5.4 The scanning tunnelling microscope
The scanning tunnelling microscope (STM) is a device of such extraordinary sensitivity that it can reveal the distribution of individual atoms on the surface of a sample. It can also be used to manipulate atoms and even to promote chemical reactions between specific atoms. The first STM was developed in 1981 at the IBM Laboratories in Zurich by Gerd Binnig and Heinrich Rohrer. Their achievement was recognised by the award of the 1986 Nobel prize for physics.
In an STM the sample under investigation is held in a vacuum and a very fine tip, possibly only a single atom wide, is moved across its surface (see Figure 25). Things are so arranged that there is always a small gap between the tip and the surface being scanned. An applied voltage between the tip and the sample tends to cause electrons to cross the gap, but the gap itself constitutes a potential energy barrier that, classically, the electrons would not be able to surmount. However, thanks to quantum physics, they can tunnel through the barrier and thereby produce a measurable electric current. Since the current is caused by a tunnelling process, the magnitude of the current is very sensitive to the size of the gap (detailed estimates can again be obtained using Equation 7.56). This sensitivity is the key to finding the positions of tiny irregularities in the surface, including individual atoms.
In practice, the STM can operate in two different ways. In constant-height mode, the tip moves at a constant height and the topography of the surface is revealed by changes in the tunnelling current. In the more common constant-current mode the height of the tip is adjusted throughout the scanning process to maintain a constant current and the tiny movements of the tip are recorded. In either mode the structure of the sample's surface can be mapped on an atomic scale, though neither mode involves imaging of the kind that takes place in a conventional optical or transmission electron microscope.
STMs have now become a major tool in the developing field of nanotechnology. This is partly because of the images they supply, but even more because of their ability to manipulate individual atoms and position them with great accuracy. One of the products of this kind of nano-scale manipulation is shown in Figure 26, the famous ‘quantum corral’ formed by positioning iron atoms on a copper surface.