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Scanning tunneling microscopy, often abbreviated as STM, is a powerful type of microscopy used for imaging surfaces at the atomic level. Developed in the early 1980s by Gerd Binnig and Heinrich Rohrer, who were awarded the Nobel Prize in Physics for their invention, STM relies on the quantum mechanical phenomenon of electron tunneling.

The core principle of STM involves a sharp conducting tip, ideally ending in a single atom, which is brought extremely close to the surface of a conducting or semiconducting sample. When a small voltage bias is applied between the tip and the sample, electrons can "tunnel" across the vacuum gap. This tunneling current is highly sensitive to the distance between the tip and the surface, varying exponentially with separation.

An STM instrument scans this tip across the sample surface. Two primary modes of operation are common. In constant height mode, the tip is kept at a fixed height above the average surface, and the variations in tunneling current are recorded to create an image. In constant current mode, a feedback loop adjusts the tip's vertical position to maintain a constant tunneling current, and the recorded height variations of the tip directly map the surface topography.

STM has revolutionized surface science by allowing researchers to visualize and manipulate individual atoms. It is used in diverse fields, including materials science, nanotechnology, and semiconductor research, to study surface structure, observe atomic processes like diffusion and adsorption, and even to engineer atomic-scale structures. While limited to conducting or semiconducting samples, its resolution and capabilities remain unparalleled for certain surface investigations.