Elastic mapping of heterogeneous nanostructures with ultrasonic force microscopy (UFM)

Ultrasonic force microscopy (UFM) is an atomic force microscopy (AFM)-related technique originally introduced to study the surface elastic properties of stiff materials. We report elastic images of heterogeneous nanostructures with a lateral resolution of the order of a few nanometers. One of the main intentions of this paper is not only to show the capability of UFM to allow one to image surface elastic properties of stiff materials but also to show that UFM can be applied to relatively soft materials with reproducible and interpretable results. The samples presented were chosen over a wide range of stiffness values (with Young's modulus E = 0.1-400 GPa): very stiff silicon carbide fibres embedded in a mullite matrix, less stiff carbon fibres embedded in an epoxy matrix and relatively compliant rubber inclusions in a polymethylmethacrylate matrix. A discussion of the conditions required to obtain unambiguous data is also provided. Results obtained using the more traditional force modulation mode are also presented and compared with the UFM images of the same samples