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A measure of growth
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06/15/2006
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RESEARCH NEWS
A measure of growth CHARACTERIZATION Researchers from Georgia Institute of Technology have developed a technique that can provide information about growing carbon nanotubes (CNTs) and other nanostructures [King et al., Appl. Phys. Lett. (2006) 88, 033107]. Instead of using a large furnace to grow nanotubes, the researchers grew them directly onto the cantilever of an atomic force microscope. The tip contains a built-in current-controlled microheater, providing highly localized heating for selective growth of CNTs. Prior to initiating growth, a 10 nm layer of Fe catalyst is deposited onto the cantilever using electron beam evaporation. When heated, this layer forms islands, which serve as catalytic sites for the CNTs. As the nanotubes grow, the resonant frequency of the cantilever tip changes. These changes can be used to determine the mass of the structure being grown. The researchers calculate that the CNTs they grew were ~4 x 10-14 kg (in a scanning electron microscope, this equates to nanotubes 5-10 µm long and 10-30 nm in diameter). “We are working on integrating the growth and weighing of the nanotubes so that we can do both at the same time,” says William P. King. The researchers also suggest that it should be possible to have a number of cantilevers operating simultaneously at different temperatures. “This is a platform for materials discovery, so we could test tens or even thousands of different chemistry or growth conditions in a very short period of time,” says King. Many of the other materials that are grown using thermal techniques could be analyzed in the same way. “This could provide a new tool for investigating the growth of these structures under different conditions,” says King.
Cordelia Sealy
Dynamic truth about dislocations CHARACTERIZATION
Researchers from Risø and Argonne National Laboratories have devised an X-ray diffraction method that gives an insight into the dynamics of dislocation patterns during plastic deformation [Jakobsen et al., Science (2006) 312, 889]. When a material is plastically deformed, dislocations are produced in the lattice of each grain and organize into patterns: typically regions of dislocations forming boundaries around defect-free regions. These patterns exhibit a characteristic behavior, e.g. their size is inversely proportional to the applied stress. Although theory can explain the properties of individual dislocations, their collective behavior has proved more difficult to understand. Observation is also tricky: electron imaging is not able to provide an insight into the dislocation dynamics and X-ray diffraction traditionally provides averaged information over many grains and subgrains. The researchers from Risø and Argonne, however, have devised an X-ray diffraction setup that allows information on dislocation behavior in Cu to be gathered on the grain and subgrain scale. By using a highly penetrating beam and two detectors to take a set of images while rotating the sample, the researchers can build up a three-dimensional reciprocal space map of the volume of interest. “A number of long-standing and very basic questions about the creation, growth, and annihilation of dislocation structures [can] be addressed,” says Henning Friis Poulsen of Risø National Laboratory in Denmark. “Perhaps the most striking result is the revelation that dislocation-free regions show intermittent dynamics.” This is in direct contradiction to current models.
Image from in situ video of evolution of X-ray diffraction pattern (below). In TEM, the spots originate from individual dislocation free regions (above).
The technique provides a new insight into the dynamics of dislocations at a local scale. “We believe such an insight is vital to guide a new generation of physically-based plasticity modeling,” says Poulsen. It should also provide a direct test of existing workhardening models and those underpinning the interpretation of X-ray linebroadening data. The technique is also sufficiently broad to be applicable to any crystalline materials. “In particular,” says Poulsen, “knowledge of the emergence of dislocation structures are of critical importance for a variety of applications based on semiconductors and large optical components.” Cordelia Sealy
TEM has a magnetic attraction CHARACTERIZATION Until recently, it was thought impossible to detect magnetic circular dichroism in a transmission electron microscope (TEM). Now, however, researchers from Technische Universität Wien in Austria, Institute of Physics ASCR in the Czech Republic, TASC INFM-CNR National Laboratory, the Università degli Studi di Trieste, and the Università di Modena e Reggio Emilia in Italy have achieved just this [Schattschneider et al., Nature (2006) 441, 486]. If the photon absorption spectrum of a material depends on the polarization of the incident radiation, it is said to exhibit dichroism. If the absorption cross-section of a ferromagnet or paramagnet in a magnetic field depends upon the helicity of a circularly polarized photon relative to that magnetic field, the material is said to show X-ray magnetic circular dichroism (XMCD). Despite their similarities, until now it has not been thought possible to determine XMCD from electron energy-
loss magnetic chiral dichroism (EMCD). However, Schattschneider and coworkers show that chiral atomic transitions can be determined from inelastic electron scattering under certain conditions. The researchers looked at a 10 nm thick Fe single crystal layer on a GaAs [001] substrate using both X-ray absorption spectroscopy and TEM. Comparable spectroscopic features are seen in both EMCD in the TEM and XMCD. The researchers suggest that EMCD could, therefore, become a new method for the analysis and characterization of magnetic materials at the nanometer scale. In particular, in contrast to X-ray techniques, which are mainly surface-sensitive, EMCD could provide detailed depth information. The existing advantages of TEM mean that such a technique could prove an invaluable microscopic method in spintronics and nanomagnetism.
Cordelia Sealy
JULY-AUGUST 2006 | VOLUME 9 | NUMBER 7-8
9
06/15/2006
15:13
Page 9
RESEARCH NEWS
A measure of growth CHARACTERIZATION Researchers from Georgia Institute of Technology have developed a technique that can provide information about growing carbon nanotubes (CNTs) and other nanostructures [King et al., Appl. Phys. Lett. (2006) 88, 033107]. Instead of using a large furnace to grow nanotubes, the researchers grew them directly onto the cantilever of an atomic force microscope. The tip contains a built-in current-controlled microheater, providing highly localized heating for selective growth of CNTs. Prior to initiating growth, a 10 nm layer of Fe catalyst is deposited onto the cantilever using electron beam evaporation. When heated, this layer forms islands, which serve as catalytic sites for the CNTs. As the nanotubes grow, the resonant frequency of the cantilever tip changes. These changes can be used to determine the mass of the structure being grown. The researchers calculate that the CNTs they grew were ~4 x 10-14 kg (in a scanning electron microscope, this equates to nanotubes 5-10 µm long and 10-30 nm in diameter). “We are working on integrating the growth and weighing of the nanotubes so that we can do both at the same time,” says William P. King. The researchers also suggest that it should be possible to have a number of cantilevers operating simultaneously at different temperatures. “This is a platform for materials discovery, so we could test tens or even thousands of different chemistry or growth conditions in a very short period of time,” says King. Many of the other materials that are grown using thermal techniques could be analyzed in the same way. “This could provide a new tool for investigating the growth of these structures under different conditions,” says King.
Cordelia Sealy
Dynamic truth about dislocations CHARACTERIZATION
Researchers from Risø and Argonne National Laboratories have devised an X-ray diffraction method that gives an insight into the dynamics of dislocation patterns during plastic deformation [Jakobsen et al., Science (2006) 312, 889]. When a material is plastically deformed, dislocations are produced in the lattice of each grain and organize into patterns: typically regions of dislocations forming boundaries around defect-free regions. These patterns exhibit a characteristic behavior, e.g. their size is inversely proportional to the applied stress. Although theory can explain the properties of individual dislocations, their collective behavior has proved more difficult to understand. Observation is also tricky: electron imaging is not able to provide an insight into the dislocation dynamics and X-ray diffraction traditionally provides averaged information over many grains and subgrains. The researchers from Risø and Argonne, however, have devised an X-ray diffraction setup that allows information on dislocation behavior in Cu to be gathered on the grain and subgrain scale. By using a highly penetrating beam and two detectors to take a set of images while rotating the sample, the researchers can build up a three-dimensional reciprocal space map of the volume of interest. “A number of long-standing and very basic questions about the creation, growth, and annihilation of dislocation structures [can] be addressed,” says Henning Friis Poulsen of Risø National Laboratory in Denmark. “Perhaps the most striking result is the revelation that dislocation-free regions show intermittent dynamics.” This is in direct contradiction to current models.
Image from in situ video of evolution of X-ray diffraction pattern (below). In TEM, the spots originate from individual dislocation free regions (above).
The technique provides a new insight into the dynamics of dislocations at a local scale. “We believe such an insight is vital to guide a new generation of physically-based plasticity modeling,” says Poulsen. It should also provide a direct test of existing workhardening models and those underpinning the interpretation of X-ray linebroadening data. The technique is also sufficiently broad to be applicable to any crystalline materials. “In particular,” says Poulsen, “knowledge of the emergence of dislocation structures are of critical importance for a variety of applications based on semiconductors and large optical components.” Cordelia Sealy
TEM has a magnetic attraction CHARACTERIZATION Until recently, it was thought impossible to detect magnetic circular dichroism in a transmission electron microscope (TEM). Now, however, researchers from Technische Universität Wien in Austria, Institute of Physics ASCR in the Czech Republic, TASC INFM-CNR National Laboratory, the Università degli Studi di Trieste, and the Università di Modena e Reggio Emilia in Italy have achieved just this [Schattschneider et al., Nature (2006) 441, 486]. If the photon absorption spectrum of a material depends on the polarization of the incident radiation, it is said to exhibit dichroism. If the absorption cross-section of a ferromagnet or paramagnet in a magnetic field depends upon the helicity of a circularly polarized photon relative to that magnetic field, the material is said to show X-ray magnetic circular dichroism (XMCD). Despite their similarities, until now it has not been thought possible to determine XMCD from electron energy-
loss magnetic chiral dichroism (EMCD). However, Schattschneider and coworkers show that chiral atomic transitions can be determined from inelastic electron scattering under certain conditions. The researchers looked at a 10 nm thick Fe single crystal layer on a GaAs [001] substrate using both X-ray absorption spectroscopy and TEM. Comparable spectroscopic features are seen in both EMCD in the TEM and XMCD. The researchers suggest that EMCD could, therefore, become a new method for the analysis and characterization of magnetic materials at the nanometer scale. In particular, in contrast to X-ray techniques, which are mainly surface-sensitive, EMCD could provide detailed depth information. The existing advantages of TEM mean that such a technique could prove an invaluable microscopic method in spintronics and nanomagnetism.
Cordelia Sealy
JULY-AUGUST 2006 | VOLUME 9 | NUMBER 7-8
9