A method to quantitatively evaluate Hamaker constant using the jump-into-contact effect in Atomic Force microscopy

TL;DR

This paper introduces a novel AFM-based method to measure the Hamaker constant by analyzing the jump-into-contact phenomenon caused by van der Waals forces, enabling precise surface and interaction characterization.

Contribution

The study presents a new quantitative approach to determine the Hamaker constant using the jump-into-contact effect in AFM, based on a simple nonlinear force model.

Findings

Validated method on mica, Si wafer, and silver samples.

Able to accurately locate the surface via cantilever deflection.

Applicable only to surfaces with van der Waals interactions.

Abstract

We find that the jump-into-contact of the cantilever in the atomic force microscope (AFM) is caused by an inherent instability in the motion of the AFM cantilever. The analysis is based on a simple model of the cantilever moving in a nonlinear force field. We show that the jump-into-contact distance can be used to find the interaction of the cantilever tip with the surface. In the specific context of the attractive van der Waals interaction, this method can be realized as a new method of measuring the Hamaker constant for materials. The Hamaker constant is determined from the deflection of the cantilever at the jump-into-contact using the force constant of the cantilever and the tip radius of curvature, all of which can be obtained by measurements. The results have been verified experimentally on a sample of cleaved mica, a sample of Si wafer with natural oxide and a silver film, using a number of cantilevers with different spring constants. We emphasize that the method described here is applicable only to surfaces that have van der Waals interaction as the tip-sample interaction. We also find that the tip to sample separation at the jump-into-contact is simply related to the cantilever deflection at this point, and this provides a method to exactly locate the surface.