Collaborative Research: Charge Transport in Helicoidal Molecular Crystals

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Non-technical abstract Crystals are straight by definition. They have sharp edges and flat faces. They are polyhedra. However, molecular crystals that twist as they grow are remarkably common, albeit little known. More than one third of simple molecular crystals are capable of forming twisted morphologies. As a largely unexplored phenomenon, crystal twisting introduces a new dimension to materials design. Plastic electronic devices, e.g. foldable LCD screens, smart phones, computers, and solar panels, depend on the shapes of tiny crystals that carry electricity. This collaborative project, supported by the Solid State and Materials Chemistry program in the Division of Materials Research at NSF, uncovers at a fundamental science level the effect of twisted morphologies on the propagation of electrical current and light through crystals comprising organic semiconductors in order to usher in the next age of personal consumer electronic devices as well as critical technologies associated with renewable energy. Early results show that twisting boosts conductivity. Outreach activities embracing literature, art, history, and education reflect the themes of crystals and chirality (items that can be distinguished from their mirror image are chiral, for example a hand) that undergird these scientific efforts, with a focus on using intriguing aspects of twisted crystals as a platform to engage K-12 students in STEM-related activities in the NY metro area. Technical AbstractHelicoidal crystals with pitches from 1-500 microns can carry charge when grown from molecules that form traditional organic semiconductors. At the level of devices, twisting on these length scales can have critical consequences on light propagation and charge injection, extraction and hopping. To elucidate the general effect of twisting on such processes, a series of semiconducting compounds are induced to twist as they crystallize from the melt into thin films as part of this collaborative research, which is supported by the Solid State and Materials Chemistry program in the Division of Materials Research at NSF. Conductive and photoconductive atomic force microscopy and charge mobility measurements using a field-effect transistor platform are performed on these helicoidal crystals as a function of pitch to determine the modulation of electric field- and photo-induced charge transport locally along and perpendicular to the twisting axes. As optoelectronic devices typically require specific crystal orientations within active layers for optimal performance, electrocrystallization is applied to molten organic conductors to collimate twisted crystals on electrode surfaces. Electrical magnetochiral anisotropy measurements, in conjunction with complete imaging polarimetry unique to the PIs' laboratories, are actualized in the search for chiral defects introduced via twisting. In doing so, this research uncovers fundamental mechanisms of crystal growth while addressing inherent limitations in the field of organic electronics, including large charge transport anisotropies along less accessible crystallographic directions and difficulties in tuning molecular interactions independent of molecular structure.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.