Mid-infrared photo-induced force microscopy (IR-PiFM/PiF-IR)- Answers to some questions

Nanoscale infrared (IR) spectroscopic imaging methods can brigde the gap between high-resolution structural imaging provided by electron microscopy and scanning probe microscopy methods and chemical information provided by conventional infrared spectroscopy using far f ield illumination and detection either in transmission or in reflection.1 These methods are complementary to surface enhanced and tip enhanced Raman spectroscopy (SERS and TERS) providing access to high-resolution chemical imaging.2 In general, spatial resolution enhancement is achieved by incorporating plasmonic nanostructures for nearfield enhancement of infrared absorption in the materials. The nanoscale spatial resolution comes along with a high spectral resolution due to powerful IR light sources, such as quantum cascade lasers (QCLs). This enables a nanoscale approach to cutting edge research in the Live and Materials Sciences, for example, by providing access to secondary structure evaluation of proteins,3 which is intimately connected to protein function at multiple levels. The emerging field of approaches can be grouped by their detection schemes in (i) methods employing far-field optical detection, such as surface enhanced infrared absorption spectroscopy (SEIRA),4 and the combination of infrared illumination with scattering scanning nearfield optical microscopy (IR s-SNOM and nano-FTIR)5 and (ii) the so-called AFM-IR methods,6 which combine IR illumination with mechanical detection via atomic force microscopy (AFM). In the context of the Faraday Discussions of Vibrations at Interfaces in Manchester, several studies involving nanoscale infrared spectroscopic imaging techniques have been presented.7–10 Among these approaches, mid-IR photo-induced force microscopy (IR-PiFM/PiF-IR) provides a particular high sensitivity for vibrations at surfaces due to its highly non-linear mechanical force detection based on tip-enhanced IR illumination in AFM. During the discussion on Friday morning related to our Faraday Discussions article,8 I received and answered several questions about the physical background, practical handling and potential applications of PiF-IR. This discussion will be included and published in a themed discussion article in Faraday Discussions. A modified version is presented in this manuscript, in particular, I re-structured the received questions and my answers aiming at enhancing clarity about method understanding and potential of PiF-IR.