Quantitative Analysis of Molecular Conductance in Molecular Junctions

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Professor C. Daniel Frisbie of the University of Minnesota-Twin Cities is supported by the Macromolecular, Supramolecular and Nanochemistry (MSN) Program of the Division of Chemistry to investigate electrical conduction in molecular semiconductors over tiny nanometer length scales and to develop molecular switching devices that may lead to new applications in nanoelectronics. Molecular semiconductors are employed in commercial OLED (organic light emitting diode) displays but their conduction properties on molecular length scales are not well understood. The project adds to the fundamental knowledge base and advances the molecular electronic principles needed for a wide range of applications in functional electronic semiconductor devices. Integrating the research with education provides the graduate students who perform the experimental work with valuable chemistry, material research and nanotechnology skills and expose them to an international research experience. The project provides summer research opportunities for undergraduate students of diverse backgrounds with special attention to females and members of underrepresented minorities. From a technical standpoint, the focus of this award is on molecular structure-conductance relationships in two mechanistic regimes, namely the quantum mechanical tunneling and the classical hopping limits. These very different transport mechanisms are defined by molecular structure, with the tunneling mechanism generally pertaining to molecules less than ~5 nm in length, and hopping persisting for molecules longer than this value, depending on bonding architecture. The principal investigator and his collaborators have shown that analytical theory called the single level model (SLM) accurately describes the current-voltage (I-V) characteristics for certain benchmark molecular junctions in the tunneling regime. In this project, the research team probe the generality of the SLM for describing molecular tunneling in a variety of systems with particular attention to the impact of intermolecular interactions and surface attachment chemistry. In the hopping regime, the emphasis is on understanding the precise nature of the intramolecular charge-transfer transition states in pi-conjugated oligomers, particularly the transition state energy (height of the free energy barrier) and the associated bond length and angle deformations. For this purpose, kinetic isotope analysis of hopping conduction is combined with quantum chemical calculations of the transition state. Both the tunneling and hopping thrusts leverage unique synthesis, measurement, and analysis strategies developed by in the principal investigator's laboratory with prior NSF supportThis 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.