Figure 2. Retention test of the accumulated charges implying ionic nature.

Topographic and EFM results 0 min (a–c), 40 min (d–f), 60 min (g–i) and 75 min (j–l) after removing the electrical stimuli, showing long retention of the charges. Scale bar, 3 μm. This suggests that the charges observed in the 1ω component of the electrostatic force gradient should correspond to oxygen ions, instead of electrons.