Photo-induced Force Microscopy (PiFM) is a novel nanoanalytical technique that combines Atomic Force Microscopy (AFM) with infrared (IR) phase identification. In order to generate the signal, PiFM uses tunable IR laser to excite the sample. When the infrared light is absorbed, it generates a localized field between the sample and a gold- or platinum-coated AFM cantilever tip, producing a photo-induced force (PiF). Due to the small interaction volume between sample and tip, PiFM achieves a spatial resolution as fine as ~5 nm, well beyond the diffraction limit of light that constrains optical methods [ 1, 2]. The resulting PiF-IR spectra are generated by recording the strength of the PiF as the laser sweeps through the wavenumbers, directly correlating with the sample's IR absorption characteristics, allowing for the utilization of existing FTIR databases for time efficient phase identification [ 1]. For homogeneous samples, PiF-IR spectra align closely with bulk Fourier Transform Infrared (FTIR) spectra, while PiFM analysis of more heterogenous or composite samples provide exciting opportunities for observing interfacing phases that are not separable with traditional FTIR.
In this study, PiFM spectra were acquired using the VistaOne system (Molecular Vista Inc., CA, USA) at the Australian National University Research School of Earth Sciences. We used three wavenumber-tunable IR lasers to cover a broad range of wavenumbers: ranges 755-1875 cm -1 (Block Engineering quantum cascade laser, USA), 2000-2400 cm -1 (DRS Daylight Solutions MIRcat quantum cascade laser, USA), and 2400-4400 cm -1 (EKSPLA PT200 optical parametric oscillator, Lithuania).
PiFM offers a combination of high sensitivity, capable of detecting molecular monolayers, and exceptional spatial resolution that together enable the visualization of phases at the nanoscale.