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How did the scanning tunneling microscope allow scientists to see individual atoms?

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The powers of 10 — nanotechnology Examples from biological and mechanical realms illustrate various “orders of magnitude” (powers of 10), from 10⁻² meter down to 10⁻⁷ meter.

The scanning tunneling microscope (STM) allows scientists to visualize individual atoms by exploiting the quantum mechanical phenomenon known as tunneling.

In this process, a sharp tungsten needle is positioned just a few angstroms from a sample surface. When a small voltage is applied, electrons "tunnel" across the gap between the tip and the surface—a movement that is forbidden under classical physics but possible due to the wavelike properties of electrons.12

Because the probability of this tunneling current is extremely sensitive to distance, the STM registers minute variations in current as the probe scans over the surface. These variations correspond to the electron density at the surface:
* In semiconductors, the density peaks at atomic sites, appearing as bright spots in the resulting image.1
* In metals, the interaction between the tip and the sample perturbs the electron density, causing the tunneling current to increase when the tip is positioned directly above an atom.1

By processing this data, the STM creates a topographical map that reveals the precise spatial distribution and structure of individual atoms.1