New technique offers better atomic resolution imaging

Scientists at the University of Illinois at Urbana-Champaign have developed a new imaging technique that uses electron diffraction waves to improve both image resolution and sensitivity to small structures.

The technique is said to work on the same principle as X-ray diffraction, but can record structure from a single nanostructure or macromolecule.

Determining the structure of materials – such as protein crystals — is currently performed using X-ray diffraction. However, many small structures used in nanotechnology have not been accessible to crystallography, so their structures remain unknown.

‘Nature is full of objects that cannot be easily crystallised, including many proteins and nano-sized objects that lack a periodic structure,’ said Jian-Min (Jim) Zuo, a professor of materials science and engineering at Illinois. ‘Our technique has the potential to image nonperiodic nanostructures, including biological macromolecules, at atomic resolution.’

To demonstrate the effectiveness of their imaging technique, Zuo and his colleagues recorded and processed the diffraction pattern from a double-wall carbon nanotube.

‘Carbon nanotubes are of special interest because the mechanical and electrical properties of a nanotube depend upon its structure,’ said Zuo, who also is a researcher at the Frederick Seitz Materials Research Laboratory on the Illinois campus. ‘However, only the outermost shell of a carbon nanotube has been imaged by scanning tunnelling microscopy with atomic resolution.’

Because carbon possesses few electrons, the scattering from an electron beam is inherently weak and typically results in an image with low contrast and poor resolution, Zuo said. Imaging carbon atoms has been a special challenge.

‘While conventional electron microscopes can achieve a resolution approaching one angstrom for many materials,’ Zuo said, ‘the resolution limit for carbon in nanotubes is only three angstroms.’

To image a double-wall carbon nanotube, the researchers first selected a single nanotube target in a transmission electron microscope. Then they illuminated the nanotube with a narrow beam of electrons about 50 nanometers in diameter.

After recording the diffraction pattern, they used an oversampling technique and iterative process to retrieve phase information and construct an image with a resolution of one angstrom.

‘Since this process does not use a lens to form the image, the resolution is not limited by lens aberration,’ Zuo said. ‘Lens aberration is the factor that has been limiting the resolution of the best electron microscopes. It’s like the blur when you look through the bottom of a wine bottle.’

The complexity of the nanotube image was surprising, Zuo said. ‘The double-wall nanotube consists of two concentric nanotubes of different helical angles. Like two screws with different pitch, sometimes the nanotube structures line up and sometimes they don’t. This results in a complicated pattern of both accidental coincidences and mismatches.’

The ability to generate images from nanoscale diffraction patterns offers a way to determine the structure of nonperiodic objects, from inorganic nanostructures to biological macromolecules, much like X-ray diffraction does for crystals, Zuo said. ‘Since diffraction is a standard method for determining structure, our nanoarea electron diffraction technique opens a door to examining the structure of individual and highly irregular molecules and nanostructures like clusters and wires.’