Jeremy M. Beebe

Measuring relative barrier heights in molecular electronic junctions with transition voltage spectroscopy.

Though molecular devices exhibiting potentially useful electrical behavior have been demonstrated, a deep understanding of the factors that influence charge transport in molecular electronic junctions has yet to be fully realized. Recent work has shown that a mechanistic transition occurs from direct tunneling to field emission in molecular electronic devices. The magnitude of the voltage required to enact this transition is molecule-specific, and thus measurement of the transition voltage constitutes a form of spectroscopy. Here we determine that the transition voltage for a series of alkanethiol molecules is invariant with molecular length, while the transition voltage of a conjugated molecule depends directly on the manner in which the conjugation pathway has been extended. Finally, by examining the transition voltage as a function of contact metal, we show that this technique can be used to determine the dominant charge carrier for a given molecular junction.

Tracing electronic pathways in molecules by using inelastic tunneling spectroscopy

Using inelastic electron tunneling spectroscopy (IETS) to measure the vibronic structure of nonequilibrium molecular transport, aided by a quantitative interpretation scheme based on Greens function-density functional theory methods, we are able to characterize the actual pathways that the electrons traverse when moving through a molecule in a molecular transport junction. We show that the IETS observations directly index electron tunneling pathways along the given normal coordinates of the molecule. One can then interpret the maxima in the IETS spectrum in terms of the specific paths that the electrons follow as they traverse the molecular junction. Therefore, IETS measurements not only prove (by the appearance of molecular vibrational frequencies in the spectrum) that the tunneling charges, in fact, pass through the molecule, but also can be used to determine the transport pathways and how they change with the geometry and placement of molecules in junctions.

Nanoscale switch elements from self-assembled monolayers on silver

Au/molecule/Ag junctions are shown to behave as voltage-controlled two-state switches. In the open state, the current-voltage behavior is consistent with a metal-molecule-metal tunnel junction. At a negative bias threshold, silver filaments bridge the gap between the two electrodes, resulting in direct metal-metal contact and an increase in current of several orders of magnitude. Under positive bias, the filaments dissolve, returning the switch to an open state. Switching rates of up to ≈10kHz have been observed. Because the only required components are silver and a self-assembled monolayer, this switch element can be incorporated into a wide array of device architectures.