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

TL;DR

This study investigates whether substrate adatoms are incorporated into organic monolayers formed by strong electron acceptors on metal surfaces, revealing that thermodynamic and kinetic factors influence adatom inclusion, with detailed experimental and theoretical analysis.

Contribution

It provides the first detailed experimental and theoretical analysis of substrate adatom incorporation in organic monolayers formed by strong electron acceptors.

Findings

F4TCNQ on Ag(100) incorporates Ag adatoms in the monolayer.

TCNQ forms an adatom-free inverted bowl configuration.

Annealing induces incommensurate phases with adatom inclusion.

Abstract

A quantitative structural investigation is reported, aimed at resolving the issue of whether substrate adatoms are incorporated into the monolayers formed by strong molecular electron acceptors deposited onto metallic electrodes. A combination of normal-incidence X-ray standing waves, low energy electron diffraction, scanning tunnelling microscopy and X-ray photoelectron spectroscopy measurements demonstrate that the systems TCNQ and F4TCNQ on Ag(100) lie at the boundary between these two possibilities and thus represent ideal model systems with which to study this effect. A room-temperature commensurate phase of adsorbed TCNQ is found not to involve Ag adatoms, but to adopt an inverted bowl configuration, long predicted but not previously identified experimentally. By contrast, a similar phase of adsorbed F4TCNQ does lead to Ag adatom incorporation in the overlayer, the cyano endgroups of the molecule being twisted relative to the planar quinoid ring. Density functional theory (DFT) calculations show that this behaviour is consistent with the adsorption energetics. Annealing of the commensurate TCNQ overlayer phase leads to an incommensurate phase that does appear to incorporate Ag adatoms. Our results indicate that the inclusion (or exclusion) of metal atoms into the organic monolayers is the result of both thermodynamic and kinetic factors.