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Sub-Ångstrom Resolution Achieved by Scanning Transmission Electron Microscopy

P. D. Nellist1, S. J. Pennycook1

Phys. Rev. Lett., 81, 4156 (1998)

Full Article (PDF 356 KB)

The highest resolution ever recorded in an image has been achieved with the ORNL 300-kV scanning transmission electron microscope (STEM) installed in the Solid State Division. Z-contrast electron microscopy is an incoherent imaging technique, which theoretically gives a factor of two increase in resolution compared to the traditional coherent methods based on axial bright field imaging. The attached results from Si in a <110> projection show definitively that information is present in the image down to a spacing of only 0.78 Å.

This achievement of sub-angstrom resolution clearly demonstrates the advantage of incoherent imaging for enhanced resolution. The interpretable (Scherzer) resolution limit of this machine is a modest 1.93 Å in the coherent bright field mode, but reduces to 1.26 Å in the incoherent Z-contrast mode, sufficient to resolve the "dumbbells" in the Si <110> projection (upper left). The Fourier transform of the image (upper right) shows an (004) spot, proving that information is indeed being passed at below the 1.36 Å spacing of the dumbbells. Under these conditions the image is a direct map of the projected atomic structure, with peak intensities close to the atomic positions. Images of this sort are directly invertable, avoiding the phase problem of conventional structure determination methods. However, if we are willing to sacrifice the intuitive nature of the image and the ability for direct inversion, it is possible to achieve a much higher image resolution. By using a larger than optimum objective aperture to pass higher spatial frequencies, and enhancing their transfer with a higher objective lens defocus, the image at the lower left is produced. Although it is no longer interpretable by eye, the Fourier transform (lower right) shows the definite presence of a {444} spot. This is proof that the microscope is passing image information at the unprecedented level of 0.78 Å.

The disadvantage of enhancing image resolution in this way is that we lose the ability to determine atomic positions directly from the image. Like the conventional methods to sub-angstrom microscopy, we must interpret the image through the use of image simulations and through-focal series. However, a major advantage of the STEM approach is that we can always return to Scherzer conditions to obtain the direct image. This is critical for determining defect configurations where the lattice periodicity is broken, for example at dislocation cores, and it is precisely these structures that are so important in materials science, since only with electrons can isolated defects be studied. In the last year, advances in objective lens design have made direct imaging technically feasible at a landmark 1 Å resolution. This would significantly increase the accuracy of the structure determination, especially for light elements, and simultaneously allow individual impurity atoms and their electronic structure to be detected in chosen atomic columns at dislocations, finally revealing the origins of embrittlement in metals and alloys. By the STEM techniques we have shown here, an ultimate image resolution of ~0.6 Å is anticipated for such a microscope.Solid State Division, ORNL.

 
 
Figure 1. Imaging of Si <110> and information Transfer to 0.78 Å, with 18 mrad objective aperture.
   
 
  1. Solid State Division, Oak Ridge National Laboratory

 Oak Ridge National Laboratory