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Microscopy - Scanning Tunneling Microscopy

current surface tip air

In the early 1980s, a new technique in microscopy was developed which did not involve beams of electrons or light to produce an image. Instead, a small metal tip is scanned very close to the surface of a sample and a tiny electric current is measured as the tip passes over the atoms on the surface. The microscope that works in this manner is the scanning tunneling microscope (STM). When a metal tip is brought close to the sample surface, the electrons that surround the atoms on the surface can actually "tunnel through" the air gap and produce a current through the tip. This physical phenomenon is called tunneling and is one of the amazing results of quantum physics. If such phenomenon could occur with large objects, it would be possible for a baseball to tunnel through a brick wall with no damage to either. The current of electrons that tunnel through the air gap is very much dependent on the width of the gap and therefore the current will rise and fall in succession with the atoms on the surface. This current is then amplified and fed into a computer to produce a three dimensional image of the atoms on the surface.

Without the need for complicated magnetic lenses and electron beams, the STM is far less complex than the electron microscope. The tiny tunneling current can be simply amplified through electronic circuitry similar to that which is used in other electronic equipment, such as a stereo. In addition, the sample preparation is usually less tedious. Many samples can be imaged in air with essentially no preparation. For more sensitive samples which react with air, imaging is done in vacuum. A requirement for the STM is that the samples be electrically conducting, such as a metal.


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