Signature of tip electronic states on tunneling spectra

The influence of STM tip electronic states on the electron transport through an atomic object on a surface is studied both experimentally and theoretically. We present scanning tunnelling spectroscopy (STS) experimental results on Ag islands with two, blunt and sharp, STM tips. The data taken with the sharp tip have an additional peak at positive bias which corresponds to the tip apex atom state. We show that sudden tip sharpness variation and corresponding I(V) characteristic change may help to differentiate between electronic states of the tip and the sample. The experimental data are discussed and compared with theoretical calculations performed for two different tips. The current and differential conductance calculations are carried out by means of the Green's function technique and a tight-binding Hamiltonian.