Reports: G5
48229-G5 Understanding Electronic and Thermal Transport at the Single Molecule Scale
Our group has focused, on measuring conductance through single molecules using an STM-based break-junction technique that was pioneed by Xu and Tao1, but using chemical link groups that bind molecules to gold metal electrodes selectively, enabling reproducible and reliable single molecule conductance measurements2.
The focus of this project has been to develop and build a modified scanning tunneling microscope (STM) to perform single molecule conductance measurements, but in addition to be able to modify the substrate temperature to enable measurements of thermoelectric current as a function of tip/substrate temperature difference. Starting in the summer of 2008, my second-year graduate student Jonathan Widawsky, first built an STM set-up duplicating an existing system in our lab. As a proof of concept, Jonathan measured transport through a series of methyl-sulfide and methyl-selenide linked benzene molecules. The results of these measurements were published in the Journal of the American Chemical Society as a communication3. He then focused on measurements of current-voltage characteristics through single molecule junctions, developing a method to carry out these measurements in a statistically significant way. His work focused on a comparison of three molecules, 4,4’-diaminostilbene, bis-(4-aminophenyl)-acetylene, and 1,6-diaminohexane, and is currently in press in a special issue of Nanotechnology4. In the past few months, Jonathan has added the capability to modify the substrate temperature from room temperature up to about 80C. He has also preformed single molecule conductance measurements where the tip is held at room temperature and the substrate is at higher temperature. In this situation, we expect a thermoelectric current to flow through the single molecule attached to gold electrodes, where the sign of the current depends on whether the molecular HOMO or LUMO is closer to the metal Fermi level. Jonathan’s preliminary results show that the sign of the thermoelectric current depends on whether amines or pyridines are used as link groups to bind to the gold metal electrodes, in agreement with calculations. We hope to carry out a few more detailed measurements and some control experiments before publishing these results.
The funds from this grant were used to jump-start this project by allowing us to obtain some capital equipment and other supplies, and also to pay for Jonathan Widawsky’s summer stipend as well as an undergraduate student’s (Valla Fatemi) wages during the year.
References:
[1] B. Q. Xu and N. J. J. Tao, "Measurement of single-molecule resistance by repeated formation of molecular junctions", Science 301, 1221-1223 (2003).
[2] L. Venkataraman, J. E. Klare, I. W. Tam, C. Nuckolls, M. S. Hybertsen, and M. L. Steigerwald, "Single-Molecule Circuits with Well-Defined Molecular Conductance", Nano Letters 6, 458 - 462 (2006).
[3] Y. S. Park, J. R. Widawsky, M. Kamenetska, M. L. Steigerwald, M. S. Hybertsen, C. Nuckolls, and L. Venkataraman, "Frustrated Rotations in Single-Molecule Junctions", Journal of the American Chemical Society 131, 10820-10821 (2009).
[4] J. R. Widawsky, M. Kamenetska, J. E. Klare, C. Nuckolls, M. L. Steigerwald, M. S. Hybersten, and L. Venkataraman, "Measurement of Voltage Dependent Electronic Transport Across Amine-Linked Single Molecular Wire Junctions", To Appear in Nanotechnology 20 (2009).
