Patterning Mesoscale Chirality by Guided Crystal Twisting

Silicon-based semiconductor crystals for computer chips and solar panels epitomize our reliance on crystalline materials and underscore their centrality for information technologies and renewable energies. While defect-free single crystals are the gold standard for most applications, bundles of imperfect, interacting crystals can be superior in some cases. This grant supports research on how organic semiconductor crystals can be fabricated as networks of twisted braids that counterintuitively outperform single crystals in the transport of electrons and detection and emission of different types of light. These twisted crystals are formed from the melt, making it possible to leverage high throughput fabrication methods. These novel materials support plastic electronics industries based on flexible light emitting diodes and displays, transistors, and wearable electronics for sensing and health monitoring thus augmenting the U.S. economy. At the intersection of solid-state physics, materials science, and process engineering, this multidisciplinary research serves as a rich training platform for undergraduate and graduate students, contributing to workforce development as called for by the CHIPS and Science Act. The colorful patterns created by twisted crystals form the basis of interactive STEAM workshops at the K-12 level. Through this activity, students learn about the concepts of crystals and light-matter interactions. Designing materials for chiroptoelectronics has relied on the synthesis of chiral molecules enantioselectively or the labor- and atom-intensive resolution of racemic mixtures of enantiomers. Twisted crystals that form spontaneously from the melt can manifest mesoscale chiroptical properties. The challenges facing the advancement of twisted crystals for chiroptoelectronics are the lack of spatial control over the twist sense and direction, and the incompatibility of current batch processing with high throughput manufacturing. This research articulates mechanisms governing twisted crystal formation and how the organization of crystal fibers affects electron and photon transport. This understanding guides the development of scalable processing methods to fabricate twisted crystal films with prescribed orientations and twist sense. Hybrid screw extrusion and roll-to-roll coating of molten material into patterned films are investigated as a scalable manufacturing method for these chiroptoelectronic materials. This continuous processing method involves inline mixing of molecular compounds and twisting additives prior to extrusion; precise control over film thickness, crystallization temperature, and shear profile; and tunable patterning using corrugated rollers. Newly developed rapid differential polarization imaging for inline characterization during film coating permits the screening and optimization of hundreds of electro-optical thin films made of collimated twisted crystal films. Examples of the applications of twisted crystals are circularly polarized light detectors and waveguides. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.