Fig. 4.

Electronic structure evolution of ${\mathrm{N}\mathrm{d}}_{1-x}{\mathrm{S}\mathrm{r}}_x{\mathrm{N}\mathrm{i}\mathrm{O}}_2$ with hole doping $x\le$ 0.225. (A) A spectral feature at $\sim$528 eV emerges with increased Sr (hole) doping in the O-K edge, attributed to $d^9\underset{\_}{L}$ states. The dotted line marks the $d^9$ peak position in ${\mathrm{N}\mathrm{d}\mathrm{N}\mathrm{i}\mathrm{O}}_3$. (B) The integrated signal over the $d^9\underset{\_}{L}$ feature spectral range (527 to 529 eV), highlighted gray in A, increases with hole doping. (C and D) The Ni-${\mathrm{L}}_3$ edge systematically shifts to high energies and broadens with hole doping. The dotted line marks the ${\mathrm{L}}_3$ peak position in ${\mathrm{N}\mathrm{d}\mathrm{N}\mathrm{i}\mathrm{O}}_3$. (E) The Nd-${\mathrm{N}}_{\mathrm{2,3}}$ edge (5$d$ states) shows a small shift to higher energies upon Sr doping of the perovskite phase. A further shift is observed upon O reduction to infinite-layer ${\mathrm{N}\mathrm{d}}_{0.8}{\mathrm{S}\mathrm{r}}_{0.2}{\mathrm{N}\mathrm{i}\mathrm{O}}_2$. The dotted line marks the ${\mathrm{N}}_{\mathrm{2,3}}$ peak position in ${\mathrm{N}\mathrm{d}\mathrm{N}\mathrm{i}\mathrm{O}}_3$. For comparison, reference spectra are plotted in gray for ${\mathrm{N}\mathrm{d}\mathrm{N}\mathrm{i}\mathrm{O}}_2$ in A and C and for ${\mathrm{N}\mathrm{d}\mathrm{N}\mathrm{i}\mathrm{O}}_3$ in E.