Interface Oxygen Induced Internal Structures of Ultrathin MgO Islands Grown on Ag(100)

Abstract

When imaged by scanning tunneling microscopy (STM) at constant current under nontopographic conditions, nanosized MgO islands grown on Ag(100) may show a fractal dark internal structure and/or appear embedded in the surface. On the contrary, when imaged under topographic conditions, the same islands appear as nearly flat plateaus. Islands showing different contrast and a different contrast dependence on sample bias have been often reported in the literature, but a thorough explanation is still missing. Here we demonstrate, on the basis of of experimental data and computational results, that such structure is due to the accumulation of oxygen at the MgO islands− substrate interface. Interestingly, interfacial oxygen concentrates at the center of the islands.

Growth-mode and interface structure of epitaxial ultrathin MgO/Ag(001) films

“…The discrepancy between the interface distance obtained here and most of the DFT calculations is in favor of the presence of extra oxygen at the interface. Accumulation of oxygen at the interface was recently demostrated, under well defined growing conditions, by a carefull comparison of STM data and DFT calculations [36]. Oxygen atoms would be located in the hollow site of the Ag top layer.…”

Bonding Properties of Isolated Metal Atoms on Two-Dimensional Oxides

“…58 This scheme (DFT+D2′) improves the description of adsorption phenomena on ionic oxide surfaces with respect to the original D2 but also leads to metal−oxide interfacial properties similar to those obtained with the less empirical van der Waals functionals, as proven for the paradigmatic case of MgO/Ag. 37,56 The magnetic moment of the adsorbed adatoms is determined recurring to spin-polarized calculations. Dipole corrections to the energy are applied along the nonperiodic direction of the cell.…”

“…The long-range dispersion forces are added according to the two-body semiempirical method (DFT+D2) originally proposed by Grimme, where the C6 and R0 parameters of the metal cations in the oxides are rescaled accounting for their partial ionic character . This scheme (DFT+D2′) improves the description of adsorption phenomena on ionic oxide surfaces with respect to the original D2 but also leads to metal–oxide interfacial properties similar to those obtained with the less empirical van der Waals functionals, as proven for the paradigmatic case of MgO/Ag. , …”

“…It is worth remembering that the adsorption and stabilization of single adatoms on oxidic substrates is a topic whose relevance goes beyond the proof of concept on the charge transfer and electron tunnelling hereby described. On the contrary, single metal atoms stabilized on oxidic or carbon-based supports are seen as the ultimate form of dispersed catalytically active metal particles in heterogeneous catalysis, the so-called single atom catalyst. − Similarly, the magnetic properties of transition metal and rare earth single atoms supported on MgO/Ag have been intensively studied, representing, in principle, the smallest possible switchable memory device. − …”

“…As for the oxide/metal substrates, the present study includes, beyond the aforementioned structures (MgO ML and BL supported on Ag(100) and Mo(100) ,− and ZnO BL supported on (111) coinage metal surfaces), other systems that we have already characterized in the past: the SiO 2 BL supported on Pt(111) and Ru(0001), the TiO 2 lepidocrocite BL on Pt(111) and Ag(100), and the ZrO 2 ML supported on Pt(111) and on a ZrPt alloy …”