The country's best microscopists are teaming up to build the highest resolution microscope in the world, and researchers in Argonne's Materials Science Division (MSD) are playing a lead role.
The U.S. Department of Energy's Office of Basic Energy Sciences is funding this $100 million microscopy project, called the Transmission Electron Aberration-corrected Microscope (TEAM). One of the goals is to achieve a resolution of 0.5 ?ngstrom resolution – about one-million times smaller than the diameter of a human hair -- by the end of the decade. Another objective is to acquire three-dimensional images at atomic resolution. Today's best microscopes can only image a two-dimensional projection of columns of atoms.
Aberration correction is crucial for the TEAM goals. This concept will serve as a platform for four more microscopes which will have unprecedented abilities for in situ experiments and analytical studies. This new type of microscope will assist researchers studying the enhanced properties of nanomaterials that are built on the one-billionth-of-a-meter scale.
Five major electron microscopy centers are teaming up on this project:
- Argonne National Laboratory
- Brookhaven National Laboratory
- Lawrence Berkeley National Laboratory
- Oak Ridge National Laboratory, and
Frederick Seitz Materials Research Laboratory at the University of Illinois at Urbana-Champaign.
Each laboratory has a separate role. "Argonne scientists are designing the Ultracorrector," said Materials Science Division Director George Crabtree. This is the electron optical lens system that is the heart of the new approach. "The vision is to use TEAM to dramatically enhance the impact of electron microscopy on materials science."
Electron microscopes allow scientists to see much deeper into materials than optical microscopes can. Optical microscopes use glass lenses and light, and this technology's limit is about 0.5 micrometers, which is about 200 times smaller than the diameter of a human hair.
To move beyond this barrier, electron microscopes use electrons and magnetic fields instead of light and glass lenses. But there are still problems. "Lens aberration is the most significant limitation to resolution in electron microscopy," said MSD TEAM leader Dean Miller. According to optics theory, the resolution limit for electron microscopes is equivalent to the wave length of the electrons which is a few picometers, or 100 times smaller than an atom, for present instrumentation. But many challenges must be overcome to reach that level.
The challenge for Bernd Kabius, who is responsible for developing the Ultracorrector, is to develop a complex system of lenses to correct the aberrated images, which are created by the optical system, or "objective lens," of present microscopes. "Without aberration correction, looking at samples would be like looking through the dimpled bottom of a wine bottle – everything looks distorted," he said. "We need improvements in aberration correction to get a clear view of the atomic world."
Aberrations are caused by the illumination traveling and focusing differently through the edges and center of the objective lens. Furthermore electrons with different energies, equivalent to colors, degrade the resolution. In optical microscopes this is overcome by shaping the glass lenses. The magnetic lenses used in electron microscopes cannot be "shaped" in the same way as glass lenses. So, Kabius is designing a combination of magnetic lenses, which are flawed individually, but together they perform as a perfect lens for aberration- free imaging.
To correct this, Kabius is designing an electron optical system in cooperation with CEOS, a small German company, containing at least 13 lenses for the Ultracorrector. They are currently performing the calculations to determine the optimum arrangement of the lenses and determining the design feasibility.
If the design is successful, and after three months of work he is optimistic that it will be, a prototype will be built and tested at Argonne over the next three years. Then a second version of the Ultracorrector will be built and integrated into the first TEAM instrument at Lawrence Berkeley.
While Argonne is building and testing the Ultracorrector, other TEAM laboratories will be contributing from their areas of expertise to improve
- detectors for analytical and imaging purposes,
- the size and current in a small electron beam,
- experimental equipment inside the microscope (for example, tomography), and
- software for remote control and data handling (some measurements create a Terabyte, or a trillion bytes, of data).
The laboratories will also be working with industry in anticipation of the day when similar microscopes are marketed.
The TEAM project will build the first aberration-corrected microscope platform at Berkeley. That basic platform will be customized so each laboratory can build one for its particular research interests.
Argonne's microscope will be optimized for studying nanomaterials in situ. One application is visualization of magnetic devices in operation to improve knowledge of magnetic elements in electronics and magnetic memory for computer data storage.
Argonne microscopist Nestor Zaluzec is looking forward to working on the Argonne TEAM. "We are eager to perform experiments in situ," said Zaluzec, "where we can watch the sample respond in real time to external conditions like changing magnetic fields."
Kabius explains that the lenses in Argonne's TEAM will be configured for maximum experimental space around the sample rather than maximum resolution. "More space around the sample will allow Argonne researchers to place samples in environmental cells," said Kabius. "Scientists can observe samples in a gaseous environment or strain a material and watch it rupture."
The TEAM project complements third-generation synchrotron sources, such as the Advanced Photon Source at Argonne. The Advanced Photon Source is the nation's most brilliant source of research X-rays. It provides precise information about the distances between atoms in a sample. The TEAM cannot provide the same precision but the information can be obtained from a region which is about 300 times smaller.
"Electron sources are brighter than third generation synchrotrons such as the APS, and significantly higher than that of neutron sources," explained Crabtree.
The five TEAM instruments will be available to users worldwide through telepresence, a pioneering technology developed at Argonne. Researchers can mail a sample to one of the laboratories and control the experiment from their computer in conjunction with the TEAM microscopist.
The TEAM project is expected to yield such results as:
- the first three-dimensional atomic imaging of defect structures ,
- the first atomic structure determination of a glass
- understanding of magnetism and ferroelectricity in nanostructures at the microscopic level,
- visualizing dislocation interactions in nanostructures under controlled stress,
- advancing interface science to the level of surface science,
- understanding grain boundary motion under stress in nanocrystals, and
- imaging defects in the oxygen sub-lattice of complex oxides.