Dynamic Modes of Atomic Force Microscopy

Abstract

This chapter presents an introduction to the concept of the dynamic operational modes of the atomic force microscope (AFM). While the static (or contact-mode) AFM is a widespread technique to obtain nanometer-resolution images on a wide variety of surfaces, true atomic-resolution imaging is routinely observed only in the dynamic mode. We will explain the jump-to-contact phenomenon encountered in static AFM and present the dynamic operational mode as a solution to avoid this effect. The dynamic force microscope is modeled as a harmonic oscillator to gain a basic understanding of the underlying physics in this mode. Under closer inspection dynamic AFM comprises a whole family of operational modes. A systematic overview of the different modes typically found in force microscopy is presented, and special attention is paid to the distinct features of each mode. Two modes of operation dominate the application of dynamic AFM. First, the amplitude-modulation mode (also called the tapping mode) is shown to exhibit an instability, which separates the purely attractive force interaction regime from the attractive–repulsive regime. Second, the self-excitation mode is derived and its experimental realization is outlined. While the tapping mode is primarily used for imaging in air and liquid, the self-excitation mode is typically used under ultrahigh vacuum (UHV) conditions for atomic-resolution imaging. In particular, we explain the influence of different forces on spectroscopy curves obtained in dynamic force microscopy. A quantitative link between the experimental spectroscopy curves and the interaction forces is established. Force microscopy in air suffers from the small quality factors of the force sensor (i.e., the cantilever beam), which are shown to limit the resolution. Also, the aforementioned instability in the amplitude-modulation mode often hinders imaging of soft and fragile samples. A combination of the amplitude modulation with the self-excitation mode is shown to increase the quality, or Q-factor, and extend the regime of stable operation. This so-called Q-control module allows one to increase as well as decrease the Q-factor. Apart from the advantages of dynamic force microscopy as a nondestructive, high-resolution imaging method, it can also be used to obtain information about energy-dissipation phenomena at the nanometer scale. This measurement channel can provide crucial information on electric and magnetic surface properties. Even atomic-resolution imaging has been obtained in the dissipation mode. Therefore, in the last section, the quantitative relation between the experimental measurement channels and the dissipated power is derived.