from the poke-it-again-Sam dept.
Researchers have written 0.8 nm (presumably diameter) conductive marks in a thin organic film with an STM. The marks were stable for at least the 2 hour scanning session. They attribute the marks to polymerisation of the film under the STM tip. D.X. Shi, L.P.Ma, S.S. Xie, and S.J. Pang, writing in [J. Vac. Sci. Technol B. 18:1187-1189 May/June 2000], describe writing 0.8 nm conductive marks in a thin organic film with an STM, improving on their previous 1.4 nm marks.
In this paper, the authors' organic film is 3-phenyl-1-ureidonitrile, C6H5-NH-CO-NH-CN. They vacuum evaporated a 10 nm thick layer of this compound on to a HOPG (highly oriented pyrolytic graphite) substrate. They wrote their conductive marks under ambient conditions, applying 10 millisecond 4.0 volt pulses to their STM tip. Their imaging was performed at 0.8 volts, with a typical current of 0.3nA, though they imaged "in constant height mode", so either the current or voltage must have varied. Their STM tip was a mechanically cut Pt-Ir wire.
After applying their 4.0 volt pulses, the authors see substantially increased conductivity under their STM tip. They report I(V) curves before and after the pulse. Before the pulse, less than 1 nA flows for biases below 1 volt. After the pulse, the resistance near zero bias drops to ~500 megohms, and the current jumps steeply upwards at biases above ~0.25 volts, so the effective bandgap seems to have dropped. The authors attribute the gain in conductivity to polymerization of the cyano groups. The authors explicitly rule out the possibility that they had blown a hole through the film to the HOPG substrate. They did a control experiment where they used higher voltages and currents to expose the HOPG substrate, and they saw the normal atomic image of a graphite surface under those circumstances.
This authors demonstrated that they could write a 6 X 8 array of their marks in a 64 nm X 64 nm field, which was the objective of their current study. Presumably they will see how closely their marks can be placed without mutual interference in a later study.
The authors wrote that they were recording their marks "using single [crystal???] films". If these were single crystal films, it would be helpful to know the spacing of the cyano groups and their orientation relative to relative to their STM's electric field. This paper's STM images do not display atomic resolution, and it would be useful to know if this is for fundamental reasons or only due to limitations in their current STM.
From the viewpoint of molecular nanotechnology, this technique could prove useful if it allows the fabrication of atomically precise wires in precise locations. The process must be quite anisotropic, given the formation of 0.8 nm marks in a 10 nm thick film, but it would be helpful to know more about the mechanism in order to see to what precision it can be pushed. Perhaps most crucially, we need to know if the polymerization if caused by the electric field or by the tunneling current. If it is field driven, then smaller electrodes (perhaps fullerene tubes or Tour wires) could impose more local fields. If it is current driven, then we are probably already at the physical limit, since their STM tip can image a graphite lattice.
