Spectroscopy of Two Dimensional Molecular Carbon

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Eric Heller of Harvard University is supported by an award from the Chemical Theory, Models and Computational Methods program in the NSF Division of Chemistry, to better understand the amazing potential of carbon materials, especially graphene. Graphene is an extended honeycomb sheet of carbon atoms only a single atom thick. It has both solid state and molecular characteristics. The list of useful devices and applications involving graphene as an essential component is already very long, for example transistors, photodetectors, solar cells, and electrodes. Two layers can be stacked to give a superconductor, i.e. a material with no electrical resistance. Graphene sheets are impervious to all atoms and molecules. The Heller group develops new theoretical understanding of what the graphene carbon lattice and the conductive electrons surrounding it are doing before, during, and after interaction with light, including bright pulses only femtoseconds in duration. Probing graphene with light is the most powerful experimental tool for revealing its secrets, but the experiments must be properly interpreted. The Heller group seeks to reveal the fundamental quantum properties of graphene using new theoretical and computational methods that it has developed. The Heller group is revising the understanding of fast electron relaxation following light absorption in graphene This new understanding is relevant for developing practical devices based on graphene and related carbon-based materials. The Heller group is developing a strongly revised understanding of fast electron processes in graphene and related carbon materials, based on properly incorporating certain important but previously neglected indirect (phonon producing or destroying) electronic light induced transitions. The key has been to restore the full physics of condensed matter spectroscopy, including the phonon coordinate dependence of the electronic transition moments. The transition moments had previously universally been taken to be constant, independent of phonon coordinate, but Heller and coworkers has shown that neglecting this coordinate dependence sacrifices the correct physical understanding. After importing the full Kramers-Heisenberg-Dirac Raman theory to graphene spectroscopy for the first time, many opportunities open up because of a fuller and more correct understanding. The Heller group is exploring these new implications including those which have experimental relevance and suggesting experiments to test their theoretical predictions. They are also probing the role of the Born-Oppenheimer approximation in these key indirect transitions. The group is exploring wide implications of this new understanding of indirect transitions, which are key to many devices including silicon solar cells, throughout condensed matter 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.