Surface defects on MgO thin films: Their detection using metastable impact electron spectroscopy and interaction with probe molecules

MgO thin films having different defect densities are explored in this study using metastable impact electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS), temperature programmed desorption (TPD), and scanning tunneling microscopy (STM). Surface point defects on MgO exhibit themselves in both the MIES and UPS spectra as a feature approximately 2 eV above the valance band, whereas extended defects are only observed spectroscopically as a broadening of the O 2p band. The interaction of NO and N₂O with the MgO surface as a function of surface defect density is explored. Upon adsorption on MgO thin films at 100 K, both NO and N₂O show the development of three features which coincide with a standard gas phase N₂O spectrum. The saturation coverage of N₂O from NO adsorption increases with increasing defect density, indicating that defect sites are mainly responsible for N₂O formation. STM images confirm the increase of thin film defect density upon thermal quenching.