Abstract

The electronic properties of interfaces between two different solids can differ strikingly from those of the constituent materials. For instance, metallic conductivity—and even superconductivity—have recently been discovered at interfaces formed by insulating transition-metal oxides. Here, we investigate interfaces between crystals of conjugated organic molecules, which are large-gap undoped semiconductors, that is, essentially insulators. We find that highly conducting interfaces can be realized with resistivity ranging from 1 to 30|[thinsp]|k|[Omega]||[nbsp]|per|[nbsp]|square, and that, for the best samples, the temperature dependence of the conductivity is metallic. The observed electrical conduction originates from a large transfer of charge between the two crystals that takes place at the interface, on a molecular scale. As the interface assembly process is simple and can be applied to crystals of virtually any conjugated molecule, the conducting interfaces described here represent the first examples of a new class of electronic systems.