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Instruments / Technical Area	Publication Details	Published	Actions
I09-Surface and Interface Structural Analysis	Thermodynamic driving forces for substrate atom extraction by adsorption of strong electron acceptor molecules Paul Ryan , Philip James Blowey , Billal S. Sohail , Luke A. Rochford , David A. Duncan , Tien-Lin Lee , Peter Starrs , Giovanni Costantini , Reinhard J. Maurer , David Phillip Woodruff The Journal Of Physical Chemistry C DOI: 10.1021/acs.jpcc.2c00711 Diamond Proposal Number(s): [17261, 20785] Show Abstract ▼ 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 end groups of the molecule being twisted relative to the planar quinoid ring. Density functional theory (DFT) calculations show that this behavior 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.	Mar 2022
I11-High Resolution Powder Diffraction	Donor–acceptor co-adsorption ratio controls the structure and electronic properties of two-dimensional alkali–organic networks on Ag(100) B. Sohail , P. J. Blowey , L. A. Rochford , P. T. P. Ryan , D. A. Duncan , T.-L. Lee , P. Starrs , G. Costantini , D. Phil Woodruff , R. J. Maurer The Journal Of Physical Chemistry C DOI: 10.1021/acs.jpcc.2c08688 Diamond Proposal Number(s): [17261, 20785] Open Access Show Abstract ▼ Abstract: The results are presented of a detailed combined experimental and theoretical investigation of the influence of coadsorbed electron-donating alkali atoms and the prototypical electron acceptor molecule 7,7,8,8-tetracyanoquinodimethane (TCNQ) on the Ag(100) surface. Several coadsorption phases were characterized by scanning tunneling microscopy, low-energy electron diffraction, and soft X-ray photoelectron spectroscopy. Quantitative structural data were obtained using normal-incidence X-ray standing wave (NIXSW) measurements and compared with the results of density functional theory (DFT) calculations using several different methods of dispersion correction. Generally, good agreement between theory and experiment was achieved for the quantitative structures, albeit with the prediction of the alkali atom heights being challenging for some methods. The adsorption structures depend sensitively on the interplay of molecule–metal charge transfer and long-range dispersion forces, which are controlled by the composition ratio between alkali atoms and TCNQ. The large difference in atomic size between K and Cs has negligible effects on stability, whereas increasing the ratio of K/TCNQ from 1:4 to 1:1 leads to a weakening of molecule–metal interaction strength in favor of stronger ionic bonds within the two-dimensional alkali–organic network. A strong dependence of the work function on the alkali donor–TCNQ acceptor coadsorption ratio is predicted.	Jan 2023

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