Basic Principles of Scanning Tunneling Microscopy

The scanning tunneling microscopy (STM) is based on the tunneling current between a metallic tip, which is sharpened to a single atom point and a conducting material.

A small bias voltage (mV to 3 V) is applied between an atomically sharp tip and the sample. There’s no current if the distance between the tip and the sample is large. When the tip is brought very close (<10Å) without physical contact, a tunneling current flows across the gap between the tip and the sample. It is because wavefunctions overlaps between the tip atom and the surface atom, electrons can tunnel through the vacuum barrier separating the tip and sample in the presence of a small bias voltage.

To produce images, the STM can be operated in two modes. One is constant current imaging and the other one is constant height imaging.