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Move Atoms with Scanning Tunneling Microscope (STM)

To achieve the bottom-up vision of nanotechnology (the capability to build materials by manipulating atoms) involves moving atoms and molecules to precise locations. The STM (scanning tunneling microscope) has the capability to move atoms around on a surface.

An STM tip narrows to a sharp point, ideally made up of just a single atom. When the tip is brought very close to the sample surface, with only about a 1 nm gap, an electrical current (called the tunneling current) occurs between the STM tip and the sample. The amount of tunneling current increases as the gap between the tip and the surface decreases.

This change in the tunneling current generates a topographical image of the surface. If the STM tip, as it scans across the sample’s surface, encounters an atom sitting on the surface, the gap shrinks and the tunneling current goes up.

Because the tip and the sample have no physical contact, the electrons have to tunnel across the gap between the tip and the sample to produce an electric current. The rules of quantum mechanics, which govern the behavior of subatomic particles, apply when working at this small scale, which is why this movement of electrons across a gap is called quantum mechanical tunneling.

The tip of a scanning tunneling microscope.
The tip of a scanning tunneling microscope.

So exactly how does an STM move atoms? A physicist named Johannes van der Waals discovered one of the weaker forces that acts on molecules and atoms. This van der Waals force allows the STM to move atoms around.

To move a particular atom to a different point on the surface of a sample, you position the STM tip above the atom. You then lower the tip to the point where the van der Waals force is strong enough to make the atom stick to another atom at the end of the STM tip when it’s moved latterly.

After the STM moves the atom to the desired spot, you raise the STM tip and the atom stays in place. Some industrious soul at the National Institute of Standards and Technology (NIST) tried this with cobalt atoms on a copper surface, moving the cobalt atoms to form the NIST logo.

Cobalt atoms arranged with a scanning tunneling microscope. [Credit: Image courtesy of the National
Credit: Image courtesy of the National Institute of Standards
Cobalt atoms arranged with a scanning tunneling microscope.
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