Change of the surface electronic structure of Au(111) by a monolayer MgO(001) film

Monolayer films of MgO(001) have been prepared on an Au(111) surface and explored by means of scanning tunneling microscopy (STM) and spectroscopy. The symmetry mismatch between the hexagonal substrate and the squared overlayer results in the formation of a (6 \ifmmode\times\else\texttimes\fi{} 1) superlattice, as revealed from the distinct stripe pattern observed in the STM images. The presence of the oxide film also modifies the potential situation at the interface, which induces a substantial upshift of the Shockley-type surface band on Au(111). The resulting MgO/Au interface band is characterized by a pseudogap at around 500 mV that opens at the position of the new Brillouin zone of the enlarged (6 \ifmmode\times\else\texttimes\fi{} 1) unit cell. In addition the oxide layer gives rise to a drastic decrease of the Au(111) work function, as deduced from the energy position of the first field-emission resonance on the bare and MgO-covered surface. The work-function drop is explained by an interfacial charge transfer from the oxide film into the electro-negative gold surface.