  Ultrathin MgO films on Mo(100) with a thickness up to 12 ML are studied by
scanning tunneling microscopy and spectroscopy at room temperature. The spatial
variation of the work function within the MgO film is mapped by field emission
resonance states (Gundlach oscillations) using dz/dU spectroscopy. We found
circular spots with significantly reduced work function (DeltaPhi=0.6 eV),
which are assigned to charged defects within the MgO film. On top of the MgO
films, small Co cluster are deposited with an average contact area of 4 nm^2.
These islands exhibit Coulomb oscillations in dI/dU spectra at room
temperature. Good agreement with orthodox theory is achieved showing variations
of the background charge Q_0 for islands at different positions, which are in
accordance with the work function differences determined by the Gundlach
oscillations.
