Like the Scanning Tunneling Microscope (STM), the AFM was also invented by Binnig, et al [1]. One of the main advantages of AFM over STM is that it does not require a conducting sample. Therefore, it is especially useful for imaging biological samples.

An AFM can be operated in various modes, which are classified by how much the probe contacts the surface. In "Contact mode" AFM a "topographical image" is produced by measuring the deflection of a small cantilever (a kind of microscopic diving board), which has a sharp probe attached to the bottom. Higher areas of the surface deflect the cantilever more. This deflection is detected by reflecting a laser off of the back of the cantilever onto a photodiode that is connected to a computer, which converts the signal into a number. The height of the probe above the surface is then adjusted until that deflection value reaches a constant number called "setpoint," which is determined by the person operating the AFM. This mechanism, known as the "feedback loop," is used to produce a digital image. As the cantilever scans the surface by the same piezo mechanism used for the STM, an image is produced, pixel by pixel, with the color of each pixel representing the height data taken at that point.

Intermittent-contact or non-contact mode AFM differs from contact mode in that the cantilever is driven (made to oscillate) at its resonance frequency, and the amplitude of this oscillation is measured by the laser and photodiode. As the probe approaches the surface, the amplitude of cantilever oscillation decreases due to interactions with the surface. Thus, the feedback loop adjusts the height of the probe above the surface in order to keep the amplitude at a constant value (the setpoint), and the height necessary to do so at each data point on the surface is recorded. The low force applied to the sample by this mode makes it particularly useful for imaging soft samples, for example DNA-Protein complexes [2].

References

[1] G. Binnig, C. F. Quate, and Ch. Gerber, Phys. Rev. Lett. 56, 930 (1986). abstract

[2] Niemeyer, C. M., Adler, M., Pignataro, B., Lenhert, S., Gao, S., Chi, L., Fuchs, H., Blohm, D., Nucleic Acids Res., 27, 4553-4561 (1999). abstract