G. Binning and H. Rohrer
Scanning Tunneling Microscopy (G. Binning and H. Rohrer)
In 1982 a paper describing a new technique known as scanning tunneling microscopy (STM) was published by a group of scientists from the IBM Research Laboratory in Zurich [G. Binning, H. Rohrer, Ch. Geber, and E. Weibel, Physical Review Letters, 49, 57 (1982)]. This paper was built on prior work in the same laboratory that showed that the magnitude of the current that flowed between the tip of a sharp piece of tungsten metal and the surface of platinum metal over which it was moved was very sensitive to the distance between the tip and the metal surface. In their 1982 paper, the IBM researchers showed how this information could be used to study the surface of the metal. They described the STM technique as follows.
The principle of the STM is straightforward. It consists essentially in scanning a metal tip over the surface at constant tunnel current. . . . The displacements of the metal tip . . . yield a topographic picture of the surface.
In other words, as the tungsten tip moves over the surface being studied, it is raised or lowered as needed to give a constant current. Measurements of the motion of the tip are then recorded and analyzed.
The photographs at the beginning of this chapter are STM images of individual iodine atoms absorbed onto a platinum metal surface [B. C. Schardt, S.-L. Yau, and F. Rinaldi, Science, 243, 1050 (1989)]. They give the clear impression that scientists have finally developed a technique that can "see" (or at least "feel") individual atoms. It therefore isn't surprising that Binning and Rohrer received the Nobel Prize in physics in 1986 for the development of STM. Nor is it surprising that the number of papers describing the use of STM is increasing exponentially.
STM is ideally suited to probing the structure of metal surfaces. It has been used to study the mechanism by which metals nucleate and grow, the way metal films develop on atomically flat surfaces, and the process by which metals can be deposited on a semiconductor substrate. STM has also provided useful information about the correlation between electron density on the surface of semiconductors and their structures, and it has been used to study the effect of dislocations on the growth of superconductors.
Research in the 1990s will find STM being used as a routine probe of the structure of molecules absorbed onto the surface of a solid. A recent paper, for example, reported the use of STM to study the structure of a small segment of synthetic DNA that contained 12 base pairs [Y. Kim, E. C. Long, J. K. Barton, and C. M. Lieber, Langmuir, 8, 496 (1992)]. The STM images not only showed individual molecules of the double-stranded DNA, they were able to resolve the two strands. When an Ru3+ complex was added to the oligonucleotide, the STM images provided an estimate of the distance between the site at which this complex binds and the end of the nucleotide fragment that is consistent with the position expected from studies of the site at which this complex cuts DNA.
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