Encoding Electronic and Biological Function in Hierarchical Soft Materials

Sather, Nicholas Allen

doi:https://doi.org/10.21985/n2-6nkb-4v87

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Encoding Electronic and Biological Function in Hierarchical Soft Materials

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Developing materials with comparable structural complexity and programmed hierarchy as those found in nature is a grand challenge in materials science. One way to synthesize soft materials with these complex architectures is to use bottom-up self-assembly of nanoscale building blocks, mimicking how organisms grow tissues with specific functions from peptides, carbohydrates, lipids, inorganic crystals, and nucleic acids. In this context, this work explores the encoding of small molecules to engage in specific interactions to drive their assembly into ordered structures for applications in energy and medicine. For targets in energy, organic molecules were programmed to template deposition of functional inorganic materials in hierarchical hybrid structures. In the first system, aromatic surfactants were studied as additives during electrodeposition of layered cobalt hydroxide to tune the interlayer spacing and microstructure of thin film supercapacitor electrodes. One specific surfactant was discovered to induce curvature of the inorganic layers resulting in the deposition of multi-walled organic-cobalt hydroxide nanotubes. The nanotubular architecture was found to provide high surface area and global orientation of the inorganic layers perpendicular to the current collecting substrate for efficient electron transport. The organic molecules used to template the structure also enhanced the stability of the hybrid material in the harsh environments necessary for energy storage function. A second hybrid system was investigated based on a series of aromatic cations as intercalants in layered organic-lead iodide perovskite thin films, which are of interest as solar cell active layers. Layered perovskites are inherently more environmentally stable than their three-dimensional counterparts, but they suffer from poor photovoltaic performance due to orientation of the semiconducting layers parallel to the substrate and the typically insulating organic cations between the layers. By introducing aromatic cations with tunable intramolecular interactions, enhanced out-of-plane conductivity and photovoltaic performance was achieved without compromising environmental stability. A different organic-inorganic hybrid system investigated in this work took inspiration from natural biomineralization processes. In this system supramolecular peptide amphiphile (PA) nanofibers were encoded with amino acids that could bind or nucleate gold nanoparticles to form conductive nanowires. The conductivity was found to be highly dependent on the pathway for gold metallization of the PA fibers. When spray coated in a thin layer on plastic substrates the hybrid nanowires demonstrated high transparency and conductivity, ideal for transparent conducting electrodes in flexible touch screens or solar cells. Finally for biomedical applications this work investigated 3D printing to order supramolecular assemblies into patterned hydrogels with programmed alignment via shear during extrusion. The attractive and repulsive interactions between individual PA molecules and between multiple PA assemblies were critical for determining the viscosity and printability of the aqueous inks. 3D printed hydrogels composed of aligned PA fibers were found to exhibit anisotropic ionic transport, programmable actuation, and directed cell outgrowth. In order to target potential bioelectronic functions in future systems, the 3D printing technique was extended to aqueous inks composed of semiconducting chromophore amphiphile (CA) nanofibril assemblies based on carbonyl bridged triarylamine aromatic cores. By incorporating hydrogen bonding motifs on the periphery of the CA aromatic core, π-orbital overlap between molecules was increased leading to higher conductivity. The addition of the biopolymer alginate to the ink not only allowed for shear alignment of the supramolecular CA fibrils during 3D printing, which results in further enhancement of the electronic transport, but also imparts durability to the material during cyclic bending. The soft organic scaffolds and organic-templated hybrid materials investigated in this work demonstrate the structural complexity that can be achieved via programmed molecular self-assembly, in some cases combined with additive manufacturing. These synthetic materials are reminiscent of hierarchical structures found in nature, and can be further explored in the future for novel functions in electronics, sustainability and healthcare.

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