Scratching the Surface : An Overview of Scanning Probe Microscopy ( SPM )

This scanning tunnelling microscope scanned a pointed electrode, referred to as a ‘tip’ in the context of STM, within a few nanometres of a conducting surface (Binnig & Rohrer, 1982). When a small bias voltage was applied between the sample and the tip, electron tunnelling was initiated. Tunnelling is a quantum mechanical effect that arises due to the particle and wave-like duality of electrons. Solutions of the Schrödinger wave equation reveal that the probability of electron tunnelling decays exponentially with a linearly increasing gap. In the context of STM this manifests itself in the form of exceptional spatial sensitivity toward changes in the tip–sample distance: Binnig and Rohrer’s STM utilised this sensitivity to produce an image of a conducting graphite sample surface by raster scanning the electrode tip across it; the resulting image mapped the local density of electronic states at the Fermi level of the graphite. The dependence of tunnelling current upon distance was easily capable of resolving the positions of single atoms in