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The PI, Ling Zang, moved with his group to University of Utah in August 2008.  This second-year report was made based on the research work performed at Utah during the grant period terminated on August 31 2009.

Under the one-year PRF-G support scientists at University of Utah (led by Ling Zang) continued to work on the one-dimensional self-assembly of planar, rigid organic molecules into well-defined nanofibers and nanobelts.  The major research effort was devoted to the molecular design and synthesis with the aim to optimize the intermolecular packing organization in a way to maximize the one-dimensional exciton diffusion, as well as the charge transport, along the long axis of nanofibers. 

Long range exciton diffusion is conducive to the higher efficacy of photoconversion through the more efficient charge separation at the donor-acceptor interface, while the expedient charge transport along the intermolecular stacks (through the p-p electron delocalization) usually enhances the charge collection at the electrodes. Both the two parameters can be precisely measured and optimized when the materials are fabricated into well-defined one-dimensional morphology, for which the optoelectronic properties are confined along the cofacial p-p stacking, i.e. the long axis of the nanowire.  Combined with the extensive, comparative measurements of crystalline structure using X-ray and electron diffraction, and morphological characterization by TEM, SEM, and AFM, the one-dimensionally enhanced exciton diffusion and charge transport can be correlated to the specific molecular stacking conformation, providing guidance on how to improve the optoelectronic performance of materials all the way from the molecular level. This is in contrast to the previous studies on conventional film based materials, where the polycrystalline heterogeneity of the sample system prevents such a structure-property correlation, and the measurement is averaged over large number of crystal domains aligned at different directions, and thus produces an underestimated value for the exciton diffusion length.  Our recent results showed that the exciton diffusion in one of the best cases can be as long as above 100 nm, much longer than that (~ 10 nm) usually observed for conducting polymer films.

The extended exciton diffusion as optimized by the intermolecular stacking was also consistent with the amplified fluorescent sensing of various gas molecules through interfacial charge transfer mechanism. These results have been reported in a series papers published in the flagship journals of chemistry and nanotechnology, including J. Am. Chem. Soc. and NanoLett.  Several more papers are currently in preparation and will be submitted for publicationsshortly. 

As highlighted in our recent review article (Accounts of Chemical Research, a special issue on nanoscience, 41 (2008) 1596-1608, invited), the research carried out under the PRF support not only produced the materials that are potentially suited for application in solar cells, but more importantly provided improved understanding of the structure-property relationship between the exciton diffusion and the intermolecular arrangement within the materials phase, thus helping design and synthesize new building-block molecules (regarding both conjugation size and geometry, as well as the side-chain modification) to approach further improvement of the cofacial intermolecular electronic interaction so as to enhance the coherent exciton migration.  

The one-year support also enabled us to train graduate students with a variety of cross-discipline knowledge in chemistry and materials science, as well as a broad range of instrumentation technology in nanoscale imaging and fabrication. Meanwhile, students have gained the opportunity to work as part of a coherent team in collaboration with different groups from both academia and industries. This helps familiarize them with teamwork in research, and allows students to be exposed to a range of intellectual and scientific methods to solve scientific problems.