Inelastic Electron Tunneling Spectroscopy at High-Temperatures.

Ion conducting materials are critical components of batteries, fuel cells, and devices such as memristive switches. Analytical tools are therefore sought that allow the behavior of ions in solids to be monitored and analyzed with high spatial resolution and in real time. In principle, inelastic tunneling spectroscopy offers these capabilities. However, as its spectral resolution is limited by thermal softening of the Fermi-Dirac distribution, tunneling spectroscopy is usually constrained to cryogenic temperatures. This constraint would seem to render tunneling spectroscopy useless for studying ions in motion. Here, the first inelastic tunneling spectroscopy studies above room temperature are reported. For these measurements, high-temperature-stable tunnel junctions that incorporate within the tunnel barrier ultrathin layers for efficient proton conduction are developed. By analyzing the vibrational modes of OH bonds in BaZrO3 -based heterostructures, the detection of protons with a spectral resolution of 20 meV at 400 K (full-width-at-half maximum) is demonstrated. Overturning the hitherto existing prediction for the spectral resolution limit of 186 meV (5.4 kB T ) at 400 K, this resolution enables high-temperature tunneling spectroscopy of ion conductors. With these advances, inelastic tunneling spectroscopy constitutes a novel, valuable analytical tool for solid-state ionics.