Stretch Evolution of Electronic Coupling of the Thiophenyl Anchoring Group with Gold in Mechanically Controllable Break Junctions

Abstract

The current-voltage characteristics of a single-molecule junction are determined by the electronic coupling Γ between the electronic states of the electrodes and the dominant transport channel(s) of the molecule. Γ is profoundly affected by the choice of the anchoring groups and their binding positions on the tip facets and the tip-tip separation. In this work, mechanically controllable break junction experiments on the N,N'-bis(5-ethynylbenzenethiol-salicylidene)ethylenediamine are presented, in particular, the stretch evolution of Γ with increasing tip-tip separation. The stretch evolution of Γ is characterized by recurring local maxima and can be related to the deformation of the molecule and sliding of the anchoring groups above the tip facets and along the tip edges. A dynamic simulation approach is implemented to model the stretch evolution of Γ, which captures the experimentally observed features remarkably well and establishes a link to the microscopic structure of the single-molecule junction.

Figure 1

Schematic representation of (a) dithiolated-N,N′-bis(5-ethynylbenzenethiol-salicylidene)ethylenediamine cobalt complex (Co-Salen-S) and (b) dithiolated-1,4-bis(phenylethynyl)-2,5-bis(ethoxy)benzene (PEEB-S).

Figure 2

Experimentally measured stretch evolution of the electronic coupling (Γ^E) for a single opening measurement of Co-Salen-S and theoretically determined stretch evolution of the electronic coupling () evaluated using the SLM applied to the transmission function T(E), which are calculated for 1000 thermodynamically most relevant configurations for individual tip–tip separation (S_tip–tip). (a) Γ^E - the data points with peaks are marked using dark blue arrows. (b) , , and reveal a falling trend for S_tip–tip interval (11.51 Å < S_tip–tip < 21.72 Å) and a rising trend for S_tip–tip > 21.72 Å. Peaks are visible for at S_tip–tip: 13.08, 17.79, and 21.72 Å, respectively. The four subdivisions I, II, III, and IV define the regions with the dominant anchoring position pairs: edge–edge, tip–edge, tip–edge + tip–tip, and tip–tip, respectively. Refer to the dominant anchoring positions in Figure 3(a).

Figure 3

(a) Theoretically determined stretch evolution of the mean anchoring position on the Au(111) facets () - four distinct S_tip–tip regions are discernible. For S_tip–tip < 13.87 Å, the dominating mean anchoring positions of the sulfur atoms on the Au(111) facets () are either edge–edge or tip–edge. For S_tip–tip between 14.65 and 18.58 Å, the tip-edge dominates over edge–edge . The contribution of edge–edge diminishes for S_tip–tip > 19.36 Å with the emergence of tip–tip . For S_tip–tip between 19.36 and 20.9 Å, the tip-edge dominate. For S_tip–tip > 21.72 Å, the are solely tip–tip. (b) Theoretically determined stretch evolution of the mean curvature () - an overall falling trend is discernible. Trough-like features are visible at S_tip–tip: 13.87, 17.79, and 20.9 Å, respectively. A distinct sharp peak is visible at S_tip–tip: 21.72 Å. (c) Theoretically determined stretch evolution of the mean anchoring angle () - An overall rising trend until saturation is discernible. Peaks are visible at S_tip–tip: 15.44 and 18.58 Å.

Figure 4

Real-space projection of Γ_mean (a) side view and (b) top view (not to scale) of the configuration with strongest coupling for the S_tip–tip = 12.33 Å. (c) The value of Γ_mean is represented by red circles at the sites of the anchoring sulfur atoms on the left and right electrodes in top view. The gold atoms of the electrodes are colored to depict the top, hcp-hollow, and fcc-hollow sites using yellow, brown, and salmon colored circles, respectively. The solid gray and dotted red lines as shown to indicate the anchoring of the sulfur atoms at top and bridge sites along the Au(111) facet edges of both the left and right electrodes.

Figure 5

Real-space projection of the theoretically determined stretch evolution of Γ_mean for those 50 configurations for each S_tip–tip, with the highest random walk weights. The gold electrodes are represented using yellow, brown, and salmon colored circles, representing the top, hcp-hollow, and fcc-hollow sites, respectively. The other colored circles appear in pairs, which reveal the binding sites for the anchoring sulfur atoms on the left and the right gold electrode. Higher and lower Γ_mean values are represented using a color scale spanning from red to gray, augmented together with the size of circle. Additionally, solid red and dotted gray lines are placed to differentiate between bridge sites and top sites along the Au(111) facet edges, respectively.