Probe-sample interaction-independent Atomic Force Microscopy-Infrared (AFM-IR) spectroscopy: towards robust nanoscale compositional mapping

摘要

Nanoscale topological imaging using Atomic Force Microscopy (AFM) combined with infrared (IR) spectroscopy is a rapidly emerging modality (AFM-IR) to record correlated structural and chemical images. While the expectation is that the spectral data faithfully represents the underlying chemical composition, sample mechanical properties affect the recorded data (known as the probe-sample interaction effect). Although experts in the field are aware of this effect, the contribution is not fully understood. Further, when the sample properties are not well known or when AFM-IR experiments are conducted by non-experts, there is a chance that these non-molecular properties may affect analytical measurements in an uncertain manner. Techniques such as resonance enhanced imaging and normalization of IR signal using ratios might improve fidelity of recorded data but are not universally effective. Here we provide a fully analytical model that relates cantilever response to the local sample expansion which opens several avenues. We demonstrate a new method for removing probe-sample interaction effects in AFM-IR images by measuring the cantilever responsivity using a mechanically induced out-of-plane sample vibration. This method is then applied to model polymers and mammary epithelial cells to show improvements in sensitivity, accuracy and repeatability for measuring soft matter compared to current state of the art (resonance enhanced operation). Rigorous analytical model driven sample-dependent cantilever responsivity understanding is an essential addition to AFM-IR imaging if identification of chemical features at nanoscale resolutions is to be realized for arbitrary samples.