Time-resolved tapping-mode atomic force microscopy

Abstract

Atomic force microscopy has unprecedented potential for quantitative mapping of material-specific surface properties on the nanoscale. Unfortunately, methods developed for local stiffness measurements suffer from low operational speeds and they require large forces to be applied to the surface, limiting resolution and precluding measurements on soft materials such as polymers and biological samples. On the other hand, tapping-mode AFM, which is well suited to soft materials due to its gentle interaction with the surface, cannot be used to recover information on the tip-sample interaction (and hence, on the material properties) due to limited mechanical bandwidth offered by the resonant AFM probe. In this chapter, a technique, called Time-resolved Tapping-mode Atomic Force Microscopy, designed for rapid quantitative material characterization on the nanoscale is described. The technique is based on time-resolved measurement of tip-sample interaction forces during tapping-mode AFM imaging by a specially designed micromachined AFM probe. The probe has an integrated high-bandwidth interferometric force sensor that is used to resolve tip-sample interaction forces with high sensitivity and temporal resolution. In the first part of the chapter, the theory, design, and fabrication of the probes are described in detail. Then quantitative force measurements with microsecond time resolution in tapping-mode imaging are presented. Finally, higher harmonic images based on the interaction force measurements are presented for various samples, demonstrating the range of applications of the technique.