An atomic force microscope (AFM) is an extremely precise microscope that images a sample by rapidly moving a probe with a nanometer-sized tip across its surface. This is quite different than an optical microscope which uses reflected light to image a sample. An AFM probe offers a much higher degree of resolution than an optical microscope because the size of the probe is much smaller than the finest wavelength of visible light. In an ultra-high vacuum, an atomic force microscope can image individual atoms. Its extremely high resolution capabilities have made the AFM popular with researchers working in the field of nanotechnology.
Unlike the scanning tunneling microscope (STM), which images a surface indirectly via measuring the degree of quantum tunneling between the probe and sample, in an atomic force microscope the probe either makes direct contact with the surface or measures incipient chemical bonding between probe and sample.
The AFM uses a microscale cantilever with a probe tip whose size is measured in nanometers. An AFM operates in one of two modes: contact (static) mode and dynamic (oscillating) mode. In static mode, the probe is kept still, while in dynamic mode it oscillates. When the AFM is brought close to or contacts the surface, the cantilever deflects. Usually, on top of the cantilever is a mirror which reflects a laser. The laser reflects onto a photodiode, which precisely measures its deflection. When the oscillation or position of the AFM tip changes, it is registered in the photodiode and an image is built up. Sometimes more exotic alternatives are used, such as optical interferometry, capacitive sensing or piezoresistive (electromechanical) probe tips.
Under an atomic force microscope, individual atoms look like fuzzy blobs in a matrix. To provide this degree of resolution requires an ultra-high vacuum environment and a very stiff cantilever, which prevents it from sticking to the surface at close range. The downside of a stiff cantilever is that is requires more precise sensors to measure the degree of deflection.
Scanning tunneling microscopes, another popular class of high-precision microscopes, usually have better resolution than AFMs, but an advantage of AFMs is that they can be used in a liquid or gas ambient environment whereas an STM must operate in high vacuum. This allows for the imaging of wet samples, especially biological tissue. When used in ultra-high vacuum and with a stiff cantilever, an atomic force microscope has similar resolution to an STM.
By Michael Anissimov