A new angle on electron microscopy

An elusive goal in optics is the perfectly achromatic lens—a lens that focuses every color of light to the same point. When it comes to electron microscopy, however, an imperfect lens isn’t necessarily a bad thing. So report Joanne Etheridge (Monash University, Australia) and her colleagues, who have exploited chromatic aberration—the tendency of a lens to bend rays of different wavelengths by different amounts—to devise a chemical-mapping technique for scanning confocal electron microscopes. Standard SCEMs use two lenses: one to focus an electron beam onto a small volume in a target sample and another to focus transmitted electrons onto the detector. The energies of the transmitted electrons carry clues about the chemical identity of the target. Normally, electron spectrometers are used to collect that energy information, but Etheridge and her coworkers adopt a simpler approach: They align their electron beam at an angle through the microscope lenses. Due to chromatic aberration, the lenses spatially separate electrons by energy, much like a prism separates colors of white light. An SCEM image can then be constructed exclusively with electrons of a selected energy. As a proof of principle, the team produced the image here, which shows the distribution of silver atoms in an aluminum–silver alloy. The bright, zigzagged line is a silver-rich region that precipitated out of the surrounding mixture. The million-pixel image took about 25 seconds to capture; with conventional techniques, it would have taken nearly half an hour. (C. Zheng et al., Phys. Rev. Lett., in press.)