Electron nanodiffraction technique offers atomic resolution imaging
James E. Kloeppel,
Physical Sciences Editor
(217) 244-1073; Kloeppel@illinois.edu
5/29/03
CHAMPAIGN, Ill. — A
new imaging technique that uses electron diffraction waves to improve both image
resolution and sensitivity to small structures has been developed by scientists
at the University of Illinois at Urbana-Champaign. The technique works 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 crystallized, 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 and corresponding author of a paper
to appear in the May 30 issue of the journal Science. "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 tunneling 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
1 angstrom for many materials," Zuo said, "the resolution limit for
carbon in nanotubes is only 3 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 1 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."
In addition to Zuo, the team included visiting scientist Ivan Vartanyants and
postdoctoral researcher Min Gao at Illinois, and researchers Ruth Zhang and
Larry Nagahara at Motorola Labs. The U.S. Department of Energy funded the work.