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Carbon nanotubes get diamond coat
RESEARCH NEWS
Static scanning X-ray source NANOTECHNOLOGY Current three-dimensional imaging for airport baggage screening and medical scanners (e.g. computer tomography) requires rotational motion of an X-ray source around the subject. Now, researchers at the University of North Carolina at Chapel Hill have developed a source based on carbon nanotubes (CNTs) that can generate a scanning X-ray beam made up of multiple smaller beams from different origins (a multibeam field-emission X-ray source). This allows imaging from multiple projection angles without moving the X-ray source [Zhang et al., Appl. Phys.
Lett. (2005) 86 (18), 184104]. The key components include a field-emission cathode with a linear array of five pixels (made from CNTs synthesized at spin-off company Xintek). These pixels emit electrons, which are then focused onto a Mo target to produce X-rays. Each pixel is gated by a metal-oxide-
Carbon nanotubes get diamond coat NANOTECHNOLOGY
Coworkers at the University of Rome, Italy and Argonne National Laboratory in the US have grown carbon nanotubes coated with nanosized diamond in one controllable step [Terranova et al., Chem. Mater. (2005), doi: 10.1021/cm0502018]. Chemical compatibility, and the outstanding mechanical, chemical, and thermal properties of carbon nanostructures, make hybrid materials attractive. Also, diamond nanorods are predicted to have better mechanical performance and equivalent energetic stability to carbon nanotubes at certain sizes. Previously, growth of tubular diamond-based nanosystems has mostly used multistep fabrication techniques, including growing the framework (e.g. carbon nanotubes), then coating with diamond. But this can be labor intensive and costly, and limits assembly into ordered arrays. Instead, to control the plasma chemistry in a hot-filament chemical vapor deposition reactor, carbon nanopowders are sprayed homogenously from holes along one side of a nozzle. The spray intersects with a flux of atomic hydrogen from a heated Ta filament that is just 6 mm away from a Si substrate
Scanning electron micrographs of nanotube bundles covered by diamond nanocrystals. (© American Chemical Society.)
coated with catalyst Fe nanoparticles. Bundles of single-walled carbon nanotubes grow up to 15 µm in length and self-align. After 6 mins, they start to stand perpendicular to the substrate as 20-100 nm wide diamond crystals start to form on them. After just 15 mins, coating is complete. Because of its negative electron affinity, diamond is one of the most promising wide band gap semiconductors. So, rigid, diamondcoated carbon nanotubes could find uses in cold-cathode devices, miniaturized cathoderay tubes, and light-emitting displays, as well as in micromechanics and nanoscale sensing, say the researchers. Mark Telford
semiconductor field-effect transistor (MOSFET)-based electronic circuit. When a pulsed control signal is swept
High-density, aligned nanotubes on sapphire
across the individual MOSFETs, it
NANOTECHNOLOGY
activates a sequence of electron beams from the pixels. The sequence of pulsed X-ray beams from the corresponding focal points on the anode yield a scanning X-ray beam. As well as reducing scanner cost and maintenance, when fully developed the technology should lead to “smaller and faster X-ray imaging systems”, says group leader Otto Zhou. “Scanners will produce higher-resolution images, enabling acquisition of new data.” The source can also be operated as a high-speed X-ray camera, capturing clear images of objects moving at high speed. Also, the nanotube technology allows operation at room temperature rather the 1000°C of conventional sources. Mark Telford
Using chemical vapor deposition, researchers at the University of Southern California (USC) have grown large areas of aligned, high-density single-walled carbon nanotubes (SWNTs) on a- and r-plane sapphire that could find use in nanotube integrated circuits and sensor arrays [Han et al., J. Am. Chem. Soc. (2005) 127 (15), 5294]. Aligned SWNTs are of interest because, when nanotubes are randomly ordered in devices, electrons take a circuitous path between electrodes, possibly by jumping from one nanotube to another. When SWNTs are more ordered, electrons flow more directly and there is less resistance. The team says their method is faster and easier than those currently used for orienting nanotubes, including techniques that rely on assistance from electrical fields or gas flow. “Previous fabrication methods are two-step: locating the nanotube, then patterning the device,” says USC’s Chongwu Zhou. “We eliminate the step of locating the nanotubes because we have them everywhere.”
Schematic of nanotube on a-plane sapphire substrate. (© 2005 American Chemical Society.)
Aligned nanotube arrays are not seen on sapphire’s other crystallographic faces: the m-plane reveals no preferential orientation; the c-plane yields randomly oriented nanotubes. The researchers believe that directional growth is caused by interactions between the nanotube and the substrate’s crystal structure. Patrick Cain
July/August 2005
13
Static scanning X-ray source NANOTECHNOLOGY Current three-dimensional imaging for airport baggage screening and medical scanners (e.g. computer tomography) requires rotational motion of an X-ray source around the subject. Now, researchers at the University of North Carolina at Chapel Hill have developed a source based on carbon nanotubes (CNTs) that can generate a scanning X-ray beam made up of multiple smaller beams from different origins (a multibeam field-emission X-ray source). This allows imaging from multiple projection angles without moving the X-ray source [Zhang et al., Appl. Phys.
Lett. (2005) 86 (18), 184104]. The key components include a field-emission cathode with a linear array of five pixels (made from CNTs synthesized at spin-off company Xintek). These pixels emit electrons, which are then focused onto a Mo target to produce X-rays. Each pixel is gated by a metal-oxide-
Carbon nanotubes get diamond coat NANOTECHNOLOGY
Coworkers at the University of Rome, Italy and Argonne National Laboratory in the US have grown carbon nanotubes coated with nanosized diamond in one controllable step [Terranova et al., Chem. Mater. (2005), doi: 10.1021/cm0502018]. Chemical compatibility, and the outstanding mechanical, chemical, and thermal properties of carbon nanostructures, make hybrid materials attractive. Also, diamond nanorods are predicted to have better mechanical performance and equivalent energetic stability to carbon nanotubes at certain sizes. Previously, growth of tubular diamond-based nanosystems has mostly used multistep fabrication techniques, including growing the framework (e.g. carbon nanotubes), then coating with diamond. But this can be labor intensive and costly, and limits assembly into ordered arrays. Instead, to control the plasma chemistry in a hot-filament chemical vapor deposition reactor, carbon nanopowders are sprayed homogenously from holes along one side of a nozzle. The spray intersects with a flux of atomic hydrogen from a heated Ta filament that is just 6 mm away from a Si substrate
Scanning electron micrographs of nanotube bundles covered by diamond nanocrystals. (© American Chemical Society.)
coated with catalyst Fe nanoparticles. Bundles of single-walled carbon nanotubes grow up to 15 µm in length and self-align. After 6 mins, they start to stand perpendicular to the substrate as 20-100 nm wide diamond crystals start to form on them. After just 15 mins, coating is complete. Because of its negative electron affinity, diamond is one of the most promising wide band gap semiconductors. So, rigid, diamondcoated carbon nanotubes could find uses in cold-cathode devices, miniaturized cathoderay tubes, and light-emitting displays, as well as in micromechanics and nanoscale sensing, say the researchers. Mark Telford
semiconductor field-effect transistor (MOSFET)-based electronic circuit. When a pulsed control signal is swept
High-density, aligned nanotubes on sapphire
across the individual MOSFETs, it
NANOTECHNOLOGY
activates a sequence of electron beams from the pixels. The sequence of pulsed X-ray beams from the corresponding focal points on the anode yield a scanning X-ray beam. As well as reducing scanner cost and maintenance, when fully developed the technology should lead to “smaller and faster X-ray imaging systems”, says group leader Otto Zhou. “Scanners will produce higher-resolution images, enabling acquisition of new data.” The source can also be operated as a high-speed X-ray camera, capturing clear images of objects moving at high speed. Also, the nanotube technology allows operation at room temperature rather the 1000°C of conventional sources. Mark Telford
Using chemical vapor deposition, researchers at the University of Southern California (USC) have grown large areas of aligned, high-density single-walled carbon nanotubes (SWNTs) on a- and r-plane sapphire that could find use in nanotube integrated circuits and sensor arrays [Han et al., J. Am. Chem. Soc. (2005) 127 (15), 5294]. Aligned SWNTs are of interest because, when nanotubes are randomly ordered in devices, electrons take a circuitous path between electrodes, possibly by jumping from one nanotube to another. When SWNTs are more ordered, electrons flow more directly and there is less resistance. The team says their method is faster and easier than those currently used for orienting nanotubes, including techniques that rely on assistance from electrical fields or gas flow. “Previous fabrication methods are two-step: locating the nanotube, then patterning the device,” says USC’s Chongwu Zhou. “We eliminate the step of locating the nanotubes because we have them everywhere.”
Schematic of nanotube on a-plane sapphire substrate. (© 2005 American Chemical Society.)
Aligned nanotube arrays are not seen on sapphire’s other crystallographic faces: the m-plane reveals no preferential orientation; the c-plane yields randomly oriented nanotubes. The researchers believe that directional growth is caused by interactions between the nanotube and the substrate’s crystal structure. Patrick Cain
July/August 2005
13