Molecular junctions have attracted a great deal of attention recently due to their importance in the new field of Molecular Electronics. Electron transport in such junctions is a result of a complex interplay of many factors, including molecular electronic structure, adsorption configuration, and chemical environment.
Scanning Tunneling Microscopy (STM) is a powerful approach to molecular junction characterization because of its ability to provide atomic-level detail about the electronic structures and morphologies of these junctions. By using this approach, we investigated for the first time, a number of fundamental properties of molecular junctions:
1) By measuring the current-voltage characteristics of individual molecules, spectroscopic information about their orbital structure and current-induced vibrational excitation/nuclear motion was obtained.
2) Injection of high-energy electrons into molecules leads to photon emission sensitive to the molecular adsorption configuration, enabling a novel form of molecular optical spectroscopy with spatial resolution at the atomic scale.
3) The capability of STM to manipulate individual atoms and molecules was used to assemble artificial molecular junctions, where the chemical effects associated with the molecular junction formation were probed.
These results demonstrate both the power of STM as a spectroscopic tool, and the need for atomic-scale approaches to characterization of nanoscale molecular materials and systems.