The electronic structure and morphology of ultrathin MgO films epitaxially grown on Ag(001) were investigated using low-temperature scanning tunneling spectroscopy and scanning tunneling microscopy. Layer-resolved differential conductance ( $dI/dU$) measurements reveal that, even at a film thickness of three monolayers, a band gap of about 6 eV is formed corresponding to that of the MgO(001) single-crystal surface. This finding is confirmed by layer-resolved calculations of the local density of states based on density functional theory.

Considerable progress has been made recently, using scanning tunnelling microscopy (STM), scanning tunnelling spectroscopy (STS) and local density functional theory (DFT), in examining the atomic structure and electronic properties of ultrathin insulating films. This article reviews pertinent results to date with special emphasis on ultrathin MgO films on Ag(001) surfaces. Using STS, the layer-by-layer resolved electronic structure up to 3 ML shows that the band gap of about 6 eV at the MgO surface develops within the first 3 ML confirmed by local density of states (LDOS) calculations. Using model calculations, the atomic species observed in STM on the MgO film are unambiguously identified. These results underline the importance of a combination of local spectroscopy, scanning probe techniques and local density of states calculations for the understanding of matter on the microscopic level. (Some figures in this article are in colour only in the electronic version)

Scanning tunneling microscopy at low temperature reveals that upon dissociative adsorption of oxygen on Ag(001) “hot” adatoms have separated in a far-ranged transient motion to two different intrapair distances around 2 and 4 nm, corresponding to 7 and 14 surface lattice constants, respectively. Manipulation experiments on oxygen atoms displaying different contrasts in the images and model calculations suggest that the transient motion ends up not only in the stable fourfold hollow site but also in two metastable sites.