This article was updated on the 19th August 2019.
A scanning tunneling microscope (STM) is a non-optical microscope that works by scanning an electrical probe tip over the surface of a sample at a constant spacing. This allows a 3D picture of the surface to be created.
How a STM Works
The STM sample must conduct electricity for the process to work. The STM uses a tip that ends in a single atom, and a voltage is passed through the tip and the sample.
Electrons use a quantum mechanical effect to ‘tunnel’ from the tip to the sample and vice versa. The current that results depends upon the distance between the probe tip and the sample surface. The tip is attached to a piezoelectric tube, and the voltage applied to the piezo rod is altered to maintain a constant distance of the tip from the surface. Changes in this voltage allow a three-dimensional picture of the material surface to be built up as the tip is scanned back and forth across the sample.
Figure 1. STM tip
An STM can have a lateral resolution of up to 0.1nm, and a depth resolution of up to 0.01nm, which combined is small enough to resolve individual atoms. In recent years, carbon nanotubes have been used for constructing these tips, allowing for improved resolution and reduced error in image reconstruction.
Due to the remarkable detail STM can discern about the surface of a material, they are very useful for studying friction, surface roughness, defects, and surface reactions in materials like catalysts. STMs are also very important tools in research surrounding semiconductors and microelectronics.
In addition to this, they can be used for lithography, where individual electrons are tunneled onto a layer of photoresist. This allows for more control over previously established methods like electron beam lithography. These sorts of techniques are not limited to electrons, with STM finding application in the deposition of metal atoms such as gold and tungsten, used in the fabrication of nanodevices.
Researchers at Cornell University have also demonstrated that it is possible to use STM principles to rotate individual molecules in a sample by applying an appropriate voltage. This could be used as a means of data storage in electrical circuits or chips down to molecular precision.