Scanning tunneling microscopy studies of copper nitride films

Scanning Tunneling Microscopy (STM) is used to study the geometric and electronic structure of Cu2N grown on the Cu(100) surface. Cu2N is a reconstruction of the Cu(100) surface caused by the adsorption of nitrogen atoms and shows insulating properties. A fundamental motivation for this work was to study the properties and size dependent evolution of an ultrathin insulating film. Insulating films of only a few atomic layers offer insight into the evolution of electronic structure and have been used to control electronic coupling at the nanoscale.;We find that the Cu2N structure is incommensurate with the Cu(100) lattice and the resulting strain contributes to the formation of Cu 2N islands. In atomic resolution images of the Cu2N islands, nitrogen atoms appear as protrusions at negative bias whereas at positive bias fourfold hollow sites appear as protrusions. Furthermore, at low bias both the hollow sites and nitrogen atoms are imaged as protrusions. These observations resolve uncertainties over the interpretation of STM topographs of Cu2N. We report four point defects that occur on the Cu 2N films and indentify the cause of two of the defects.;Our tunneling spectra indicate that Cu2N acts as an insulator, with a band gap that exceeds 4 eV. We study changes in this electronic structure with size, ranging from few-atom islands to complete films. We find that the conduction band edge first emerges in few-atom islands, and shifts toward lower energy with increasing island size, consistent with Density Functional Theory (DFT) calculations. Images of the local density of states show standing wave patterns consistent with the confinement of electrons to these two-dimensional islands. Measurements of the tunneling barrier height and image potential states indicate that the Cu2N work function is ∼0.9 eV larger than bare Cu. This suggests a significant surface dipole, consistent with charge transfer predicted by theory.