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Resolving Individual Light Atoms
Mar 2010
OAK RIDGE, Tenn., March 29, 2010 – Using the latest in aberration-corrected electron microscopy, researchers at the US Department of Energy’s Oak Ridge National Laboratory and their colleagues have obtained the first images that distinguish individual light atoms such as boron, carbon, nitrogen and oxygen.

The images were obtained with a Z-contrast scanning transmission electron microscope (STEM). Individual atoms of carbon, boron, nitrogen and oxygen – all of which have low atomic numbers – were resolved on a single-layer boron-nitride sample.

Individual boron and nitrogen atoms are clearly distinguished by their intensity in this Z-contrast scanning electron transmission microscope image. Each single hexagonal ring of the boron-nitrogen structure – for instance, the one marked by the green circle in (a), consists of three brighter nitrogen atoms and three darker boron atoms. The lower image (b) is corrected for distortion. (Image: Oak Ridge National Laboratory)

“This research marks the first instance in which every atom in a significant part of a nonperiodic material has been imaged and chemically identified,” said Stephen Pennycook of the Materials Science and Technology Div. “It represents another accomplishment of the combined technologies of Z-contract STEM and aberration correction.”

Pennycook and Oak Ridge colleague Matthew Chisholm were joined by a team that includes Sokrates Pantelides, Mark Oxley and Timothy Pennycook, all of Vanderbilt University and ORNL; Valeria Nicolosi at Oxford University in the UK; and Ondrej Krivanek, George Corbin, Niklas Dellby, Matt Murfitt, Chris Own and Zotlan Szilagyi, all of Nion Co. of Kirkland, Wash., which designed and built the microscope. The team’s Z-contrast STEM analysis is described in an article published March 25 in the journal Nature.

The new high-resolution imaging technique enables materials researchers to analyze, atom by atom, the molecular structure of experimental materials and to discern structural defects in those materials. Defects introduced into a material – for example, the placement of an impurity atom or molecule in the material’s structure – often are responsible for the material’s properties.

The group analyzed a monolayer hexagonal boron-nitride sample prepared at Oxford University and found and identified three types of atomic substitutions – carbon atoms substituting for boron, carbon substituting for nitrogen and oxygen substituting for nitrogen. Boron, carbon, nitrogen and oxygen have atomic numbers – or Z values – of 5, 6, 7 and 8, respectively.

This experimental electron microscope image shows a monolayer of boron nitride containing atomic substitutions. Quantitative analysis of the image produced a detailed atomic model, which is shown superposed on the image. Boron atoms are shown red, carbons atoms yellow, nitrogen atoms green and oxygen atoms blue. (Image: Nion Co., Oak Ridge National Laboratory and Vanderbilt University)

The annular dark field analysis experiments were performed on a 100-kV Nion UltraSTEM microscope optimized for operation at 60 kV. Aberration correction, in which distortions and artifacts caused by lens imperfections and environmental effects are computationally filtered and corrected, was conceived decades ago but only made possible relatively recently by advances in computing. Aided by the technology, Oak Ridge's electron microscopy group set a resolution record in 2004 with its 300-kV STEM.

The recent advance comes at a much lower voltage, for a reason. “Operating at sixty kilovolts allows us to avoid atom-displacement damage to the sample, which is encountered with low Z-value atoms above about eighty kilovolts,” Pennycook said. “You could not perform this experiment with a three hundred-kilovolt STEM.”

Materials, chemical and nanoscience researchers and theorists can use the high-resolution images to design more accurate computational simulations that will predict the behavior of advanced materials, which are key to meeting research challenges that include energy storage and energy efficient technologies.

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