Functionalization of gold is often realized via ligands with sulfur acting as head group (RS). Examples are self assembled monolayers (SAMs) of disulfides or n-alkyl thiolates on extended gold surfaces and protected gold nanoparticles (AuNP). Although a prerequisite for the stability of SAMs or protected AuNPs is the strong RS-Au bond, it has been difficult to establish fundamental issues for this interaction. In particular, several models have appeared for the stable adsorption configuration of RS on Au(111) terraces. Very recently, however, experimental evidence have appeared for the existence of Au ad-atoms at the RS-Au(111) interface [1,2]. These observations have made it imperative to establish the potential energy surface for such systems. In the present contribution, ab initio calculations are used to explore the structure of methylthiolates (MeS) on Au(111) and ad-atoms on Au(111) [3].
We find that adsorption atop the ad-atom enhances the MeS-Au bond strength by 0.4 eV with respect to the preferred (bridge-fcc) site on the terrace. Moreover, allowing for bridge adsorption between adjacent Au ad-atoms gold-thiolate polymers (MeSAu)x are formed. Interestingly, such configurations stabilize the system furhter by 0.75 eV.
The importance of (MeSAu)x structures depends on whether it may drive the required surface reconstruction (creating ad-atoms). Our results show that this indeed is the case.
The observation that (MeSAu)x complexes are relevant in the case of MeS adsorption on Au(111), is important for the general understanding of thiolate-gold interactions. Homoleptic (RSAu)x complexes are known to form zigzag rings or strands, and are polymeric in nature [4]. Moreover, for AuNP a "divide and protect" scenario was recently proposed, where (MeSAu)4 units were suggested to protect a metal core [5]. Augmented by the present results, it appears that common RS-Au bonding motifs apply for Au systems of different size.
[1] M. Yu et al., Phys. Rev. Lett. 97, 166102 (2006).
[2] P. Maksymovych et al., Phys. Rev. Lett. 97, 146103 (2006).
[3] H. Grönbeck and H. Häkkinen, J. Phys. Chem B Letter (in press).
[4] H. Grönbeck, M. Walter and H. Häkkinen, J. Am. Chem. Soc. 128, . 10268 (2006).
[5] H. Häkkinen, M. Walter and H. Grönbeck, J. Phys. Chem. B 110, 9927 (2006).
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