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Scanning tunneling microscopy (STM) is a powerful surface-sensitive technique used to image surfaces at the atomic level. It relies on the quantum mechanical phenomenon of electron tunneling. In STM, a sharp conductive tip, often made of tungsten or platinum-iridium, is brought very close to a conductive surface, typically within a few angstroms. A small voltage is applied between the tip and the surface. Due to the proximity and applied voltage, electrons can quantum mechanically "tunnel" across the vacuum gap between the tip and the surface.

The magnitude of this tunneling current is extremely sensitive to the distance between the tip and the surface. Even a change of a single atom's height can significantly alter the tunneling current. STM operates in one of two primary modes: constant height mode and constant current mode. In constant height mode, the tip is scanned across the surface at a fixed height, and variations in the tunneling current are recorded to create a topographic image. In constant current mode, a feedback loop adjusts the tip's vertical position to maintain a constant tunneling current as it scans. This feedback signal, representing the tip's height, is then used to construct the topographic image. STM can provide atomic resolution and is used in various fields, including solid-state physics, materials science, and nanotechnology, for studying surface structures, atomic manipulation, and electronic properties.