Charge Tunneling in Organic Matter

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With this award, the Macromolecular, Supramolecular and Nanochemistry (MSN) Program of the Division of Chemistry is funding Professor George Whitesides of Harvard University to investigate methods to control the flow of electrical currents through thin layers of materials that are normally electrically insulating. When these materials are sufficiently thin (1-2 nanometers, or less than 20 atoms in thickness), they allow electrical charge to cross by a fundamentally different process called tunneling. In tunneling, the charge, faced with a normally insulating barrier, simply passes through it. In understanding tunneling of charge through normally insulating organic matter, this project pursues three objectives: the design of new materials that either enhance or decrease tunneling; the introduction of students on this project to a style of science that requires integration of skills from organic synthesis and materials science to quantum physics, and strengthening the U.S. technical workforce; the design, construction, and demonstration of new tools for studying tunneling that are broadly useful, and sufficiently simple for use at the undergraduate college level in science teaching. The objective of this project is to understand charge tunneling through thin, insulating organic films. It will use a junction developed specifically for this purpose that has three parts: i) an electrode of flat gold or silver; ii) a self-assembled monolayer (SAM) of an organic or organometallic compound (or mixture of compounds as the tunneling barrier; iii) a second electrode comprising a compliant structure composed of a drop of liquid eutectic gallium-indium alloy, covered with a thin, conducting film of gallium oxide. The project will measure the tunneling current across this junction at low voltages (V ~ 0.5V), using a number of SAMs designed to test theories relating to rates of tunneling, to the molecular and electronic structure of the molecules across which tunneling is occurring. The project will have two objectives: i) to validate a level of theory adequate to explain tunneling rates; ii) to use this theory to predict new types of tunneling mechanisms, and new phenomena involving tunneling, through organic matter. The desired outcome of the work is a fundamental understanding of the orbital structures particularly the highest energy occupied molecular orbitals, or HOMOs and the electronic couplings between them, that lead to either particularly high or particularly low charge tunneling rates. Both outcomes would be interesting, and both, ultimately, may be useful in the design of molecular electronic and organic electronic devices.