Non-contact AFM measurement of the Hamaker constants of solids: Calibrating cantilever geometries.

Hypothesis: Surface effects arising from roughness and deformation can negatively affect the results of AFM contact experiments. Using the non-contact portion of an AFM deflection curve is therefore desirable for estimating the Hamaker constant, A, of a solid material. A previously validated non-contact quasi-dynamic method for estimating A is revisited, in which the cantilever tip is now always represented by an "effective sphere". In addition to simplifying this previous method, accurate estimates of A can still be obtained even though precise knowledge of the nanoscale geometric features of the cantilever tip are no longer required.

Experiments: The tip's "effective" radius of curvature, R, is determined from a "calibration" step, in which the tip's deflection at first contact with the surface is measured for a substrate with a known Hamaker constant. After R is known for a given tip, estimates of A for other surfaces of interest are then determined.

Findings: An experimental study was conducted to validate the new method and the obtained results are in good agreement with predictions from the Lifshitz approximation, when available. Since R accounts for all geometric uncertainties of the tip through a single fitted parameter, no visual fitting of the tip shape was required.