High-resolution imaging meets vibrational spectroscopy

In a scanning transmission electron microscope (STEM), a high-energy electron beam is focused to near-atomic dimensions and scanned over a thin specimen. In addition to generating images from the scattered electrons, one can simultaneously use an electron spectrometer to map the energy lost by the beam. Among the details energy-loss spectroscopy can reveal are elemental composition and chemical bonding, but the technique has traditionally suffered from poor energy resolution, typically because of fluctuations in the high voltage supplied to the electron source. Now researchers led by Ondrej Krivanek, an adjunct professor of physics at Arizona State University and president of Nion Co, which manufactures commercial STEMs, have ameliorated the problem. By implementing a newly designed monochromator that is immune to voltage variations, they achieve an energy spread of 9 meV, as shown here. That’s a nearly 30-fold improvement in energy resolution compared with the unfiltered beam and a 10-fold improvement over earlier monochromator designs. Thanks to the narrow energy spread, the researchers could resolve the low-energy excitation peaks caused by lattice vibrations in materials such as silicon dioxide, silicon carbide, and titanium hydride. Although the peaks were wider than those measured with more traditional tools such as Raman spectroscopy, energy-loss spectroscopy in a STEM benefits from extremely high spatial resolution: The researchers mapped variations in the phonon spectra at the nanometer length scale as a 2-nm-wide beam scanned the interface between silicon and silicon dioxide. (O. L. Krivanek et al., Nature 514, 209, 2014, doi:10.1038/nature13870 .)