How the atomic force microscope scans surfaces

Petri P. Lehenkari, Guillaume T. Charras, Stephen A. Nesbitt and Mike A. Horton

Figure 1. How the atomic force microscope scans surfaces. (a) A schematic illustration of the method of operation of an atomic force microscope. In this example, the sample being analysed is a multi-nucleated osteoclast derived from bone (osteoclasts, which resorb bone matrix, are large cells, with dimensions up to 100 µm x 100 µm x 8 µm). The position of the cantilever of the atomic force microscope is controlled by three piezo-ceramic controllers, which place the tip of the atomic force microscope in the x and y directions and additionally, under a controlled and known downward force, in the z or vertical direction. A laser beam is directed at the reflective, upper surface of the cantilever, and the deflected light detected by a four-quadrant photo-detector. Both the size and position of the current created in the detector are linked via a computer to a feedback circuit, which maintains the cantilever position at a defined location on the surface that is being analysed. Meanwhile the cantilever is scanned backwards and forwards across the surface (raster scanned) to produce an image (of surface topography, using the simplest form of atomic force microscopy) that reflects the change in position of the cantilever tip. Other forms of imaging and measurement using atomic force microscopy can be generated by analysing the different types of information produced in the feedback circuits and also by employing different modes of controlling the movement of the cantilever (see text, for further details). (b) An example of a three-dimensional image of the surface topography of an osteoclast that was cultured on a glass coverslip. This atomic force microscopy image was produced using the simplest mode of operation (the so-called contact mode, whereby the tip of the microscope is kept on the surface of the sample that is being analysed) (fig001mhu).