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

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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.