Scanning Probe Microscope

Scanning Probe Microscope

A scanning probe microscope (SPM) is any type of microscope that can see surfaces on a very fine scale--so fine, in fact, that individual atoms may be discerned. There are several types of SPMs, all about the size of a fingernail, but all rely on a small, extremely sharp probe that scans across a surface, recording the forces it encounters and transmitting this information back to a sensor. With such microscopes scientists have studied surfaces at the level of individual atoms.

There are three basic types of scanning probe microscopes, all of which work slightly differently: (1) the atomic force microscope (AFM), (2) the scanning tunneling microscope (STM), and (3) the near-field scanning optical microscope (NSOM). The first scanning probe microscope was an STM, invented in 1982 by Heinrich Rohrer and Gerd Bining of the IBM Zurich Research Center in Switzerland. In 1986 Binnig and Rohrer shared the Nobel Prize in Physics for their design of the scanning tunneling microscope.

The atomic force microscope is something like a phonographic needle. Using lithography, a very small tip is made of silicon nitride (similar to the way computer chips are made). The tip is tapered as narrow as possible, possibly to a single atom. Hung from a cantilever, it then moves across the surface to be investigated, touching it, bending and flexing in response to the extremely small electromagnetic and mechanical forces it feels from the atoms on the surface. A small laser beam reflected off the back of the tip is detected, and the small changes in the location of the reflected beam are used to reconstruct the surface over which the tip is moving. The AFM can make an image down to atomic dimensions, about 0.1 nanometers (nm), somewhat like a topographical map, and works best over relatively smooth surfaces.

The scanning tunneling microscope was built before the AFM, and, since the only difference is in their tips, the STM invention provided all the image reconstruction techniques needed by other SPMs. A very fine wire tip is brought to within a few tenths of a nanometer of a surface. (The surface must be an electrical conductor for the STM to work, a limitation that the AFM does not have.) At this distance electrons can tunnel across the gap, from the surface up to the tip, generating a current that is detected. This current is also fed back into the tip-controller, a piezoelectric crystal that expands or contracts slightly when it encounters current, and the controller then tells the tip where to move next--up, down, left, right, back, or forward. The STM was a major advance in the study of materials, but it was limited to analyzing only conductors, usually metals.

The near-field scanning optical microscope runs a fiber optic probe over the sample. A small laser beam shines out the end of the probe, which is reflected off the surface for most materials. For opaque samples it is transmitted all or partially through, and some samples fluoresce under the influence of the laser. The light is then gathered and used to form the image. The detail that can be seen by the NSOM is limited by the wavelength of light used, in practice about 400 - 500 nm, although techniques exist to reduce this to about 40 - 50 nm--still not nearly as accurate as the AFM or STM, but useful for larger structures.

Since AFM can analyze electrical insulators as well as conductors, it has become more widely used, especially among biologists studying the surfaces and molecules of the biological world. The placement of a small magnetic coating on the AFM tip allows it to measure magnetic properties of a sample, and other schemes work for other properties.

With the SPM, scientists have taken pictures of individual atoms and crystal surfaces, they have made movies of fundamental biological molecules like DNA and RNA bind and move together, the molecular basis of phenomena like adhesion and lubrication, and a host of other new and creative applications. Using the SPMs, scientists are also construction s mall machines like switches and batteries, a new field called nanotechnology.