Nano Focus: Al catalyst yields massive doping of Si nanowires
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ano Focus
healthy arterial tissue stuck to the wires the longer the wires remained in the body, suggesting that the zinc was not damaging the arterial walls. And because zinc is known to fight arterial plaque, a zinc-based stent could further help patients suffering from ischemic problems. Carlo Di Mario, a clinical cardiologist at Imperial College London who was not involved in the research, said that the work is interesting and it appears that zinc-based stents would last longer in the body than magnesium-based stents. However, he said that the low tensile strength of zinc is an issue: The wires work perfectly in rats, but the material
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is not strong enough to hold open human arteries. Using zinc alloys is necessary, but “the avenue to alloys seems a bit too long for a clinician to be interested at this stage,” Di Mario said. However, Drelich said they are already testing promising new zinc alloys to fix the strength issue—the invented alloys meet all of the benchmarks necessary for a bioabsorbable stent material candidate. The team is now continuing in vivo testing of new alloys and expects to complete prototype mini-stents this year. Within a few years, they should be ready to move on to clinical trials, he said. Joseph Bennington-Castro
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Al catalyst yields massive doping of Si nanowires
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emiconductor nanowires represent versatile nanoscale building blocks, finding applications as new transistors and circuits for next-generation electronics, as well as in photonics, solar cells, biosensing, and neuro-engineering technology. Silicon nanowires have attracted particular interest, where their properties are controlled by doping with foreign ions. It has recently been demonstrated that aluminum provides an effective growth catalyst for silicon nanowires, where the aluminum also provides an effective p-type dopant, being homogeneously distributed throughout the silicon. As reported in the April 4 issue of Nature (DOI: 10.1038/nature11999; p. 78), an international team of researchers from École Polytechnique de Montréal in Canada, Max Planck Institute of Microstructure Physics in Germany, and Northwestern University in Illinois have now gained an atomic-level understanding of this process. Silicon nanowires were grown by heating a silicon substrate supporting aluminum islands to a temperature where only the aluminum melts, and not the silicon. The substrate was then exposed to a vapor-phase silane reactant. Part (a) in the figure shows how the surface of an aluminum drop adsorbs silicon
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MRS BULLETIN
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VOLUME 38 • JUNE 2013
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(a) The high-resolution transmission electron microscope image shows the interface between aluminum and silicon (© Nature/MPI of Microstructure Physics). (b) Atom probe tomography reveals the atomic structure of the material: red shows the representative positions of the aluminum atoms and blue those of the silicon atoms. An analysis of the data shows the very high concentration of aluminum in the silicon and its uniform distribution. For experimental reasons, the nanowire was co
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