scanning tunneling microscope
A scanning tunneling microscope, or STM, is an instrument so sensitive that it can map individual atoms on a surface. Its working part is a metal tip sharpened to a single atom at the very end. The tip is brought to within about a nanometre of a conducting surface — close, but not touching — and a small voltage is applied between them. Although a gap of empty space should block any current, a tiny current does flow, carried by electrons that quantum-mechanically tunnel across the barrier.
What makes this current so useful is its extreme sensitivity to distance. The tunneling current falls off exponentially as the gap widens, so that moving the tip just one atom's width farther away can shrink the current roughly tenfold. The instrument exploits this knife-edge sensitivity: a feedback system nudges the tip up and down to keep the current constant as it scans across the surface, and the record of those height adjustments traces out the bumps and dips of the atomic landscape.
Invented by Gerd Binnig and Heinrich Rohrer in 1981, the STM gave humanity its first direct images of single atoms arranged on a crystal, and won them a Nobel Prize. Beyond imaging, the same tip can be used to nudge individual atoms into chosen positions, spelling out words atom by atom. The STM is one of the clearest demonstrations that quantum tunneling is not a curiosity but a reliable, measurable, technologically useful effect.
The exponential dependence of tunneling current on the gap is what gives the STM its atom-scale resolution.
An STM does not photograph atoms with light; its 'image' is a map of electron tunneling, so it shows where electrons are available, not a literal picture. It also needs a conducting sample — insulators call for related tools like the atomic force microscope.