The Scanning Tunneling Microscope (STM) images material surfaces at the atomic level. It was developed by Gerd Binnig and Heinrich Rohrer at the IBM Research Laboratory in Rüschlikon, Zürich in 1981. They were awarded the Nobel Prize in Physics for this invention in 1986. For the STM to work, the measured sample must conduct electricity i.e. be a metal or semiconductor. The STM is particular useful for studies in the field of e.g. nanoelectronics.
The scanning tunneling microscope (STM) works by scanning a very sharp metal wire tip over the sample surface. By bringing the tip very close to the surface, and by applying an electrical voltage to the tip or sample, the surface can be imaged down to individual atoms. For an STM, good resolution is considered to be 0.1 nmof lateral and 0.01 nm of depth resolution. The STM relies on the quantum tunneling effect.
In quantum mechanics, electrons have both particle-like and wave-like properties. Quantum tunneling is a wave-like effect that occurs at an electronic barrier. If the barrier is thin enough, the wave function may extend into the next region, through the barrier allowing an electron to tunnel through the barrier and, if under bias, create an electrical current called the tunneling current.
In the STM, the starting point of the electron is either the tip or sample, depending on the setup of the instrument. The barrier is the gap (air, vacuum, liquid), and the second region is the other side, i.e. tip or sample, depending on the experimental setup. By monitoring the current through the gap, we have very good control of the tip-sample distance.
The Feedback Loop
A feedback loop monitors the tunneling current and adjusts the tip to maintain a constant current. These adjustments are recorded by the computer and presented as an image in the STM software. Such a setup is called a constant current image.
Feedback loop and electron tunneling in a scanning tunneling microscope (STM).
About Heinrich Rohrer, b. 1933 Buchs (SG), d. 2013, Wollerau (SZ)
Heinrich Rohrer was educated at the ETH, Zürich and then did two years of postdoctoral research on superconductivity at Rutgers Universityin the USA. Rohrer subsequently returned to Switzerlandand in 1963 joined the IBM Research Laboratory, where he remained until his retirement in 1997.
References and further reading
- Wikipedia: https://en.wikipedia.org/wiki/Heinrich_Rohrer
- Wikipedia: https://en.wikipedia.org/wiki/Scanning_tunneling_microscope
- Nanoelectronics: https://pubs.rsc.org/content/articlehtml/2015/cs/c4cs00204k