Experimental and Theoretical Insights into Nanoscale AFM-IR Imaging of Complex Heterogeneous Structures.

Nanoscale chemical imaging enabled by atomic force microscopy-infrared spectroscopy (AFM-IR) provides valuable insights into the complex structures and chemical compositions of materials and biological samples. While AFM-IR has been applied to subsurface imaging, the underlying mechanisms, particularly in nonplanar geometries and complex heterogeneous structures, remain underexplored. This study presents a theoretical analysis and experimental validation of AFM-IR for imaging subsurface features within organic multilayer structures, uncovering how image broadening depends on whether the excitation occurs in the subsurface or the covering layer. An analytical model based on the sample geometry demonstrates that the lateral size of the absorber significantly impacts both the signal intensity and spatial resolution in AFM-IR chemical imaging. These findings are experimentally validated, and a more representative finite element method (FEM) model was subsequently created, resulting in strong agreement with the experimental data. The model reveals how irregular structures directly impact photothermal expansion, providing an explanation for the distinct image broadening observed with infrared excitation of different layers. Additionally, a linear relationship is observed between feature size, chemical images, and AFM-IR signal intensity. These findings contribute significantly to the understanding of the AFM-IR signal, providing insights into resolution and sensitivity, paving the way for more advanced nanoscale chemical imaging capabilities.