**5.3. Transmission electron microscope**

High-resolution transmission electron microscopy (HR-TEM) has improved microscopy resolution, more developed techniques, and coupling with advanced approaches will enable the understanding and engineering of grain boundaries (including twins) and intergranular films. For instance, scanning transmission electron microscopy (STEM) coupled with electron energy loss spectroscopy (EELS) is capable of simultaneously mapping the atomic/electronic structure of light elements such as oxygen at adequate spatial resolution. The electronic state of the elements across the boundary can be identified by STEM-EELS line scan crossing the grain boundary in steps of a few nanometres [32]. TEM imaging further resolves details of the crystal structure of grains and grain boundaries.
