Electron tomography reaches new benchmark
Characterization
March 4, 2008
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| 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.) |
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 energy-filtered 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. 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