Force-distance curves by atomic force microscopy

Atomic force microscopy (AFM) force-distance curves have become a fundamental tool in several fields of research, such as surface science, materials engineering, biochemistry and biology. Furthermore, they have great importance for the study of surface interactions from a theoretical point of view. Force-distance curves have been employed for the study of numerous materials properties and for the characterization of all the known kinds of surface forces. Since 1989, several techniques of acquisition and analysis have arisen. An increasing number of systems, presenting new kinds of forces, have been analyzed. AFM force-distance curves are routinely used in several kinds of measurement, for the determination of elasticity, Hamaker constants, surface charge densities, and degrees of hydrophobicity. The present review is designed to indicate the theoretical background of AFM force-distance curves as well as to present the great variety of measurements that can be performed with this tool. Section 1 is a general introduction to AFM force-distance curves. In Sections 2-4 the fundamentals of the theories concerning the three regions of force-distance curves are summarized. In particular, Section 2 contains a review of the techniques employed for the characterization of the elastic properties of materials. After an overview of calibration problems (Section 5), the different forces that can be measured with AFM force-distance curves are discussed. Capillary, Coulomb, Van der Waals, double-layer, solvation, hydration, hydrophobic, specific and steric forces are considered. For each force the available theoretical aspects necessary for the comprehension of the experiments are provided. The main experiments concerning the measurements of such forces are listed, pointing out the experimental problems, the artifacts that are likely to affect the measurement, and the main established results. Experiments up to June 1998 are reviewed. Finally, in Section 7, techniques to acquire force-distance curves sequentially and to draw bidimensional maps of different parameters are listed.