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The impurities of gold

A year ago, physicists from the University of São Paulo (USP) and the State University of Campinas (Unicamp) unveiled the details of the breaking of gold nanowires, structures that measure billionths of a meter and represent a strategic material for the manufacture of the next generation of computers. This achievement won them the cover story of the Physical Review Letters on December 17, 2001, but there was one detail that still puzzled them: the distance between the atoms of gold, which, before the break, become 25% further away from each other than the 0.29 nanometers (nm) found in crystal, a pure substance.

Experimental measurements, carried out in high resolution electron microscopes, register distances in the order of 0.36 nm, or, much larger, of 0.48 nm. In an article published on January 23 this year, once again in the Physical Review Letters, Adalberto Fazzio, Antônio José Roque da Silva and Frederico Dutilh Novaes, from USP, in conjunction with Edison Zacarias da Silva, from Unicamp, demonstrated, using computer simulations, that this variation is due to the presence of impurities. Hydrogen atoms bound to the gold atoms would explain the shorter distances, while sulfur atoms would be responsible for the larger, 0.48 nm, distances. The work was one of the highlights in the Editors’ Choice section of the February 14 issue of Science.

The physicists examined the possibility of the impurities being other chemical elements, such as boron, carbon, nitrogen and oxygen, but only hydrogen and sulfur coincided with the experimental results. “The problem is that no one can see what comes with the atoms of gold”, Fazzio comments. In electron microscopes, both hydrogen – with just one proton acting as a nucleus, around which orbits a single electron – and sulfur – 16 protons and 16 electrons – disappear in the light of the respectable atom of gold, made up of 79 protons and 79 electrons. Anyway, they appear to increase the resistance of the nanowires and therefore to delay their breaking. “Bonds between atoms of gold with impurities are more stable than those between two atoms of gold”, comments Roque da Silva.

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