AFM-IR Depth Sensitivity, the Way to Tomographic Reconstruction

The principle of AFM-IR technique is based on the coupling between a tunable infrared laser and an AFM (Atomic Force Microscope). The sample is irradiated with a pulsed nanosecond tunable laser. If the IR laser is tuned to a wavenumber corresponding to sample absorption band, the absorbed light is directly transformed into heat. This fast heating results in a rapid thermal expansion localized only in the absorption region detected by the AFM tip. Thus, the detection scheme is analogous to photo-acoustic spectroscopy, except that AFM tip and cantilever are used to detect and amplify the thermal expansion signal instead of a microphone in a gas cell. The thermal expansion induces cantilever oscillations that are rigorously proportional to the local absorption allowing to build up IR absorption spectra. These spectra use to correlate very well conventional IR absorption spectra collected by FT-IR spectroscopy. In addition, mapping oscillations amplitude versus tip position, for one specific wavenumber, gives a spatially resolved map of IR absorption that can be used to localize specific chemical functions¹.<br/>After 20 years of development and improvement the AFM-IR technique becomes now a robust and efficient tool for infrared analysis at nanometer scale. The AFM-IR system can now work in contact mode, tapping mode and peakforce tapping mode ^2,3,4with sensitivity and resolution around 5 nm with spectra bandwidth about 0.5 cm^-1 (linked to the pulsed laser properties). The domain of applications is really huge, covering many diverse research areas like materials and polymer science, life science, astrochemistry, and culture heritage^1,4.<br/>The capability of AFM-IR subsurface sensitivity has been demonstrated by the surface sensitive mode². Recently we have shown the possibility to change the probing depth of analysis and fully calibrated each operating mode with different cantilever types on soft material like polymers. The contact resonance mode is the most promising as each resonance modes possess is own specific probing depth which is inversely proportional to its frequency. This new outlook of the contact mode allows to propose a way to reconstruct the 3D shape of a non-absorbing polymer into an absorbing polymer matrix and this without destroying the sample. This opens to the AFM-IR technique a new mode of analysis and gives a unprecedent tool to characterize the polymer sample not only over the surface but also in depth.<br/><br/>References<br/>[1] A. Dazzi, C.B. Prater, <b><i>Chem. Rev.,</i></b> 117, 7, 5146–5173, (2017).<br/>[2] J. Mathurin et al., <b><i>J. Appl. Phys.</i></b> 131, 010901, (2022).<br/>[3] J. Mathurin et al. <b><i>A&A</i></b>, 622 (2019).<br/>[4] D. Kurouski et al., <b><i>Chem. Soc. Rev.</i></b> 49, 3315-3347, (2020).