Resolving oxide surfaces - From point and line defects to complex network structures

Abstract

In the following, we demonstrate the atomic-scale analysis of oxide surfaces. Essential physical properties were extracted using noncontact atomic force microscopy (nc-AFM) and scanning tunneling microscopy (STM). The main focus has been put on the determination of surface structures. A review of the recent achievements towards atomic-scale resolution from highly crystalline to amorphous materials is provided. An overview of local probe microscopy and spectroscopy to get beyond the averaging character of diffraction methods is thereby summarized. In particular, surface defects of various dimensionality were investigated. Furthermore, acquisition of information on electronic properties is detailed. The presented material covers zero-dimensional (0D) point defects, one-dimensional (1D) line defects, and two-dimensional (2D) random networks, i.e., amorphous structures. First, we present spectroscopy data taken on thin MgO films grown on Ag(001). Distance- and bias-dependent nc-AFM and STM measurements were recorded on these films. The local work-function shift and electronic structure of color centers in the MgO surface were studied. In the next section, the structure determination of ultrathin alumina/NiAl(110) is shown. Atomically resolved nc-AFM reveals a detailed picture of various line defects in the film. Finally, we discuss the atomic structure of a recently discovered ultrathin vitreous silica film on Ru(0001). The atomic arrangement in the 2D random network, resembling the classical picture of Zachariasen, is analyzed in terms of the pair correlation function and ring-size distribution. The determination of structure has always been in the focus of the scientific community. Still, diffraction methods are powerful tools in the field of surface science. However, they have limitations when it comes to the analysis of complex structures or materials without periodicity and order. This topic is clearly visible in the Feature Article by Markus Heyde et al. Here, the authors present how they have applied state of the art atomic force and scanning tunneling microscopy to verify oxide film structures ranging from zero-dimensional (0D) point defects, one-dimensional (1D) line defects to two-dimensional (2D) random networks, i.e. amorphous structures. The latter example has fully demonstrated the validity of Zachariasen's postulation and thereby unraveled for the first time the real-space structure of an amorphous solid in all of its details. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.