Scanning Tunneling Microscopy of the Ordered Water Monolayer on MgO(001)/Ag(001) Ultrathin Films

Two-dimensionally ordered monolayers of water on MgO(001) have been extensively studied in the past using diffraction and spectroscopic and computational methods, but direct microscopic imaging has not been reported so far. Here, we present a scanning tunneling microscopy (STM) study, supported by infrared and X-ray photoelectron spectroscopy, of the c(4 × 2)-10H2O and p(3 × 2)-6H2O structures prepared on ultrathin MgO(001)/Ag(001) films. For the applied tunneling conditions, the contrast in the STM images originates from the hydroxyl groups, which result from water dissociation within the monolayer. The observed periodicities match the structures for the energetically most favorable c(4 × 2) and p(3 × 2) monolayer phases obtained from density functional calculations. Although the molecular water species within the monolayers, which are essential for the stabilization of the hydroxyl groups, could not be resolved, the STM results presented in this study provide further confirmation of the predicted structural models of the c(4 × 2)-10H2O and p(3 × 2)-6H2O monolayers.