2D Encapsulation of Molecular Crystals for Close-contact Measurement of Exciton Dynamics

由共轭分子制成的分子晶体通常被称为有机半导体。这些材料可以吸收光并有效地传输能量或激发（称为激子），使它们对太阳能转换应用具有吸引力。此外，它们还支持一些独特的现象，比如一个激子分裂成两个能量更低的激子，称为单线态裂变。在这个由化学部化学结构动力学和机理（CSDM-A）项目资助的项目中，加州大学河滨市分校的Chris Bardeen教授将新型样品制备与时间分辨光学显微镜相结合，研究有机晶体中的激子。有机固体中激子的表征是具有挑战性的，因为晶体的脆弱性质使其难以进行测量。巴丁教授和他的学生们克服了这一挑战，他们将这些晶体封装在石墨烯或六方氮化硼的二维薄片下。这些原子薄层形成了晶体的光学透明和耐化学涂层。然后使用高分辨率显微镜技术研究封装的晶体，该技术可以跟踪激子在晶体中移动并经历单线态裂变的过程。从这项工作中获得的见解可以促进我们对发光二极管和有机光伏电池的理解。该项目还在探索量子计算等领域的影响。该项目为参与这项研究的研究生和本科生提供光谱、材料表征、显微镜和数据分析方面的高级培训，并支持塔夫脱小学的推广工作。正在进行的项目包括不同年级的科学演示，科学展览教程，四年级学生对加州大学洛杉矶分校为期一天的参观，以及本科生志愿者的课堂参观。一项针对三年级学生的动手实验已经被开发出来，在这个实验中，每个学生都要建造一辆定制的太阳能汽车。该项目的重点是典型的分子晶体半导体，如苝、四烯和二苯己三烯。生长具有可变厚度的片状分子晶体，然后覆盖石墨烯和六方氮化硼。有了这种使能技术，就可以进行两类实验。第一种方法包括使用显微镜和光致发光方法测量封装晶体的性质及其与环境的相互作用。可以解决的问题包括激子在分子晶体中的行为是否在20纳米的长度尺度上是不均匀的，以及是否有可能在原子薄膜上转移能量和电荷。第二类实验侧重于新的空间和时间分辨实验，以确定单线态裂变产生的三重态对的自旋纠缠态是否可以扩展到介观距离。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。

Molecular crystals made from conjugated molecules are oftentimes called organic semiconductors. These materials can absorb light and efficiently transport the energy or excitation (called excitons), making them attractive for solar energy conversion applications. In addition, they support unique phenomena, such as the splitting of a single exciton into two lower-energy excitons, called singlet fission. In this project funded by the Chemical Structure Dynamics and Mechanism (CSDM-A) program of the Chemistry Division, Professor Chris Bardeen of the University of California, Riverside is combining novel sample preparation with time-resolved optical microscopy to study excitons in organic crystals. The characterization of excitons in organic solids is challenging, since the fragile nature of the crystals makes it difficult to perform measurements. Professor Bardeen and his students overcome this challenge by encapsulating these crystals beneath two-dimensional sheets of graphene or hexagonal boron nitride. These atomically thin layers form an optically transparent and chemically-resistant coating for the crystal. The encapsulated crystals are then studied using high-resolution microscopy techniques that can follow the excitons as they move through the crystal and undergo singlet fission . Insights from the work could advance our understanding of light-emitting diodes and organic photovoltaic cells. The project is also exploring implications for fields such as quantum computing. The project is providing the graduate and undergraduate students involved in this research with advanced training in spectroscopy, materials characterization, microscopy, and data analysis, as well as supporting outreach efforts to Taft Elementary School. Ongoing projects include science demonstrations for various grades, science fair tutorials, a day-long visit to UCR by the fourth-grade, and classroom visits by undergraduate volunteers. A hands-on experiment for third graders has been developed in which each student builds a customized solar-powered car.The project focuses on prototypical molecular crystal semiconductors like perylene, tetracene ,and diphenylhexatriene. Plate-like molecular crystals with variable thicknesses are grown and then covered with graphene and hexagonal boron nitride. With this enabling technology in place, two categories of experiments are pursued. The first involves measuring the properties of the encapsulated crystal and its interaction with the environment using microscopy and photoluminescence methods. Questions that can be addressed include whether the exciton behavior in molecular crystals heterogeneous on length-scales down to 20 nm and whether it is possible to transfer energy and charge across the atomically thin membranes. The second category of experiments focuses on new space and time-resolved experiments to determine whether spin-entangled states of triplet pairs produced by singlet fission can extend over mesoscopic distances.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.