A variation of scanning probe microscopy that measures the force of interaction between a fine-tipped probe and the surface of a sample. Capable of nanometre-scale resolution, it is suitable for imaging the topography of biomolecules such as DNA and proteins, cell surfaces, and cell organelles. Essentially the apparatus consists of a silicon-tipped probe, mounted on a flexible cantilever, which is moved across the sample surface. Deflections of the probe are detected by a laser beam focused onto the back of the cantilever and reflected to a photosensor position detector. The basic technique can operate in various modes. In contact mode the tip of the probe touches the surface, so that it interacts with the attractive (van der Waals) forces exerted by the atoms of the sample surface. The probe (or sample) can be moved very precisely in three dimensions (by a piezoelectric device), and feedback from the position detector enables the probe position to be adjusted and maintained at a constant force or constant distance from the surface. Measures of these adjustments provide the raw data, which are processed by computer and converted into images. In other modes the probe is oscillated very close to the surface but not allowed to touch the sample, or to touch it only intermittently. These can give better resolution with certain types of sample.
http://www.nanoscience.com/education/AFM.html Succinct account of the technique
Subjects: Biological Sciences.