Incommensurability and atomic structure of c(2x2)N/Cu(100):: A scanning tunneling microscopy study

We use a scanning tunneling microscope operating in a low-temperature ultrahigh vacuum environment to study the atomic structure of single layer films of Cu(2)N grown on Cu(100). The c(2x2) lattice of Cu(2)N is incommensurate, with a lattice constant of 0.372 +/- 0.001 nm that is 3% larger than the bare Cu(100) surface. This finding suggests that the strain due to lattice mismatch contributes to self-assembly in this system. We find that the image contrast on Cu(2)N islands depends on bias voltage, which reconciles several interpretations in the literature. We assign features in these STM images to the Cu, N, and hollow sites in the Cu(2)N lattice with the aid of coadsorbed CO molecules. This atomic registry allows us to characterize four different defects on Cu(2)N, which influence the sticking coefficient and electronic coupling of adsorbates.