Thermodynamic Driving Forces for Substrate Atom Extraction by Adsorption of Strong Electron Acceptor Molecules

A quantitative structural investigation is reported, aimed atresolving the issue of whether substrate adatoms are incorporated into themonolayers formed by strong molecular electron acceptors deposited ontometallic electrodes. A combination of normal-incidence X-ray standingwaves, low-energy electron diffraction, scanning tunnelling microscopy, andX-ray photoelectron spectroscopy measurements demonstrate that thesystems TCNQ and F4TCNQ on Ag(100) lie at the boundary betweenthese two possibilities and thus represent ideal model systems with whichto study this effect. A room-temperature commensurate phase of adsorbedTCNQ is found not to involve Ag adatoms, but to adopt an inverted bowlconfiguration, long predicted but not previously identified experimentally.By contrast, a similar phase of adsorbed F4TCNQ does lead to Ag adatomincorporation in the overlayer, the cyano end groups of the molecule beingtwisted relative to the planar quinoid ring. Density functional theory(DFT) calculations show that this behavior is consistent with theadsorption energetics. Annealing of the commensurate TCNQ overlayerphase leads to an incommensurate phase that does appear to incorporateAg adatoms. Our results indicate that the inclusion (or exclusion) of metal atoms into the organic monolayers is the result of boththermodynamic and kinetic factors.