Charge Transport Through Single-Molecule Junctions

Abstract

In the pursuit of down-sizing electronic components, the ultimate limit is the use of single molecules as functional devices. Molecules can also provide additional functionalities compared to semiconductors. Using synthetic chemistry, an almost endless choice of molecular structures and compositions is available. With the proper knowledge, one can design single molecules that can perform a variety of tasks which would be unthinkable in solid-state devices, or would require a very high degree of complexity. Ultimately, full-scale electronic circuit may be built from single molecules, possibly by means of self-assembly. Despite the tremendous opportunities molecules offer, the research field of molecular electronics is still in its infancy, and many challenges remain to be solved, such as how to make reliable electric contact to single molecules, and what is the role of the molecule/electrode interface in determining charge transport. In the first part of this thesis, we developed a single-molecule transistor. Transistors are essential electronic components, in which the current between the source/drain electrodes can be tuned using a gate electrode. Using this transistor, we demonstrated the dominant role of image-charges in determining the alignment of the molecular level responsible for transport with respect to the Fermi energy of the electrodes. We found that image-charges lead to a renormalization of the transport gap when the molecule is brought closer to the electrodes. In the second part, we developed a quantum chemical model which allows us to describe and predict molecular charge transport properties for a specific class of molecules. With this knowledge, we fabricated the first resonant tunneling device exhibiting negative differential conductance. Our model also lead to the implementation of the first efficient single-molecule diode. A unique aspect of the measurements is the presence of a gate electrode. As the gate voltage can be used to tune the alignment of the orbitals, the rectification ratio can be tuned over a considerable range. More importantly, the gate-dependent measurements demonstrate that the proposed intrinsic rectification mechanism is operative in the molecule. Altogether, our findings provide guide-lines for designing single-molecule components, linking molecular structure to electronic functionality.