Molecular Junctions: Control and Dynamics at the Single Molecule Limit

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The overarching goal of this work is to understand nanometer scale junctions and electron transport through molecules in these junctions. Calculations detailing quantum interference in the electron transport through molecules, and the control of these features, show great potential for use as discrete electronic elements. Concurrent work on the fabrication of an ultra high vacuum scanning tunneling microscope, designed to enable Raman spectroscopy, is presented. Scanning tunneling microscopy is required to image spatially molecules and surfaces at the sub-angstrom length scale. The imaging of a number of surfaces, utilizing scanning tunneling microscopy, as well as attempts at measuring single molecule conductance, are presented. The experimental measurements of transport through single molecules strongly coupled to both electrodes entail a large uncertainty in the localized structure of the junction and consequently there is a large distribution of conductance values. Calculations addressing the variability of conductance due to small geometric changes show the sensitivity of the localized structure. Molecular dynamics and charge transport calculations are coupled to model the effects of thermal motion on conductance. Our new microscope aims to overcome the uncertainties in experimental measurements by measuring spectroscopic and electrical properties at the same time. Transport calculations on limited classes of molecules show a very large dynamic range in electron transmission probability. The large dynamic range is attributed to quantum interference between orthogonal molecular states. The dynamic range in these systems, and the synthetically common chemical motif, makes them promising candidates for further studies and future electronic devices. Quantum interference and the breakdown in the traditional 'rules of thumb' for charge transport open a new window of possibility into the design of molecular devices with switching speed and dynamic range that rival solid state devices. In an example of a single molecule transistor, we calculate a change in conductance of 8 orders of magnitude with an applied gate voltage. In designing a molecule with multiple interference features, we propose and calculate the current/voltage behavior of a molecular rectifier with a rectification ratio of > 150,000. Unexplored chemical space should yield new and promising candidates for future electronic devices.

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  • 10/01/2018
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