Pushing the limits of SPM

Breaking the speed limit One of the serious, inherent limitations in all forms of SPM is the low imaging rate. This is because each image is built up pixel by pixel in a sequential scan of the surface. The scanning motion involves electronically controlled mechanical displacements, and usually some form of feedback is applied between a control parameter, such as tunneling current or force, and the height of the probe, e.g. the tip. Each of these elements introduces its own characteristic time restrictions, and together they conspire to make the acquisition time of STM, AFM, and other SPM images lengthy – typically between several seconds and several minutes per image. Faster scanning would bring several distinct advantages. The most obvious of these is the possibility of following dynamic processes in real time2-4. There are already numerous examples of SPM studies on surface dynamics in which a series of images is recorded to generate a movie of the dynamic process5. For example, Fig. 1 shows three images taken from an STM movie on a Cu(001) surface in which four embedded In atoms are followed in their motion by Joost W. M. Frenken†*, Tjerk H. Oosterkamp†, Bas L. M. Hendriksen†, and Marcel J. Rost†,‡