A sum rule for inelastic electron tunneling spectroscopy: an ab initio study of a donor (TTF) and acceptors (TCNE, TCNQ and DCNQI) parallelly oriented on Cu(100)

Abstract

We performed first-principles simulations of Inelastic Electron Tunneling Spectroscopy (IETS) for horizontally lying individual molecules that form popular donor–acceptor pairs (the TTF donor and its possible partner acceptors TCNE, TCNQ and DCNQI) on Cu(100). We find that the highest frequency C–H stretching modes are highly active for the (electron-rich) donor molecule but inactive for the (electron-poor) acceptors. We explain this contrasting response by the spatial extension of sp³ rehybridization upon adsorption: the donor molecule entirely deforms into sp³ while the acceptors rehybridize only at their outer ends leaving the central spacer unaffected. The sp³-induced buckling permits in-plane vibration modes to overlap with the π-type tunneling states and hence to be detected in IETS. In addition, the IET-spectra of a family of cyano-group acceptors, TCNE, TCNQ and DCNQI, show a recurring pattern of signals from vibrations involving their common CN outer ends plus a set of compound-dependent signals arising from the spacing moiety. The IET-response of individual chemical groups thus adds up for these flat-lying acceptor molecules, evidencing a sum rule that may facilitate their identification.