Atomic-Resolution AFM in Liquids

The aim of this thesis was to modify an existing AFM to allow for highest-resolution FM imaging in liquids. Based on a commercially available atomic force microscope, the new setup now provides true atomic-resolution imaging in various liquid environments. In order to achieve the required high force sensitivity, the first step is to understand the details of FM AFM theory. Based on an in-depth theoretical consideration of the maximum achievable force sensitivity, presented in Chap. 2, the key parameters influencing the performance of an FM AFM system are identified. In a next step, the instrumentation of FM AFM will be presented from a more engineering point of view. The original design of the used commercial microscope has been designed to suit a brought range of AFM applications, which is of great advantage when using one instrument for different kinds of experiments. To achieve the highest force sensitivity, however, it is necessary to specialize the instrument with the high-resolution application in mind. Care was taken to develop a setup that can be changed to the original setup within a few minutes. In Chap. 3, all improvements of the entire instrument are explained in detail. All modifications were performed with the aim to preserve the instrument's ease of use. This was demonstrated in industry, where the developed setup was tested by technicians with no experience in FM AFM, revealing true atomic resolution in a liquid environment. This clearly demonstrates the high usability of the described instrument in industrial research. The high-resolution capability of the modified instrument was then exploited to resolve the atomic structure of two different systems, being of utmost importance within both, fundamental as well as application-oriented research. One substrate of interest is calcite, the most stable polymorph of calcium carbonate (CaCO 3 ), presented in Chap. 4. Calcite is one of the most abundant minerals on earth and plays an important role in a wide range of different fields including e.g. biomineralization 8-10 and environmental geochemistry. Especially the solid-liquid interface is of greatest interest. The performance of the developed setup was demonstrated, resolving single atomic-scale defects and kinked monatomic steps on the calcite surface. Beside performance check, the surface structure was analyzed and both sublattices, the carbonate as well the calcium sublattice, could be identified simultaneously. In Chap. 5, a second sample system is presented. Lithium niobate LiNbO3, is a man-made material, which exhibits unusually pronounced optical, piezoelectric, electro-optic, elastic, photoelastic and photorefractive properties 11. Besides traditional applications in fields of high-frequency signal processing, LiNbO3 constitutes an important material of choice for the fabrication of surface acoustic wave (SAW) devices. Thus, lithium niobate is used in microfluidic systems such as pumps, mixers and centrifuges 12, but also in chemical or biological sensors 13. Having these applications in mind, a fundamental research interest elucidating the surface structure and composition is directly evident. Consequently, many studies exist, dedicated to understanding the surface structure of LiNbO3. So far, however, these studies have been limited to length scales not capable of resolving individual atoms. Previous efforts to directly resolve the atomic-scale structure have failed because of the ferroelectric nature of LiNbO3. The ferroelctricity results in high surface charges, which cannot be screen or compensated in a UHV environment. The liquid environment naturally screens the long range electrostatic forces, thus, enabling true atomic-resolution imaging. The presented results are, thus, the first atomic scale images obtained on LiNbO3 surfaces. This clearly demonstrated the high impact of this new technique in fields of both fundamental and applied research.