The fabrication and characterization of metallic nanometer-sized gaps suitable for conductivity measurements of single molecules were studied. Controlled gap formation by electromigration (EM) is demonstrated in contiguous and ultrathin Ag structures wetting the Si(100) substrate. The gaps obtained are in the range of nanometers or even subnanometers, as revealed by lateral conductivity measurements and scanning tunneling microscopy carried out under ultrahigh vacuum conditions. Annealing to 300K closes the gap by enabling surface diffusion of Ag, and another cycle of opening by EM at 80K can be performed. The functionality of the contacts is demonstrated by insertion of ferrocenedithiol molecules into the gap.

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Single and double barrier structures have been realized with Ag nanostructures grown on Si͑111͒ in combination with scanning tunneling microscopy. The series connection of a Schottky ͑SB͒ and tunneling barrier mimics a double barrier structure showing Coulomb blockade oscillations as revealed by scanning tunneling spectroscopy ͑STS͒. Although the SB remains in presence of a Ag ͱ 3 ϫ ͱ 3 reconstruction, the dI / dV characteristic turns into a single barrier structure. The Ag reconstruction provides a sufficiently high electron mobility capturing intrinsic defects which shortens the resistance of the SB. These results show that vertical transport properties, as measured with STS, are not only controlled by the structure and the bonding on the atomic scale, but depend strongly on the lateral properties of the interface as well.

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