Steric and Chain Length Effects in the (${\sqrt {(3)} }$×${\sqrt {(3)} }$)R30° Structures of Alkanethiol Self‐Assembled Monolayers on Au(111)

Abstract

The translational and orientational potential energy surfaces (PESs) of n-alkanethiols with up to four carbon atoms are studied for (√(3)×√(3))R30° self-assembled monolayers (SAMs). The PESs indicate that methanethiol may form SAM structures that are not accessible for long-chain thiols. The tilt of the thiol molecules is determined by a compromise between the preferred binding geometry at the sulfur atom and the steric requirements of the alkane chains. The Au-S bond lengths, offset from the bridge position (brg), and the Au-S-C bond angles result in tilt angles of the S-C bond in the range of 55-60°. As DFT/generalized gradient approximation systematically underestimates chain-chain interactions, the binding energies are corrected by comparison to MP2 interaction energies of alkane dimers in SAM-like configurations. The resulting thiol binding energies increase by approximately 1 kcal mol(-1) per CH(2) group, which results in a substantial stabilization of long-chain SAMs due to chain-chain interactions. Furthermore, as the chain length increases, the accessible range of backbone tilt angles is constrained due to steric effects. The combination of these two effects may explain why SAM structures with long-chain thiols exhibit higher order in experiments. For each thiol two favorable SAM structures are found with the sulfur head group at the fcc-brg and hcp-brg positions, respectively. These domains may coexist in thermal equilibrium. In combination with the symmetry of the gold (111) surface, this raises the possibility of up to six different domains on single-crystal terraces. Reconstructions by an adatom or vacancy of ethanethiol SAMs with (√(3)×√(3))R30° lattice are also studied using PES scans. The results indicate that adsorption of thiols next to a vacancy is favorable and may lead to point defects inside SAMs.

Alkylthiol self-assembled monolayers on Au(111) with tailored tail groups for attaching gold nanoparticles

“…mol. -1 ) [42] and (ii) an increase of the steric effects confining the accessible range of tilt angles, for NDT during the process of the self-organization [42]. Notice that the concentration of the molecules during SAM formation was equivalent for both molecules.…”

“…In the case of the dithiol solutions of NDT and BDT, the gold substrates were immersed in the aqueous AuNPs suspension for 1 hour. We checked visually that no aggregation occurred [42].…”

“…This side is termed the head group, whereas the outward moiety is the tail group. The quality of self-organization processes not only depends on the gold-thiol bond but also on the interaction between the neighbouring alkyls due to lateral van der Waals interaction (~1 kcal·mol −1 per −CH2− group) [8,42]. The increase of alkyl chain length leads to its tilting within a SAM instead of standing upright [21,25,36,41] or in some cases to laying phase [8] even when concentration is sufficient to form dense packing.…”

Thiolate adsorption on Au(${\bm {hkl}}$hkl) and equilibrium shape of large thiolate-covered gold nanoparticles

“…Although this excludes steric repulsions between chains, it captures all essential features of the general RSAu bond. This is a reasonable choice as shown by the recent work of Torres et al 40 These authors show that the adsorption energy of n-alkanethiols on Au(111) changes by only 2 kcal/mol (0.09 eV/molecule) when the length of chain increases from one to six carbon atoms. Steric interactions between chains have a minor impact on binding energies; for example, Torres et al 40 find that the binding energies of ethanethiolate and propanethiolate are affected by chain interactions by only 0.1 kcal/mol and 0.7 kcal/mol, respectively.…”

Effect of van der Waals interactions in the DFT description of self-assembled monolayers of thiols on gold

“…The formation of the SAMs is affected by the presence of surface defects [33,37], by the thiol chain length which can distress the SAMs functionality [38][39][40][41], by the thiol terminal groups [42] which can lead to lateral interactions, by the formation of surface oxides [43][44][45] or by the thiol concentration [31,46,47]. Furthermore, the presence of adatoms on the surface has been claimed as an important factor toward the formation of thiol SAMs on gold surfaces [48][49][50][51], and, in particular, it has been found that methanethiol leads to a strong surface reconstruction during its adsorption [52].…”