Atomic force microscopy improved resolution employing large scanning speeds: Effects of the double relaxation time

The minimum scanning speed of atomic force microscopes for improved atomic resolution has been measured in liquid media, and shown to be equal to 100 nm/s for mica immersed in water corresponding to the time spent scanning the distance between two neighbor ions (∼0.52 nm) of ∼5 ms. The scanning velocity dependence of the force acting on the tip in the double-layer region (∼135 nm) when it approaches the surface was also measured. The stationary component of this force, for scanning speeds up to 30 μm/s, was identified as the exchange of the liquid media with ε≈80 by the tip with ε≈6. As the tip approaches the surface and as well as when the tip images atomic features, this repulsive force shows a relaxation time of a few milliseconds, corresponding to the shielding of the surface charge by the solution, i.e., the double-layer relaxation time. Scanning surfaces at speeds higher than the ratio of the atomic features distance and this relaxation time results in a variable repulsive force acting on the tip, as a function the scanning speed, which might be used to improve the atomic imaging resolution.