Fig. 2: AFM tip calibration using a PDMS reference elastomer.

a Surface topography of the PDMS sample (5:1) used for the characterisation of the probe. Scale bar = 1 μm. b Representative indentation loading and unloading curve for the PDMS in 0.1% aqueous SDS showing a typical elastic deformation with minimal adhesion. c Assumption of a hard standard TM-AFM tip indenting a soft elastic material as a paraboloid of revolution, where \({h}_{c}\) is the contact depth, \(h\) is the total indentation depth, \(R\) is the end radius of curvature of the parabola, and \(a\) is the contact radius. d Representative power law fit to the initial 60 nm of a PDMS loading curve presented in b. e Average tip radius function obtained from Eq. 10 (also see Supplementary Fig. S2). f Zoom-in of the tip geometry for a maximum contact depth of 20 nm. The tip geometry was determined from a minimum \({h}_{c}\) = 5 nm, as indicated by the horizontal dotted line. The dashed line shows the power law for \({h}_{c}\) = 5 nm, where n = 7.55 ± 0.20 and c = (7.90 ± 3.90) × 10⁻¹¹µm^−6.55; see Eq. (6) and Supplementary Table S1.