Nanomechanics of Bidentate Thiolate Ligands on Gold Surfaces

The effect of the chain length separating sulfur atoms in bidentate thiols attached to defective gold surfaces on the rupture of the respective molecule-gold junctions has been studied computationally. Thermal desorption always yields cyclic disulfides. In contrast, mechanochemical desorption leads to cyclic gold complexes, where metal atoms are extracted from the surface and kept in tweezer-like arrangements by the sulfur atoms. This phenomenon is rationalized in terms of directional mechanical manipulation of Au-Au bonds and Au-S coordination numbers. Moreover, the flexibility of the chain is shown to crucially impact on the mechanical strength of the junction.

Figure 1

(a) Pathway of mechanical desorption of $\mathbf{C}\mathbf{2}$ (see Figs. S4, S5, and S6 for the $\mathbf{C}\mathbf{1}$, $\mathbf{C}\mathbf{3}$, and $\mathbf{C}\mathbf{4}$ species, respectively): Total electronic energy along the mechanical desorption pathway (filled black circles), total electronic energy of the fragmentation products Frag-0 (empty black circles), Frag-1 (empty blue triangles), and Frag-2 (empty red diamonds), as illustrated in Fig. S8. The insets illustrate the products (molecule only) for the three fragmentation scenarios at $D=7.8\AA$. Force versus distance curves for regions of elastic deformation (solid red lines, right axis) are shown; the connecting broken red lines are merely guides to the eye through discontinuous plastic deformation events (see the SM for details). The filled red circle indicates the $F_{\mathrm{rup}}$ value reported in Table 1. (b) Evolution of the average vertical distance of those Au atoms that are initially in the top layer (empty squares) relative to this distance (7.29 Å) at $D=0\AA$, and average Au-S distance (filled circles) relative to this distance (2.40 Å) at $D=0\AA$ along the mechanical desorption pathway. The shaded areas indicate the different Au-S coordination numbers (see text for definition) in the three distinct regimes along the detachment pathway, (I) $2:2$, (II) $2:1$, and (III) $1:1$, whereas the insets are representative snapshots at $D$ values of 2.4, 6.4, and 7.8 Å, respectively. (c) Final products for the mechanical desorption of the studied adsorbates. Note that the two bottom gold layers are omitted for clarity.

Figure 2

(Top) S-Au-S angle for parts of the mechanical desorption pathways (as parametrized by $D$) of $\mathbf{C}\mathbf{2}$, $\mathbf{C}\mathbf{3}$, and $\mathbf{C}\mathbf{4}$. The insets introduce the atom labels and illustrate the relevant structural changes at $D=6.4\AA$ and 7.2 Å ($\mathbf{C}\mathbf{2}$), 6.6 Å and 7.2 Å ($\mathbf{C}\mathbf{3}$), 7.2 Å and 8.2 Å ($\mathbf{C}\mathbf{4}$). (Bottom) Atoms-in-molecules charges for ${\mathrm{S}}_A$ (empty circles), ${\mathrm{S}}_B$ (filled circles), ${\mathrm{Au}}_1$ (empty triangles), and ${\mathrm{Au}}_2$ (filled triangles) for the same part of the mechanical desorption pathways of $\mathbf{C}\mathbf{2}$, $\mathbf{C}\mathbf{3}$, and $\mathbf{C}\mathbf{4}$ (see top insets for atom labeling).

Figure 3

Projected density of states for (a) $\mathbf{C}\mathbf{2}$ at $D=6.4\AA$, (b) $\mathbf{C}\mathbf{3}$ at $D=6.6\AA$, and (c) $\mathbf{C}\mathbf{4}$ at $D=7.2\AA$. The depicted orbitals are $3p$ for ${\mathrm{S}}_A$ (black line) and ${\mathrm{S}}_B$ (red line) and $5d$ for ${\mathrm{Au}}_1$ (blue shaded area) and ${\mathrm{Au}}_2$ (green line). The labeling of the atoms corresponds to that introduced in Fig. 2, and the insets illustrate relevant molecular orbitals corresponding to the pDOS states indicated by the arrows (isosurface representation at $0.00015e{\mathrm{Bohr}}^{-3}$).