Atomic Force Microscopy Infrared Spectroscopy Method for Multisample Comparison of Topographic, Infrared Imaging, and Stiffness Domains

Atomic force microscopy-infrared spectroscopy can simultaneously acquire topographical, chemical, and stiffness information with nanoscale resolution. One approach to measuring relative stiffness is to use the phase-locked loop frequency channel to assess the probe-sample contact resonance. A stiffer sample domain results in higher contact resonance frequency, and more compliant domains result in lower frequency contact resonance; however, in practice, these data are difficult to compare between samples and can even be problematic from tip-engagement to tip-engagement on the same sample. This challenge is due to cantilever and tip variations that give rise to contact resonance frequency offsets, which can be large compared to the width of the frequency manifold associated with the surface. To overcome this challenge, we present a standard procedure for contact resonance frequency comparison between samples that circumvents the problem of the absolute frequency variation. Application of the technique is demonstrated on a carbon-epoxy composite, an enzyme-functionalized cellulose amide compressed disk, and human cadaver ligament samples.