Direct experimental evidence for substrate adatom incorporation into a molecular overlayer

TL;DR

This study provides definitive experimental evidence using surface X-ray diffraction that metal substrate adatoms incorporate into a molecular overlayer, clarifying previous ambiguous interpretations based on microscopy.

Contribution

It introduces surface X-ray diffraction as a definitive method to identify substrate adatoms in molecular overlayers, supported by combined experimental and theoretical analysis.

Findings

SXRD confirms presence and position of Au adatoms

DFT shows adatom incorporation is energetically favored

Molecular geometry is twisted, not inverted bowl shape

Abstract

While the phenomenon of metal substrate adatom incorporation into molecular overlayers is generally believed to occur in several systems, the experimental evidence for this relies on the interpretation of scanning tunnelling microscopy (STM) images, which can be ambiguous and provides no quantitative structural information. We show that surface X-ray diffraction (SXRD) uniquely provides unambiguous identification of these metal adatoms. We present the results of a detailed structural study of the Au(111)-F4TCNQ system, combining surface characterisation by STM, low energy electron diffraction and soft X-ray photoelectron spectroscopy with quantitative experimental structural information from normal incidence X-ray standing waves (NIXSW) and SXRD, together with dispersion corrected density functional theory (DFT) calculations. Excellent agreement is found between the NIXSW data and the DFT calculations regarding the height and conformation of the adsorbed molecule, which has a twisted geometry rather than the previously supposed inverted bowl shape. SXRD measurements provide unequivocal evidence for the presence and location of Au adatoms, while the DFT calculations show this reconstruction to be strongly energetically favoured.