Small-Amplitude Atomic Force Microscopy
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Small-amplitude atomic force microscopy (AFM) is a dynamic AFM technique, in which the AFM cantilever is vibrated at very small amplitudes, typically less than ≈ 1 Å (= 10− 10 m = 0.1 nm), and the amplitude, phase, and/or resonance frequency of the lever are measured. Small amplitudes serve to linearize measurements of tip–surface interactions. Consequently, measurements can be directly related to interaction stiffness (negative if the force gradient) and energy dissipation. Because of the small amplitudes, the measurement is local, and interactions can be mapped point by point. This eliminates ambiguities present in large-amplitude techniques.
Historically, scanning tunneling microscopy (STM) was the first “easy” technique to reliably provide atomic resolution images of a variety of conductive crystalline surfaces. Early on it was realized that forces play an important role in STM measurements. This inspired the invention of the atomic force microscope in 1986. In terms of applications, it was hoped that AFM would provide atomic resolution on insulators, which was not possible with STM. This goal was finally achieved in 1995, when true atomic resolution imaging was demonstrated by several groups using dynamic, noncontact AFM at large amplitudes. Researchers also realized early on that AFM can be used to measure forces as a function of tip–surface separation in a variety of environments. However, issues such as drift, mechanical instabilities, low sensitivity, and nonlinearities made the direct, local, and linear measurement of force interactions a difficult prospect in many situations.
Small-amplitude AFM techniques were developed to address the issues of nonlinearity and nonlocality that continue to plague large amplitude measurements. Moreover, combined with high-sensitivity deflection sensors, such as fiber interferometry, and the use of tailored lever structures, these techniques were able to overcome sensitivity and instability issues as well. In this chapter we will discuss the underlying theory of small-amplitude AFM, conditions and limitations for its use, and recent examples of small-amplitude AFM applied to atomic-scale imaging, force–distance measurements, atomic-scale dissipation, and force measurements in liquids.