Photophysics of Conjugated Polymer Thin Films and Semiconducting Single-Walled Carbon Nanotubes

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TECHNICAL SUMMARY:The Division of Materials Research and the Chemistry Division contribute funds to this award. This award supports theoretical and computational research and education on the photophysics of (1) thin films of -conjugated polymers, and (2) semiconducting single-walled carbon nanotubes.In (1), the PI aims to understand the mechanism of charge generation in thin films of (a) single-component polymers as well as (b) heterostructures consisting of polymer blends. The PI will investigate the roles of morphology and interchain interactions, and develop a theory that will describe simultaneously excitons, excimers and exciplexes, and free and bound polaron pairs. In the single-component polymers the PI aims to resolve an old controversy regarding the origin of polaron-derived states in thin films near the optical threshold. The PI hypothesizes that photoexcitation in the presence of significant interchain interactions leads to branching and generates a polaron-pair state directly.The PI will perform calculations on simplified models to support this key idea and propose further detailed calculations. In the polymer blends the PI notes that the binding energy of the interchain exciton generated by charge-transfer or the wavelength dependence of the charge generation cannot be understood within the simple wisdom that the band offset be larger than the binding energy of the intrachain exciton. Calculations for the single-component systems will be extended to the blends.In (2) the PI has two broad goals. The first is to reach quantitative understanding of the nonlinear and modulation optical spectroscopic experiments that are being performed for chirally enriched specific nanotubes. The PI aims to quantitatively understand (a) the energetics of the low-energy dark excitons, (b) the Aharonov-Bohm effect in semiconducting nanotubes and the transfer of oscillation strength from the bright to the dark exciton in magneto-optic spectroscopy, (c) optical absorptions polarized transverse to the nanotube axis, and (d) electroabsorpion. The second goal is to develop sophisticated theoretical and computational techniques to understand the physics of higher energy excitons and even multiexcitons.Graduate students and the postdoctoral fellow working on the project will gain experience at the interface of physics, chemistry and optics. An undergraduate student will also be involved in the research. Exciting new research results will be incorporated into an interdisciplinary graduate course. The P.I. participates as a mentor in the Student Exit Project in the Cienaga High School in Vail, Arizona. In addition, one to two presentations per year on careers in science in the same high school are planned. The P.I. also participates in a Binational Consortium in Optics, which involves exchange of information with scientists and students from Institutes in Mexico. Existing international cooperation with scientists in India will be continued. New international collaborations with scientists from India as well as Italy are being initiated.NON-TECHNICAL SUMMARY:The Division of Materials Research and the Chemistry Division contribute funds to this award. This award supports theoretical and computational research and education seeking to understand how light interacts with nanostructures, particularly large molecules and carbon nanotubes. The PI uses computer simulation and theory to gain insight at a microscopic level. The interactions between electrons in these nanostructures can be very strong and lead to new effects, novel microscopic processes, or unusual optical properties. During the past twenty years conjugated polymers and molecules have evolved from laboratory curiosities to key new optical materials. The work supported by this award is interdisciplinary involving condensed matter physics and materials chemistry. It contributes to the intellectual base from which of new technologies for nanoscale electronics, computation, and nanostructured materials may emerge.Graduate students and the postdoctoral fellow working on the project will gain experience at the interface of physics, chemistry and optics. An undergraduate student will also be involved in the research. Exciting new research results will be incorporated into an interdisciplinary graduate course. The P.I. participates as a mentor in the Student Exit Project in the Cienaga High School in Vail, Arizona. In addition, one to two presentations per year on careers in science in the same high school are planned. The P.I. also participates in a Binational Consortium in Optics, which involves exchange of information with scientists and students from Institutes in Mexico. Existing international cooperation with scientists in India will be continued. New international collaborations with scientists from India as well as Italy are being initiated.