Gold film grown on quartz.
The surface of gold film grown on quartz.
Photo: prof. Leo Kouwenhoven
Surface of flame annealed gold film.
The surface of flame annealed gold film.
Photo: prof. Leo Kouwenhoven

The Scanning Tunneling Microscope
- Preparation of Specimen

An STM specimen needs a substrate that is extremely flat, down to the atomic level. If the specimen is uneven, the STM probe will have difficulties in scanning very steep pits or ridges. Other defects, such as single atoms sitting on an otherwise flat substrate, will also be a problem since the atoms themselves may function as unwanted STM probes, destroying the desired image.

One of the most commonly used STM substrates is a special form of graphite (highly oriented pyrolytic graphite or HOPG). It is a naturally layered material that is easy to prepare and relatively inert. A fresh surface can be obtained as easily as pressing a piece of adhesive tape to the surface and peeling away the top layer. The resulting surface will have large flat areas useful for scanning.

Other Substrates
Other popular materials that provide large, atomically flat surfaces include mica, quartz, and silicon. These materials are insulators, so to be used for STM a thin layer of noble metal (mainly gold or platinum) is deposited on the surface. Annealing (heating and then slowly cooling) the metal layer helps to smooth the surface and produce large flat areas.

Growing Gold
A thin gold film can also be grown while deposited on a flat mica surface and then stripped away. The surface of the gold that was in contact with the mica will be very flat over large areas.

Avoiding Contamination
Once a flat surface has been achieved, it is important to keep it free of contamination. A typical dust particle consists of millions of atoms, and could easily destroy the STM probe. Other hazards include chemical reactions between the specimen and the surrounding air that result in impurities on the substrate. These impurities could eventually make the STM probe crash. To prevent these problems, many STMs operate in high vacuum. Other techniques include periodic heating of the sample in a neutral atmosphere to remove impurities from the surface.

  Related Laureates:
The Nobel Prize in Physics, 1986
- Gerd Binnig and Heinrich Rohrer »

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