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Electron tomography reaches new benchmark
RESEARCH NEWS
Electron tomography reaches new benchmark CHARACTERIZATION
The shape, size, and atomic structure of nanomaterials is crucial to understanding and, even more importantly, controlling their function. Transmission electron microscopy (TEM) is able to characterize materials down to the atomic scale, as is required with such materials, but only provides two-dimensional information. To characterize materials in three dimensions, a technique like tomographic structure reconstruction, commonly used in the biosciences, is needed. However, despite advances in high-angle, annular dark-field and energyfiltered tomography, only resolution down to 1 nm3 has been achieved in the physical sciences. Now, however, researchers have reconstructed fullerene-like particles on the atomic scale using low-voltage TEM with aberration-corrected phase contrast imaging [Bar Sadan et al., Nano Lett. (2008), doi: 10.1021/nl073149i]. The team from the Weizmann Institute of Science, Israel, Research Centre Jülich, and Technische Universität Dreseden in Germany base their approach on bright-field electron tomography. While bright-field TEM can achieve resolution down to 2 Å, this has only been possible at high accelerating voltage. Now, however, the researchers have used negative spherical aberration imaging (NCSI) at 80 kV in an FEI Titan 80-300 TEM to achieve a resolution of 2 Å.
(a)
(c)
(b)
Model of a four-shell nanooctahedron (a), a 3-Å-thick slice through the model (b), and a corresponding slice in the experimental tomogram. The nested shells of the octahedron with a closest separation of 6.15 Å are resolved in the tomogram. (Courtesy of Lothar Houben, Research Centre Jülich.)
To demonstrate the capabilities of the approach, the team took a series of high resolution images of MoS2 nested-shell nanooctahedrons at 3° intervals over a tilt range of ±60°. “[Our approach] directly delivers a close representation of the electrostatic potential of the particle, without any elaborate data processing,” says Lothar Houben of Research Centre Jülich. The tomograms provide information on the scale of <3 Å in all three dimensions. The nested shells of the nanooctahedrons can be seen, and the hollow structure of the nanoparticles is also revealed. The overall resolution achieved is 0.11 nm3, an order of magnitude better than achieved to date.
The researchers believe that a wider tilt angle range and smaller increments could improve the achievable three-dimensional resolution further. “It is certainly a remarkable achievement to get tomographic data at this scale and, I believe, the best spatial resolution thus far achieved,” says Andrew L. Bleloch of the University of Liverpool. However, there are some issues with the approach, cautions Bleloch. “The technique is best applied to structures that are only a few atomic layers thick. For thicker samples, the projection requirement is likely to breakdown.” Nevertheless, the results set a new benchmark for electron tomography. Cordelia Sealy
Engineering light emission with nanoparticle dopants OPTICAL PROPERTIES Researchers from Clemson University and the École Nationale Supérieure de Physique de Strasbourg in France have developed rare-earth-doped, core-shell LaF3 nanoparticles that could enable spectral design of luminescent materials [DiMaio et al., Proc. Natl. Acad. Sci., USA (2008) doi: 10.1073/pnas.0711638105]. The broad spectral range of emission from such nanoparticles could find application in light-emitting diodes (LEDs), solar cells, lasers and amplifiers, and biological assaying. For applications, such nanoparticles should, ideally, have the ability to tailor independent emissions from a codoped material. Until now, it was thought that this could only be achieved by using nanoparticles incorporated into a host matrix where each nanoparticle is doped with a particular rare earth element (i.e. a lanthanide). Instead, however, the researchers use 10 nm, core-shell LaF3 nanoparticles with Tb3+ and Eu3+ dopants constrained to specific
10
A model of a complex core-shell nanoparticle. The yellow regions denote undoped LaF3, while the green and red denote Tb3+- and Eu3+-doped LaF3, respectively. individual shells. The distance between shells, and therefore dopants, can be carefully controlled. This enables, in turn, control over the energy transfer to varying degrees – from zero to partial to total – between the dopants within an individual nanoparticle. Using three-shell particles, the ratio of the 540-nm
FEB-APR 2008 | VOLUME 3 | NUMBER 1-2
Tb3+ peak to that of the 590-nm Eu3+ peak can be varied from 0.2–2.4. This variation only arises from changes in the internal structure of the nanoparticles, not any compositional or external dimensional changes. The separation between the shells has to be a minimum of >2 nm. “This work shows that nanoparticles ~10 nm in diameter can have complex core-shell architectures that allow significant tuning of their light emissive properties,” explains Jeffrey R. DiMaio, now at Tetramer Technologies. These nanoparticles, and composites made from them, could be used to engineer specific spectral features. “The key issue here is that nanoparticles can be engineered to emit at multiple tunable wavelengths from a single optical source,” comments Jean Pierre Leburton of the Beckman Institute for Advanced Science and Technology.
Cordelia Sealy
Electron tomography reaches new benchmark CHARACTERIZATION
The shape, size, and atomic structure of nanomaterials is crucial to understanding and, even more importantly, controlling their function. Transmission electron microscopy (TEM) is able to characterize materials down to the atomic scale, as is required with such materials, but only provides two-dimensional information. To characterize materials in three dimensions, a technique like tomographic structure reconstruction, commonly used in the biosciences, is needed. However, despite advances in high-angle, annular dark-field and energyfiltered tomography, only resolution down to 1 nm3 has been achieved in the physical sciences. Now, however, researchers have reconstructed fullerene-like particles on the atomic scale using low-voltage TEM with aberration-corrected phase contrast imaging [Bar Sadan et al., Nano Lett. (2008), doi: 10.1021/nl073149i]. The team from the Weizmann Institute of Science, Israel, Research Centre Jülich, and Technische Universität Dreseden in Germany base their approach on bright-field electron tomography. While bright-field TEM can achieve resolution down to 2 Å, this has only been possible at high accelerating voltage. Now, however, the researchers have used negative spherical aberration imaging (NCSI) at 80 kV in an FEI Titan 80-300 TEM to achieve a resolution of 2 Å.
(a)
(c)
(b)
Model of a four-shell nanooctahedron (a), a 3-Å-thick slice through the model (b), and a corresponding slice in the experimental tomogram. The nested shells of the octahedron with a closest separation of 6.15 Å are resolved in the tomogram. (Courtesy of Lothar Houben, Research Centre Jülich.)
To demonstrate the capabilities of the approach, the team took a series of high resolution images of MoS2 nested-shell nanooctahedrons at 3° intervals over a tilt range of ±60°. “[Our approach] directly delivers a close representation of the electrostatic potential of the particle, without any elaborate data processing,” says Lothar Houben of Research Centre Jülich. The tomograms provide information on the scale of <3 Å in all three dimensions. The nested shells of the nanooctahedrons can be seen, and the hollow structure of the nanoparticles is also revealed. The overall resolution achieved is 0.11 nm3, an order of magnitude better than achieved to date.
The researchers believe that a wider tilt angle range and smaller increments could improve the achievable three-dimensional resolution further. “It is certainly a remarkable achievement to get tomographic data at this scale and, I believe, the best spatial resolution thus far achieved,” says Andrew L. Bleloch of the University of Liverpool. However, there are some issues with the approach, cautions Bleloch. “The technique is best applied to structures that are only a few atomic layers thick. For thicker samples, the projection requirement is likely to breakdown.” Nevertheless, the results set a new benchmark for electron tomography. Cordelia Sealy
Engineering light emission with nanoparticle dopants OPTICAL PROPERTIES Researchers from Clemson University and the École Nationale Supérieure de Physique de Strasbourg in France have developed rare-earth-doped, core-shell LaF3 nanoparticles that could enable spectral design of luminescent materials [DiMaio et al., Proc. Natl. Acad. Sci., USA (2008) doi: 10.1073/pnas.0711638105]. The broad spectral range of emission from such nanoparticles could find application in light-emitting diodes (LEDs), solar cells, lasers and amplifiers, and biological assaying. For applications, such nanoparticles should, ideally, have the ability to tailor independent emissions from a codoped material. Until now, it was thought that this could only be achieved by using nanoparticles incorporated into a host matrix where each nanoparticle is doped with a particular rare earth element (i.e. a lanthanide). Instead, however, the researchers use 10 nm, core-shell LaF3 nanoparticles with Tb3+ and Eu3+ dopants constrained to specific
10
A model of a complex core-shell nanoparticle. The yellow regions denote undoped LaF3, while the green and red denote Tb3+- and Eu3+-doped LaF3, respectively. individual shells. The distance between shells, and therefore dopants, can be carefully controlled. This enables, in turn, control over the energy transfer to varying degrees – from zero to partial to total – between the dopants within an individual nanoparticle. Using three-shell particles, the ratio of the 540-nm
FEB-APR 2008 | VOLUME 3 | NUMBER 1-2
Tb3+ peak to that of the 590-nm Eu3+ peak can be varied from 0.2–2.4. This variation only arises from changes in the internal structure of the nanoparticles, not any compositional or external dimensional changes. The separation between the shells has to be a minimum of >2 nm. “This work shows that nanoparticles ~10 nm in diameter can have complex core-shell architectures that allow significant tuning of their light emissive properties,” explains Jeffrey R. DiMaio, now at Tetramer Technologies. These nanoparticles, and composites made from them, could be used to engineer specific spectral features. “The key issue here is that nanoparticles can be engineered to emit at multiple tunable wavelengths from a single optical source,” comments Jean Pierre Leburton of the Beckman Institute for Advanced Science and Technology.
Cordelia Sealy