DC16-14-309-a. Left: ratio of the AFMIR maps at 1710 and 1600 cm⁻¹. The white isocontour delineates the height corresponding to 5% of the maximum height measured by the AFM tip. Top right: AFMIR spectral maps in the 1900–1350 cm⁻¹ range (labelled S1 to S4, performed in selected locations shown on the map). Vertical lines mark the 1710 and 1600 cm⁻¹ wavenumber positions. Bottom right: histogram of the ratio of the AFMIR maps at 1710 and 1600 cm⁻¹.