New Depths in Infrared Nanospectroscopy

Illustration of the principle of subsurface infrared nanoimaging. (Source: CIC nanoGUNE)

Researchers from the Nano­optics Group at CIC nanoGUNE in San Sebastian demonstrate that nanoscale infrared imaging – which is established as a surface-sensi­tive technique – can be employed for chemical nanoidenti­fication of materials that are located up to 100 nm below the surface. The results further show that the infrared signa­tures of thin surface layers differ from that of subsurface layers of the same material, which can be exploited to distinguish the two cases. The findings push the technique one important step further to quanti­tative chemo­metrics at the nanoscale in three dimensions.

Optical spectroscopy with infrared light, such as Fourier transform infrared (FTIR) spectro­scopy, allows for chemical identi­fication of organic and inorganic materials. The smallest objects which can be distinguished with conven­tional FTIR microscopes have sizes on the micrometre-scale. Scientists at CIC nanoGUNE, however, employed nano-FTIR to resolve objects, which can be as small as a few nanometres.

In nano-FTIR – based on near-field optical micro­scopy – infrared light is scattered at a sharp metallized tip of a scanning-probe micro­scope. The tip is scanned across the surface of a sample of interest and the spectra of scattered light are recorded using Fourier transform detection principles. Recording of the tip-scattered light yields the sample’s infrared spectral properties and thus the chemical compo­sition of an area located directly below the tip apex. Because the tip is scanned across the sample surface, nano-FTIR is typi­cally considered to be a surface-charac­terization technique.

Importantly though, the infrared light that is nano-focussed by the tip does not only probe a nano­metric area below the tip, but in fact probes a nanometric volume below the tip. Now, the researchers showed that spectral signa­tures of materials located below the sample surface can be detected and chemically identified up to a depth of 100 nm. Further­more, the researchers showed that nano-FTIR signals from thin surface layers differ from that of subsurface layers of the same material, which can be exploited for deter­mination of the materials distri­bution within the sample. Remarkably, surface layers and subsurface layers can be distin­guished directly from experimental data without involving time-consu­ming modelling. (Source: Elhuyar Fund.)

Reference: L. Mester et al.: Subsurface chemical nanoidentification by nano-FTIR spectroscopy, Nat. Comm. 11,. 3359 (2020); DOI: 10.1038/s41467-020-17034-6

Link: CIC nanoGUNE BRTA, Donostia-San Sebastián, Spain

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