When excited thermally or by a photon, atoms that make up a solid material vibrate in ways that embody essential information about the material’s composition, structure, and optical and electrical properties. Known as phonons, the characteristic vibrations are typically measured using Raman scattering, IR spectroscopy, and other optical techniques. However, those measurements are diffraction limited, so they cannot detect fine spatial features. Now Fredrik Hage of SuperSTEM, an electron microscopy user facility in the UK, and his colleagues have devised a method to measure phonons with an atomic resolution probe.
The researchers adapted a technique called electron energy-loss spectroscopy or EELS. In EELS, electrons of a known initial energy are fired at a sample to scatter off the material’s atoms and reach a detector, which records the electrons’ energy. The difference between the initial energy and the measured energy reveals the energy lost due to inelastic scattering processes. Because EELS can focus electron beams down to angstrom-scale spots, it already provides atomic resolution. Hage and his colleagues boosted the sensitivity of EELS to extend it to phonon scattering, which leads electrons to lose as little energy as tens of meV. The researchers accomplished that feat with a simple trick: Instead of positioning the detector to look directly down at the sample surface, they rotate it by tens of milliradians. Though tiny, the change in viewing angle significantly reduces the signals arising from elastic scattering and delocalized inelastic contributions, which can overwhelm the smaller signals from more localized inelastic scattering associated with the creation of phonons. The researchers demonstrated their technique on hexagonal boron nitride, an insulator with a layered structure similar to that of graphene. Scanning the electron beam over the sample yielded energy-loss spectra whose peaks, which are associated with exciting phonons, differed depending on whether the beam was over an atom or between atoms. With those spectra, the researchers could build a map of the material’s atomic structure and phonon response.
Hage and colleagues say the ability to resolve atomic-scale variations in the vibrational response is especially important for understanding the influence of defects or interfaces, which often alter only the local behavior. Such localized changes are often not possible to detect using conventional methods. (F. S. Hage et al., Phys. Rev. Lett. 122, 016103, 2019.)
