Study of metallic adhesion using scanning tunneling microscopy

Scanning tunneling microscopy provides an ideal tool for studying interaction forces, in particular metallic adhesion, at distances of the order of a few Å, a range that has not been accessible by other experimental techniques. The tip of the tunnel microscope serves both for tunneling and as a force probe, thus enabling direct correlation of tunneling and force data. Experiments were conducted in uhv using Ir tips and polycrystalline Ir, Au and Al samples. The interaction force gradient and tunnel conductivity were measured as a function of tunnel gap width. The observed characteristics are significantly different for the three metals. In the case of Ir, tunneling and force data are consistent with adhesion theory. Interpretation of the results obtained for Au and Al is more difficult since positive force gradients were observed in the tunneling regime. This indicates that the foremost atoms at the tip apex experienced a repulsive force long before electrical contact is established. © 1990 Pergamon Press plc.