A simple system of molecular conductors has been built on a silicon surface and can be controlled via the charge in a single silicon atom.

Researchers in Canada have demonstrated a surprisingly simple system of molecular conductors that can be built on a silicon surface and controlled via the charge in a single silicon atom. 

Robert Wolkow and his group at the University of Alberta at Edmonton studied the behaviour of styrene-like organic molecules forming a straight line on a silicon surface. The molecules stop at a specific silicon atom with a so-called dangling bond, a site that can easily pick up electrical charges. The researchers used scanning tunnelling microscopy (STM) to image such lines and found that, in a certain voltage range, the row of identical molecules seems to get taller towards the end facing the dangling bond. This implies that the molecules closer to the charged site display a higher conductance than their identical twins further from the site. 

Wolkow’s team interpret this ’sloping effect’ as a charge-induced gating of the molecular conductivity. The magnitude of a charge in the dangling bond silicon atom appears to gate the current that flows between the silicon surface and the STM tip. The sloping effect, which typically works across a distance of up to 8nm, can be abolished by choosing a relatively high voltage (more than 2V) or by capping the dangling bond by covalently binding it to a free radical. 

The latter observation suggests potential for such arrangements as chemically controlled transistors or as sensors with single-molecule sensitivity, conclude the researchers. Michael Gross


P G Piva et alNature, 2005, 435, 658