Electron confinement effects on Ni-based nanostructures

Abstract

Scanning tunnelling microscopy (STM) and scanning tunnelling spectroscopy(STS) are very efficient techniques for studying confinement effects onnanostructured surfaces. Noble metal (Cu, Ag, Au) and near-noble metal (Ni, Pd,Pt) (111) surfaces present a dispersive surface state (the Shockley state)that allows a direct investigation of these effects in real space. Both theenergy spectrum and the spatial distribution of the wavefunctions can beinvestigated using spectroscopic imaging techniques. We present a verysimple model for the confinement of surface states which is sufficient toperform a first-order analysis of our experimental data on two kinds ofsurface resonators (quantum boxes and linear resonators). We summarizethe different acquisition modes in STM/STS that allow the study of theconfinement effects via the mapping of the local density of states. Thesepoints are illustrated for the well-known case of the Cu(111) surface.The main difference between Ni-based nanostructures and similar objectson the clean Cu(111) surface is that d states are present in the energyrange where the Shockley surface state is expected. We report a combinedexperimental and theoretical analysis of the dispersion of the surface stateon the clean Ni(111) surface and on a mixed Ni–Cu(111) phase (βphase). In both cases the analysis of the electronic structure of the surface ispossible only in situations of strong confinement. The presence of d states is foundto affect the characteristics of the Shockley state, which opens interestingperspectives.