Nanoscale resolution

Proc. Natl Acad. Sci. USA 111, 7191–7196 (2013)

Infrared spectroscopy is a powerful method for characterizing compounds and solids, but the long wavelength of infrared radiation has limited the spatial resolution of infrared spectroscopic imaging. Now, by combining infrared scattering-scanning near-field microscopy with broadband infrared synchrotron radiation, researchers in the USA have realized spectroscopic imaging over the whole mid-infrared wavelength range (700–5,000 cm⁻¹) with a sub-40-nm spatial resolution and a high sensitivity. They refer to this technique as synchrotron infrared nanospectroscopy. Using a modified commercial Fourier-transform infrared spectrometer, the researchers were able to obtain images on a timescale of minutes. They demonstrated its effectiveness by using it to investigate a variety of samples, including surface phonon polaritons in SiO₂, surface-adsorbed dried proteins and CaCO₃ polymorph heterogeneity in a mussel shell. Unlike many other nanoscale microscopy techniques, synchrotron infrared nanospectroscopy can be used to investigate nanoscale phenomena in soft matter under ambient conditions. The near-field confinement of infrared radiation results in tip-limited spatial resolutions, which are two to three orders of magnitude better than those of Fourier-transform infrared spectroscopy. Furthermore, the high spectral irradiance and spatial coherence of synchrotron radiation enable vibrational spectroscopy to be performed with a high signal-to-noise ratio across the whole mid-infrared wavelength range.