Structural Regulation of Mechanical Gating in Molecular Junctions

Abstract

In contrast to silicon-based transistors, single-molecule junctions can be gated by simple mechanical means. Specifically, charge can be transferred between the junction's electrodes and its molecular bridge when the interelectrode distance is modified, leading to variations in the electronic transport properties of the junction. While this effect has been studied extensively, the influence of the molecular orientation on mechanical gating has not been addressed, despite its potential influence on the gating effectiveness. Here, we show that the same molecular junction can experience either clear mechanical gating or none, depending on the molecular orientation in the junctions. The effect is found in silver-ferrocene-silver break junctions and analyzed in view of ab initio and transport calculations, where the influence of the molecular orbital geometry on charge transfer to or from the molecule is revealed. The molecular orientation is thus a new degree of freedom that can be used to optimize mechanically gated molecular junctions.

Keywords: break junction; ferrocene; mechanical gating; molecular junction; orbital hybridization; transition voltage spectroscopy.

Figure 1

Break junction setup. Schematics of the used break junction setup, in which the distance between the Ag electrode tips can be adjusted in sub-angstrom resolution, and an illustration of a Ag–ferrocene–Ag single-molecule junction.

Figure 2

Current–voltage, differential conductance spectra, and transition voltage spectroscopy (TVS) plots. (a,b) Four spectra of current as a function of voltage measured at different interelectrode displacements (for color code and displacement see insets of (c,d) in Ag–ferrocene–Ag molecular junctions with (a, type 1) and without (b, type 2) mechanical gating response. (c) Differential conductance as a function of applied voltage for the junction studied in (a). (d) Same as (c) but with data collected for the molecular junction studied in (b). Insets (c,d): Absolute values of peak position (marked with arrows in (c,d) as a function of interelectrode displacement. (e,f) TVS plots constructed from the same I–V spectra presented in (a,b), showing ln(I/V²) versus 1/|V| for spectra with (e, type 1) and without (f, type 2) mechanical gating response. For consistency, the negative side of the I–V curves is considered for TVS analysis. Inset (i): Zoomed view of the TVS plots to better present the change of transition voltage upon squeezing. Inset (ii): Transition voltage (absolute values) as a function of interelectrode displacement for type 1 and 2.

Figure 3

Transport calculations. (a,b) Calculated transmission for parallel (a) and perpendicular (b) molecular orientations in the junction at a varying distance between the electrode tips. Insets: Ball-and-stick models of the calculated structures (only small parts of the electrodes are shown). (c,d) Differential conductance for the parallel (c) and perpendicular (d) configurations at the same varying electrode tip distances as in (a,b). (e) Charging of the ferrocene molecule in the parallel (blue) and perpendicular configurations (orange). (f) Total energy as a function of interelectrode displacement. (⊥,∥) Denotes perpendicular and parallel molecular orientation with respect to the long junction axis.

Figure 4

Isosurfaces of the calculated orbitals that dominate electronic transport. Left: Isosurface plots of the two (degenerate) LUMOs of an isolated ferrocene. Center and right, respectively: isosurface plots of selected electron wave functions of the Ag–ferrocene–Ag junction and their energies (with respect to Fermi energy) for the parallel and perpendicular configurations (at interelectrode separations of 9.81 and 6.2 Å, which correspond to the blue curves in Figure 3a,b). These energies lie in the immediate vicinity of the unoccupied transmission resonances. All isosurfaces contain 93% of the wave function. The plots also contain ball-and-stick models of the structures (color coding of the atoms: white (H), black (C), pink (Fe), and silver (Ag)).