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Electron microscope attains 50-picometer resolution

26 February 2009

Electron microscope attains 50-picometer resolution

Ultimately, the resolution of an electron microscope is limited by the electron's de Broglie wavelength. For the 300-keV electrons typical in scanning transmission electron microscopy, that limit is about 2 pm, or 1/25th of the radius of hydrogen's 1s orbital. But STEM images are formed by focusing a billion or so electrons per second onto a sample. The spherical aberration of the electromagnetic lenses and the finite size of the electron source cause the electrons to lose phase coherence, lowering the resolution to about 100 pm, or about twice the distance between atoms in many crystals. Now, a team from Lawrence Berkeley National Laboratory in California has halved the STEM resolution limit to 50 pm. The boost in performance comes from two novel components: an electron source that emits copious electrons from a region just 25 pm across and a hexapole corrector that can compensate for phase aberrations up to fifth order. Using their new microscope, the LBNL researchers looked at a piece of germanium foil. According to x-ray crystallography, Ge atoms are arranged in rows of dumbbell pairs aligned end-to-end. Ordinarily, the dumbbells are too small to be resolved with STEM. But, as the accompanying figure shows, the LBNL microscope could resolve the 47-pm separation between two paired atoms. The resolution is so fine that the thermal jiggling of the atoms during the room-temperature measurement acts as an additional source of blur. (R. Erni, M. D. Rossell, C. Kisielowski, U. Dahmen, Phys. Rev. Lett., in press.) — Charles Day

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