Electron microscopy is a workhorse of nanomaterials characterization, famed for its ability to image at the atomic scale. Researchers have now coupled this capability with improved vibrational spectroscopy, which could enable scientists to better understand the nanostructures involved in processes such as catalysis, heat transfer, and solar energy harvesting (Nature 2014, DOI: 10.1038/nature13870).
Scanning tunneling electron microscopes have induced atomic lattice vibrations for decades, but the instruments could not detect low-energy jitters. Scientists had to rely on Raman or infrared spectroscopy to analyze these vibrations and characterize atoms and bonds within a sample.
“If you ask an organic chemist to tell you what’s in some goo you know nothing about, they will take a Raman spectrum,” says Ondrej L. Krivanek, a cofounder of Nion, a firm that develops electron microscopes. “We’re going after a signal that’s been very powerful in other fields but really hasn’t been used in electron microscopy.”
To detect these vibrations with an electron microscope, Krivanek and a team of researchers boosted the sensitivity of a technique known as electron energy-loss spectroscopy. By measuring how much energy electrons lose as they shoot through a sample, researchers can determine how much of that energy goes into exciting vibrations.
Most electrons, however, zip through the sample without shedding any energy. This creates broad and intense peaks in spectra that can bury low-energy vibration signals, Krivanek says. But the new instrument slims the breadth of these peaks by more than a factor of 20. Combined with a high-resolution spectrometer, the microscope can discern vibrational modes that the researchers say were “hidden in plain sight” until now.
“This is the next level,” says Nigel D. Browning of Pacific Northwest National Laboratory, who was not involved with the study. “The ability to look at and characterize single nanoparticles is going to be huge.”
But Krivanek believes his team can still improve the tool’s energetic and spatial resolution to retrieve vibrational spectra from single atoms. “This is just the start of the road,” he says.