Mechanism of atomic force microscopy imaging in solution

The recent advent of high-resolution imaging and force spectroscopy using atomic force microscopy (AFM) in aqueous and organic solutions opens the door to imaging a wide range of surfaces and their solvation structure. However, to fully benefit from the high resolution, and provide a significant new analytical ability, a detailed understanding of the underlying contrast mechanisms leading to atomic and molecular resolution is crucial. Without a theory that connects the measured force to atomistic models of the surface and microscope tip, the information that can be distilled from these measurements is limited.

In a recent paper in Physical Review Letters researchers from the London Centre for Nanotechnology (Matt Watkins, Alexander Shluger) used modern molecular simulation techniques to provide a general framework for the interpretation of AFM images in water. They present analysis of the atomistic mechanisms that produce the variations in force that the microscope measures and that determine the image contrast.

The molecular dynamics simulations demonstrate that the forces acting on a microscope tip result from the direct interaction between a tip and a surface, and forces entirely due to the water structure around both tip and surface. The observed force depends on a tip structure and is a balance between largely repulsive potential energy changes as the tip approaches a surface and the entropic gain when water is sterically prevented from occupying sites near the tip and surface. Understanding the interplay of these different components that contribute to the force the microscope measures will be vital for interpretation of high resolution images of interfaces in solution.

This work has been published in Physical Review Letters (M Watkins and AL Shluger, Phys. Rev. Lett. 105, 196101 (2010)) and featured on the cover of the journal

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Figure: Structure of water between and atomic force microscope tip    and  surface

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Structure of Water