Tip steering for fast imaging in AFM

In atomic force microscopy a sharp tip, supported by a cantilevered beam and interacting locally with a sample, is raster-scanned over a surface to build a three dimensional image. Typical scan times are on the order of minutes or longer, depending on the size, resolution, and image quality desired. For a variety of reasons it is of great interest to reduce the time to gather an image. One of the most exciting applications is the imaging in real time of dynamic phenomena such as the motion of single molecules in molecular biology. In many cases the sample to be imaged is string-like, such as nanowires, actin, and DNA strands. In this case most of the imaging time is wasted gathering data about the substrate rather than about the sample. In this work we propose a high-level control algorithm to steer the tip along the string, thereby imaging only the area directly around the sample. This approach focuses the resolution directly where desired and greatly reduces the time to gather an image by reducing the area to be scanned. Depending on the sample, an order of magnitude or better improvement in the imaging time can be achieved. As the algorithm makes no demands on the low level control of the tip it can be combined with approaches aimed at increasing the allowed scanning speed, resulting in even greater reductions in the imaging time. Furthermore, the chances of damaging the tip due to interaction with stray particles on the substrate is greatly reduced since the tip is kept near to the sample. In addition to a simulation study, we present a physical experiment in which a carbon nanotube is imaged using an atomic force microscope controlled by the tip-steering algorithm. To the authors knowledge this is the first reported instance of an image obtained by such high-level feedback control.