- Email: [email protected]
Nanowires get into the groove
RESEARCH NEWS
Microscope breaks angstrom barrier CHARACTERIZATION
(Left) A direct image of a Si crystal looking down the 112 zone axis resolving pairs of atom columns just 0.78 Å apart. Recorded with the ORNL 300 kV STEM with Nion aberration corrector. (Courtesy Matthew F. Chisholm.) (Right) Image of a catalyst specimen revealing individual Pt atoms on the surface of an alumina support. Pt3 trimers are circled. (Sample courtesy Richard D. Adams and Michael Amiridis, University of South Carolina; photo courtesy Albina Borisevich, ORNL.)
Researchers from Nion Company and Oak Ridge National Laboratory (ORNL) have demonstrated unequivocal evidence of subangstrom resolution in a scanning transmission electron microscope (STEM) [Nellist et al., Science (2004) 305, 1741]. Previous demonstrations of subangstrom resolution in STEMs have relied on spots in a Fourier transform and the intensity profiles of single atoms. These measurements are not unambiguous, say the researchers, because they can be sensitive to noise, instabilities in the microscope, or incorrect adjustment of the detector black level. To get around this, the researchers from Nion and ORNL used a Nion aberration
corrector on a VG Microscopes HB603U 300 kV STEM to look at a Si crystal in the [112] orientation in annular darkfield imaging mode. “Looking down on a Si crystal, we can see atoms that are only 0.78 Å apart, which is the first unequivocal proof that we’re getting subangstrom resolution,” explains Stephen J. Pennycook of ORNL. “This is the first direct image of a bulk material at subangstrom resolution.” The Fourier transforms of the image shows information down to 0.71 Å and even apparent lattice information at 0.6 Å. The researchers say that there is no evidence that the observed spots are caused by distortions or incorrect background levels. The experimental images also agree well with simulated profiles. “With aberration correction you can see everything better, basically,” says Pennycook. “One important application of these machines is to the electronics industry,” he explains, “where we are able to probe across a gate dielectric with unprecedented resolution.” The microscope’s smaller beam also improves sensitivity, making it possible to image individual dopant or impurity atoms within materials, at defects or interfaces, and on their surfaces. Similarly, it will also be possible to see individual catalyst atoms on real catalysts, says Pennycook. “We therefore have the tool to connect structure to properties on the atomic scale,” he adds. Pennycook believes that we may not have reached the ultimate resolution of the electron microscope yet. “Next generation corrector devices are already under design and it may be possible to push down toward 0.3 Å resolution, at which point the diameter of the atom itself will become the limiting factor.” The work was funded by the Basic Energy Sciences program of the US Department of Energy’s Office of Science. Cordelia Sealy
Nanowires get into the groove NANOTECHNOLOGY The fabrication of nanowires, particularly using a bottom-up rather than a top-down approach, is generating growing interest for integrated circuits. Now researchers from the University of Cambridge and the National University of Singapore have fabricated self-assembled coaxial crystalline SiC nanowires for the first time [Ho et al., Nano Lett. (2004) doi: 10.1021/nl0491733]. Starting with a polycrystalline Cu substrate and Si in a horizontal tube furnace, thermal annealing at 1000°C
produces grooves along the grain boundaries of the Cu. The researchers use this as a natural template for the subsequent growth of SiC nanowires. In the next stage of the process, methane gas is admitted to the same furnace containing the Cu template and Si under a background of Ar gas. Isolated nanocrystals ~20-50 nm in size initially nucleate and then coalesce along the grain boundaries. Sintering occurs, fusing the nanocrystals together. Unidirectional growth of the nanowires continues as long as a favorable gas
ambient is maintained, resulting in densely packed or radial nanowires tens of microns long. The researchers believe that a vapor-solid nucleation mechanism is responsible for the nanowire growth. First, SiC radicals are produced as decomposed methane gas reacts with the Si. SiC condensates onto the Cu surface at the thermally exposed grain boundaries, where nucleation of the nanowires begins spontaneously. The tips of the nanowires are flat and free from catalyst particles. Scanning
electron microscopy reveals that each wire consists of numerous (3 to 200) nanowires encapsulated within a larger diameter (0.2-2 µm) wire. Highresolution transmission electron microscopy shows a crystalline structure with a high density of planar defects. The researchers suggest that the coaxial nanowires could be combined with conductive polymers for use in photovoltaics or as interconnects in functional devices. Cordelia Sealy
November 2004
7
Microscope breaks angstrom barrier CHARACTERIZATION
(Left) A direct image of a Si crystal looking down the 112 zone axis resolving pairs of atom columns just 0.78 Å apart. Recorded with the ORNL 300 kV STEM with Nion aberration corrector. (Courtesy Matthew F. Chisholm.) (Right) Image of a catalyst specimen revealing individual Pt atoms on the surface of an alumina support. Pt3 trimers are circled. (Sample courtesy Richard D. Adams and Michael Amiridis, University of South Carolina; photo courtesy Albina Borisevich, ORNL.)
Researchers from Nion Company and Oak Ridge National Laboratory (ORNL) have demonstrated unequivocal evidence of subangstrom resolution in a scanning transmission electron microscope (STEM) [Nellist et al., Science (2004) 305, 1741]. Previous demonstrations of subangstrom resolution in STEMs have relied on spots in a Fourier transform and the intensity profiles of single atoms. These measurements are not unambiguous, say the researchers, because they can be sensitive to noise, instabilities in the microscope, or incorrect adjustment of the detector black level. To get around this, the researchers from Nion and ORNL used a Nion aberration
corrector on a VG Microscopes HB603U 300 kV STEM to look at a Si crystal in the [112] orientation in annular darkfield imaging mode. “Looking down on a Si crystal, we can see atoms that are only 0.78 Å apart, which is the first unequivocal proof that we’re getting subangstrom resolution,” explains Stephen J. Pennycook of ORNL. “This is the first direct image of a bulk material at subangstrom resolution.” The Fourier transforms of the image shows information down to 0.71 Å and even apparent lattice information at 0.6 Å. The researchers say that there is no evidence that the observed spots are caused by distortions or incorrect background levels. The experimental images also agree well with simulated profiles. “With aberration correction you can see everything better, basically,” says Pennycook. “One important application of these machines is to the electronics industry,” he explains, “where we are able to probe across a gate dielectric with unprecedented resolution.” The microscope’s smaller beam also improves sensitivity, making it possible to image individual dopant or impurity atoms within materials, at defects or interfaces, and on their surfaces. Similarly, it will also be possible to see individual catalyst atoms on real catalysts, says Pennycook. “We therefore have the tool to connect structure to properties on the atomic scale,” he adds. Pennycook believes that we may not have reached the ultimate resolution of the electron microscope yet. “Next generation corrector devices are already under design and it may be possible to push down toward 0.3 Å resolution, at which point the diameter of the atom itself will become the limiting factor.” The work was funded by the Basic Energy Sciences program of the US Department of Energy’s Office of Science. Cordelia Sealy
Nanowires get into the groove NANOTECHNOLOGY The fabrication of nanowires, particularly using a bottom-up rather than a top-down approach, is generating growing interest for integrated circuits. Now researchers from the University of Cambridge and the National University of Singapore have fabricated self-assembled coaxial crystalline SiC nanowires for the first time [Ho et al., Nano Lett. (2004) doi: 10.1021/nl0491733]. Starting with a polycrystalline Cu substrate and Si in a horizontal tube furnace, thermal annealing at 1000°C
produces grooves along the grain boundaries of the Cu. The researchers use this as a natural template for the subsequent growth of SiC nanowires. In the next stage of the process, methane gas is admitted to the same furnace containing the Cu template and Si under a background of Ar gas. Isolated nanocrystals ~20-50 nm in size initially nucleate and then coalesce along the grain boundaries. Sintering occurs, fusing the nanocrystals together. Unidirectional growth of the nanowires continues as long as a favorable gas
ambient is maintained, resulting in densely packed or radial nanowires tens of microns long. The researchers believe that a vapor-solid nucleation mechanism is responsible for the nanowire growth. First, SiC radicals are produced as decomposed methane gas reacts with the Si. SiC condensates onto the Cu surface at the thermally exposed grain boundaries, where nucleation of the nanowires begins spontaneously. The tips of the nanowires are flat and free from catalyst particles. Scanning
electron microscopy reveals that each wire consists of numerous (3 to 200) nanowires encapsulated within a larger diameter (0.2-2 µm) wire. Highresolution transmission electron microscopy shows a crystalline structure with a high density of planar defects. The researchers suggest that the coaxial nanowires could be combined with conductive polymers for use in photovoltaics or as interconnects in functional devices. Cordelia Sealy
November 2004
7