MATERIAL TRANSFER BETWEEN METALLIC TIPS AND SURFACE IN THE STM

Recently it has been demonstrated that metallic tips can be used to reliably deposit nanometer-scale hillocks onto metal surfaces. Field evaporation of the tip during the voltage pulse has been suggested as the mechanism for feature formation. We have studied the deposition with W, Pt-Ir, Mo and Ni tips onto gold substrates. In these systems we have observed that the threshold pulse amplitude has a linear dependence on the logarithm of the gap resistance, in agreement with the field-evaporation mechanism. However, the threshold pulse amplitude was essentially independent of the tip material, in disagreement with the field-evaporation mechanism. The threshold for field evaporation for W and Ni is different by more than a factor of two. We will also present tip position versus time data that clearly show the transfer of material between the tip and the substrate. We are proposing that large electrostatic forces between the tip and substrate may be responsible for feature formation.