INVESTIGATING MESOSTRUCTURED SUPERCONDUCTORS ENGINEERED USING BLOCK COPOLYMER SELF-ASSEMBLY

Abstract

The synthesis and study of quantum materials, e.g., materials showing strong electronic correlations such as superconductors, is of increasing importance to areas including information and energy technology. The vast majority of investigations in quantum materials science are focused at the level of atomic order, but a growing body of work demonstrates that mesoscale (10s-100s of nm) structure can modulate quantum level derived properties. In particular, the field of superconductivity is rich in mesoscale phenomena, such as periodic mesostructure dependent magnetic vortex lattices, thermomagnetic flux avalanche behavior, or effects associated with the Cooper pair coherence length. In this thesis, routes to quantum metamaterials are described, which make use of a particular type of self-assembling soft matter components, i.e., block copolymers (BCPs), to engineer the mesoscopic architecture and associated macroscopic properties of superconductors in a highly tunable fashion. After an introduction chapter, in which the particular challenges addressed in this thesis are summarized, the second chapter of this thesis provides a broad overview over soft matter enabled quantum materials, both in terms of what has already been accomplished as well as a discussion of particular opportunity spaces for future work. This overview presents the wide spectrum of novel structures and physical phenomena made assessible through the use of soft matter self-assembly for the study of different classes of quantum materials, i.e., superconductors, topologically protected quantum materials, and magnetic quantum materials. The remaining chapters of this thesis describe experimental studies assessing multiple pathways to BCP self-assembly derived superconductors in an effort to bring new capabilities and insights to the field. Throughout all these chapters, confinement and mesostructure in BCP derived superconductors is reported to affect fundamental, quantum level properties. Further, solution processible routes to superconductors are shown to enable new methods for defining macroscopic shape and form. This thesis investigates BCP templated superconductors though these two lenses: understanding the fundamental role of mesostructure in defining quantum materials properties, and exploring the technological benefits of solution processibility. To these ends, several synthetic routes are developed which provide new scientific insights and/or demonstrate improved compatibility with fabrication by a larger community. In the third chapter, superconducting mesoporous niobium carbonitride (NbCN) thin films are prepared using a triblock terpolymer as a structure directing agent for niobia sol precursors. The final material structure after thermal processing is consistent with a mesoscale distorted alternating gyroid composed of highly granular, crystalline NbCN. Transport measurements determine the critical magnetic field in these mesoporous thin film superconductors, which is equal to or higher than in bulk material or non-porous films. It is conjectured that the porosity of the BCP templated NbCN is responsible for improved properties due to better penetration of reactive gases during processing. The work also successfully demonstrates transitioning BCP self-assembly derived superconductors from the bulk to thin films allowing integration of photolithographic top-down methods with bottom-up self-assembly to direct superconducting mesostructures on silicon substrates. This promises the generation of solution accessible device structures from a combination of ideas from the soft and hard condensed matter sciences. In the fourth chapter, hexagonally mesoporous, crystalline NbCN superconductors are prepared using Pluronics-family BCPs. This achievement overcomes a long-standing hurdle in the mesoporous materials field, creating phase-pure transition metal nitride-type materials without loss of mesostructural order. It is further demonstrated, that small molecule and polymeric pore-expanding agents as well as choice of BCP molar mass and composition lends the ability to finely tune mesostructural parameters like pore size and wall thickness. The wide availability of Pluronics BCPs is expected to accelerate the investigation of this superconducting class of mesoporous non-oxides, which should further enable a wealth of studies, e.g., into host-guest interactions and mesostructure-property correlations. In the fifth chapter, an alternative route to BCP self-assembly derived superconductors is pursued, which involves backfilling of a BCP templated mesoporous ceramic material with a superconducting metal, i.e., indium, under high pressure to generate thick macroscopic bulk materials. To that end, triblock terpolymers together with oligomeric polysilazanes are first self-assembled and processed to create double gyroid structured silicon oxynitride (SiON) ceramic monoliths of order 60 µm in thickness. After infiltration of these mechanically robust porous ceramic templates with molten indium under high pressure, high-fidelity replication of the double gyroid structure is observed throughout the thickness of the resulting nanocomposites. Analysis determines that the superconducting coherence length of nanoconfined indium is reduced to the length scale of the gyroid strut thickness, causing a switch from type I to type II superconductor behavior and a large enhancement of the critical magnetic field. Finally, in a conclusion chapter the results of this thesis are summarized in terms of key insights obtained from the study of BCP self-assembly derived superconductors. Furthermore, an outlook is presented of possible future work that is enabled by the findings described herein for the next generation of Ph.D. students continuing with this line of research at the intersection of the soft and hard condensed matter sciences.