Collaborative Research: Charge Transport in Helicoidal Molecular Crystals

根据定义，非技术抽象晶体是直白的。他们有锋利的边缘和平坦的脸。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。作为一种在很大程度上未被探索的现象，晶体扭曲为材料设计引入了一个新的维度。塑料电子设备，如可折叠的LCD屏幕、智能手机、计算机和太阳能电池板，依赖于携带电能的微小晶体的形状。该合作项目由NSF材料研究部的固态和材料化学计划支持，在基础科学水平上揭示了扭曲形态对电流和光通过包含有机半导体的晶体传播的影响，以开创个人消费电子设备以及与可再生能源相关的关键技术的下一个时代。早期的研究结果表明，扭转可以提高电导率。涵盖文学、艺术、历史和教育的宣传活动反映了水晶和手性的主题(可以从镜像中区分出来的物品是手性的，例如一只手)，这些主题支撑了这些科学努力，重点是利用扭曲的水晶的有趣方面作为一种平台，让K-12学生参与纽约大都市区与STEM相关的活动。技术摘要螺距为1-500微米的螺旋晶体在从形成传统有机半导体的分子中生长时可以携带电荷。在器件层面上，这些长度尺度上的扭曲可能会对光传播和电荷注入、提取和跳跃产生严重影响。为了阐明扭曲对这些过程的一般影响，作为这项合作研究的一部分，一系列半导体化合物在从熔体结晶成薄膜时被诱导扭曲，这项合作研究得到了NSF材料研究部固态和材料化学计划的支持。电导和光导原子力显微镜和使用场效应晶体管平台的电荷迁移率测量作为螺距的函数在这些螺旋晶体上进行，以确定电场和光致电荷沿扭曲轴和垂直于扭曲轴的局部传输的调制。由于光电子器件通常需要在有源层内特定的晶体取向才能获得最佳性能，因此将电结晶应用于熔融的有机导体，以准直电极表面的扭曲晶体。电磁各向异性测量与PI实验室独有的完整成像偏振测量相结合，用于寻找通过扭曲引入的手性缺陷。在此过程中，这项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限制，包括沿较难接近的晶体方向的大电荷传输各向异性，以及调节独立于分子结构的分子相互作用的困难。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.