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

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.