Quantum Physics

27.9 The Scanning Tunneling Microscope 895x piezoz piezoIy piezoBased on a drawing from P. K. Hansma, V. B. Elings, O. Marti, andC. Bracker, Science 242:209, 1988, Copyright 1988 by the AAAS.Figure 27.20 A schematic view of an STM.The tip, shown as a rounded cone, is mountedon a piezoelectric x, y, z scanner. A scan of thetip over the sample can reveal contours of thesurface down to the atomic level. An STM imageis composed of a series of scans displacedlaterally from each other.tances z greater than 1 nm (that is, beyond a few atomic diameters), essentially notunneling takes place. This exponential behavior causes the current of electronstunneling from surface to tip to depend very strongly on z. This sensitivity is thebasis of the operation of the STM: by monitoring the tunneling current as the tipis scanned over the surface, scientists obtain a sensitive measure of the topographyof the electron distribution on the surface. The result of this scan is used tomake images like that in Figure 27.20. In this way the STM can measure theheight of surface features to within 0.001 nm, approximately 1/100 of an atomicdiameter!The STM has, however, one serious limitation: it depends on electrical conductivityof the sample and the tip. Unfortunately, the surfaces of most materials arenot electrically conductive. Even metals such as aluminum are covered with nonconductiveoxides. A newer microscope—the atomic force microscope, or AFM—overcomes this limitation. It measures the force between a tip and the sample,rather than an electrical current. This force depends strongly on the tip–sampleseparation just as the electron tunneling current does for the STM. The AFM hascomparable sensitivity for measuring topography and has become widely used fortechnological applications.Perhaps the most remarkable thing about the STM is that its operation is basedon a quantum mechanical phenomenon—tunneling—that was well understoodin the 1920s, even though the first STM was not built until the 1980s. What otherapplications of quantum mechanics may yet be waiting to be discovered?