Tip-induced deformation of polystyrene latex reference nanoparticles in atomic force microscopy

The measurement approach typically prescribed for sizing polystyrene latex (PSL) reference nanoparticles by atomic force microscopy (AFM) is based on a single value of the height obtained from a three-dimensional image of the particle. A major challenge is that PSLs reportedly deform during sample preparation as well as imaging. While the particle-substrate deformation is controlled by adhesive contact forces, the tip-induced deformation may vary to a large degree depending on the operational conditions and the geometrical constraints imposed by the instrument. In this paper, we are mainly concerned with the effect of cantilever tilt, tip shape and setpoint voltage on tip-induced deformation of PSL reference nanoparticles. Tips are characterized by scanning electron microscopy and using a multilayer Si/SiO₂ tip characterizer. We find that the plastic flow of materials, induced by the asymmetric imaging geometry, shifts the position of the maximum particle height away from the center. This shape-change often leads to an initial height increase for PSLs that are smaller than 40 nm. Since AFM height analysis algorithms often equate apparent particle height with the highest measured point of the particle, these results have implications from the perspective of AFM-based nanoparticle metrology.