Quantum State Resolved Spectroscopy of Excitonic and Multi-Excitonic Dynamics in Quantum Confined Nanostructures and Heterojunctions

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TECHNICAL SUMMARYMultiresonant Coherent Multidimensional Spectroscopy (CMDS) is a new and potentially transformative approach for characterizing complex nanostructures. It is based on using multiple tunable laser beams to excite different quantum states to form multiple quantum coherences (MQCs) that re-emit output beams during the time the MQCs retain their quantum mechanical phase coherence. This program, supported by the NSF Solid State and Materials Chemistry Program, uses the resonances with the quantum states to create multidimensional signatures of the individual substructures within complex nanostructures. It isolates the individual coherence pathways that contribute to the output intensity and uses these pathways to obtain the coherent and incoherent dynamics with quantum state resolution. The coherent dynamics includes both dephasing interactions and coherence transfer and the incoherent dynamics includes charge transfer and population relaxation. The program is particularly interested in developing methods for quantum state resolution of the charge transfer dynamics between donor-acceptor substructures within larger nanostructures. The quantum states of interest include the quantum confined excitonic and multiexcitonic states of the substructures as well as surface states. The multidimensional spectra and the coherent and incoherent dynamics of the multiexcitonic states identify the mechanisms responsible for multiexciton generation (MEG). Higher order wave mixing probes the potential energy surface of different energetic excitons. Alternative CMDS methodologies include 3-color pathways and multiplex detection. The nanostructures used in this program are simple, well-characterized model systems that represent the different quantum confined substructures and morphologies that are of interest in developing new nanotechnologies. NON-TECHNICAL SUMMARYProviding the energy required for the future is an enormous challenge that requires new technologies that efficiently harvest solar energy and turn it into the electrical power and solar fuels needed for growing economies. Nanotechnology is a promising direction for providing this capability because the quantum effects that occur at small dimensions provide opportunities for engineering complex nanostructures that are efficient and robust solar converters. The small size of these nanostructures dictates the creation of new technologies that can access the individual quantum states within the individual substructures and follow the flow of energy from the initial absorption of light to the final conversion into electricity or solar fuels. The laser methods developed in this program will provide these capabilities. Not only will this methodology define the fundamental scientific principles controlling how the energy is harvested and used but it will be disseminated to the wider scientific community through web site tutorials, scientific conferences and public lectures, on-line course materials, and the training of undergraduate and graduate students. The dissemination will be transformative because this new methodology provides deeper insights into how materials function and can address questions that cannot be answered by current technologies.