Aberration Correction Enables 3-D Imaging Akin to Confocal Optical Technique
Daniel S. Burgess
Aberration-corrected scanning transmission electron microscopy offers users the ability to perform 3-D imaging similar to confocal microscopy, a group at Oak Ridge National Laboratory in Tennessee has reported. The technique can probe sample volumes that are 500 million times smaller than those interrogated using the optical technique, and further improvements in resolution are expected with the release of fifth-order aberration correctors in 2007.
A third-order aberration-corrected 300-kV scanning transmission electron microscope demonstrates the feasibility of generating 3-D images with nanoscale resolution in a manner similar to confocal optical microscopy. Courtesy of Oak Ridge National Laboratory.
Aberration correctors null spherical aberration in the system through the deliberate application of quadrupole and octupole fields using non-round magnetic lenses. Previously, the laboratory researchers demonstrated the improvements in resolution that are obtained by using their third-order aberration-corrected 300kV instrument to image 78-pm spacings between pairs of silicon columns (see “Direct Subangstrom Resolution Demonstrated,” Photonics Spectra, October 2004, page 18).
Stephen J. Pennycook, leader of Oak Ridge’s electron microscopy group, said that confocal microscopy revolutionized the biological sciences by enabling depth sections to be reconstructed into 3-D images of cells and other biological structures. Unless techniques such as near-field imaging or multiphoton excitation are employed, however, its resolution is limited to approximately 150 nm laterally and 400 nm in depth. In contrast, third-order aberration-corrected scanning transmission electron microscopy offers a lateral resolution of less than 0.1 nm and a depth resolution on the order of 6 nm.
The images of the TiO2 powder and metal nanoparticles were used to create a 3-D rendering. The white features are high-contrast regions from the bright-field series and correspond to the powder. The yellow features are high-intensity regions from the dark-field series and correspond to the nanoparticles. The nanoparticles appear elongated as a result of the microscope’s probe intensity profile.
“This advance should open up many new fields in biology, materials science, catalysis and nanoscience,” he said, citing the team’s efforts to produce nanoscale-resolution 3-D images of proteins and cells, to offer atomic explanations for the performance of macroscopic semiconductor devices, to image catalyst clusters inside mesoporous supports and to study individual grains in nanophase materials.
To validate the approach, the scientists used the 300-kV microscope, equipped with a third-order aberration corrector from Nion Co. of Kirkland, Wash., to collect depth series of a TiO2 powder impregnated with 1to 3-nm platinum and gold nanoparticles and of an Al2O3 powder impregnated with Pt2Ru4(CO)18 and mounted on a carbon film. They generated a 3-D image from the former, which comprised 50 frames and required less than five minutes to produce, compared with several hours if they had used tilt-series scanning transmission electron microscopy. In the latter, they could resolve individual atoms over several adjacent frames.
A 50-frame depth series of high-angle annual dark-field (left) and bright-field (right) images was collected of TiO2 powder impregnated with 1- to 3-nm platinum and gold nanoparticles at various defocus values.
Pennycook said that the next-generation, fifth-order correctors are expected to be available next year. The lab will acquire models from Nion and from Corrected Electron Optical Systems GmbH of Heidelberg, Germany, for use on new microscopes from Nion and from FEI Co. of Hillsboro, Ore. The systems could im-prove the lateral resolution by a factor of two and depth resolution by a factor of four.
The team hopes to demonstrate atomic-resolution 3-D spectroscopy as well as imaging, he said, with the goal of observing the electronic states around an atom in a high-k dielectric that can result in leakage or reduced carrier mobility. Other plans include the in situ imaging of proteins undergoing folding and self-assembly processes, catalysts, growing nanostructures, and energy conversion processes in fuel cells and photovoltaics.
PNAS, Feb. 28, 2006, pp. 3044-3048.
MORE FROM PHOTONICS MEDIA