Materials Science

1…2…3…I Love to Count

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Science  26 Nov 2010:
Vol. 330, Issue 6008, pp. 1157
DOI: 10.1126/science.330.6008.1157-a

Scanning transmission electron microscopy (STEM) methods are highly sensitive to atom type and arrangement and thus can be used for precise analysis of the structure of materials at the nanometer scale. However, current applications have been limited by the need for calibration standards that exhibit complex relationships between the image intensity and sample thickness, orientation, and crystallinity, or depend on comparisons of relative contrast. LeBeau et al. have developed a method to perform a column-by-column count of the atoms in an arbitrarily shaped sample without prior knowledge of the shape or thickness, by making direct comparisons with simulated images. Position-averaged convergent beam electron diffraction patterns were used to measure the local thickness and ensure accurate tilting of a gold foil sample. The raw data were filtered to obtain the location of each atom column, and comparisons between experimental and simulated column intensities were used to determine the number of atoms within each column to within ±1 atom. By varying the value used in the simulations for the finite effective source of the microscope, a parameter that is difficult to measure in non–aberration-corrected STEM, it was also possible to determine when the simulated and observed intensities did not agree, and thus to obtain limits for this parameter. In conjunction with other atomic-level microscopy techniques, it should be possible to count the atoms in more complex samples without the need for calibration standards.

Nano Lett. 10, 4405 (2010).

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