Scanning Tunneling Microscopy of an Organic Conductor

We report a scanning tunneling microscopy (STM) study of the surface of a TTF-TCNQ crystal. Individual organic molecules at the surface of an organic molecular crystal are resolved in STM images for the first time. The high electrical conductivity of TTF-TCNQ allows individual molecules to be imaged with STM without depositing them on a conducting substrate. We show that the appearance of the molecules in these STM images can be explained qualitatively in terms of the electron density distributions of the molecular orbitals that are involved in conduction. The TTF-TCNQ surface is well ordered, with flat terraces that are separated by steps of a single molecular layer. The observed periodicity on the surface and the observed step heights agree with the bulk crystal structure.