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This technique involves moving a very sharp tip across the surface of the sample under investigation. The tip is controlled so that it is very close to the surface but not touching it, and as the tip moves, a constant distance is maintained by applying and measuring a small electrical current between the tip and the surface. The "up and down" movement of the tip needed to keep the current constant makes it possible to construct a contour map of the surface. The precision of this method allows pictures to be generated with sufficient resolution to show individual atoms. False colours are used to indicate the surface contours
Using this technique it has been possible, for example, to find out more about the catalyst used in the desulphurisation of fuels. Molybdenum (Mo) or tungsten sulphides are used, with a nickel or cobalt (Co) promoter, but the role of the promoter is unclear.
Work at the Center for Atomic-scale Materials Physics in Denmark has used STM to show the effect the cobalt promoter has on the molybdenum sulphide clusters.











STM produces some stunning images, and further examples can be found at:


Scanning Tunnelling Microscopy (STM)