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

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.

Direct Experimental Evidence for Substrate Adatom Incorporation into a Molecular Overlayer

“…The results reveal a strong energetic advantage for the adatom model with an adsorption energy per unit surface area of 3.35 eV/nm 2 relative to a value of 2.57 eV/nm 2 in the absence of the adatom. This situation is similar to that of F 4 TCNQ on Ag(100) for which we showed that the strong intralayer bonding in the 2D-MOF that is formed is the origin of the preference for the adatom model. The results show the optimum lateral registry of the overlayer to the underlying Au(111) surface to correspond to the Au adatoms occupying local atop sites; shifting this registry to a hollow site reduced the adsorption energy by 0.43 eV/nm 2 , while starting in a bridge site, the calculation converged on the hollow site geometry.…”

“…The low coherent fraction of the F atoms also indicates the co-occupation of multiple heights. Higher-resolution XP F 1s spectra recorded from F 4 TCNQ adsorbed on Ag(100) show the presence of a second component, weakly resolved at the higher energies of the NIXSW measurements, that appears to be related to radiation damage. This suggests that on both of these surfaces the F NIXSW results may be influenced by the presence of a coadsorbed atomic F species.…”

Donor–Acceptor Co-Adsorption Ratio Controls the Structure and Electronic Properties of Two-Dimensional Alkali–Organic Networks on Ag(100)

“…STM images, recorded in constant current mode using electrochemically etched polycrystalline tungsten tips, were plane corrected and flattened using the open-source image-processing software Gwyddion . To form the alkali-TCNQ coadsorption phases, TCNQ was first deposited on a clean Ag(100) surface to form the commensurate ( 1 4 − 3 − 1 ) phase using the preparation method described previously . The alkali atoms were then deposited to increasing coverages onto the sample at room temperature.…”

“… 52 To form the alkali-TCNQ coadsorption phases, TCNQ was first deposited on a clean Ag(100) surface to form the commensurate phase using the preparation method described previously. 41 The alkali atoms were then deposited to increasing coverages onto the sample at room temperature. The highest K coverage ordered phase required annealing to ∼300 °C to produce a well-ordered surface (see Table S1 ).…”

“…A particularly relevant molecule in this context is 7,7,8,8-tetracyanoquinodimethane (TCNQ), which is a prototypical electron acceptor that can be used as an additive to interfaces to lower the energy barrier for electron transfer from the metal into the semiconductor. TCNQ is also capable of forming highly conductive charge transfer salts in combination with suitable electron donor molecules. − Moreover, TCNQ adsorbed on Ag surfaces has been shown to form a diverse range of phases on the (111) , and (100) terminations, some of which have been found to incorporate Ag metal adatoms into the adsorbate network, inducing the formation of a two-dimensional metal–organic framework (2D-MOF). In particular, when co-deposited with alkali metals, TCNQ forms a number of different two-dimensional networks as a function of overall coverage and alkali/TCNQ ratio, representing an ideal system to study the controllable synthetic parameters for 2D-MOF formation.…”