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Trapping of charged gold adatoms by dimethyl sulfoxide on a gold surface

Owing to their inertness, single crystal Au(111) substrates are commonly adopted to support the growth of 2D supramolecular architectures, where specific inter-molecular, rather than molecule-substrate interactions are exploited to drive the assembly. Moreover, the (111) face of gold is the most stable one, and molecular interactions on and with this substrate are of great importance since, in most applications, where polycrystalline gold electrodes or clusters are involved, (111)-terminated facets predominate. At the opposite, despite being inert (“noble”) in its bulk form, gold exhibits rich catalytic properties and ligand chemistry when in the form of small clusters composed by tens of atoms down to single atoms. At the boundary between the inertness of bulk gold and its nanoscale reactivity, in this work we show how a simple polar molecule, dimethyl sulfoxide (DMSO, (CH3)2S, see Fig. 1a), can trap single gold adatoms natively available on the Au(111) surface, thus suggesting a novel motif for anchoring organic adlayers of polar molecules on metal substrates and providing new insights into the interaction of DMSO (and solvents in general) with gold.
When DMSO is deposited at saturation on Au(111) at temperatures ranging from 173 K to 233 K, LT-STM (Low Temperature Scanning Tunneling Microscopy) images reveal a compact phase (Fig. 1b) coexisting with characteristic square complexes (Fig. 1b, c) of DMSO. In these phases, LT-STM images and DFT (Density Functional Theory) calculations suggest that DMSO is sitting in an “inverted umbrella” geometry, i.e., with the S atom bound to the Au surface on terminal sites. Moreover, the molecules interact with their neighbors via hydrogen bonds between the O atom and the methyl groups (CH3). The XPS (X ray Photoelectron Spectroscopy, not shown) and NEXAFS (Near Edge X ray Absorption Fine Structure, see Fig. 1d) measurements, performed at the ALOISA beamline, support this DMSO geometry: the S 2p XPS spectra are always characterized by single component, suggesting that all the S atoms are in the same environment, while the linear dichroism NEXAFS spectra at the O 1s edge suggest that the S=O bond is tilted by 25 ± 10° with respect to the surface plane, in optimum agreement with the structural parameters obtained by DFT calculations. The work has been done in collaboration with the University of Trieste, the CNR, the INSTM and the King’s College London.

Figure 1. (a) Ball and stick model of DMSO. (b) STM image (12.75 × 12.75 nm2) showing the high coverage DMSO phase coexisting with some square complexes. (c) High resolution image of a square complex. Transparent ball and tick models are superimposed to guide the eye in the identification of the single molecules. (d) Linear dichroism NEXAFS spectra at the O 1s edge for two different synchrotron light polarizations (s polarization, parallel to the surface; p polarization, perpendicular to the surface) of DMSO/Au(111) after annealing at 213 K. A dichroism is evident between the main peaks 3 and 4 (in red and yellow, respectively).

After annealing to higher temperatures (T > 233K), the compact phase and the square complexes are no more observed, and new complexes are found (see Fig. 2), namely three different rectangle complexes (Fig. 2a, b, c) and a triangle complex (Fig. 2d). The triangle complex is clearly made of three DMSO molecules in an “inverted umbrella” geometry, with the O atoms located close to each other. This specific configuration should intuitively be extremely unfavorable due to the mutual repulsion between the negatively-charged O atoms. Indeed, the DFT simulations indicate that this complex is stable if, and only if, a charged gold adatom is present at the center of the complex, thereby linking the O atoms via a bond of ionic nature. It is to be noted that in the experimental STM images, no protrusion related to the adatom can be observed within the complex, and the simulated STM images confirm that, even if present, the adatom is “invisible”.
With concern to the three different rectangle complexes (Fig. 2a, b, c), namely the symmetric rectangle, asymmetric rectangle, and chiral rectangle, to explain their stability, two gold adatoms are hypothesized at the center of the complexes, linking the O atoms of the four DMSO molecules. Also in this case, the simulated STM images (not shown) confirm that no protrusion related to the adatom can be observed, in agreement with the experimental images.
These linker adatoms are under-coordinated with respect to bulk gold and DFT calculations show that they are positively charged, thereby acting as an ionic linker between the otherwise repelling oxygen terminations of the molecules.

Figure 2. STM image (center, 13 × 13 nm2) showing the various complexes that form on Au(111) after annealing to 273K. (a-d) panels show high resolution details of the complexes together with ball models of geometries obtained by DFT simulations, where the gold adatoms are painted in green to ease their identification.

 

This research was conducted by the following research team:

Z. Feng1,3, S. Velari2, A. Cossaro3, C. Castellarin-Cudia3, A. Verdini3, E. Vesselli1,3, C. Dri1,3, M. Peressi1,4,5, A. De Vita2,6, G. Comelli1,3
 
1Physics Department, University of Trieste, Trieste, Italy
Engineering and Architecture Department, University of Trieste, Trieste, Italy
CNR-IOM, Laboratorio TASC, Trieste, Italy
CNR-IOM DEMOCRITOS Theory@Elettra Group, Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy
Consorzio Interuniversitario Nazionale per la Scienza e la Tecnologia dei Materiali (INSTM), Trieste, Italy
Department of Physics, King’s College London, London, United Kingdom



Contact person:

Carlo Dri, Email: dri@iom.cnr.it

 

Reference

Z. Feng, S. Velari, A. Cossaro, C. Castellarin-Cudia, A. Verdini, E. Vesselli, C. Dri, M. Peressi, A. De Vita, G. Comelli  “Trapping of charged gold adatoms by dimethyl sulfoxide on a gold surface”, ACS Nano 9, 8697 (2015); DOI: 10.1021/acsnano.5b04985

 

Last Updated on Monday, 26 October 2015 15:04