A scanning tunneling microscope (STM) is a device that obtains images of the atoms on the surfaces of materials. The STM is not an optical microscope; instead, it works by detecting electrical forces with a probe that tapers down to a point only a single atom across.
The probe in the STM sweeps across the surface of which an image is to be obtained. The electron shells, or clouds, surrounding the atoms on the surface produce irregularities that are detected by the probe and mapped by a computer into an image. The resolution of the image is on the order of one nanometer (1 nm) or less, where 1 nm = 0.000000001 meter = 10-9 m.
The STM has applications in molecular science, also known as Nanotechnology. It can be used to move atoms individually, as well as to generate high-resolution maps of material surfaces.