Alkali Doping Leads to Charge-Transfer Salt Formation in a Two-Dimensional Metal–Organic Framework

Abstract

Efficient charge transfer across metal–organic interfaces is a key physical process in modern organic electronics devices, and characterization of the energy level alignment at the interface is crucial to enable a rational device design. We show that the insertion of alkali atoms can significantly change the structure and electronic properties of a metal–organic interface. Coadsorption of tetracyanoquinodimethane (TCNQ) and potassium on a Ag(111) surface leads to the formation of a two-dimensional charge transfer salt, with properties quite different from those of the two-dimensional Ag adatom TCNQ metal–organic framework formed in the absence of K doping. We establish a highly accurate structural model by combination of quantitative X-ray standing wave measurements, scanning tunnelling microscopy, and density-functional theory (DFT) calculations. Full agreement between the experimental data and the computational prediction of the structure is only achieved by inclusion of a charge-transfer-scaled dispersion correction in the DFT, which correctly accounts for the effects of strong charge transfer on the atomic polarizability of potassium. The commensurate surface layer formed by TCNQ and K is dominated by strong charge transfer and ionic bonding and is accompanied by a structural and electronic decoupling from the underlying metal substrate. The consequence is a significant change in energy level alignment and work function compared to TCNQ on Ag(111). Possible implications of charge-transfer salt formation at metal–organic interfaces for organic thin-film devices are discussed.

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

“…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 generality of this phenomenon.…”

“…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.…”

“…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 Xray 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].…”

“…phase, aided by dispersion-corrected density function theory (DFT) calculations [16]. The stoichiometries of these two phases have been identified on the basis of SXP spectra as KTCNQ and K2TCNQ, respectively.…”

“…The arrangement of the molecules differs in the K2TCNQ phase (Fig. 1b), which we refer to as the 'head-to-tail' phase, for which a full quantitative structure determination has been reported [16]. .…”