Force-distance curves by atomic force microscopy

Atomic force microscopy (AFM) force-distance curves have become a fundament al tool in several fields of research, such as surface science, materials e ngineering, biochemistry and biology. Furthermore, they have great importan ce for the study of surface interactions from a theoretical point of view. Force-distance curves have been employed for the study of numerous material s properties and for the characterization of all the known kinds of surface forces. Since 1989, several techniques of acquisition and analysis have ar isen. An increasing number of systems, presenting new kinds of forces, have been analyzed. AFM force-distance curves are routinely used in several kin ds 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 AF M force-distance curves as well as to present the great variety of measurem ents that can be performed with this tool. ection 1 is a general introduction to AFM force-distance curves. In Section s 2-4 the fundamentals of the theories concerning the three regions of forc e-distance curves are summarized. In particular, Section 2 contains a revie w of the techniques employed for the characterization of the elastic proper ties of materials. After an overview of calibration problems (Section 5), t he different forces that can be measured with AFM force-distance curves are discussed. Capillary, Coulomb, Van der Waals, double-layer, solvation, hyd ration, hydrophobic, specific and steric forces are considered. For each fo rce the available theoretical aspects necessary for the comprehension of th e experiments are provided. The main experiments concerning the measurement s of such fords are listed, pointing out the experimental problems, the art ifacts that are likely to affect the measurement, and the main established results. Experiments up to June 1998 are reviewed. Finally, in Section 7, t echniques to acquire force-distance curves sequentially and to draw bidimen sional maps of different parameters an listed. (C) 1999 Elsevier Science B. V. All rights reserved.