Transport properties of touching molecules

Advances in the understanding of transport through individual molecules make the next critical issue to characterize charge transport from one single molecule to another one. This talk addresses recent results on electron transport between two touching C$_{60}$ molecules [Phys. Rev. Lett. 103, 206803 (2009)]. The tip of a scanning tunneling microscope was used to pick up single C$_{60}$ molecules from C$_{60}$ islands prepared on Au or Cu substrates. With such C$_{60}$-tips contact experiments were performed on both clean surfaces and on other C$_{60}$ molecules. We found that while the contact conductance of a single molecule between two Cu electrodes can vary up to a factor of 3 depending on electrode geometry, the conductance of a C$_{60}$-C$_{60}$ contact is consistently lower by 2 orders of magnitude. First-principles transport calculations reproduce the experimental results, allow a determination of the actual C$_{60}$-C$_{60}$ distances, and identify the essential roles of the molecule-metal interface and the intermolecular link on the contact conductance. The theoretical analysis thereby provides insight into the limiting factors for transport in C$_{60}$ junctions.