Elsevier

Surface Science

Volume 701, November 2020, 121687
Surface Science

The structure of 2D charge transfer salts formed by TCNQ/alkali metal coadsorption on Ag(111)

https://doi.org/10.1016/j.susc.2020.121687Get rights and content

Highlights

  • Coadsorption phases of TCNQ with K and Cs studied by STM, LEED, NIXSW.

  • KTCNQ and CsTCNQ phases both incommensurate and have very similar structures.

  • K2TCNQ is commensurate, Cs2TCNQ is incommensurate, but structures very similar.

  • Role of commensuration and relative heights of alkalis and TCNQ in all phases reported and discussed.

Abstract

The structure of coadsorption phases formed on Ag(111) by TCNQ (7,7,8,8-tetracyanoquinodimethane) with Cs are compared with previously reported coadsorption phases formed with K, following investigation by scanning tunnelling microscopy (STM), low energy electron diffraction, soft X-ray photoelectrons spectroscopy and normal incidence X-ray standing waves (NIXSW). For each alkali we identify two ordered phases, one with an alkali: TCNQ stoichiometry of 1:1 and the other 2:1. STM images show the molecular organisation is the same for Cs and K, although only the K2TCNQ phase is commensurate with the substrate. A previously-published detailed structure determination of the K2TCNQ phase, complemented by density function theory calculations that identify bonding strengths, showed that the binding within the layer is much stronger than that of the layer to the substrate. Insensitivity to commensuration is thus to be expected. The situation for KTCNQ and CsTCNQ is less clear; these ordered incommensurate overlayers clearly have strong intralayer bonding, but the relative strength of the average overlayer-substrate bonding is unknown. NIXSW data show that the alkalis in these phases occupy adsorption sites far more distant from the substrate than the TCNQ molecules when compared to the near coplanar alkali-TCNQ geometry of K2TCNQ and Cs2TCNQ. Ultraviolet photoelectron spectra show increasing bonding shifts of TCNQ orbital states with alkali coverage.

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 (4.284.933.222.09) 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 (3015) 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).

References (31)

  • I.G. Torrente et al.

    Int. J. Mass. Spectrom.

    (2008)
  • M.M. Kamna et al.

    Surf. Sci.

    (1998)
  • M.B. Trzhaskovskaya et al.

    At. Data Nucl. Data Tables

    (2001)
  • Y. Geng et al.

    Coord. Chem. Rev.

    (2017)
  • Y. Wang et al.

    Chem. Commun.

    (2012)
  • D. Stradi et al.

    RSC Adv.

    (2016)
  • L. Romaner et al.

    Phys. Rev. Lett.

    (2007)
  • T.-C. Tseng et al.

    Nat. Chem.

    (2010)
  • S. Barja et al.

    J. Phys. Condens. Matter.

    (2013)
  • J.I. Martínez et al.

    Phys. Status Solidi B

    (2011)
  • M.N. Faraggi et al.

    J. Phys. Chem. C

    (2012)
  • A. Della Pia et al.

    Nanoscale

    (2016)
  • P.J. Blowey et al.

    Nanoscale

    (2018)
  • C. Wäckerlin et al.

    Chem. Commun.

    (2011)
  • H. Yamane et al.

    J. Pnys. Chem. Lett.

    (2017)
  • View full text