Self-Assembly and Electronic Properties of Conjugated Molecules: Towards Mono-Molecular Electronics

The use of single molecules as active components in electronic devices is presently considered a potential alternative to semiconductor-based nanoscale electronics since it directly provides precisely-defined nano-scale components for electronic devices which eventually allows for simple processing and device fabrication. In the present thesis the self-assembly and electron transport properties of conjugated molecules are investigated by means of scanning tunneling microscopy (STM) and spectroscopy (STS) at a solid-liquid interface as well as under ultrahigh vacuum conditions and low temperatures. The use of the molecules in hybrid-molecular electronic devices and potential approaches to a mono-molecular electronics are explored. In particular, electron-donor-acceptor-multiads are shown to exhibit a nano-phase-segregation at the solid-liquid interface which allows for the integration of different electronic functions, i.e. opposite rectification of tunneling currents, at the nano-scale. Furthermore, the dependence of the electronic coupling of stacked disk-like molecules on the lateral off-set in the stack is demonstrated experimentally which offers new possibilities for the control of the electronic properties of these three-dimensional architectures. In addition the first STM/STS experiments on single organic donor-acceptor complexes are presented which provide evidence for charge transfer processes at the single molecule level. Finally, charge transfer complexes are combined with the approach of nano-phase-segregation to realize the first single-molecule transistor with integrated nanometer-sized gates. In this prototypical device the current through a hybrid-molecular diode made from a hexa-peri hexabenzocoronene (HBC) in the junction of the STM is modified by charge transfer complexes covalently attached to the HBC in the gap. Since the donor which complexes the covalently attached acceptor comes from the ambient fluid the set-up represents a single-molecule chemical field-effect transistor with nanometer-sized gates. This is considered a major step towards mono-molecular electronics.