Donor-acceptor co-adsorption ratio controls structure and electronic properties of two-dimensional alkali-organic networks on Ag(100)

TL;DR

This study combines experimental and theoretical methods to explore how the ratio of alkali donors to TCNQ acceptors influences the structure and electronic properties of 2D alkali-organic networks on Ag(100), revealing charge transfer and dispersion effects.

Contribution

It provides a detailed analysis of how coadsorption ratios control the structure and electronic properties of alkali-organic networks, integrating experimental data with DFT calculations.

Findings

Coadsorption ratio significantly affects network structure.

Work function varies strongly with coadsorption ratio.

Density functional theory aligns well with experimental structural data.

Abstract

The results are presented of a detailed combined experimental and theoretical investigation of the influence of coadsorbed electron-donating alkali atoms and the prototypical electron acceptor molecule TCNQ (7,7,8,8-tetracyanoquinodimethane) on the Ag(100) surface. Several coadsorption phases were characterised by scanning tunnelling microscopy, low energy electron diffraction, and soft-X-ray photoelectron spectroscopy. Quantitative structural data were obtained using normal incidence X-ray standing wave (NIXSW) measurements and compared with the results of density functional theory (DFT) calculations using several different methods of dispersion correction. Generally good agreement between theory and experiment was achieved for the quantitative structures, albeit with prediction of the alkali atom heights being challenging for some methods. The adsorption structures depend sensitively on the interplay of molecule-metal charge transfer and long-range dispersion forces, which are controlled by the composition ratio between alkali atoms and TCNQ. The large difference in atomic size between K and Cs has negligible effects on stability, whereas increasing the ratio of K:TCNQ from 1:4 to 1:1 leads to a weakening of molecule-metal interaction strength in favour of stronger ionic bonds within the two-dimensional alkali-organic network. A strong dependence of the work function on the alkali donor-TCNQ acceptor co-adsorption ratio is predicted.