Dependence of the tip–surface interaction on the surface electronic structure

We compare the results of ab initio calculations of the interaction of Si tips with the surfaces of CaCO3, MgO, and CaF2. The calculations were performed using the density functional theory and the SIESTA code. We used a conventional Si tip model with a dangling bond at the apex. The results demonstrate a considerable electron density redistribution between the tip and surface, which depends on the energy offset of the Si states with respect to the occupied and empty states of the insulator. The tip–surface interaction has two main components. One is due to the polarisation of the neutral Si tip by the surface electric field. The stronger electrostatic component originates from the electron redistribution effect mentioned above. As a result the strength of the tip–surface interaction is comparable to that for ionic tips (e.g. modelled by the MgO cube).