Nano-IR spectroscopy -- atomic force microscopy (AFM) based infrared (IR) spectroscopic detection -- brings significant value as a spatially-resolved surface analysis technique for disordered oxide materials such as glasses, but additional development and fundamental understanding of governing principles is needed to interpret nano-IR spectra since the existing theory described for organic materials does not work for materials with high extinction coefficients for IR absorption. This talk presents two recent advancements of nano-IR spectroscopic imaging of subsurface structural changes in oxide glasses. The first is the application of scattering-based scanning near-field optical microscopy (s-SNOM) study of silica glass surfaces. An algorithm to convert near-field scattering signals of silica to its optical constants was developed and used to reveal the elongation of the Si-O bond length distribution in the topographically elastic contact region. The second study is the calculation of a transient temperature profile inside the IR-absorbing material considering IR refraction at the interface as well as IR adsorption and heat transfer inside the sample, which can be compared with the topographic height change of the surface detected by photothermal AFM-IR. This calculation explains differences in peak positions and amplitudes of AFM-IR spectra from those of specular reflectance and extinction coefficient spectra. It also addresses the information depth of the AFM-IR characterization of bulk materials.

Imaging invisible subsurface structural damages in silica and silicate glass with nano-IR spectroscopy

ACS Spring 2022

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