Illustration of QPCM on a Cu(111) surface. (A) Schematic model demonstrating the working principle of QPCM. The gray circles and arrows indicate the motion of the tip and the Cu atomic contact. (B) Conductance G as a function of tip approaching distance d acquired with the tip on top of a Cu adatom. (C) QPCM image with the same scan size as the inset in (B); the forward scan (from left to right) is shown. (D) Backward scan (from right to left) acquired simultaneously with the image shown in (C). (E) Constant current image of a step edge on Cu(111). Standing wave patterns originating from the surface state are clearly visible in the image. (F) QPCM image of the same area as shown in (E). The conductance decrease from top to bottom of the image is due to the plane in which the tip scans being slightly tilted with respect to the surface. Reprinted with permission from Quantum Point Contact Microscopy, Yong-hui Zhang et al., Nano Letters, July 26, 2011, Copyright © 2011 American Chemical Society
Since the first optical microscopes appeared in the late 1600s – an exact date and original inventor elude precise identification – microscopy has evolved dramatically. Scanning tunneling microscopy (STM), atomic force microscopy (AFM) and (although not generally recognized as an established method) point contact microscopy (PCM) allow scientists to view objects inaccessible to optical microscopes, with images of atoms now commonplace. Nevertheless, even this inexorable march towards ever-smaller scales has encountered limitations. (For example, STM does not provide information on local chemistry, while PCM cannot adequately image individual atoms due to it not having a single-atom contact.)….
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