Frequency-sweep force volume AFM-IR: Decoupling infrared mapping from mechanical properties

Infrared nano-spectroscopy by atomic force microscopy-infrared (AFM-IR) couples an atomic force microscope (AFM) to tunable infrared (IR) laser radiation to perform infrared signature mapping of complex samples at a nanometric spatial resolution. Recently, the new frequency-sweep force volume AFM-IR operating mode was introduced, offering a way to measure the full frequency response of the cantilever-sample system during IR mapping. Such operating mode enables to integrate the frequency-dependent IR signal over resonance modes, thus incorporating into the IR response the resonance line shape and hence the magnitude of the mechanical damping of the system. Unlike conventional resonance-tracking methods (as implemented in AFM-IR contact, tapping, and peak force tapping), the frequency-sweep force volume AFM-IR mode performs IR mapping without requiring active resonance adjustment. This makes it particularly suitable for mechanically heterogeneous samples with substantial frequency shifts and low signal-to-noise ratios. In this work, we present a systematic AFM-IR study on standard polymer samples to showcase the IR mapping capabilities of the frequency-sweep force volume mode compared to resonance-enhanced contact and resonance-enhanced force volume modes. This study highlights that frequency spectra are a crucial tool to evaluate the suitability of a specific resonance mode to perform IR mapping and thus to interpret AFM-IR data. Integrating the frequency response of the system furthermore allows to improve the IR contrast on heterogeneous regions.