Understanding the signal we observe with AFM is at the heart of making conclusions about the systems we study. And when we perform non-contact AFM, there are a number of different interaction mechanisms that are possible, including van der Waals (sometimes referred to as London dispersion forces) or electrostatics.
We cleaned off a metal tip and used a single CO molecule to count the number of atoms at the apex. Then we transferred to look at calcium flouride - an ionic crystal that has a rather complex unit cell. What we found, in contrast to previous studies, is that we could understand our data (when we're far enough from the surface!) in terms of Coulomb interaction between the apex of the tip and the surface atoms.
We also showed measurements where the contrast is due to forces on the order of piconewtons. To put that in context, the force of gravity that acts on a 50 kg person on the surface of the Earth is 490.5 Newton. We would need to move that person 100 billion km away from the Earth for the force to be 2 piconewtons. That's a very sensitive measurement!