Observation of individual organic molecules at a crystal surface with use of a scanning tunneling microscope

A scanning tunneling microscopy (STM) study of the surface of a tetrathiafulvalene-tetracyanoquinodimethane) TTF-TCNQ crystal is described. The surface of this conducting organic molecular crystal is well ordered, with flat terraces separated by steps of a single molecular layer and with periodicity that agrees with the bulk crystal structure. Individual organic molecules at the surface are resolved in the STM images. The principal features of the experimental STM images are reproduced in simulations in which contours of constant electron density are calculated from the molecular orbitals that are involved in conduction.