The electrostatic contribution to the long-range interactions between tungsten and oxide surfaces under ultrahigh vacuum

The force gradient between a tungsten tip and a flat oxide substrate has been measured as a function of tip-to-surface distance, D, using an oscillating cantilever technique under Ultrahigh Vacuum. We analyze the relative contribution of van der Waals and electrostatic forces between a W tip and SiO2, TiO2and MgO substrates. The observed Hamaker constants are in agreement with the Lifshitz theory, while we tentatively correlate the surface charge densities with the gap of the oxides. q1999 Elsevier Science B.V. All rights reserved.