Atomic layer deposition techniques for building massive thermal gradients and advanced bioelectronic interface devices
Keywords: Materials Science and Engineering.

Nanoscale fabrication methods have enabled many modern advancements such as the integrated circuit, but still have the potential to unlock more advantages in a wide variety of fields, from medicine to thermal management and energy systems. Atomic layer deposition (ALD) is not only critical to modern nanofabrication developments, but is the only tool that can currently grow films conformally with angstrom-level resolution on a wide variety of templates, including high aspect ratio details. With the ability to control films on an atomic level, ALD is positioned as the best method for depositing thin, conformal films on planar or high aspect ratio structures, resulting in the potential for many unique structures and capabilities. Additionally, because of its flexibility in deposition, it can easily be integrated with a variety of materials systems, substrates and devices. First, in this thesis, I discuss the use of ALD to create thin-shelled, ultra-high aspect ratio (> 100:1) nanostructures. I motivate their use as micron-scale thermal insulators by discussing their application in vacuum thermionic energy converters (TECs). These structures enable a 3-micron vacuum gap between metallic films with negligible surface area coverage and can sustain a temperature gradient of at least 150 Kelvin. I show the benefits of these ALD nanostructures, starting with ease and versatility of fabrication. I then examine the mechanical properties of individual nanostructures in compression using nanoindentation to better characterize the densities needed to support TEC electrodes. Through these studies, we find that thin-shelled alumina structures irreversibly fail under 0.1 mN of compressive force per structure. There are additional failure events before this point, but we anticipate a margin of reversibility until final failure. I conclude this section of my thesis with a discussion of device assembly and thermal characterization. Using two forms of thermal characterization, I verify the high thermal resistivity of the interlayer at both low and high temperatures. At the high temperatures, we are additionally able to verify that massive thermal gradients can be maintained across micron-scale vacuum gaps. This ability to hold these thermal gradients in micron-scale gaps could enable unprecedented efficiencies in TEC devices as well as benefit other high temperature gradient applications such as thermal protection systems for extreme environments. Secondly, due to their unique properties, these structures may offer promising advancements in other applications such as bioelectronic interfaces. Here, we discuss their use in interfacing with electrogenic cells such as neurons and cardiomyocytes. Because these cells are at the basis of human function, understanding normal and diseased states relies on a knowledge of the dynamics in which they operate. Recording many cells in parallel with high signal-to-noise is still a challenge for scientists due to device limitations, namely poor coupling between electrode and cell. Using the nanoelectrodes and ALD nanostructures, we show devices capable of recording extended periods of electrical activity with stimulation. By incorporating chemical or other modifications, these nanostructures have the potential to offer a more intimate and stable seal for extended and sensitive recordings. Lastly, I touch on the ability to create these ultra-high aspect ratio nanostructures with electrode materials at low temperatures. Building off the idea of "nanostraws", nanofluidic channels for intracellular sampling and delivery, we can extrapolate the benefits of integrating metallic materials into these polymeric substrates. Through these various applications of vertical ALD nanostructures, we show the ways in which nanofabrication tools can be used to create unique structures that are impossible through other means. The thin-shelled structures that are built through this technique show several remarkable properties at thermal interfaces, including a 50,000 K/mm temperature gradient and large compressive strengths, and promising applications in bioelectronic devices.