**Acknowledgements**

The authors would like to acknowledge R. Zazpe and L. Hueso for providing the samples that motivated this chapter and M.J. Sánchez for fruitful discussions on the impedance measurements. Special thanks has to be given to H. Zhang, who performed AFM measurements, and A. Savenko, who was in charge of in-depth SIMS measurements.

C.P. Quinteros also wants to acknowledge DAAD and Argentinian Education Ministry for funding her stay at the Forschungszentrum Jülich.
