Dr. Matthew Sfeir, CUNY
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Abstract: The promise of nanotechnology lies in the emergence of novel electronic and photonic phenomena with the potential for new device technologies. For example, optical transitions at the nanoscale frequently result from strongly absorbing excitons, whose electronic structure and dynamics are shape, size, and morphology dependent. However, since excitons are bound states, device concepts that exploit the unique photophysics of excitons, for example, organic photovoltaics or multiple exciton generation solar cells, depend critically on the ability to direct specific favorable conversion processes and suppress unfavorable ones. Here I will discuss how ultrafast optical spectroscopy is an important tool to understand the success (and failure) of nanoscale device concepts. I will demonstrate how the unique optical signature of excitons allow for dynamical tracking of energy conversion processes on ultrafast time scales. We have used these optical signatures to understand the harvesting of excitons in nanoscale lasers, photocatalytic water splitting devices, and photovoltaics. I will focus on recent work from my group that has identified, for the first time, classes of individual molecules that exhibit unusual electronic phenomena stemming from many-body interactions. This includes the first stable and efficient examples of biexciton formation in individual molecules and polymer chains (“singlet fission”) that occurs when the biexciton (triplet pair or 4 spin exciton) is lower in energy than the singlet exciton (2 spin state). Since it is optically dark, biexciton formation involves a complex energy conversion process based on a series of energy and spin conversion phenomena. I will discuss our work to systematically identify these energy conversion steps, enhance the formation rate of the biexciton, increase its overall excited state lifetime, and optimize these dynamics for device purposes.
** Refreshments served from 2:45pm – 3:00pm in PAS 218. Thank you. **