Advancing nanoscale chemical imaging for subsurface complex structures via AFM-IR

Abstract

Nanoscale chemical imaging using atomic force microscopy-infrared spectroscopy (AFM-IR) is pivotal for investigating the intricate structures and chemical compositions of diverse materials and biological samples. The integration of atomic force microscopy (AFM) and infrared (IR) spectroscopy in AFM-IR allows for simultaneous high-resolution imaging and chemical analysis, surpassing the limitations of conventional IR spectroscopy. This powerful technique enables researchers to unravel the nanoscale details of complex systems, shedding light on fundamental properties and functionalities. Despite the invaluable utility of AFM-IR, its potential for probing subsurface structures has largely remained unexplored. In a recent theoretical study [1], our group investigated the depth dependence of signal intensity and spatial resolution, highlighting the potential of AFM-IR for subsurface imaging. To bridge this gap, we conducted an experimental and theoretical investigation to assess AFM-IR's capabilities in measuring subsurface complex structures. Tapping mode AFM-IR was employed for its enhanced signal detection and exceptional spatial resolution, achieving resolutions as fine as 10 nm. Test samples were meticulously prepared using clean room facilities, involving the coating of a silicon substrate with negative photoresist SU-8, patterning with E-beam lithography, and overlaying with a layer of positive photoresist PMMA （as shown in Figure 1A）. Analytical and FEM models were developed to gain a comprehensive understanding of the signal generation and probing process. Our investigation yielded exciting results, demonstrating the imaging of subsurface complex structures using absorption bands from either the underlying or covering layer （Figure 1B, E）. Notably, we observed that when the excitation laser is tuned to the absorption band of the covering layer, the imaged subsurface structures appear larger compared to when it's tuned to the underlying layer（Figure 1F）. To comprehend this phenomenon, we focused on individual nanopillars (see Figure 2A, C) with a series of designed diameters and employed analytical and FEM models. These models provided insights into the relationship between structure size and signal intensity, as well as spatial resolution. Our results show that both analytical and FEM models exhibit good agreement with the experiments (see Figure 2B, D). This analysis presents the first experimental demonstration that the spatial resolution of AFM-IR is directly proportional to the diameter of the absorber. Additionally, we observe that the signal intensity increases roughly linearly with the diameter of the absorber, rather than being proportional to the sample thickness.