Inelastic electron tunneling spectroscopy: Intensity as a function of surface coverage

Inelastic election tunneling spectroscopy (IETS) is a sensitive technique for obtaining vibrational spectra of molecules adsorbed on an oxide surface and incorporated into a metal-oxide-metal tunnel junction. IETS energy data are used routinely. However, IETS intensities contain additional information which, for theoretical and experimental reasons, has not been used systematically. This paper examines the variation of IETS intensity with surface coverage of dopant molecules in the junction, a relationship of practical and theoretical importance. IET spectra are taken using standard experimental techniques and a liquid doping technique which allows the surface coverage to be determined independently. From an analysis of a large number of modes of benzoic acid on alumina, it is found that IETS intensity, defined in the usual way as the normalized change in conductance, $\text{Δ σ}\text{σ}$, is a nonlinear function of surface coverage. A physical model is presented which attributes this behavior to a difference in elastic tunneling conductances through empty or filled regions of the dopant layer in a junction with a fraction of a monolayer coverage. In addition, the liquid and vapor doping techniques in common use in IETS are discussed in terms of statistical mechanics and are shown to be manifestations of the same basic phenomenon.