We chose to polish the sample without using any liquid, which led to a better control of the angle between the polishing plate and the sample but also increased the chance of crosscontamination and of reaching locally high temperatures that can alter the composition of the sample. For minimizing contamination, the sample is polished on the same area of the polishing paper for a limited time. No oscillation was applied to not alter the targeted geometry. Indeed, oscillation would have varied the angle between the normal axis of the sample and the tangent of the polishing paper. It can, however, be observed by eye that some of the removed materials accumulate on the sample and might alter the results of the XPS line scan measurements (see Figure ).