STM probes beneath the surface

In less than ten years, scanning tunnelling microscopy (STM) has developed into a technique of unsurpassed surface sensitivity allowing vacuum–solid, gas–solid and liquid–solid interfaces to be probed on an atom-by-atom basis. On the other hand, solid–solid interfaces buried in the bulk cannot be directly studied with the STM since the tunnel current that flows between tip and sample – the basis of the technique – is governed by the vacuum gap conductivity of the tunnel barrier. This conductivity is determined by the overlap of the wave-functions between the tip and sample which is a surface effect.