Fig. 1. Representation of materials and methods used in this study.

Schematic crystal structures for precursor phase and reduced a n = ∞ and b n = 5 layered square-planar nickelate compounds. c Schematic of the ToF-SIMS measurement technique. d SIMS spectra measured separately for positive and negative ions are analyzed by identifying peaks by mass-to-charge ratio (m/z) and extracting integrated area.