Understanding AFM-IR Signal Dependence on Sample Thickness\nand Laser Excitation: Experimental and Theoretical Insights

Photothermal\ninduced resonance (PTIR), also known as atomic force\nmicroscopy-infrared (AFM-IR), enables nanoscale IR absorption spectroscopy\nby transducing the local photothermal expansion and contraction of\na sample with the tip of an atomic force microscope. PTIR spectra\nenable material identification at the nanoscale and can measure sample\ncomposition at depths >1 μm. However, implementation of quantitative,\nmultivariate, nanoscale IR analysis requires an improved understanding\nof PTIR signal transduction and of the intensity dependence on sample\ncharacteristics and measurement parameters. Here, PTIR spectra measured\non three-dimensional printed conical structures up to 2.5 μm\ntall elucidate the signal dependence on sample thickness for different\nIR laser repetition rates and pulse lengths. Additionally, we develop\na model linking sample thermal expansion dynamics to cantilever excitation\namplitudes that includes samples that do not fully thermalize between\nconsecutive pulses. Remarkable qualitative agreement between experiments\nand theory demonstrates a monotonic increase in the PTIR signal intensity\nwith thickness, with decreasing sensitivities at higher repetition\nrates, while signal intensity is nearly unaffected by laser pulse\nlength. Although we observe slight deviations from linearity over\nthe entire 2.5 μm thickness range, the signal’s approximate\nlinearity for bands of sample thicknesses up to ≈500 nm suggests\nthat samples with comparably low topographic variations are most amenable\nto quantitative analysis. Importantly, we measure absorptive undistorted\nprofiles in PTIR spectra for strongly absorbing modes, up to ≈1650\nnm, and >2500 nm for other modes. These insights are foundational\ntoward quantitative nanoscale PTIR analyses and material identification,\nfurthering their impact across many applications.