Directed assembly of single molecules in electronic junctions: basic electronic conduction paths and switching mechanism

Single-molecule electronics aims at realizing electronic functions by integration of suitable single molecules. The approach is geared towards the final limit of size reduction as molecules are the smallest objects still providing the required structural diversity. Utmost importance has to be paid to distinguish intrinsic molecular properties from effects due to contacts or the measurement. In most cases single-molecule contacts are created in a scanning tunneling microscope (STM) or in mechanically controlled break junctions (MCBJ). Both methods differ in the control of and the access to parameters that influence electronic transport. Furthermore, different requirements have to be fulfilled by the molecules used in these experiments. Therefore, the molecules used in the setups are usually very different, and even the results of transport measurements of similar molecules obtained in different settings are often not comparable.

In this project we develop a molecular platform that is suited for both investigation methods and can serve as a base module for functional units. The contacts of this platform to the electrodes should be reproducible and well-conducting. In addition it should form a self-sustained protruding post that enables flow of electronic current along the subunits designated for that purpose only.

Scanning tunneling microscopy & spectroscopy and measuring the elastic and inelastic electron transport in break junctions gives information about the contact geometry and about the parts of the molecules that contribute to electron transport. Furthermore the vibrational modes excited by the measurement carry information about the molecule-electrode conformation and about the interaction of conduction electrons and the vibrational modes.

In addition, the modular form of the molecule allows for an investigation of fundamental functional mechanisms on the molecular scale, e.g. molecular switches, by integration of functional moieties into the base molecule.

This new, interdisciplinary approach in combination with the complementary measurement techniques will lead to a detailed insight into the molecular transport channels and their coupling to the contacting electrodes. This knowledge can then be used to establish basic design rules for tailoring of molecular functional units.