Synthesis and Preparation of Oxide Ultrathin Films

There is a steadily growing interest in the research on oxide surfaces and films, due to their challenging fundamental properties and to their actual and potential applications in catalytic systems, chemical sensors, electronic and magnetic devices, and functional and aesthetic coatings. Special efforts have been directed at the study of ultrathin oxide films [1–11]. Such two-dimensional systems are emerging as important new materials where the relevant phenomena are induced by the extreme vertical confinement, and new phases and structures are stabilized, which cannot be obtained in bulk form. Last but not least, the possible use of thin oxide films as model systems to substitute for bulk oxides has been shown to be very appealing. Critical properties basically depend on the reduced dimensionality, as well as on the stoichiometry, defectivity, and morphology of the films, on the extent of the crystalline order, and on the sharpness of the interfaces between film and substrate or between different films in multilayers, which are to a great extent determined by the preparation method. Therefore a main driving force in the development of oxide materials in the form of ultrathin films has been the progressive improvement of the fabrication procedures. Since the 1950s, studies of finite size effects in ferroelectrics pointed to a critical film thickness (typically 10 nm) below which ferroelectricity disappears [12]. Improved fabrication techniques have completely changed the picture. Studies on singlecrystal, “perfect” ultrathin films have led to the conclusion that perovskite layers down to a very few nanometers in thickness remain ferroelectric [13]. New fabrication methods, alternatives to the conventional thermal oxidation, have enabled the thickness of the gate silicon oxide in metal–oxide–semiconductor field effect transistors (MOSFETs) to decrease to sub-nanometric dimensions, thus reducing both power consumption and power dissipation [14]. The expected, unprecedented properties of ultrathin NiO remained undisclosed until the stabilization of a stoichiometric (1 1)–1ML NiO/Ag(001) phase with