Cross-sectional high-resolution transmission electron microscopy of the microstructure of electrochromic nickel oxide

Electrochromic nickel oxide films on indium tin oxide (ITO) were investigated by cross-sectional high-resolution transmission electron microscopy and energy dispersive X-ray analysis. Microstructural features and the differences between high and poor-quality samples were studied and compared. The dominant phase of the NiO film has cubic structure, but selected area electron diffraction patterns revealed many extra diffraction spots for the high-performance samples which may result from additional hydrated nickel oxide phases. The NiO grains do not show clear shapes in the cross-sectional plane nor are there signs of a preferred orientation. The mean grain size is larger and there are more defects and superlattices in samples with good electrochromic properties. There was a clear correlation between grain size distribution and performance. The high-quality samples had a mean grain size of about 6.5 nm, whereas the poor-quality samples exhibited significantly smaller mean grain sizes of 5.0 and 3.8 nm.