Between peaks and trenches

Theoretical model determines the geometric arrangement that would make the use of nanotubes possible

ADALBERTO FAZZIO / USPElectrons circulate between the carbon (lilac) and the siliconADALBERTO FAZZIO / USP

In 1991, Japanese physicist Sumio Iijima discovered the microscopic cylinder made up of carbon atoms, the same chemical elements as the lead in pencils. From then until now, interest has not stopped growing in these tubes, whose thickness is thousands of times less than a strand of hair: they measure a few billionths of a meter, or nanometers, hence the name nanotube. Due to their rigidity and to their properties to conduct of electricity, armchair nanotubes are seen as an alternative material for a new generation of chips (integrated circuits) and connectors wires capable of joining together electronic components that tend to shrink more than a hundredfold, which therefore makes impracticable the use of present-day devices, made of gold on silicon.

But, in practice, there has not yet been any success in getting the nanotubes to preserve their capacity for carrying electricity when deposited on a surface, a fundamental property for make their use possible. The solution for this problem seems to be closer. Adalberto Fazzio and Walter Orellana, from the University of São Paulo (USP), and Roberto Hiroki Miwa, from the Federal University of Uberlândia (UFU), have identified the factors that are fundamental for a nanowire to make a good electrical contact: the orientation of the nanotube in relation to the geometric arrangement of the atoms on the surface of the silicon (and the distance between the tube and the surface). In computer simulations, they little by little brought a nanotube with 72 carbon atoms closer to a slide of a few hundred silicon atoms, which in the electronic microscope recalls a succession of long parallel chains of mountains, intercalated by valleys or trenches.

Studying the interaction between the atoms of the nanotube and those of the surface, the physicists noted that a chemical bonding occurs between the silicon and the carbon when the carbon cylinder slots into one of these valleys, parallel to the two chains of mountains formed by the silicon atoms. In this region, in which the tube almost touches the silicon surface in reality, the carbon atoms and the silicon remain 0.211 nanometers from each other, electrons accumulate. Like the water of a river, these electrons can flow freely along the whole extension of the nanotube. This channel makes it possible for an electric current to pass, as the Brazilian physicians show in a study published in the Physical Review Letters, of October 17. “The channel of electrons increased the property of conducting electricity in the nanotubes, which now show this characteristic”, says Fazzio. “They are results that indicate the possibility of using carbon nanotubes as wires.”

Shortly before, Peter Albrecht and Joseph Lyding, from the University of Illinois, United States, with the assistance of an atomic force microscope, succeeded in putting a nanotube on top of hydrogenated silicon surface. It now remains to overcome another challenge: separating the electricity-conducting nanotubes from those that behave like semiconductors, characteristics determined at random by the form how the carbon atoms join together when an electric discharge is applied in carbon vapor. “Should an effective form of differentiating these two types of nanotubes be achieved”, says Orellana, “it will be possible to integrate them with a silicon surface and construct chips and nanowires.”

The Project
Computer Simulation of Nanocomputing Materials; Modality Thematic Project; Coordinator Adalberto Fazzio IF/USP; Investment R$ 913,029.43