Donor–Acceptor Co-Adsorption Ratio Controls the Structure and Electronic Properties of Two-Dimensional Alkali–Organic Networks on Ag(100)

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 7,7,8,8-tetracyanoquinodimethane (TCNQ) on the Ag(100) surface. Several coadsorption phases were characterized by scanning tunneling 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 the 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 favor of stronger ionic bonds within the two-dimensional alkali–organic network. A strong dependence of the work function on the alkali donor–TCNQ acceptor coadsorption ratio is predicted.

Structure Determination of F4TCNQ on Ag(111): A Systematic Trend in Metal Adatom Incorporation

“…This necessarily means that the C–N, and to a lesser extent the C–C atoms must occupy slightly different adsorption heights, consistent with the slightly reduced f values for these atoms. These results are closely similar to those obtained previously for F 4 TCNQ on Ag(100), 32 Cu(111) 20 and Au(111) 19 and for TCNQ on Ag(111), 18 indicating that the molecule adopts a twisted conformation on the surface, although the fact that the C–C atoms are marginally higher than the C–F atoms (by 0.14 ± 0.10 Å) differs from these earlier studies. The origin of the low coherent fraction for the fluorine is unclear, but this has also been seen in the earlier studies of F 4 TCNQ adsorption.…”

“…These results are closely similar to those obtained previously for F 4 TCNQ on Ag(100), Cu(111) and Au(111) and for TCNQ on Ag(111), indicating that the molecule adopts a twisted conformation on the surface, although the fact that the C–C atoms are marginally higher than the C–F atoms (by 0.14 ± 0.10 Å) differs from these earlier studies. The origin of the low coherent fraction for the fluorine is unclear, but this has also been seen in the earlier studies of F 4 TCNQ adsorption. ,, In some cases a chemically shifted F 1s component was attributed to the presence of atomic fluorine (at a binding energy of 682 eV) due to radiation damage (e.g., ref ), but no such shifted peak was seen in the present study. We can only surmise that radiation damage leads to a separate molecular F species that does not have a detectable 1s XPS chemical shift relative to F 4 TCNQ and does not have C atoms at significantly different heights to those of the coadsorbed intact F 4 TCNQ.…”

“…The origin of the low coherent fraction for the fluorine is unclear, but this has also been seen in the earlier studies of F 4 TCNQ adsorption. 19,20,32 In some cases a chemically shifted F 1s component was attributed to the presence of atomic fluorine (at a binding energy of 682 eV 33 ) due to radiation damage (e.g., ref 20), but no such shifted peak was seen in the present study. We can only surmise that radiation damage leads to a separate molecular F species that does not have a detectable 1s XPS chemical shift relative to F 4 TCNQ and does not have C atoms at significantly different heights to those of the coadsorbed intact F 4 TCNQ.…”

Probing the role of surface termination in the adsorption of azupyrene on copper

“…31,63,64 Recently, PBE + MBD-NL provided an accurate prediction of the adsorption height of 7,7,8,8-tetracyanoquinodimethane (TCNQ) on Ag(100) when compared to XSW measurements. 65 Replacing the PBE functional with the range-separated hybrid functional HSE06, 30 combined with the MBD method, was able to fully resolve the adsorption structure and energetics of benzene on Ag(111). 59 The optimised HSE06 + MBD-NL structures were used for all further analysis and are presented in Fig.…”