Collaborative Research: Charge Transport in Helicoidal Molecular Crystals

根据定义，晶体是直的。它们有锋利的边缘和平坦的面。它们是多面体。然而，分子晶体在生长过程中扭曲是非常常见的，尽管鲜为人知。超过三分之一的简单分子晶体能够形成扭曲的形态。作为一种尚未被探索的现象，晶体扭曲为材料设计引入了一个新的维度。塑料电子设备，如可折叠液晶屏、智能手机、电脑和太阳能电池板，都依赖于携带电流的微小晶体的形状。这个由美国国家科学基金会材料研究部固态和材料化学项目支持的合作项目，在基础科学水平上揭示了扭曲形态对电流和光通过由有机半导体组成的晶体传播的影响，以迎来个人消费电子设备的下一个时代，以及与可再生能源相关的关键技术。早期的研究结果表明，扭转可以提高导电性。包括文学、艺术、历史和教育在内的推广活动反映了晶体和手性的主题（可以与其镜像区分的物品是手性的，例如手），这些主题巩固了这些科学努力，重点是利用扭曲晶体的有趣方面作为一个平台，让纽约都会区的K-12学生参与stem相关活动。技术摘要：当从形成传统有机半导体的分子中生长时，具有1-500微米间距的环状晶体可以携带电荷。在器件水平上，这些长度尺度上的扭曲会对光传播和电荷注入、提取和跳变产生关键影响。为了阐明扭曲对这一过程的一般影响，作为合作研究的一部分，一系列半导体化合物在从熔体结晶成薄膜时被诱导扭曲，这是由美国国家科学基金会材料研究部固态和材料化学项目支持的。利用场效应晶体管平台对这些螺旋晶体进行导电和光导原子力显微镜和电荷迁移率测量，作为螺距的函数，以确定沿和垂直于扭转轴的局部电场和光诱导电荷传输的调制。由于光电器件通常需要在有源层内特定的晶体取向以获得最佳性能，因此电结晶应用于熔融有机导体以准直电极表面上的扭曲晶体。电磁手性各向异性测量，结合pi实验室独有的完整成像偏振法，在寻找通过扭转引入的手性缺陷时实现了。在此过程中，本研究揭示了晶体生长的基本机制，同时解决了有机电子学领域的固有局限性，包括沿不易接近的晶体学方向的大电荷输运各向异性以及独立于分子结构调节分子相互作用的困难。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.