Insulators at the ultrathin limit: electronic structure studied by scanning tunnellingmicroscopy and scanning tunnelling spectroscopy

Abstract

Considerable progress has been made recently, using scanning tunnelling microscopy (STM),scanning tunnelling spectroscopy (STS) and local density functional theory (DFT), inexamining the atomic structure and electronic properties of ultrathin insulating films. Thisarticle reviews pertinent results to date with special emphasis on ultrathin MgO films onAg(001) surfaces. Using STS, the layer-by-layer resolved electronic structure up to 3 MLshows that the band gap of about 6 eV at the MgO surface develops within the first 3 MLconfirmed by local density of states (LDOS) calculations. Using model calculations, theatomic species observed in STM on the MgO film are unambiguously identified. Theseresults underline the importance of a combination of local spectroscopy, scanning probetechniques and local density of states calculations for the understanding of matter on themicroscopic level.