EELS Finds Atoms

Researchers in Japan have pushed to the single-atom limit the sensitivity of the chemical spectroscopy method called electron energy loss spectroscopy (EELS). The advance in EELS’s analytical resolving power provides scientists the ability to pinpoint in solids the locations of lone atoms such as impurities and identify them chemically (Nat. Chem., DOI: 10.1038/nchem.282).

In an EELS experiment, researchers irradiate a solid specimen with an electron beam and measure the element-specific decrease in beam energy (the energy loss) caused by interactions between the beam and sample atoms. Commonly used in conjunction with transmission electron microscopy (TEM), EELS can often reveal the chemical identity of atoms in the nanometer-sized area probed by the TEM beam.

A standard way to boost the spatial resolution of both methods is to increase the beam energy (up to about 400 keV), which narrows the electron beam toward atomic dimensions. But therein lies a trade-off: Raising the acceleration voltage focuses the beam but typically destroys sample structures. Lowering the beam energy spares the specimen but destroys the focus. Both problems dash chances for single-atom analysis.

To sidestep those problems, Kazu Suenaga and Yuta Sato of the National Institute of Advanced Industrial Science & Technology, Tsukuba, and coworkers modified their microscope with special electron focusers known as aberration correctors and then tuned the TEM beam energy to just 60 keV, an uncommonly low magnitude. With that setup, the group probed carbon nanotubes loaded with a few fullerene cage molecules that had each been doped with one atom of a foreign element such as calcium or cerium. The team reports that the method revealed the identity and positions of the individual foreign atoms within the nanotubes and differentiated between Ce³⁺ and Ce⁴⁺.

“This is an important advance for imaging the chemical state of dopant atoms in fullerene and other carbon materials such as graphene,” says David A. Muller, a professor and TEM-EELS expert at Cornell University. As beam correctors improve and even lower beam voltages are used, he adds, it may be possible to extend this approach to more weakly bonded molecular crystals.