Near-Field Scanning Optical Microscopy Applied to the Study of Ferroelectric Materials

Abstract

During the last century, the spatial resolution of the optical microscope reached its theoretical limit which is given by diffraction of light and hence close to its wavelength (Abbe, 1882). As a result, several techniques like electron microscopy, tunnel effect or Scanning Probe Microscopy (SPM), were proposed as alternatives to improve spatial resolution. Despite several optical techniques are beating the diffraction limit (Hell & Stelzer, 1992; Harke et al., 2008) in general optical microscopy resolution is considerably limited at the fast growing field of Nanotechnology. In fact, the optimization of electrooptical devices sometimes requires the study of material properties below the nanometre scale. For that purpose, the resolution of the Transmission Electron Microscope (TEM) is the highest one being able to observe the atomic structure (and composition) of a vast number of compounds. The inconveniences of this powerful technique are related with the preparation of the material under study since the sample thickness must be reduced to 100 nm. Alternatively, the Scanning Electron Microscope (SEM) is often used for structural characterization when atomic resolution is not mandatory. The SEM is a valuable tool for imaging metallic or semiconductor samples, but it results less useful for characterizing dielectric materials. For this reason, Atomic Force Microscopy (AFM) could be considered the best option for studying ferroelectric surfaces. Other than morphology, the measurement of physical magnitudes can be also performed by means of the AFM (Asenjo et al., 2006; Cefali et al., 2003; Kwak et al., 2000), which leads to another important advantage with respect to electron microscopes. In fact, most of the experiments previously performed in different fields of Material Science have been revisited at the nanometer scale by means of advanced SPM techniques. Given the great variety of them, in this chapter we focus our attention in the Near-field Optical Scanning Microscope (NSOM), because of during last years it has been revealed as a powerful technique for studying ferroelectric domains and domain walls in a non-invasive way, (Eng & Gutherodt, 2000; Lamela et al., 2007, 2009; Lifante et al., 2008). The AFM can be considered the simplest but also the most versatile scanning probe instrument, since most of the SPMs are developed on the basis of an AFM, as described in Section 2. The NSOM is one of these rather recent techniques, which allows the microscope user to study optical features and correlate with the topography that is being registered