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Carbon onions
RESEARCH NEWS
Carbon onions High-yield, easy, and low-cost fabrication of carbon nanostructures would be a huge boost for research. Now a team at the University of Cambridge has found a simple way of producing large numbers of spherical carbon nano-onions, without having to resort to high vacuum equipment [Nature (2001) 414, 506-507]. Using a basic apparatus of two graphite electrodes submerged in deionized water, an arc discharge (16-17 V and 30 A) passed between the electrodes generates C60 cores surrounded by ‘nested’ particles. High purity nanoonions float to the surface. The nano-onions are 25-30 nm in diameter on average, but range in size from 5-40 nm. The researchers found nano-
onions with 7, 10, or 15 walls, and cores of 7-8 Å in diameter. Such dimensions could be ideal for lubrication applications, where nanoparticles similar to nanoonions but consisting of halogens, tungsten, and sulfur are currently used. Although the production rate of 3 mg per minute is faster than conventional processes, problems remain to be addressed before the new fabrication technique could be rolled out on an industrial scale, say the researchers. Simple adjustments – such as increasing the discharge current to 50 A, chilling and recirculating the water, and auto-feeding the carbon anode – could make the process suitable for mass production, the researchers believe.
A pillar of nanotechnology Researchers from Rensselaer Polytechnic Institute have found a way to grow pillars of carbon nanotubes [Carbon (2002) 40(1), 47-51]. Using chemical vapor deposition (CVD), cylindrical pillar-like structures of densely packed aligned multiwalled carbon nanotubes are grown on silicon (Si) substrates. The 10-100 µm nanotube pillars form when (Ni) nickel-coated oxidised Si(001) substrates are exposed to a xyleneferrocene mixture, and can reach lengths of several tens of micrometers. The researchers believe that the growth process starts with
12
February 2002
circular cracks forming on the surface during the 800°C CVD process, caused by nickelferrocene interactions. The cracks allow ferrocene and xylene to reach the SiO2, providing catalyst and carbon fluxes for the emerging carbon nanotubes. This self-organized growth is very attractive for fabricating interconnected architectures – and has the advantage of being a bottom-up approach compatible with traditional microelectronics processing. Potentially, the pillars could be used for energy storage, as electrodes or composite reinforcements.
Polymer formation is not crystal clear Crystallizing polymers from solution is highly complex and is not well understood at the molecular level. Simulations by researchers at the University of Massachusetts shed new light on the problem and challenge existing explanations. The nucleation and growth of crystals from a solution of small molecules proceeds with formation of crystal nuclei by thermal fluctuations, which if larger than a certain critical size, provide a surface for further crystallization. The molecules are much smaller than the crystal nuclei and are involved with only one nucleus at a time. Polymers, however, are much more gregarious. Different regions of single polymers can participate in multiple nuclei, leading to incomplete polymerization. The polymer chains fold back and forth, forming crystalline lamellae (10 nm thick platelets with regular facets). However, the lamellae that form in experiments are two orders of magnitude smaller than predicted thermodynamically and it is not well understood how growth proceeds for sufficiently large lamellae. In the November 19 print issue of Physical Review Letters [(2001) 87, 218302], P. Welch and Murugappan Muthukumar describe the results of Langevin dynamics simulations, based on a simple ball and spring model, that tackle these two problems. The researchers suggest that a nucleation and growth mechanism – not spinodal
dynamics – creates initial, or 'baby' nuclei. The initial lamellar thickness is spontaneously selected, which then thickens by negotiating free energy barriers, explains Muthukumar. The equilibrium lamellar thickness is smaller than the extended chain limit, which is significant for the analysis of crystallization kinetics, and opens up for re-analysis data accumulated over the past 40 years, says Muthukumar. Polymer chains diffuse to the baby nuclei where they absorb and crystallographically attach at the growth front. In a departure from existing theories, the researchers believe that this stage is not subject to an energy barrier. As folded polymer chains are added to the baby nuclei, they rearrange at the growth front to form 'stems'. The thickening of the lamellae can be put down to chain dynamics within the crystal. This account of lamellar thickness, nucleation mechanism, and lamellar thickening challenges current models such as LauritzenHoffman and its generalizations, say the researchers. Crucially, the simulations also seem to agree well with experimental observations. Muthukumar hopes these latest results will lead to more extensive simulations on larger systems under different conditions, new models, and time-resolved experiments – and indicates that a new theory of polymer crystallization is needed.
Carbon onions High-yield, easy, and low-cost fabrication of carbon nanostructures would be a huge boost for research. Now a team at the University of Cambridge has found a simple way of producing large numbers of spherical carbon nano-onions, without having to resort to high vacuum equipment [Nature (2001) 414, 506-507]. Using a basic apparatus of two graphite electrodes submerged in deionized water, an arc discharge (16-17 V and 30 A) passed between the electrodes generates C60 cores surrounded by ‘nested’ particles. High purity nanoonions float to the surface. The nano-onions are 25-30 nm in diameter on average, but range in size from 5-40 nm. The researchers found nano-
onions with 7, 10, or 15 walls, and cores of 7-8 Å in diameter. Such dimensions could be ideal for lubrication applications, where nanoparticles similar to nanoonions but consisting of halogens, tungsten, and sulfur are currently used. Although the production rate of 3 mg per minute is faster than conventional processes, problems remain to be addressed before the new fabrication technique could be rolled out on an industrial scale, say the researchers. Simple adjustments – such as increasing the discharge current to 50 A, chilling and recirculating the water, and auto-feeding the carbon anode – could make the process suitable for mass production, the researchers believe.
A pillar of nanotechnology Researchers from Rensselaer Polytechnic Institute have found a way to grow pillars of carbon nanotubes [Carbon (2002) 40(1), 47-51]. Using chemical vapor deposition (CVD), cylindrical pillar-like structures of densely packed aligned multiwalled carbon nanotubes are grown on silicon (Si) substrates. The 10-100 µm nanotube pillars form when (Ni) nickel-coated oxidised Si(001) substrates are exposed to a xyleneferrocene mixture, and can reach lengths of several tens of micrometers. The researchers believe that the growth process starts with
12
February 2002
circular cracks forming on the surface during the 800°C CVD process, caused by nickelferrocene interactions. The cracks allow ferrocene and xylene to reach the SiO2, providing catalyst and carbon fluxes for the emerging carbon nanotubes. This self-organized growth is very attractive for fabricating interconnected architectures – and has the advantage of being a bottom-up approach compatible with traditional microelectronics processing. Potentially, the pillars could be used for energy storage, as electrodes or composite reinforcements.
Polymer formation is not crystal clear Crystallizing polymers from solution is highly complex and is not well understood at the molecular level. Simulations by researchers at the University of Massachusetts shed new light on the problem and challenge existing explanations. The nucleation and growth of crystals from a solution of small molecules proceeds with formation of crystal nuclei by thermal fluctuations, which if larger than a certain critical size, provide a surface for further crystallization. The molecules are much smaller than the crystal nuclei and are involved with only one nucleus at a time. Polymers, however, are much more gregarious. Different regions of single polymers can participate in multiple nuclei, leading to incomplete polymerization. The polymer chains fold back and forth, forming crystalline lamellae (10 nm thick platelets with regular facets). However, the lamellae that form in experiments are two orders of magnitude smaller than predicted thermodynamically and it is not well understood how growth proceeds for sufficiently large lamellae. In the November 19 print issue of Physical Review Letters [(2001) 87, 218302], P. Welch and Murugappan Muthukumar describe the results of Langevin dynamics simulations, based on a simple ball and spring model, that tackle these two problems. The researchers suggest that a nucleation and growth mechanism – not spinodal
dynamics – creates initial, or 'baby' nuclei. The initial lamellar thickness is spontaneously selected, which then thickens by negotiating free energy barriers, explains Muthukumar. The equilibrium lamellar thickness is smaller than the extended chain limit, which is significant for the analysis of crystallization kinetics, and opens up for re-analysis data accumulated over the past 40 years, says Muthukumar. Polymer chains diffuse to the baby nuclei where they absorb and crystallographically attach at the growth front. In a departure from existing theories, the researchers believe that this stage is not subject to an energy barrier. As folded polymer chains are added to the baby nuclei, they rearrange at the growth front to form 'stems'. The thickening of the lamellae can be put down to chain dynamics within the crystal. This account of lamellar thickness, nucleation mechanism, and lamellar thickening challenges current models such as LauritzenHoffman and its generalizations, say the researchers. Crucially, the simulations also seem to agree well with experimental observations. Muthukumar hopes these latest results will lead to more extensive simulations on larger systems under different conditions, new models, and time-resolved experiments – and indicates that a new theory of polymer crystallization is needed.