Bottom-illuminated photothermal nanoscale chemical imaging with a flat silicon ATR in air and liquid

We demonstrate a novel approach for bottom-illuminated atomic force microscopy infrared spectroscopy (AFM-IR). This nearfield technique is suitable for AFM-IR measurements in liquids, taking advantage of an attenuated total reflection (ATR) setup where the developing evanescent wave is used for photothermal excitation of the sample of interest. Conventional bottom-illuminated measurements are conducted using high-refractive index prisms. We showcase the advancement of instrumentation through the introduction of novel, flat silicon substrates as replacements for prisms. We illustrate the feasibility of this technique for bottom illuminated AFM-IR in both, air and liquid. We also show how modern rapid prototyping technologies enable commercial AFM-IR instrumentations to accept these new substrates. This new methodic approach paves the way for a wide range of experiments since virtually any established protocol for Si surface functionalization can be applied to this sample carrier and the low unit cost enables rapid iteration of experiments.