Part I. High resolution spectroscopy of conjugated organic semiconductors. Part II. Determination of transport domain connectivity in proton exchange membrane fuel cells using advanced imaging techniques

We have investigated two technologically important polymeric materials, conjugated organic semiconductors and perfluorosulfonic acid ionomers. The common theme in our research is the application of novel high resolution imaging techniques to determine how nanoscopic interactions effect material function.Part I presents studies of organic semiconductor molecules whose optical, electronic, and mechanical properties make them advantageous materials for the active layer of optoelectronic devices. The performance of these devices critically relies on interactions between neighboring molecules to promote facile charge transport. Our approach to studying intermolecular interactions in organic semiconductors has been to use a novel oligomeric tetramer structure wherein four oligomer "arms" are bound to a tetrahedrally coordinated carbon core. We perform single molecule spectroscopy (SMS) of spatially isolated tetramers which allows us to directly probe the interactions of a small number of closely bound oligomers. Single molecule photon statistics show intensity fluctuations over several magnitudes in time. On microsecond timescales, fluctuations are observed which we attribute to time dependent interarm coupling from random diffusive motion. Fluctuations of the photon emission on timescales of the radiative lifetime are measured using photon pair correlation spectroscopy (PPCS). These short time correlations provide insight into the quantum mechanical nature of photon emission from single molecules. We also use femtosecond transient absorption spectroscopy to study ultrafast excited state processes in the tetramer.Part II is dedicated to our research of proton exchange membranes (PEM) for fuel cell applications. The extent of aqueous domain interconnectivity or percolation is directly related to the fuel cell performance as it influences important factors such as catalyst accessibility, membrane utilization, and water management. We use a combination of nanoscale electrochemical methods to determine the interconnectivity of aqueous domains in perfluorosulfonic acid ionomers. The first technique uses pore mediated electrochemical nanolithography wherein the PEM is used as a lithography masked during anodic etching of silicon. Secondly, we have developed a series of novel conductive probe atomic force microscopy (CP-AFM) techniques which use a platinum probe as a nanoscale cathode in an operating PEM fuel cell (PEMFC). This allows us to directly map the electrochemically active domains responsible for proton transport.