... scale accuracy and can be brought within a few Amstroms of the conducting sample surface. The metallic tip and the conducting substrate are in very close proximity but are not in actual physical contact, (http://www. chem.qmw.ac.uk/surfaces/scc/, 7/10/03). At separations as small as this, the outer electron orbitals of the tip and sample overlap. On the application of a bias voltage between the tip and the surface, electrons are able to tunnel through the vacuum barrier via the quantum mechanical tunnelling effect. The direction of current flow is determined by the polarity of the bias. If the sample is biased negative with respect to the tip, then electrons will flow from the surface to the tip as shown above, whilst if the sample is biased positive with respect to the tip, then electrons will flow from the tip to the surface as shown below. (http://www.chem.qmw.ac.uk/surfaces/scc/, 7/10/03). The exponential (in vacuum) decay of ...

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