Observation of the effect of tip electronic states on tunnel spectra acquired with the scanning tunneling microscope

We have investigated the effect of electronic states in the tip of a scanning tunneling microscope on measured tunnel-junction spectra by obtaining spectroscopic data with a tunneling tip terminated by two closely spaced microtips. The tip was such that the two microtips were approximately 9 A{} apart, one of which was about 0.5 A{} closer to the surface than the other. When single atomic-size protrusions were imaged, they appeared doubled, and separate spectroscopic data could then be obtained over the same point with each microtip. Differences in the current-voltage characteristics obtained with each microtip suggest that one of the microtips possessed a strong electronic feature at about 1.4 eV above the Fermi energy. The data show the strong effect tip states can have on the measured occupied-state density, while having relatively little effect on the measured unoccupied-state density.