What does a scanning tunneling microscope do? How does it work?

Imagine trying to roll a ball up a hill, but instead of needing enough energy to get to the top, the ball magically appears on the other side. This is analogous to quantum tunneling, a fascinating concept central to the operation of a scanning tunneling microscope (STM). In classical physics, particles like electrons would need sufficient energy to overcome a barrier. However, quantum mechanics tells us that particles can sometimes 'tunnel' through barriers even if they don't have enough energy, much like that ball phasing through the hill.

This phenomenon occurs because particles exhibit both particle-like and wave-like characteristics. When the wave-associated with an electron encounters a barrier, there's a non-zero probability that it can exist on the other side, allowing what seems like an impossible transition. It's this quantum mechanic quirk that STMs exploit, detecting the presence of electrons that have 'tunneled' across a gap from the microscope's tip to the sample's surface, without the two ever physically touching.

The ability to see the world at an atomic scale has revolutionized many fields of science and technology. Atomic-level imaging, as achieved by STMs, allows scientists to visualize individual atoms and molecules, which was once the realm of science fiction. Atoms are the building blocks of matter, and by imaging them, we gain insights into the fundamental structure of materials.

How is this done? STMs take advantage of the fine control over the position of a very sharp metallic tip, often ending in a single atom. This tip scans across the sample's surface, with the quantum tunneling effect enabling the measurement of the tunneling current at various points. The STM collects data that reflects the surface's topography - essentially feeling the bumps and valleys of atoms and molecules. The resulting images are maps of the surface at the atomic level, providing a depth of detail that is critical for fields such as materials science, chemistry, and nanotechnology.

Tunneling current is the heartbeat of the scanning tunneling microscope's functionality. This current represents the flow of electrons that occurs due to quantum tunneling between the microscope's tip and the sample. To maintain a constant tunneling current, the STM uses a feedback loop that adjusts the tip's height as it scans across the surface. This incredibly sensitive process can detect variations in the surface topography down to the atomic scale.

But why does the current decay exponentially with distance? It's due to the wave nature of electrons. The probability of an electron being on the other side of a barrier decreases exponentially the thicker or higher the barrier becomes. Therefore, even a tiny increase in the gap between the tip and the surface can greatly reduce the tunneling current. By precisely controlling this gap, the STM tracks the surface contours and converts the positional changes into visual data, creating images that let us see the previously unseen world of atoms and molecules.