Scanning tunneling microscopy (STM)

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The Scanning Tunneling Microscope (STM), developed in the 80's, allows imaging of surface topographies down to atomic resolution. In an STM, a sharp metallic tip is positioned very close (a few Å) away from the surface under study, so that a tunneling current flows between tip and surface when a voltage is applied. Because of the strong (exponential) variation of the tunneling current with the distance, the latter can be controlled by keeping the tunneling current at a constant value using a feedback loop. For imaging, the tip is scanned across the surface, while the tip height is measured as a function of lateral position. The data from these line scans are usually converted to a grey-shade image, with the higher tip positions appearing with brighter color.