An experimental investigation of resonance curves on metallic surfaces in dynamic force microscopy: the influence of frozen versus mobile charges

Amplitude resonance curves of a driven dynamic force microscope cantilever interacting with a Cu(100) substrate in ultra high vacuum are derived and analysed, extending a previous study on Al2O3(0001) (Polesel-Maris et al 2003 Nanotechnology 14 1036). It is shown that the charges that are trapped on the oxidized n+-doped silicon tip give rise to long-range electrostatic forces that dominate the van der Waals forces, due to the metallic nature of the substrate. These electrostatic forces cannot be compensated by the usual procedure consisting of applying a constant bias voltage between the tip and the sample. Indeed, the trapped charge does not remain constant on the timescale of the experiment due to charge leakage across the tip oxide layer. Removal of this oxide by electron field emission solves this problem and allows the access of a pure van der Waals tip–substrate interaction regime.