Formation of Metal−Molecule−Metal Tunnel Junctions:  Microcontacts to Alkanethiol Monolayers with a Conducting AFM Tip

We report the characterization of metal-molecule-metal tunnel junctions made by contacting Au-supported self-assembled monolayers (SAMs) of alkanethiols with a conducting atomic force microscope (AFM) tip, Scheme 1. The electrical properties of individual molecules and molecular assemblies are currently of heightened interest because of potential applications in molecular electronics and new opportunities for understanding charge transport in organic systems.1 Molecular level electrical transport studies require innovative approaches for making electrical contacts to oriented molecules. Scanning tunneling microscopy (STM)2 and electrochemical methods3 have been used for a number of years to examine transport in surface-confined molecules. In STM and electrochemistry, the molecules are in direct contact with one metal electrode and charge is delivered by vacuum tunneling or redox molecules, respectively. More recently, metal-molecule-metal junctions have been fabricated by assembling molecules inside metal-capped nanopores4 and mechanical “break junctions”,5 or between mercury drops,6 nanofabricated electrodes,7 and crossed wires.8 Metallic nanoparticles have also been used as electrical contacts to molecular monolayers supported on metal surfaces.9 Self-assembly or Langmuir-Blodgett techniques are commonly employed in these studies because they are convenient approaches to immobilizing molecules at metal surfaces.