Polaron and Spin Transport in Nanoscale Molecular Junctions

This award is funded by the Macromolecular, Supramolecular and Nanochemistry Program in the Division of Chemistry. Professor C. Daniel Frisbie of the University of Minnesota-Twin Cities is supported to develops techniques both for making ultrathin films (1-10 nm, a tiny fraction of a human hair in width) of organic semiconductors and for sensitively measuring their conduction properties using an atomic force microscope (AFM). The organic films are made in such a way that molecular orientation is precisely controlled so that electrical conduction is measured in the most favorable direction. These methods advance the frontiers of organic semiconductor science that underpin display technology by providing new knowledge about how charges move in oriented molecules. Organic semiconductors belong to a new class of functional molecular materials that have achieved widespread commercial success in organic light emitting devices (OLEDs) employed in smart phone displays and televisions. Understanding the electrical conduction properties of these molecular semiconductors on microscopic 'even nanoscopic' length scales is vital to their continued improvement. Integrating the research with education provides research opportunities for graduate students and postdoctoral researchers, training them to be the next generation of experts in molecular synthesis and characterization, electrical conductance measurements, and nanotechnology.

The plan leverages unique experimental approaches developed by the PI and his students over the last 15 years including (i) conducting probe atomic force microscopy (CP-AFM) for forming metal-molecule-metal junctions and (ii) oligoimine click chemistry for building surface-anchored pi-conjugated wires with monomer-by-monomer control over chain architecture. There are four project goals: (1) to use isotopic labeling to understand the nature of polarons and polaron hopping transition states in pi-conjugated molecular wires; (2) to investigate polaron size effects on hopping transport; (3) to measure spin transport as a function of both molecular wire length and tailored architecture using CP-AFM spin valve junctions with ferromagnetic (FM) contacts; (4) to demonstrate electronic structure control and current switching. The team collaborates with two quantum chemistry groups that provide theoretical and computational support for the project. Overall, the research plan balances lower risk experiments that are certain to yield important results with higher risk ideas that open up new directions.