ACEM Instrument Achieves Significant Performance Milestone
In 2004, Oak Ridge National Laboratory (ORNL) began installing one of the nation's first aberration-corrected electron microscopes. An aberration-corrected electron microscope (ACEM) is a combined scanning transmission and conventional transmission instrument. The microscope is being installed at the Laboratory's new Advanced Microscopy Laboratory, which is part of ORNL's High Temperature Materials Laboratory (HTML).
While still in the installation phase, the HTML's JEOL 2200FS-AC ACEM has achieved sub-Ångstrom image resolution, and has demonstrated excellent imaging of single atoms. The ACEM operates as both a conventional transmission electron microscope (TEM) and a scanning transmission electron microscope (STEM). In TEM imaging mode, experiments showed that the ACEM has an "information limit" (that is, a potentially achievable resolution limit) of 0.9Å. An Ångstrom is about the diameter of a single atom (there are 10 million Ångstroms in a millimeter). This limit meets the theoretically achievable limit as controlled by parameters such as the stabilities of the high voltage and lens power supplies, and the physics of the emission of electrons in the electron gun. The excellent environment provided by the specially constructed Advanced Microscopy Laboratory permits the ultimate microscope performance to be achieved.
In STEM operation, using the annular dark-field (ADF) imaging mode, high atomic number elements image in bright contrast relative to lower atomic number elements. This is an important imaging mode, for example, to show the distribution of single atoms and small clusters of atoms of heavy elements (such as platinum) in catalytic materials. An experimental catalyst material comprising clusters of three rhenium atoms on gamma aluminum oxide (γ-Al2O3) is being studied (with Professor Bruce Gates and students at the University of California Davis). The advantages of ADF imaging are seen in Figures 1 and 2.
The imaging of single atoms and small clusters such as demonstrated here is one of the primary thrust areas for ACEM research, to support ORNL's catalyst characterization programs. These images, it should be noted, have been obtained using non-ideal optical conditions that yield an ADF image resolution of just under 1Å. Improvements in the imaging geometry have recently been made, and researchers anticipate achieving even better resolution.