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

Area of Science:

• Spectroscopy
• Materials Science
• Nanotechnology

Background:

• Photothermal induced resonance (PTIR), or atomic force microscopy-infrared (AFM-IR), offers nanoscale IR absorption spectroscopy.
• PTIR enables material identification and composition analysis at the nanoscale, even at depths exceeding 1 μm.

Purpose of the Study:

• To investigate the dependence of PTIR signal intensity on sample thickness.
• To understand the influence of laser parameters (repetition rate, pulse length) on PTIR measurements.
• To develop a model for PTIR signal transduction and validate it experimentally.

Main Methods:

• Measurements of PTIR spectra on 3D-printed conical structures of varying thicknesses (up to 2.5 μm).
• Systematic variation of IR laser repetition rates and pulse lengths during measurements.
• Development of a theoretical model correlating sample thermal expansion dynamics with cantilever excitation amplitudes.

Main Results:

• PTIR signal intensity increases monotonically with sample thickness.
• Higher laser repetition rates lead to decreased sensitivity, while pulse length has minimal effect.
• Approximate signal linearity observed for thicknesses up to ~500 nm, suggesting suitability for quantitative analysis with low topographic variation.
• Undistorted absorptive PTIR spectra achieved for various absorption modes at significant depths.

Conclusions:

• Established foundational insights for quantitative nanoscale PTIR analysis and material identification.
• Demonstrated the critical role of sample thickness and laser parameters in PTIR measurements.
• Highlighted the potential of PTIR for advanced material characterization across diverse applications.