Microscopic Charge Transport in Molecularly Doped Organic Materials

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Next generation electronics require materials that can transport electricity like metals but also offer new properties such as being flexible, light weight, low-cost, and recyclable. In this regard, all-carbon materials such as plastics are attractive, but they lack the electrical transport ability of traditional metals. Unlike metals, carbon-based plastics are rarely perfectly structured (crystalline). Their disorder makes it difficult to determine how electricity flows through them, a necessary first step in improving their electrical performance. In this project funded by the Chemical Structure, Dynamics, and Mechanisms (CSDM-A) program of the Chemistry Division, Professor Aaron Massari of the University of Minnesota-Twin Cities is using sophisticated laser techniques to study the movement of electrical charges through complex molecular materials that could act as future electrical conductors. The interdisciplinary nature of the project supports a training environment for undergraduate and graduate students that produces next-generation scientists who are skilled in benchtop chemistry, but also electrical, microscopic, and spectroscopic techniques. Ultimately, the research may not only lead to new perspectives on existing problems, but also inform the rational design of electrical materials leading to devices that are less toxic and require less energy to produce. Professor Massari serves as the Director of the Energy and U Show, which brings science to over 10,000 3rd-6th graders each year. The show is filled with vibrant scientific demonstrations and the First Law of Thermodynamics. For many, the event introduces the idea of attending college for the first time as more than half of the attendees are members of underrepresented groups. Professor Massari also directs the chemistry portion of the University on the Prairie program, where he and his research group travel to work with 60 students in grades 7-12 and their teachers from rural, geographically isolated portions of the state. These outreach and educational activities encourage STEM careers. The project focuses on the roles of structural parameters, such as polymorphism, donor-acceptor spacing, partial charge transfer, and covalent attachment of molecular dopants, in the transport of charge through ground-state doped molecular thin films. The measurements leverage the ability of two-dimensional infrared (2D-IR) spectroscopy to isolate the rates of charge transfer within specific molecular geometries from highly heterogeneous chemical environments. Short pulses of infrared light excite molecular vibrations on neutral and charge (doped) molecules in a film and then a second pair of pulses probe the evolution of that excitation as a marker for the location of the charge as a function of time. Resolving the vibrational spectrum into a second dimension allows one to monitor the kinetics of charge transfer for specific molecular configurations independent of the others so that the transport kinetics can themselves be resolved. The work systematically tests the impact of microscopic molecular spacing and order on macroscopic electrical transport by using molecules with particular structural modifications that affect their packing in known ways. Electrical transport is also driven in-situ in samples while they are characterized by 2D-IR spectroscopy.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.