Charge transport in single molecular junctions at the solid/liquid interface.

Charge transport characteristics in metal-metal nanocontacts and single molecular junctions were studied at electrified solid-liquid interfaces employing a scanning tunneling microscope-based break junction technique, in combination with macroscopic electrochemical methods, in non-conducting solvents and in an electrochemical environment. We aim to demonstrate recent attempts in developing fundamental relationships between molecular structure, charge transport characteristics, and nanoscale electrochemical concepts. After an introduction and brief description of the experimental methodology, a case study on the electrical and mechanical properties of gold atomic contacts in aqueous electrolytes is presented. In experiments with alkanedithiol and α,ω-biphenyldithiol molecular junctions the role of sulfur-gold couplings and molecular conformation, such as gauche defects in alkyl chains and the torsion angle between two phenyl rings, are addressed. The combination with quantum chemistry calculations enabled a detailed molecular-level understanding of the electronic structure and transport characteristics of both systems. Employing the concept of "electrolyte gating" to 4,4'-bipyridine and redox-active molecules, such as perylene bisimide derivatives, the construction of "active" symmetric and asymmetric molecular junctions with transistor- and diode-like behavior upon polarization in an electrochemical environment will be demonstrated. The latter experimental data could be represented quantitatively by the Kutznetsov/Ulstrup model, assuming a two-step electron transfer with partial vibration relaxation. Finally, we show that (individual) surface-immobilized gold clusters within the quantum-confined size range exhibit features of locally addressable multistate electronic switching upon electrolyte gating, which appears to be reminiscent of a sequential charging through several redox states. The examples addressed here demonstrate the uniqueness and capabilities of an electrochemical approach for the fundamental understanding and for potential applications in nano- and molecular electronics.