Direct Experimental Evidence for Substrate Adatom Incorporation into a Molecular Overlayer

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 tunneling 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)-F 4 TCNQ system, combining surface characterization by STM, low-energy electron diffraction, and soft X-ray photoelectron spectroscopy with quantitative experimental structural information from normal incidence X-ray standing wave (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 favored.

Role of Adatoms for the Adsorption of F4TCNQ on Au(111)

“…The twist of F4TCNQ and the uplift of the adatom highlight the importance of the vdW contributions, since they are only reported by groups that include the dispersion corrections. 32 , 49 Our adsorption geometry for the case with adatoms is in good agreement with the values determined via normal-incidence x-ray standing waves (NIXSW) (3.45 ± 0.20 Å for the carbon backbone) and surface x-ray diffraction (SXRD) (3.29 ± 0.04 Å for the carbon backbone and 2.95 ± 0.08 Å for the adatom) reported by Mousley et al 32 …”

“…We find strong indications that the incorporation of adatoms is driven by the fact that the molecule, specifically its cyano groups, can form more efficient covalent bonds with adatoms than with Au atoms in the first layer. This finding is supported by the observation that the adatom is moved away from its initial equilibrium position at the Au(111) hollow site toward an Au(111) atop site (also in agreement with literature 44 , 46 ) while being lifted up to the adsorption height of the F4TCNQ adlayer. 46 The finding is furthermore corroborated by the observation that the interaction between the surface and F4TCNQ alone, as well as with a hypothetical F4TCNQ–Au network, is driven mostly by vdW forces.…”

“…To study the role of adatoms, we chose F4TCNQ on Au(111), since multiple independent studies have found that it readily and reproducibly forms well-ordered structures containing adatoms as part of the adlayer. 35 , 44 , 46 , 47 The surface structure consists of only one F4TCNQ molecule and one adatom per unit cell, which reduces the computational effort and facilitates the analysis of this system (compared to larger unit cells). The strong interactions between the adsorbate and substrate lifts the herringbone reconstruction of Au(111), and the F4TCNQ molecule-Au-adatom network is arranged in a Au(111) surface supercell, as shown in Figure 1 .…”

“…This causes a twist of the F4TCNQ molecule in the adsorbed state. 32 , 49 Conversely, if adatoms are present, they attach to the cyano groups of two of the four neighboring F4TCNQ molecules. This causes the adatoms to be lifted from the substrate toward the F4TNCQ backbone (see Figure 1 a), which further corroborates the conclusion that the electronic coupling of the adatoms to the surface is weakened.…”

“…They are also generally invisible to most spectroscopy methods (such as core spectroscopy), because their signal is too weak compared to the signal from the bulk substrate. 32 Hence, they often need to be indirectly inferred. 33 Yet, recent studies, based on the determination of adsorption heights and ab initio calculations, 34 indicate that adatom-containing structures may be more prevalent than hitherto thought, even calling previous studies explicitly into question.…”