One- and two-photon photoelectron spectroscopy used to study molecular electronics systems.

The drive to introduce organic molecular materials into electronic device applications, "organic electronics," is motivated by a number of potentially attractive features, such as ease of fabrication, ability to fabricate on flexible substrates, and the wide extent to which organic materials can be functionalized via organic synthetic methods. A range of applications is foreseen for organic field effect transistors and organic light emitting diodes including, for instance, flexible displays and other low-cost flexible electronics.

In addition to replacing inorganic semiconductors in more or less conventional device architectures, organic systems also are of interest in the more speculative, and potentially more revolutionary area of "molecular electronics." Here, it is envisioned that the non-linear characteristics of individual molecules, or small ensembles, will provide the required device functionality, allowing low-cost chemical synthetic methods to, at least partially, replace multibillion dollar semiconductor fabrication lines in the production of nanoscale device structures.

In either application format, charge injection at a molecule-contact interface plays a vital role in controlling transport and, thus, potential device performance. Interfacial charge injection is dictated by chemical bonding and the resulting band lineup between the Fermi level of the contact and transport levels (HOMO and LUMO) of the molecule. Information relating to band lineup is difficult to obtain using conventional techniques in the case of the single-molecule length-scale systems of interest in molecular electronics.

Researchers at NIST have employed a combination of one-photon (He I resonance excitation) and two-photon photoelectron spectroscopy, using subpico-second Ti:sapphire laser-based excitation, to determine the electronic structure, including injection barriers to both occupied and unoccupied levels, of 4,4'-bis-(phenylethynyl)benzenethiol self-assembled mono-layers, a system that has become a benchmark for theoretical and experimental studies in the area of molecular electronics.

Further work on the influence of chemical substitutions and measurements of electron dynamics are under way or planned for the future.

CONTACT: Steven Robey, [phone]; [e-mail] or Roger van Zee, [phone]; [e-mail] or Christopher Zangmeister, [phone]; [e-mail].