What is a Scanning Tunneling Microscope?
A scanning tunneling microscope (STM) is an innovative type of microscope which, instead of using reflecting light like conventional optical microscopes, uses quantum tunneling between a sample and a probe tip to image the surface. The resolutions achieved by an STM can be as high as 0.1 nm lateral resolution and 0.01 nm depth resolution. This is a few times higher than the resolutions achievable using the best electron microscopes.
An STM can work in a variety of environments: besides from ultra high vacuum, it also works in environments saturated with water, air, etc. This makes the microscope very flexible. However, the surface must be very clean and the STM tip very sharp, causing practical challenges in imaging. The STM was developed by Gerd Binnig and Heinrich Rohrer in 1981. In 1986, they won a Nobel Prize in Physics for their work on STMs.
An STM tip is so sharp that it consists of only a single atom. When the tip is "dull" and consists of two atoms rather than one, this leads to fuzzier images. The challenge of creating sufficiently sharp tips has led researchers to explore the use of carbon nanotubes as STM tips, as they are very rigid and easy to produce. The tip is sometimes called the "stylus", and a platinum-iridium combination is among the most widely used tip materials.
Like many other microscopes, advanced vibration dampening is often required to create a useful STM. In the earliest systems, magnetic levitation schemes were used, though today spring-based systems are most popular. Shortly after STMs became common knowledge, a High School student was able to create a crude one using only about $100 US Dollars (USD) of materials. An oscilloscope was used as an imaging screen.
The tip of an STM is guided by a "piezo," or piezoelectric crystal, which bends in a small but very predictable way in response to an electric field. In an STM, tip movement is completely computer controlled.
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