Scanning Tunneling Electron Microscopy (STEM) is a powerful technique that provides magnification of conducting surfaces to the atomic level. My recent experience as a member of a group of twenty teachers selected to work for eight summer weeks with scientists at Xerox Corporation and professors at the University of Rochester has opened new vistas for me and my high school students. We learned to use the Burleigh Instructional Scanning Tunneling Microscope (ISTM), an instrument designed for educational use by the University of Rochester, Xerox Corporation, and Burleigh Instrument Inc. While the images we can produce are not of the same quality as are often seen in the research literature, care in making a sharp, uncontaminated scanning tip and data collection in a vibration-free area, can result in pictures that are more than adequate for the introduction of "atomic eyes" to my classroom. The ISTM is useful in the study of the effect of acids and bases on metals and the surfaces of semiconductors (some experiments are described below). Cutting the platinum-iridium tip or preparing the tungsten tip and mounting the sample are hands-on activities that give students a glimpse of the technological nanoworld. This state-of-the-art instrumentation is making it possible for students to actually view atoms in their own classroom. What is truly amazing, however, is that the ISTM can be set up and atomic resolution images obtained in about an hour.
Instructional Scanning Tunneling MicroscopeWhile I am delighted to see scanning tunneling microscopy brought into the high school classroom, the article by Carl Rapp (1) perpetuates a common misconception about the beautiful images of the graphite surface that are obtained by STM. Inspection of Figures 4-7 shows that these images cannot be a true picture of the way atoms are disposed on the surface of graphite. First, the distance between the "graphite atoms" is too large, about 2 Å; the C-C bond length in bulk graphite is known from crystallographic data to be only 1.415 Å (2). Second, the six-membered rings in graphite do not contain central atoms!This oddity in STM images of graphite was noted by Binnig et al. in 1986 (3). A simple explanation is based on the fact that STM does not produce pictures of atoms, but of the tunneling current. In the most common form of graphite, the sheets of six-membered rings are offset, so that half of the atoms in the top sheet lie atop the center of a six-membered ring in the sheet underneath. Thus, graphite contains two inequivalent types of carbon atoms, which happen to transmit different tunneling currents. A more rigorous treatment of electronic structure effects is given by Tersoff (4), who concludes that for one-or two-dimensional semiconductors such as graphite, "the image has no direct relation to the positions of the atoms within the unit cell." Literature Cited
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