AFM-IR-based nanoscale chemical Imaging

Abstract

Mid-infrared (IR) spectroscopy is a well established method for analysis of polymers due to its ability to provide molecular specific IR information non-destructive, quickly and with little sample preparation. However, conventional IR spectroscopy is limited to spatial resolutions of several microns, thus preventing its meaningful application to samples with nanoscale components. This is where atomic force microscopy-infrared spectroscopy (AFM-IR) comes in. In AFM-IR a pulsed IR laser is aimed at the sample area under the AFM tip. The thermal expansion is detected by the AFM cantilever in the near-field region allowing spatial resolutions below the diffraction limit and down to 20 nm. Tuning the laser wavelength while keeping the AFM tip in one location provides local infrared spectra. Scanning the AFM tip while keeping the laser wavelength constant provides infrared absorption images and AFM topography images, concurrently. Infrared bands can be assigned to vibrations of functional groups using well established spectra-structure correlations, taking advantage of the direct comparability between the AFM-IR obtained photothermal induced resonance (PTIR) spectra and their far-field FTIR counterparts. AFM-IR has successfully been applied to biological, inorganic and organic samples such as polymer films and fibres. In this work, we demonstrate the application of AFM-IR to a polymer sample a sample consisting of a mixture of three different polymers was analyzed, the three chemically different domains. The three different polymers were identified through their AFM-IR spectra and their spatial distribution was determined at nanoscale resolution.