An optical technique that combines Fourier transform infrared (FTIR) spectroscopy and scattering-type scanning near-field optical microscopy (s-SNOM) now allows nanoscopic quantities of materials to be identified chemically and mapped.
The technique of nano-FTIR developed has been developed by scientists from the nanoscience research centre NanoGUNE in San Sebastian, Spain, the University of Munich, LMU, Germany and Neaspec GmbH in Martinsried, Germany.
The non-invasive identification of materials and the mapping of their features on the nanometre scale of resolution is a critical problem in modern materials science. Various approaches have been taken to address this problem including the development of high-resolution imaging techniques, including electron microscopy and scanning probe microscopy. Unfortunately, while these techniques are powerful in their own right, their low chemical sensitivity does not meet the demands of modern analytical chemistry that must today function on the nanoscale. Conversely, optical spectroscopy offers suitably high chemical sensitivity but it is limited to the diffraction boundary, approximately half the wavelength of light, which means it is not a viable technique for nanoscale mapping.
The European team has now combined the advantages of both approaches and sidestepped their limitations in nano-FTIR by bringing together sensitivity and resolution in a single hybrid technique. The team explains that by illuminating the metalized tip of an atomic force microscope (AFM) using a broadband infrared laser they can and analyse the backscattered light with a unique FTIR spectrometer. This allowed the researchers to localise the infrared spectra obtained with a spatial resolution of below 20 nanometres. 20 nm is equivalent to a probed volume of down to 10 zeptolitres.
“Nano-FTIR thus allows for fast and reliable chemical identification of virtually any infrared-active material on the nanometre scale,” explains team member Florian Huth. Importantly, the nano-FTIR spectra obtained correlate extremely well with the conventional FTIR spectra for the same materials in bulk but the resolution is a factor of 300 times greater than with conventional infrared spectroscopy.
“Nano-FTIR can thus make use of standard infrared databases of molecular vibrations to identify organic materials in ultrasmall quantities and at ultrahigh spatial resolution,” the team says.
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