Nano Focus: Defects in carbon nanotubes heal themselves under the right conditions
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Defects in carbon nanotubes heal themselves under the right conditions
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ollowing reports on the experimental growth of carbon nanotubes up to one meter in length, computational simulations have now shown why this is possible. Using density functional theory (DFT) calculations of the energy landscape and the kinetics of carbon nanotube growth, researchers at Hong Kong Polytechnic University, Rice University in Houston, and Tsinghua University have shown that “healing” mechanisms exist at the carbon/catalyst interface that heal and restore potential topological defects—pentagons, heptagons, and pentagon-heptagon (5|7) pairs—to hexagons before they move more than an atomic layer or two beyond the interface. “In our theoretical analysis we show that at practical temperatures and given the values of energy barriers, the rates of formation and removal of defects are balanced,” said Boris Yakobson of Rice University. “They form with some probability and then at a higher rate they are deleted or ‘healed.’” According to Feng Ding from Hong Kong Polytechnic University, this finding, reported in the June 15 issue of Physical Review Letters (DOI: 10.1103/ PhysRevLett.108.245505; 245505), puts to rest the theory that nanotubes form
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MRS BULLETIN
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VOLUME 37 • AUGUST 2012
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The researchers suggested a phase diagram by plotting the absolute temperature T versus the hole-doping concentration p for the cuprate system. If pairing is suppressed down to T = 0, a quantum critical point pc defines the starting point for a quantum phase transition. In the absence of pairing, the quantum critical regime starts at this point and includes the area between the dotted lines in the figure, representing extensions of the Fermi liquid phase and the pseudogap phase of the system. The occurrence of pairing extends the range of quantum criticality into the superconductivity and pseudogap regimes.
As far as what this discovery might mean to experimentalists, Ashkenazi said, “I think this work is an important breakthrough toward a more focused way to give instruction to experimentalists to look for where to go to find high Tc superconductors and, hopefully, maybe even room-temperature superconductors. In general you have to look close to phase transitions—in particular, the proximity of a metal–insulator Mott transition is a good place to look for higher Tc materials—but our work sets the stage for more work to be done.” Tim Palucka
initially with lots of defects, which are then annealed out at elevated temperatures over time. In fact, any defect that makes it beyond a couple of atomic layers from the interface without being healed is permanently embedded in the carbon structure, leading to nano-cones, nano-horns, or other defective shapes. Such defective shapes are undesirable for carbon nanotubes intended for use in practical devices because defects change the chiral indices, which are related to the circumference, of the nanotube along its length. The chiral indices define the electronic structure and bandgap of the material, which mu
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