The atomic force microscope (AFM) is a widely utilized instrument for measuring nanometer-scale variations in mechanical properties. Most AFM techniques only measure these properties at a single high frequency (e.g. hundreds of kHz on resonance), or at low frequencies (e.g. few Hz in the case of force spectroscopy). Many polymers and biological samples have properties that vary dramatically as a function of frequency. To fully characterize these types of samples there is a need for AFM techniques that can determine material properties across many decades of frequency. Force modulation microscopy (FMM) is a sub-resonant dynamic AFM technique that relates the cantilever vibration amplitude to the tip-sample contact stiffness. Because FMM is not reliant on resonant vibration of the AFM cantilever, stiffness at any frequency below the resonance and above the low frequency noise floor can be measured. Here, experimental considerations, such as non-idealized excitation transfer functions, are elucidated through proof-of-concept measurements on a silicon bridge. By operating with high-frequency ultra-small cantilevers, we show that it is possible to increase the frequency range of FMM by an order of magnitude. These innovations in FMM result in the ability to perform broadband viscoelastic characterization on a range of polymeric materials.
Society for Experimental Mechanics
June 8-11, 2015
Costa Mesa, CA
atomic force microscopy, nanomechanical properties