Fig. 6

APT analysis and SIMS comparison. a APT analysis of the three films as seen from different orthogonal views. Cu isosurfaces (blue) at 19 at.% ([Cu] = 19 at.%) for untreated and Se-only and at 15 at.% ([Cu]= 15 at.%) overlayed with Na distribution (green) for Se+Na₂Se. The composition of the regions labelled with roman numerals is shown in the table. The two regions enclosed by the red circles are discussed in the main text. b Scaled ⁶³Cu (blue) and ²³Na (green, Se+Na₂Se) SIMS signal depth profiles and APT maps and STEM cross section of Se+Na₂Se plotted on the same scale and position for comparison. c [²³Na]/([⁶³Cu] + [²³Na]) SIMS signal depth profile ratio (black) and corresponding [Na]/([Cu] + [Na]) atomic depth profile ratio obtained from APT (purple) of Se+Na₂Se. d Averaged APT elemental quantification of different regions of the Se+Na₂Se tip identified in a from the surface (I) towards the back of the film (VI)