Constant height scanning tunneling spectroscopy using an alternating voltage signal

A method of performing scanning tunneling spectroscopy (STS) at a constant height while maintaining feedback control is described. By using sinusoidal voltage oscillations the ac tunneling current can be separated from the di splacement current present in current-voltage (i-V) measurements. The metho d allows statistically well averaged spectra spanning both occupied and uno ccupied states of the sample to be obtained from a continuous set of i-V cu rves. The unoccupied pi* state on graphite is observed using the technique described, and the utility of the technique is further demonstrated by the detection of several bulk states on NiTe2 and PdTe2, two transition metal d ichalcogenides. A weighting towards k perpendicular states is seen in the S TS spectra. These results show that the fast alternating current form of ST S is suitable for probing low amplitude electron states on metal surfaces w ithin 2 eV of the Fermi level. (C) 1999 American Institute of Physics. [S00 21-8979(99)02608-0].