Subsurface chemical nanoidentification by nano-FTIR spectroscopy

Abstract

Nano-FTIR spectroscopy based on Fourier transform infrared near-field spectroscopy allows for label-free chemical nanocharacterization of organic and inorganic composite surfaces. The potential capability for subsurface material analysis, however, is largely unexplored terrain. Here, we demonstrate nano-FTIR spectroscopy of subsurface organic layers, revealing that nano-FTIR spectra from thin surface layers differ from that of subsurface layers of the same organic material. Further, we study the correlation of various nano-FTIR peak characteristics and establish a simple and robust method for distinguishing surface from subsurface layers without the need of theoretical modeling or simulations (provided that chemically induced spectral modifications are not present). Our experimental findings are confirmed and explained by a semi-analytical model for calculating nano-FTIR spectra of multilayered organic samples. Our results are critically important for the interpretation of nano-FTIR spectra of multilayer samples, particularly to avoid that geometry-induced spectral peak shifts are explained by chemical effects.

Roadmap of Terahertz Imaging 2021

“…It is worth mentioning that s-SNOM is not purely surface-sensitive, but probes a volume down to about 50 nm in dielectric media [294], which gives it tomographic capabilities [295][296][297][298]. This capability is still to be exploited in the THz frequency regime, e.g., for the (also quantitative) characterization of electrically conductive layers buried under a dielectric layer [299].…”

Extracting the Infrared Permittivity of SiO2 Substrates Locally by Near-Field Imaging of Phonon Polaritons in a van der Waals Crystal

“…We achieved this by combining the virtues of monochromatic near-field imaging of polaritons, such as sensitivity to phonon damping and LO phonon energies, with advanced theoretical and numerical approaches. The extension of this procedure to broadband sources, such as those used in Fourier transform infrared nanospectroscopy [ 21 , 22 , 23 ], holds great promises to extracting dielectric functions of isotropic and anisotropic nanomaterials.…”

“…On the other hand, the measurement of the IR dielectric permittivity has relied on traditional far-field techniques (diffraction-limited), such as ellipsometry or Fourier-transform infrared spectroscopy (FTIR), with a micrometer resolution in the best of cases. In contrast, by beating the diffraction limit of light, near-field optical microscopy (s-SNOM) exhibits instrumental advantages with respect to traditional techniques, such as extreme sensitivity to the optical losses of the material and nanometer spatial resolutions capable of determining the complex dielectric permittivity of nanomaterials with extraordinary accuracy [ 21 , 22 , 23 ].…”

Recent Progress on the Characterization of Cellulose Nanomaterials by Nanoscale Infrared Spectroscopy

“…Thickness-induced chemical shifts of IR s-SNOM resulted in chemical and structural analysis more complex than those for conventional bulk spectra. The versatility of nanoscale IR spectroscopy and imaging is at the cost of the ability to directly correlate the obtained spectra information to the quantitative characterization of chemical and structural properties [ 70 , 71 ]. Complex data processing methods with certain degrees of signal degradation still cannot eliminate background interferences.…”