Reversible short-range electrostatic imaging in frequency modulation atomic force microscopy on metallic surfaces

The mechanism of atomic-scale image formation in non-contact AFM on metallic surfaces is analysed using total-energy pseudopotential calculations. Depending on the tip–apex configuration, we find two different imaging modes. Both clean and metal contaminated Si tips provide atomic resolution arising from the very strong covalent tip–sample interaction, in striking similarity with the imaging mechanism found on semiconductor surfaces. A completely new mechanism, reversible short-range electrostatic imaging, due to subtle charge-transfer interactions is identified for oxidized Si tips. Contrary to the strong covalent-bond imaging, this new mechanism causes only negligible surface perturbation and can account for recent experimental results.