly tuned structures spanning the nanoscopic to macroscopic length scales. Synthetically recapitulating this degree of complexity has become a unifying goal connecting the fields of chemistry, nanoscience, biology, and materials science. One common strategy is to direct the bottom up assembly of nanoparticle building blocks into hierarchical structures using stimuli such as electric fields. Despite the promise and great versatility of electric fields, there are many knowledge gaps surrounding their use to assemble highly complex structures. In this thesis, we explore the assembly of nanoparticles into hierarchical structures through dielectrophoresis (DEP), or the motion of polarizable objects in non-uniform electric fields. Critically, through a systematic approach, we study the fundamental polarizability of individual particles, the assembly of particle dimers, and finally the emergence of macroscopic structure from nanoscopic particles. Interweaving these explorations are instrumentation advances that broaden our ability to measure fundamental particle properties and explore hierarchical structures. Initially, we measure the polarizability of nanoparticles in solution using fluorescence microscopy. Specifically, we quantify the polarizability of solution-phase semiconductor quantum dots (QDs) for the first time. Through analyzing the thermodynamic distribution of particles in a microfluidic device with a non-uniform electric field profile, we identify a striking 30-fold increase in polarizability in the presence of low salt conditions due to the Debye screening length being commensurate with the particle size. This increase in polarizability indicates that nanoparticles assemble far more rapidly and easily than previously predicted. Next, we study the assembly of nanoparticles in the vicinity of anisotropic template particles as a path to realizing hierarchical structures. Specifically, we explore eight particle geometries using finite element analysis and find a >10-fold local field enhancement near some shapes, potentially promoting hierarchical assembly. We subsequently introduce a framework for predicting the assembly outcome of particles with multiple distinct sizes and shapes that includes thermodynamic and kinetic considerations. Then, we perform experiments demonstrating the hierarchical assembly of QDs into macroscopic structures. Despite theory predicting the formation of chains, we observe a macroscopic foam-like cellular phase when the QDs experience a combination of alternating current (AC) and direct current (DC) voltages. The resulting materials are both highly hierarchical in that they are 200 µm thick materials comprised of 20 nm particles, but they also represent extremely low-density materials. Finally, we report the invention of a novel instrument for imaging hierarchical materials. Specifically, we describe a massively parallel atomic force microscope with >1000 probes that is made possible through the combination of a new cantilever-free probe architecture and a scalable optical method for detecting probe-sample contact that provides sub-10 nm vertical precision. === 2022-09-28T00:00:00Z