STM and STS of oxide ultrathin films

Ultrathin oxide films consist of a few monolayers of oxide material on a support and are generally no more than a nanometer thick. For example, a typical oxide ultrathin film might consist of a monolayer of titanium oxide that wets the surface of a gold crystal. The interaction of the film with the substrate is a critical element in determining the stability of the film and its atomic and electronic structure. Because ultrathin oxide films are so thin, the atoms that make up the film are simultaneously both surface and interface atoms. This gives rise to material structures that would otherwise not be formed, and results in unique environments for atoms or molecules interacting with the film in a gas or liquid environment. The study of oxide ultrathin films is important due to their relevance in catalysis, corrosion, and microelectronics. The properties of the films are determined to a significant extent by their defects. Scanning tunneling microscopy (STM) offers a unique way to image at atomic resolution the films and their defects, which may manifest themselves as point defects such as vacancies and impurities or as extended defects related to structural boundaries. These atomic structure studies can be complemented by scanning tunneling spectroscopy (STS), which provides local electronic structure information within a few eV around the Fermi energy