What is the role of voltage between a scanning tunneling microscope tip and a sample?

In a scanning tunneling microscope (STM), the application of a small voltage between the probe tip and the sample surface is essential for operation. This voltage creates the necessary conditions for tunneling, a quantum mechanical phenomenon that allows electrons to cross the gap between the tip and the sample.1

By applying this voltage, electrons are induced to tunnel across the gap, generating a tunneling current. As the probe scans the surface, the instrument registers variations in this current, which are then processed to create a topographical image of the surface at the atomic level.1

Beyond imaging, the voltage plays a functional role in manipulating matter. A sufficiently strong electric field generated by the voltage can be used to move atoms along the sample surface or to enhance etching rates. In specific applications, such as the fabrication of nanostructures, a higher voltage can be used to remove atoms from the tip and deposit them onto a substrate.1