Sensitizers in inelastic electron tunneling spectroscopy: a first-principles study of functional aromatics on Cu(111).

Low sensitivity is a key problem in inelastic electron tunneling spectroscopy (IETS) with the scanning tunneling microscope. Using first-principles simulations, we predict different means to tune the IETS sensitivity of symmetrical functional aromatics on a Cu(111) surface. We show how the IET-spectra of phenyl-NO₂ compounds can be greatly enhanced as compared to pristine phenyl. More precisely, the NO₂ substituent qualifies as a sensitizer of low-frequency wagging modes, but also as a quencher of high-frequency stretching modes. At variance, the CO₂ substituent is found to suppress the whole IET-activity. The head-up (non-anchoring) and head-down (anchoring) configurations of the functional group lead to minor changes in the signals, nevertheless allowing access to discriminate configurational features. It is shown how to disentangle the electronic and steric effects of the substituent in the STM junction.