Tiny Crystals Located and Identified by Infrared Nanoscopy
Clemens Liewald, Fritz Keilmann
Microscopy Today, Volume22, Issue 06 Nov 2014, pp 24 – 29 (2014)
Infrared (IR) spectroscopic analysis of materials is based on molecular vibrations that label any chemical compound with a characteristic “fingerprint” spectrum in the 3–30 μm wavelength region. No labeling of samples is needed for IR spectrometers (usually abbreviated FTIR for Fourier-transform infrared spectrometer) to readily acquire IR spectra of any material in any form, be it in gas, liquid, or solid state. Infrared spectra identify chemicals in a straightforward manner, and therefore most suppliers bundle their FTIR instruments with spectral databases that offer automatic chemical recognition and even quantitative information (chemometrics) on the relative abundance of individual constituents of mixtures and composite materials. No wonder IR spectrometers are helpful in all fields of science and engineering and are common workhorses in university and industrial laboratories. The single worst shortcoming of IR chemical recognition has been its inability to perform submicrometric microscopy, owing to the diffraction limit of spatial resolution, at half the wavelength of light analyzed. In the world of sub-μm microscopy, IR was a “no go” in spite of the inherent rich information available. But there is no fundamental-physics reason why light could not be concentrated to a volume much smaller than a wavelength, even down to atomic size. Fortunately an 85-year-old idea on super-focusing was turned into reality two decades ago when antenna-based light concentration to a few nm size was demonstrated in theory and experiment. In consequence light optical microscopy and IR near-field microscopy could be successfully developed in a single instrument, and a commercial optical/IR/THz nanoscope product was introduced (neaspec.com) that offers a routine spatial resolution of 20 nm. Named NeaSNOM (for new scanning near-field optical microscope) it earned an Innovation Award from Microscopy Today in 2013. Here we highlight the real-world application of NeaSNOM, including recent searches for sub-μm crystals in a biological context, and sub-μm coexisting structural phases within organic thin films.