Gundlach oscillations and Coulomb blockade of Co nanoislands on MgO/Mo(100) investigated by scanning tunneling spectroscopy at 300 K

Abstract

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 in 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 $(\ensuremath{\Delta}\ensuremath{\Phi}=0.6\text{ }\text{eV})$, which are assigned to charged defects within the MgO film. On top of the MgO films, small Co clusters are deposited with an average contact area of ${A}_{\text{Co}}\ensuremath{\simeq}4\text{ }{\text{nm}}^{2}$. These islands exhibit Coulomb oscillations in $dI/dU$-spectra at room temperature. Good agreement with orthodox theory is achieved showing variations in the background charge ${Q}_{0}$ for islands at different positions, which are in accordance with the work function differences determined by the Gundlach oscillations.