The motion and storage of ions provide key functionalities in solid-state devices. Analytical tools are therefore sought that allow the behavior of ions in solids, in particular along interfaces, to be monitored and analyzed with high spatial resolution and in real time. In principle, inelastic electron tunneling spectroscopy (IETS) offers these capabilities [1-4].
However, while the ion dynamics well above room temperature is of key interest for solid-state ionics, IETS has been mostly limited to the liquid helium temperatures due to the thermal smearing of the electron distribution in metal contacts at high-temperatures .
As reported in Advanced Materials 2007299 (2021), we have broken this limit arising from the thermal softening of the tunnel electrodes' Fermi–Dirac distributions. This breakthrough now allows for the use of IETS for high-resolution analysis of ion populations well above room temperature.
To experimentally investigate ionic diffusion along interfaces  and explore the potential of IETS to study ionic species at high-temperatures, we have used the same BaZrO3-based heterostructures as proton conductors and electron tunnel barriers (Figure 1).
By analyzing O–H bond vibrations, the existence of protons in the tunnel barriers is confirmed (Figure 2). The tunnel junctions yield high-resolution IETS spectra of diffused protons along interfaces in BaZrO3–BaYOx-based tunnel barriers up to at least 400 K (Figure 3). We demonstrate IETS spectral resolution that is a factor of nine better than the established theoretical limit. With these advances, IETS constitutes a viable high-resolution analytical tool for further nanoionic and nanoelectronic studies at high-temperatures in nanostructured tunnel junctions (Figure 4).