Photothermal Atomic Force Microscopy Coupled with InfraredSpectroscopy (AFM-IR) Analysis of High Extinction CoefficientMaterials: A Case Study with Silica and Silicate Glasses

Photothermal atomic force microscopy coupledwith infrared spectroscopy (AFM-IR) brings significant value as aspatially resolved surface analysis technique for disordered oxidematerialssuchasglasses,butadditional development andfundamental understanding of governing principles is needed tointerpret AFM-IR spectra, since the existing theory described fororganic materials does not work for materials with high extinctioncoefficients for infrared (IR) absorption. This paper describestheoretical calculation of a transient temperature profile inside theIR-absorbing material considering IR refraction at the interface aswell as IR adsorption and heat transfer inside the sample. Thiscalculation explains the differences in peak positions andamplitudes of AFM-IR spectra from those of specular reflectanceand extinction coefficient spectra. It also addresses the information depth of the AFM-IR characterization of bulk materials. AFM-IRapplied to silica and silicate glass surfaces has demonstrated novel capability of characterizing subsurface structural changes andsurface heterogeneity due to mechanical stresses from physical contacts, as well as chemical alterations manifested in surface layersthrough aqueous corrosion.