The atomic force microscope (AFM) is used widely to image surfaces and measure interfacial forces with atomic/molecular scale resolution. In this talk, I will discuss our recent work that explores the foundations of quantitative AFM force measurements.
Measurement of the force between two atoms is now performed routinely by dynamically exciting the cantilever, in a frequency modulation (FM) mode. This has led to remarkable discoveries including the ability to chemically identify individual atoms based on the force that they exert on other atoms. I will show that the shape of this interatomic force law directly controls this capacity. Rapidly varying interatomic force laws, which are common in nature, can corrupt their own measurement leading to spurious and unphysical results. Conditions under which reliable interatomic force measurements can be achieved are given.
Essential to all quantitative force measurements is calibration and standardisation of the measured force. Users of the AFM often calibrate the spring constants of its cantilevers using functionality built into individual instruments. This calibration is performed without reference to a global standard, hindering the robust comparison of force measurements reported by different laboratories. I will describe a virtual instrument (sadermethod.org) whereby users from all laboratories can instantly and quantitatively compare their calibration measurements to those of others – standardizing AFM force measurements – and simultaneously enabling non-invasive calibration of AFM cantilevers of any geometry.