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Scanning Probe Microscopes

In 1981 two Swiss Physicists G Binnig and H Röhrer invented the first of these instruments, the Scanning Tunneling Microscope (STM), at the IBM laboratories in Zurich. For this they were awarded a Nobel Prize in 1986. Unlike the light microscope and the electron microscope these instruments employ a finely pointed mechanical probe that is scanned across the surface. This scanning is used to map the surface structure. The probe which may be composed of a tip with a single atom at the end does not touch the surface in the Scanning Tunneling Microscope (STM). If it did irreparable damage might be done to the tip

The STM is capable of visualizing individual atoms but will only work with conducting materials. This instrument can even be used to move single atoms on the surface of a specimen. Electrons may also be added or removed from atoms or molecules to create anions or cations.  The STM depends upon quantum phenomena for its operation. When the probe, in practice a single atom on the end of a fine needle, is scanned across the surface of a conducting specimen, at atomic distances away, a current will flow from the electron cloud around the atoms on the specimen surface to the probe when a small voltage is applied to the probe. This ‘tunneling current’ may actually flow from the probe tip, or to the tip depending on the polarity, and it changes rapidly as the probe moves over the surface. The closer the tip atom to the electron cloud around the atoms on the surface, the greater the tunneling current. By applying electrical feedback this current can be kept constant and the tip is moved up or down to achieve this state. The movement of the tip (Z) is used to map the (X, Y) surface topography of the specimen. Single atoms and arrays (lattices) can easily be displayed in an image of the specimen surface. An STM instrument is always operated in an ultra high vacuum environment. The surface of the specimen, besides needing to be extremely flat, has to be absolutely clean and free of any contaminating molecules which might prove fatal to the fine probe.