Infrared Imaging Beyond the Diffraction Limit

There are a variety of tools for characterizing nanoscale morphology and structure, such as scanning probe, scanning electron, and transmission electron microscopies. In contrast, the typical tools for analyzing chemical composition yield information at a larger length scale, often missing critically important nanoscale information.  For example, the most widely used infrared technique, Fourier transform infrared spectroscopy, generates chemical data with micrometer spatial resolution.  To address this gap, we installed a multifunctional instrument capable of simultaneously providing correlated topological, chemical (via infrared spectroscopy), thermal, and mechanical property maps with a spatial resolution below the diffraction limit of infrared radiation. We are currently working to further improve the instrument performance in terms of spectral range, spectral resolution and sensitivity.

The size dependent properties of nanomaterials give them functionality not found in their macroscopic counterparts. Determining the relationship between the structural, physical and chemical properties of nanomaterials is important for technological applications including electronics, photovoltaics, catalysis, biology and therapeutics. While infrared (IR) spectroscopy provides rich chemical and structural information, the wavelengths used to excite molecular vibrations, typically in the 3 μm - 15 μm range, are far larger than the characteristic dimensions of nanomaterials or cellular sub-components. Due to these longer wavelengths, IR imaging beyond the diffraction limit at the length scales typical of nanomaterials is more challenging than with visible light.

IR microscopy with lateral resolution beyond the diffraction limit of IR radiation has been successfully applied to imaging biological samples, including single cells, tissue sections, and bacteria. The measurements have improved insight into the metabolic functions of these complex systems. Information from IR microscopy shows great promise for diagnosing cancer and other diseases. IR microscopy has also been useful for understanding phase separation in polymer blends (fig. 1) and for studying polymer thin films, including those used in organic solar cells.

We are currently working on improving our multifunctional instrument’s sensitivity to measure nanoparticle properties. This will allow us to determine the relationships between the structure and properties of nanomaterials as a function of size. We believe that when developed, our instrument could impact many nanotechnology applications in fields such as material science, energy, biology, and medicine. 

Fig. 1. Polymer sample made from polymethylmethacrylate (PMMA, large particles) and polystyrene (PS, small particles) embedded in an epoxy matrix as seen in an AFM height image (left). A simultaneous IR image (right) recorded at 1720 cm^-1 adds chemical contrast, allowing for material identification. The carboxylic groups in PMMA absorb light at 1720 cm^-1 while the PS and the epoxy matrix do not.