'Hanky' vision explained

November 1, 1996

Scanning tunnelling microscopes (STMs) have provided scientists with the ability to see and even manipulate individual atoms. But do the images they produce actually reflect reality?

Andrew Fisher, a University College London researcher, has been working on the problem as part of a project backed by the Engineering and Physical Sciences Research Council. He believes the research has made an important contribution to scientists' understanding of why molecules appear to have the shapes they do when seen "through" STMs.

STMs work by making electrons jump or "tunnel" between a sharp metallic tip and the surface of the material. The atomic-scale image results from a measurement of how rapidly these electrons tunnel into a material. Very small particles such as electrons behave like waves because of the strange laws of quantum physics. Dr Fisher said: "Often there are several routes through the molecule and each wave-like electron samples all the possible routes rather than just choosing one of them."

The waves combine to produce a complicated pattern because their peaks and troughs do not arrive together. This effect is analogous to the view of a street lamp seen through a handkerchief.

Dr Fisher said: "The light passing through each pore in the hanky has peaks and troughs at different places and takes different amounts of time to arrive at the eye, producing the characteristic streaky pattern around the light."

This effect is called interference and the observation by Dr Fisher that it is an integral part of the operation of STM probes helps to explain why the tunnelling image that results does not necessarily have the same shape as the true arrangements of atoms in the molecule: the places where the tunnelling process is strongest is not necessarily where the atoms are located.

Dr Fisher added: "We have shown for example why the benzene molecule appears as a triangle in STM images of certain surfaces even though its real shape is hexagonal.

"It was very exciting to see that such an apparently complex phenomenon in a new area of physics could be understood on the same basis as explaining the rainbow colours of an oil film on a garage forecourt."

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