I am interested in various aspects of electrochemical charge transfer, in particular at the single-molecule level. This includes redox systems as diverse as small transition metal complexes, metal nanoparticles and biomolecules. We mainly employ electrochemical scanning tunnelling microscopy (ECSTM), which combines ultimate spatial resolution (atoms!) with the detection of ultra-low currents (~ pA). Other approaches are chip-based nanoelectrodes and various electrochemical techniques using single-crystal metal substrates and microelectrodes. Such a well-equipped toolbox gives the opportunity to study both fundamental and applied aspects of single-molecular electron transport. How does the immediate environment of a molecule influence its electron transport properties? Can one use such a configuration as device components in nanoscale electronic circuitry ("molecular electronics")? Is it possible to use such a concept in innovative sensor applications?
Redox molecules can be brought to display tunnelling current characteristics that are reminiscent of electronic device components, such as diodes, transistors, and switches. These features could thus be exploited in nanoscale and molecular electronics on the way to ever smaller electronic circuitry. The figure illustrates the experimental setup, an electrochemical scanning tunnelling microscope (STM), along with the expected tunnelling current response for different tunnelling experiments. To the left is an example of diode-like behaviour in bias spectroscopy. The tunnelling current is recorded while the difference between tip and substrate potentials is changed (called “bias voltage”).
The figure describes the tunnelling current characteristics similar to a transistor for an electrochemical gate configuration. Both tip and substrate potentials are changed at the same time, keeping the bias voltage constant. These experiments can be performed on individual molecules bound to a conducting substrate. While it remains to be seen if the performance of electrochemically controlled devices can compete with established electronic components and circuit architectures, the above features open up the perspective of true single-molecule electrochemistry. This includes the study of fundamental charge transport in a wide range of electrochemically active systems and their exploitation in innovative sensor and catalyst design.
For the first time ever, we could demonstrate single-molecule diode, transistor, and switching characteristics in an ionic liquid.
This is a detailed study of redox-mediated single-molecule charge transport in aqueous electrolyte. We determine the conductance per molecule, interfacial tunnelling rate constants, and bridge the gap between monolayer and single-molecule charge transport.
We identify inorganic transition metal complexes as a new class of redox-active switch molecules and show that the electron transport mechanism is hopping.