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.
Highlights
Nano-FTIR spectroscopy based on Fourier transform infrared near-field spectroscopy allows for label-free chemical nanocharacterization of organic and inorganic composite surfaces
We demonstrate (1) that nano-FTIR peaks of subsurface layers are shifted to lower frequency as compared with that of bulk materials or thin surface layers, and (2) that surface and subsurface layers can be differentiated by analyzing the ratio of peak heights obtained at different demodulation orders n, without theoretical modeling or simulations
For subsurface infrared near-field spectroscopy we use a setup based on a commerical s-SNOM
Summary
Nano-FTIR spectroscopy based on Fourier transform infrared near-field spectroscopy allows for label-free chemical nanocharacterization of organic and inorganic composite surfaces. The technique—named nano-FTIR spectroscopy—yields near-field phase spectra that match well the absorptive properties of organic samples[19,20,21], and allows for nanoscale chemical identification based on standard FTIR references[22]. Despite s-SNOM and nano-FTIR being surface scanning techniques, the finite penetration depth of near fields into the sample allows for subsurface probing of nanoscale structures and defects up to a depth of 100 nm[23,24,25,26,27]. We demonstrate validity and applicability of our findings for a large variety of materials, by summarizing and discussing the results of an extended theoretical and experimental study of the nano-FTIR peak characteristics of differently thick subsurface layers exhibiting various molecular vibrational modes
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