Mechanism of the enhanced conductance of a molecular junction under tensile stress

Despite its fundamental importance for nanophysics and chemistry and potential device applications, the relationship between atomic structure and electronic transport in molecular nanostructures is not well understood. Thus the experimentally observed increase of the conductance of some molecular nanojunctions when they are stretched continues to be counterintuitive and controversial. Here we explore this phenomenon in propanedithiolate molecules bridging gold electrodes by means of ab initio computations and semiempirical modeling. We show that in this system it is due to changes in Au-S-C bond angles and strains in the gold electrodes, rather than to the previously proposed mechanisms of Au-S bond stretching and an associated energy shift of the highest occupied molecular orbital and/or Au atomic chain formation. Our findings indicate that conductance enhancement in response to the application of tensile stress should be a generic property of molecular junctions in which the molecule is thiol-bonded in a similar way to gold electrodes.

Figure 1

Relaxed structures of the Au-PDT-Au molecular junction for various distances $d$ between the outermost atoms of the Au clusters. (a) $d=21.75$ Å, (b) $d=25.70$ Å, (c) $d=26.50$ Å, (d) $d=29.00$ Å, (e) $d=30.00$ Å, and (f) $d=33.40$ Å. As the junction is stretched, Au-Au bonds near the molecule break in succession until in (f) the molecule is connected to each Au cluster by a chain of Au atoms.

Figure 2

Calculated conductance at zero temperature of trans (gauche) Au-PDT-Au junctions vs junction length for different structures is shown in red (blue). Red circles between $a_1$ and $a_2$, $b_1$ and $b_2$, $c_1$ and $c_2$, $e_1$ and $e_2$, and between $f_1$ and $f_2$ show the conductances for structures similar to Figs. 1, 1, 1, 1, and 1, respectively. Point d indicates the structure shown in Fig. 1. $A$ and $C$ are explained in the inset of Fig. 3. $U$ is the unstrained junction.

Figure 3

Variation of the C-S-Au (blue), S-Au-Au (red), and Au-Au-Au (green) angles for trans-PDT junctions with junction length $d$. C-S-Au angles for gauche structures are maroon. Solid (hollow) circles show angles to the left (right) of the molecule. The solid (dashed) lines in the inset show the conductance vs C-S-Au angle for junctions obtained from those marked $A$ and $C$ in Figs. 2 and 3 by rigidly rotating the 14 (1) -atom gold clusters about the S atoms.