Abstract
The archetypal electron acceptor molecule, TCNQ, is generally believed to become bent into an inverted bowl shape upon adsorption on the coinage metal surfaces on which it becomes negatively charged. New quantitative experimental structural measurements show that this is not the case for TCNQ on Ag(111). DFT calculations show that the inclusion of dispersion force corrections reduces not only the molecule-substrate layer spacing but also the degree of predicted molecular bonding. However, complete agreement between experimentally-determined and theoretically-predicted structural parameters is only achieved with the inclusion of Ag adatoms into the molecular layer, which is also the energetically favoured configuration. The results highlight the need for both experimental and theoretical quantitative structural methods to reliably understand similar metal-organic interfaces and highlight the need to re-evaluate some previously-investigated systems.
Highlights
It is well-established that molecular adsorption on metal surfaces can lead to significant alterations in the electronic, chemical, and geometrical structure of both the adsorbed molecules and the underlying surface
We illustrate the limitations of this approach for a system in which we find that only a combined experimental and theoretical investigation methodology that includes quantitative structural measurements is capable of solving the complexity of metal–organic interfaces involving π-bonded molecules. We apply this methodology to an archetypal molecular adsorbate system, namely 7,7,8,8-tetracyanoquinodimethane (TCNQ) on a coinage metal surface, and show that the molecular conformation is significantly different from that accepted as conventional wisdom in the literature. This has arisen in part because of earlier failures to account for dispersion forces in density functional theory (DFT) calculations but, more significantly, because quantitative experimental structural data highlight the need to account for adsorbate-induced substrate reconstruction in the calculations
Experimental characterisation of the single-layer TCNQ adsorption phases formed by vacuum deposition of the molecule onto a clean Ag(111) surface at room temperature was undertaken by scanning tunnelling microscopy (STM) and low-current low energy electron diffraction (LEED) in an ultra-high vacuum (UHV) chamber at the University of Warwick, and by low-current LEED, soft X-ray photoelectron spectroscopy (SXPS) and normal incident X-ray standing wavefield (NIXSW) in the UHV end-station installed on beamline I09 of the Diamond Light Source storage ring
Summary
We illustrate the limitations of this approach for a system in which we find that only a combined experimental and theoretical investigation methodology that includes quantitative structural measurements is capable of solving the complexity of metal–organic interfaces involving π-bonded molecules We apply this methodology to an archetypal molecular adsorbate system, namely 7,7,8,8-tetracyanoquinodimethane (TCNQ) on a coinage metal surface, and show that the molecular conformation is significantly different from that accepted as conventional wisdom in the literature. The theoretical analysis demonstrates that several of these models are almost degenerate in energy, so their coexistence should be considered in order to interpret the NIXSW measurements correctly These results highlight the need for both quantitative experimental structural information and DFT calculations to establish the true molecular structure and demonstrate the need for careful interpretation of NIXSW data
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