Scanning Tunneling Microscopy (STM)
Updated: Apr 5
With the invention of the scanning probe microscope (STM) in 1981, the family of scanning probe microscopes was born. While working at IBM Zurich Research Laboratories in Switzerland, Gerd Binnig and Heinrich Rohrer created the first operational STM. Binnig and Rohrer were awarded the Nobel Prize in Physics in 1986 for their apparatus.
How an STM works?
The scanning tunneling microscope (STM) scans a surface with a razor-sharp metal wire tip. We can scan the surface at an incredibly fine scale – down to resolving individual atoms – by bringing the tip very close to the surface and delivering an electrical charge to the tip or sample.
A quantum mechanical phenomena known as tunneling. When electrons pass through a barrier that they shouldn't be able to get through, this is known as a tunneling current. You won't be able to move "over" a barrier if you don't have enough energy. Electrons, on the other hand, exhibit wavelike qualities in the quantum mechanical universe. These waves don't stop abruptly at a wall or barrier, but instead gradually fade away. The probability function may expand into the next zone, through the barrier, if the barrier is thin enough! Because there is a slight chance that an electron will be on the other side of the barrier, some will pass through and emerge on the other side if there are enough electrons. When an electron moves through the barrier in this fashion, it is called tunneling.
According to quantum physics, electrons contain both wave and particle-like features. Tunneling is a result of the wavelike aspect of the environment.
To apply this to the STM, the electron's starting point is either the tip or the sample, depending on the instrument's arrangement. The gap (air, vacuum, liquid) is the barrier, while the opposite side, i.e. tip or sample, is the second zone, depending on the experimental setup. We have excellent control over the tip-sample distance by measuring the current via the gap.
Pierre Curie discovered the piezoelectric effect in 1880. Squeezing the sides of certain crystals, such as quartz or barium titanate, produces the appearance. As a result, opposing charges are created on both sides. The action can also be inverted; a voltage applied across a piezoelectric crystal causes it to elongate or compress.
Scanning tunneling microscopy (STM) and most other scanning probe techniques use these materials to scan the tip. PZT is a common piezoelectric material in scanning probe microscopy (lead zirconium titanate).
To measure the current, scan the tip, and transfer this information into a format that can be used for STM imaging, we'll need electronics. The tunneling current is constantly monitored by a feedback loop, which makes modifications to the tip to maintain a constant tunneling current. The computer records these adjustments and displays them as a picture in the STM program. A constant current image is the name for such a configuration.
In addition, the feedback loop can be switched off for highly flat surfaces, leaving only the current to be displayed. This is an image with a constant height.