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Oxide ultra-thin films on metals represent a new class of materials with promising properties for applications in microelectronics, magnetic devices and as support in catalysis. Films of a few atomic layers may exhibit structures that are completely different from those of bulk oxides and display many different phases, depending on the growth conditions. We report the results obtained for silica films grown on Mo(112) and for FeO/Pt(111). , , In this latter case, the structural and electronic properties responsible for the Moiré pattern experimentally observed have been adressed.
In some cases, even though the structural and electronic properties of the ultra-thin films are similar to those of the massive material, the films may display unexpected properties. In particular, when the oxide film contains a few atomic layers, charge transfer can occur from the metal substrate-oxide interface to the adsorbed species through the oxide film. , Metal atoms with high electron affinity become negatively charged on MgO supported on Mo(100) and Ag(100), while the electropositive K atoms become K+. This effect has been confirmed experimentally by the long-range order and by the spot appearance observed in the STM images for gold atoms on MgO/Ag(100). ,
These findings have been rationalized in terms of change of the metal work function upon deposition of the oxide films. The charge transfer through the MgO film is possible because the oxide induces a decrease of the metal substrate work function. For Au atoms on ultra-thin crystalline films of other oxides, like SiO2/ Mo(112), no charge transfer occurs because of the high work function of this system.
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