Scanning probe microscope is any of a variety of microscopes that scan the surface of materials with a sharp point and display the results on a televisionlike screen. The point, called a probe, can be as tiny as 0.3 billionth of a meter (about 1/80,000,000 inch) across—about the width of an atom. Scanning probe microscopes can provide three-dimensional images of individual atoms. They have become important research tools in physics, engineering, and chemistry.
The first of these microscopes was the scanning tunneling microscope (STM), invented in 1981 by physicists Gerd Binnig of West Germany and Heinrich Rohrer of Switzerland. Binnig and Rohrer won shares of the 1986 Nobel Prize in physics for their invention.
The STM creates an atomic map of surfaces. It works with materials that can carry an electric current. The STM brings its probe to within 1 nanometer (1 billionth of a meter) of the sample (the surface being studied) and creates a voltage (difference in charge) between the probe and the sample. The voltage creates a tunneling current, an electric current consisting of a flow of electrons between the probe and the sample.
The strength of the tunneling current depends on the distance between the probe and the sample. As the probe scans the sample, the STM measures the current and keeps it constant by raising or lowering the probe. A computer records the probe’s movements and uses them to create the image. The image can be up to about 100 million times as large as the sample. One use of the STM has been to study the growth of silicon crystals, which are used as computer chips.
Another major type of scanning probe microscope is the atomic force microscope (AFM). The AFM does not use a tunneling current, so researchers can use it to study samples that do not conduct electricity. In most applications, the probe gently touches the sample. As the probe scans the sample, the AFM measures the mechanical force between the probe and the sample. When the force increases due to a bump on the sample, the AFM moves the probe upward. When the force decreases due to an indentation in the sample, the AFM moves the probe downward. A laser device measures the up-and-down movements. A computer uses the measurements to create the image.
Another type of AFM has a magnetic probe. The probe does not touch the sample but moves up and down in reaction to magnetic forces between itself and the sample. Researchers use this AFM with such samples as the surfaces of magnetic disks for computers.
Other scanning probe microscopes include the scanning thermal profiler, which reacts to temperature differences on the sample. The scanning near field optical microscope measures light reflected from the sample. The scanning electrochemical microscope reacts to chemical changes on the sample.
