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Our group has focused, on measuring conductance through single molecules using an STM-based break-junction technique that was pioneed by Xu and Tao [1], but using chemical link groups that bind molecules to gold metal electrodes selectively, enabling reproducible and reliable single molecule conductance measurements [2].

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 communication [3]. 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 Nanotechnology [4]. In the past few months, Jonathan has added the capability to modify the substrate temperature from room temperature up to about 100C. He has also preformed single molecule conductance measurements where the tip is held at room temperature and the substrate is at higher temperature.

Over the last summer, Jonathan worked along with undergraduate Radha Parameswaran, and they measured conductance of alkanes terminated with diphenyl phosphine linkers. This work resulted in a publication in the Journal of Physical Chemistry Letters this spring [5].

In this past year, Jonathan, working along with a first year graduate student Scott Berkley, has measured the thermo-electric current across single molecule junctions in response to a temperature difference between the two electrodes. This work is currently being prepared for publication. Our results show that molecules that conduct through the HOMO orbital have a negative thermo-electric current while those that conduct through the LUMO have a positive thermoelectric current. We are collaborating with the theory group of Jeff Neaton at the Molecular Foundry in Lawrence Berkeley National Labs to support our measurements with density functional theory based calculations.

The funds from this PRF grant were also used to pay an undergraduate Valla Fatemi. His research focused on the effect of solvents in the conductance of single molecule junctions. Results from Valla’s work have been submitted for publication last month.

This grant has enabled us to buy some lab equipment and pay the summer salary of graduate students and undergraduate students. The work funded by this grant has resulted in three publications so far, with a fourth one that has been submitted and a fifth is being prepared.

References:

[1] B. Q. Xu and N. J. J. Tao, Science 301, 1221 (2003).

[2] L. Venkataraman, J. E. Klare, I. W. Tam, et al., Nano Lett. 6, 458 (2006).

[3] Y. S. Park, J. R. Widawsky, M. Kamenetska, et al., J. Am. Chem. Soc. 131, 10820 (2009).

[4] J. R. Widawsky, M. Kamenetska, J. Klare, et al., Nanotechnology, 434009 (2009).

[5] R. Parameswaran, J. R. Widawsky, H. Vazquez, et al., J. Phys. Chem. Lett. 1, 2114 (2010).