The structure of 2D charge transfer salts formed by TCNQ/alkali metal coadsorption on Ag(111)
Graphical abstract
Introduction
The important role of metal-organic interfaces in determining the electronic properties of devices based on organic semiconductors has led to a number of surface science studies of related model systems. One molecule of particular interest is 7,7,8,8-tetracyanoquinodimethane (TCNQ), which is a strong electron acceptor capable of forming highly conducting charge transfer salts in combination with suitable electron donor molecules. As such there have been a number of studies of TCNQ adsorption on coinage metal (111) surfaces. (e.g. [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]). Coadsorption of TCNQ with alkali metals offers a way of producing 2D salt layers, as well as a way of modifying the surface work function and thus the electronic energy alignment at the metal-organic interface [13]. Knowledge of the structure of these salts is a prerequisite to understanding their properties. To explore this idea we report here on the full characterisation of the structures formed by coadsorption of TCNQ with Cs and with K on the Ag(111) surface, including quantitative structural data on the adsorption heights using the technique of normal incidence X-ray standing waves (NIXSW) [14]. The results of our investigation of TCNQ/K coadsorption phases on this surface, identifying two distinct phases of different TCNQ/K stoichiometry, have already been reported in detail [15], [16], [17]; these include a particularly complete study of a phase of K2TCNQ stoichiometry, which is commensurate with the substrate and therefore accessible to density functional theory (DFT) calculations. These calculations, aided by the benchmark NIXSW data, gave insight into the nature of the bonding within the overlayer and to the underlying Ag(111) [16]. For coadsorption of Cs with TCNQ we have identified no commensurate coadsorption phases accessible to DFT slab calculations, but find two ordered incommensurate Cs/TCNQ phases that appear to have similar organisation of the coadsorbed species to the two K/TCNQ phases. This provides an interesting base for comparison of the effect of the different alkali metal ion on the unit mesh dimensions and the height of the molecules above the surface, as determined by NIXSW. The fact that similar alkali/TCNQ layers may readily form without substrate commensurability, apparently insensitive to the lateral corrugation of the overlayer-substrate potential, might be explained by the fact that all of these layers are actually 2D charge transfer salts with strong bonding within the layer and weaker bonding to the underlying surface. We recently demonstrated this very precisely in the specific case of the commensurate Ag(111)-K2TCNQ system [16]. Here we present the experimental characterisation of all four phases, providing the basis for a possible generalisation of this phenomenon.
Section snippets
Experimental details
Identification and characterisation of the adsorption phases was conducted in a UHV chamber at the University of Warwick equipped with facilities for both room temperature scanning tunnelling microscopy (STM), low-current (and hence low radiation damage) microchannel plate low energy electron diffraction (MCP-LEED) and ultraviolet photoelectron spectroscopy (UPS). Further characterisation and quantitative structural data were obtained using synchrotron radiation X-ray photoelectron spectroscopy
Characterisation of coadsorption phases
Details of the identification and properties of K/TCNQ coadsorption phases have been published elsewhere. Briefly, an incommensurate phase was identified by LEED and STM and details of this phase and the associated NIXSW data were reported [15], as have similar data and a complete analysis of the structure and electronic properties of a commensurate phase, aided by dispersion-corrected density function theory (DFT) calculations [16]. The stoichiometries of these two
Conclusions
A comparison of our (previously published) structural characterisation of the coadsorption phases of TCNQ and K on Ag(111) with new results for the coadsorption phases of TCNQ and Cs on this surface shows very strong similarities indicating that the detailed structures and bonding character are largely independent of which alkali is incorporated. Of particular note is that this is true for both the K2TCNQ and Cs2TCNQ phases, which are, respectively, commensurate and incommensurate. This
CRediT authorship contribution statement
P.J. Blowey: Conceptualization, Data curation, Formal analysis, Investigation, Writing - original draft, Writing - review & editing. L.A. Rochford: Investigation, Writing - review & editing. D.A. Duncan: Investigation, Writing - review & editing. P.T.P. Ryan: Investigation, Writing - review & editing. D.A. Warr: Investigation, Writing - review & editing. T.-L. Lee: Investigation, Writing - review & editing. G. Costantini: Conceptualization, Writing - review & editing. D.P. Woodruff:
Declaration of Competing Interest
None
Acknowledgements
The authors thank Diamond Light Source for allocations SI15899 and NT18191 of beam time at beamline I09 that contributed to the results presented here. P.J.B. acknowledges financial support from Diamond Light Source and EPSRC. G.C. acknowledges financial support from the EU through the ERC Grant “VISUAL-MS” (Project ID: 308115).
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