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

Published

October 4, 2024

Author(s)

Devon Jakob, Jeffrey Schwartz, Georges Pavlidis, Karen Grutter, Andrea Centrone

Abstract

Photothermal induced resonance (PTIR), also known as AFM-IR, enables nanoscale IR absorption spectroscopy by transducing the local photothermal expansion and contraction of a sample with the tip of an atomic force microscope. PTIR spectra enable material identification at the nanoscale and can measure sample composition at depths > 1 µm. However, implementation of quantitative, multivariate, nanoscale IR analyses requires improved understandings of PTIR signal transduction and of the intensity dependence on sample characteristics and measurement parameters. Here, PTIR spectra measured on 3D-printed conical structures up to 2.5 µm tall elucidate the signal dependence on sample thickness for different IR laser repetition rates and pulse lengths. Additionally, we develop a model linking sample thermal expansion dynamics to cantilever excitation amplitudes that includes samples that do not fully thermalize between consecutive pulses. Remarkable agreement between experiments and theory demonstrates a monotonic increase of the PTIR signal intensity with thickness, with decreasing sensitivities at higher repetition rates, while signal intensity is nearly unaffected by laser pulse length. Although we observe slight deviations from linearity over the entire 2.5 µm thickness range, the signal's approximate linearity for bands of sample thicknesses up to ≈ 500 nm suggests that samples with comparably low topographic variations are most amenable to quantitative analysis. Importantly, we measured undistorted, absorptive profiles and negligible peak shifts in PTIR spectra for strongly absorbing modes, up to ≈ 1650 nm, and > 2500 nm for other modes. These insights are foundational for quantitative nanoscale PTIR analyses and material identification, furthering their impact across many applications.

Citation

Analytical Chemistry

Volume

96

Issue

41

Pub Type

Journals

Download Paper

https://doi.org/10.1021/acs.analchem.4c02834

Local Download

Keywords

Photothermal induced resonance, PTIR, AFM-IR, nanoscale spectroscopy, chemical imaging, mid-IR.

Citation

Jakob, D. , Schwartz, J. , Pavlidis, G. , Grutter, K. and Centrone, A. (2024), Understanding AFM-IR Signal Dependence on Sample Thickness and Laser Excitation: Experimental and Theoretical Insights, Analytical Chemistry, [online], https://doi.org/10.1021/acs.analchem.4c02834, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=957948 (Accessed December 11, 2024)