Energy level alignment at organic semiconductor/metal interfaces: effect of polar self-assembled monolayers at the interface.

We determined the shifts in the energy levels of approximately 15 nm thick poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] films deposited on various substrates including self-assembled monolayer (SAM) modified Au surfaces using photoelectron spectroscopy. As the unmodified substrates included Au, indium tin oxide, Si (with native oxide), and Al (with native oxide), a systematic shift in the detected energy levels of the organic semiconductor was observed to follow the work function values of the substrates. Furthermore, we used polar SAMs to alter the work function of the Au substrates. This suggests the opportunity to control the energy level positions of the organic semiconductor with respect to the electrode Fermi level. Photoelectron spectroscopy results showed that, by introducing SAMs on the Au surface, we successfully increased and decreased the effective work function of Au surface. We found that in this case, the change in the effective work function of the metal surface was not reflected as a shift in the energy levels of the organic semiconductor, as opposed to the results achieved with different substrate materials. Our study showed that when a substrate is modified by SAMs (or similarly by any adsorbed molecules), a new effective work function value is achieved; however, it does not necessarily imply that the new modified surface will behave similar to a different metal where the work function is equal to the effective work function of the modified surface. Various models and their possible contribution to this result are discussed.

Molecular wires, switches, and memories.

Design and measurements of molecular wires, switches, and memories offer an increased device capability with reduced elements. We report: Measurements on through-bond electronic transport properties of nanoscale metal-1,4-phenylene diisocyanide-metal junctions are reported, where nonohmic thermionic emission is the dominant process, with isocyanide-Pd showing the lowest thermionic barrier of 0.22 eV; robust and large reversible switching behavior in an electronic device that utilizes molecules containing redox centers as the active component, exhibiting negative differential resistance (NDR) and large on-off peak-to-valley ratio (PVR) are realized; erasable storage of higher conductivity states in these redox-center-containing molecular devices are observed; and a two-terminal electronically programmable and erasable molecular memory cell with long bit retention time is demonstrated.

Conductance switching in single molecules through conformational changes.

We tracked over time the conductance switching of single and bundled phenylene ethynylene oligomers isolated in matrices of alkanethiolate monolayers. The persistence times for isolated and bundled molecules in either the ON or OFF switch state range from seconds to tens of hours. When the surrounding matrix is well ordered, the rate at which the inserted molecules switch is low. Conversely, when the surrounding matrix is poorly ordered, the inserted molecules switch more often. We conclude that the switching is a result of conformational changes in the molecules or bundles, rather than electrostatic effects of charge transfer.

Synthesis and preliminary testing of molecular wires and devices.

Presented here are several convergent synthetic routes to conjugated oligo(phenylene ethynylene)s. Some of these oligomers are free of functional groups, while others possess donor groups, acceptor groups, porphyrin interiors, and other heterocyclic interiors for various potential transmission and digital device applications. The syntheses of oligo(phenylene ethynylene)s with a variety of end groups for attachment to numerous metal probes and surfaces are presented. Some of the functionalized molecular systems showed linear, wire-like, current versus voltage (I(V)) responses, while others exhibited nonlinear I(V) curves for negative differential resistance (NDR) and molecular random access memory effects. Finally, the syntheses of functionalized oligomers are described that can form self-assembled monolayers on metallic electrodes that reduce the Schottky barriers. Information from the Schottky barrier studies can provide useful insight into molecular alligator clip optimizations for molecular electronics.