One-Angstrom Microscope Images Carbon Atoms

Using the One-Ångstrom Microscope (OÅM), researchers at the DOE's Lawrence Berkeley National Laboratory (LBNL; Berkeley, CA; 510-486-4610) have made unprecedented images of columns of carbon atoms in a diamond lattice that are only 0.89Å apart. The OÅM has also resolved nitrogen atoms in the presence of gallium atoms in columns spaced 1.13Å apart.

Imaging Achievement
About the OÅM
Tool Minimizes Sample Hassle

Imaging Achievement (Back to Top)
"The ability to make images of light elements such as carbon, nitrogen, and oxygen in solids at atomic resolution is a very big step forward," says Christian Kisielowski, who was a member of the LBNL research team that achieved this resolution. "As it was achieved by a technique that can be a routine tool in the future, it is of great interest to science and industry."

Many of the most promising solids under investigation today, including superhard materials, high-temperature superconductors, and semiconductors with large band-gap energies, incorporate light elements in crystal lattices at short interatomic distances.

Using traditional tools, it is difficult to see small atoms at atomic resolution because the atoms don't scatter the electrons in the microscope's beam very strongly. When the light atoms are close to heavy ones, it is almost impossible to resolve them because the heavy atoms create a complex interference pattern.

The OÅM overcomes this difficulty by making a through-focal series of images. In the case of gallium nitride, 20 different images are made. In each of these, the scattered electrons interfere with different relative phases. Computer processing is then used to unscramble the electron waves and combine them into a single high-resolution image in which all electrons are in phase, Kisielowski says.

"We're aiming to investigate materials with even shorter bond lengths," Kisielowski says. "We want to have procedures in place that work reliably and fast to make the experiments available to our user community as soon as possible...colleagues from other laboratories have already started to share our excitement by investigating their own samples with the OÅM."

Other members of Kisielowski's microscopy research team at LBNL's Materials Sciences Division were Michael O'Keefe, Christian Nelson, Chengyu Song, and Roar Kilaas.

About the OÅM (Back to Top)
The OÅM produces images from a mid-voltage, high-resolution transmission electron microscope (TEM). The basic instrument is a Philips CM300FEG/UT, a TEM with a field-emission electron source and an ultra-twin objective lens with low spherical aberration (Cs = 0.65mm). The device also has a native resolution of 1.7Å. Images are recorded digitally with a charge-coupled device (CCD) camera or on plates.

The microscope can be used for three different methods of extending the resolution to the desired 1.0Å. These are holography, focal series restoration, and EREM. In addition, the instrument is capable of energy-filtered imaging.

The OAM is located at LBNL's National Center for Electron Microscopy (NCEM). The tool had its genesis in the early 1990s, when NCEM's three-story, million-volt Atomic Resolution Microscope (ARM), was the world's most powerful with a practical resolution of 1.6Å. Soon after this, a Japanese-built, one-and-a-quarter-million-volt machine in Germany achieved 0.95-Å resolution. However, the achievement cost more than 10 million Deutschmarks.

At about the same time, O'Keefe proposed a way to computer-process through-focus images to achieve higher resolution from a medium-voltage microscope, an approach first suggested in the late 1960s. Through-focus methods depend on beams in which all of the electrons have nearly the same energy level.

"Such a microscope can be designed so that its 'information limit'—the limit to which it produces phase-scrambled information—lies well beyond its traditionally defined nominal resolution, with all the transferred information in phase," he says. "By combining information from many images, a single image with resolution approaching the information limit can be achieved."

In 1993, NCEM secured funds to acquire a Philips CM300 that was optimized through O'Keefe's information-limit specifications. Although the typical resolution limit of a CM300 is 1.7Å, recent results confirm the OÅM's capacity to produce phase-scrambled information far beyond this level. In the case of diamond, Kisielowski, O'Keefe showed that the OÅM's information limit can extend to at least 0.89Å. Computer programs that process the focal-series images allow OÅM to reconstruct images with resolutions near its information limit.

Tool Minimizes Sample Hassle (Back to Top)
Meanwhile the ARM, NCEM's "grandfather" microscope, is far from being outmoded by its diminutive descendant. The OÅM can only produce ultra-high resolution with samples that are less than a hundred angstroms thick. These are prepared through an arduous process through which layer after layer of atoms are chiseled away using a low-angle, low-energy beam of argon atoms.

In contrast, the ARM can use samples that are three times thicker and composed of heavy atoms, yet still achieve a respectable resolution. Because it is a high-voltage microscope, the ARM can accommodate larger sample holders which are required to perform dynamic experiments such as in-situ straining or heating. It also allows for larger tilt angles than the OÅM, which are key for materials science experiments.

For more information, call O'Keefe at 510-486-4610, or e-mail Paul Preuss at LBNL at paul_preuss@lbl.gov.

Written by Paul Preuss