Dynamic atomic force microscopy methods

Abstract

In this report we review the fundamentals, applications and future tendencies of dynamic atomic force microscopy (AFM) methods. Our focus is on understanding why the changes observed in the dynamic properties of a vibrating tip that interacts with a surface make possible to obtain molecular resolution images of membrane proteins in aqueous solutions or to resolve atomic-scale surface defects in ultra high vacuum (UHV). Our description of the two major dynamic AFM modes, amplitude modulation atomic force microscopy (AM-AFM) and frequency modulation atomic force microscopy (FM-AFM) emphasises their common points without ignoring the differences in experimental set-ups and operating conditions. Those differences are introduced by the different feedback parameters, oscillation amplitude in AM-AFM and frequency shift and excitation amplitude in FM-AFM, used to track the topography and composition of a surface. The theoretical analysis of AM-AFM (also known as tapping-mode) emphasises the coexistence, in many situations of interests, of two stable oscillation states, a low and high amplitude solution. The coexistence of those oscillation states is a consequence of the presence of attractive and repulsive components in the interaction force and their non-linear dependence on the tip–surface separation. We show that key relevant experimental properties such as the lateral resolution, image contrast and sample deformation are highly dependent on the oscillation state chosen to operate the instrument. AM-AFM allows to obtain simultaneous topographic and compositional contrast in heterogeneous samples by recording the phase angle difference between the external excitation and the tip motion (phase imaging). Significant applications of AM-AFM such as high-resolution imaging of biomolecules and polymers, large-scale patterning of silicon surfaces, manipulation of single nanoparticles or the fabrication of single electron devices are also reviewed. FM-AFM (also called non-contact AFM—NC-AFM) has achieved the long-standing goal of true atomic resolution with AFM in UHV. Our analysis starts with a discussion of the relation between frequency shifts and tip–surface interactions, emphasising the ability of perturbation theory to describe the measured frequency shift. We discuss the role of short-range chemical interactions in the atomic contrast, with particular attention to semiconductor and ionic (alkali halides and oxides) surfaces. Also included is a detailed quantitative comparison between theoretical simulations and experiment. Inversion procedures, the determination of the tip–sample interaction from the frequency shift versus distance curves above specific sites, are also reviewed. We finish with a discussion of the optimal range of experimental operation parameters, and the use of damping (excitation amplitude) as a source of atomic contrast, including the possible interpretation in terms of microscopic dissipation mechanisms.