Electron Resonances in Sharp Tips and Their Role in Tunneling Spectroscopy

The influence of the shape and electronic structure of the tip in scanning tunneling spectroscopy is studied by a combined theoretical and experimental approach. Tunneling conductance spectra of a Cu(111) surface are used to detect and characterize the electronic states of W tips. The characteristic tunneling conductance of the Cu(111) surface state is the only feature present in spectra recorded with a blunt tip, while spectra obtained with sharp tips display also tip resonances. By means of a self-consistent tight-binding model of realistic tip structures we show that a sharp W tip exhibits, in contrast with blunt tips, narrow resonant states around the Fermi level.