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Atomic Resolution Chemical Analysis

Browning ND1, Chishom MF1, Pennycook SJ1

Nature 366, 143 (1993)

Full Article (PDF 460 KIB)

Chemical analysis at atomic resolution has been the major long-term goal of analytical electron microscopy. The atomic structure of internal interfaces and the nature and distribution of impurities govern many of the technologically important properties of materials. Atomic scale determination of structure, composition and chemical bonding therefore represents the ultimate tool with which to probe the most critical regions of advanced materials. The recent addition of a sensitive parallel detection electron energy loss spectrometer to the VG HB501 UX scanning transmission electron microscope in the Solid State Division has now achieved this goal. With this facility, which is unique in the world, it is possible to identify unknown elements and determine chemical bonding in materials at their most fundamental level, the atomic scale.

The basis for atomic resolution chemical analysis is the Z-contrast image. This provides a direct image in which atomic columns with higher atomic number (Z) appear brighter, a unique identification of the atomic structure. By eliminating the need for image simulations to interpret the image, (required with conventional phase contrast imaging), the Z-contrast image can be used as a map to position the probe for electron energy loss spectroscopy. As only high-angle scattering is used for the Z-contrast image, the low-angle scattering can be used simultaneously for spectroscopy. The energy loss spectrum contains absorption edges which are characteristic of particular elements, thus allowing chemical species in the atomic column to be identified from the energy loss fingerprint. For large energy losses (above ~300 eV), the detected signal is generated from the single atomic column selected from the image. Figure 1 shows a series of cobalt L2,3 core-loss spectra obtained in single atomic plane steps across a CoSi2-Si {111} interface. The disappearance of cobalt in moving one atomic plane (2.7Å) is clearly seen. Figure 2 shows a composition profile, obtained from integrating the intensity in each cobalt edge, which drops almost 90% in moving just one atomic plane across the interface, a striking demonstration of atomic resolution chemical analysis.

This technique opens up many longstanding areas of materials research. Segregation at dislocation cores is known to have strong effects on the electronic properties of semiconductors and on the mechanical properties of high-temperature intermetallics and nanophase ceramics, but has so far been inaccessible to fundamental study. Similarly, the strength of metal/ceramic interfaces, the transport properties of grain boundaries and tunnel junctions in high-temperature superconductors, and many other fields of materials research are all opened up to an entirely new level of insight by this advance.

 

Ultradispersed Pt on g-alumina

 
 
Figure 1. Co L2,3 core-loss spectra obtained in single atomic plan steps across a CoSi2-Si {111}.
 
 
  Figure 2. Composition profile showing a 90% drop moving a single atomic plane across the interface.
   
 
  1. Solid State Division, Oak Ridge National Laboratory

 Oak Ridge National Laboratory