James G. Kushmerick

Contact-induced crystallinity for high-performance soluble acene-based transistors and circuits

The use of organic materials presents a tremendous opportunity to significantly impact the functionality and pervasiveness of large-area electronics. Commercialization of this technology requires reduction in manufacturing costs by exploiting inexpensive low-temperature deposition and patterning techniques, which typically lead to lower device performance. We report a low-cost approach to control the microstructure of solution-cast acene-based organic thin films through modification of interfacial chemistry. Chemically and selectively tailoring the source/drain contact interface is a novel route to initiating the crystallization of soluble organic semiconductors, leading to the growth on opposing contacts of crystalline films that extend into the transistor channel. This selective crystallization enables us to fabricate high-performance organic thin-film transistors and circuits, and to deterministically study the influence of the microstructure on the device characteristics. By connecting device fabrication to molecular design, we demonstrate that rapid film processing under ambient room conditions and high performance are not mutually exclusive.

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.

Tuning current rectification across molecular junctions

We demonstrate the ability to tune the current rectification in metal-molecule-metal junctions through control of the interaction strength of one of the two metal-molecule contacts. Current-voltage characteristics of thiolate bound molecular wires with a nitro or pyridine termination show that the extent of current rectification in a molecular junction correlates well with the extent of coupling between the chemical linker and metal electrode.

Magnetic directed assembly of molecular junctions

We present a technique for fabricating molecular junctions for molecular electronic devices. Silica microspheres are rendered magnetically susceptible and electrically conductive by the sequential deposition of nickel and gold films. The metallized microspheres undergo directed assembly into lithographically defined magnetic arrays functionalized with self-assembled monolayers of prototypical molecular wire candidates. We characterize the resulting junctions by scanning electron microscopy and measure their current-voltage characteristics. Magnetic directed assembly provides a wafer-level route for the fabrication of molecular junctions and opens up the potential for hybrid complementary metal-oxide semiconductor∕molecular electronic applications.

Origin of discrepancies in inelastic electron tunneling spectra of molecular junctions

We report inelastic electron tunneling spectroscopy (IETS) of multilayer molecular junctions with and without incorporated metal nanoparticles. The incorporation of metal nanoparticles into our devices leads to enhanced IET intensity and a modified line shape for some vibrational modes. The enhancement and line-shape modification are both the result of a low lying hybrid metal nanoparticle-molecule electronic level. These observations explain the apparent discrepancy between earlier IETS measurements of alkane thiolate junctions by Kushmerick et al. [Nano Lett. 4, 639 (2004)] and Wang et al. [Nano Lett. 4, 643 (2004)].

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.

Nanotechnology: Molecular transistors scrutinized.

Transistors have been made from single molecules, where the flow of electrons is controlled by modulating the energy of the molecular orbitals. Insight from such systems could aid the development of future electronic devices.

Metal-molecule contacts

Metal-molecule contacts play a key role in determining the current-voltage ( I - V ) characteristics of a molecular junction. This article reviews specific ways that metal-molecule contacts can be controlled to impart a desired functionality, and highlights some unanswered questions.

Probing stress effects in single crystal organic transistors by scanning Kelvin probe microscopy

We report scanning Kelvin probe microscopy (SKPM) of single crystal difluoro bis(triethylsilylethynyl) anthradithiophene (diF-TESADT) organic transistors. SKPM provides a direct measurement of the intrinsic charge transport in the crystals independent of contact effects and reveals that degradation of device performance occurs over a time period of minutes as the diF-TESADT crystal becomes charged.