Large Conductance Variations in a Mechanosensitive Single-Molecule Junction

Abstract

An appealing feature of molecular electronics is the possibility of inducing changes in the orbital structure through external stimuli. This can provide functionality on the single-molecule level that can be employed for sensing or switching purposes if the associated conductance changes are sizable upon application of the stimuli. Here, we show that the room-temperature conductance of a spring-like molecule can be mechanically controlled up to an order of magnitude by compressing or elongating it. Quantum-chemistry calculations indicate that the large conductance variations are the result of destructive quantum interference effects between the frontier orbitals that can be lifted by applying either compressive or tensile strain to the molecule. When periodically modulating the electrode separation, a conductance modulation at double the driving frequency is observed, providing a direct proof for the presence of quantum interference. Furthermore, oscillations in the conductance occur when the stress built up in the molecule is high enough to allow the anchoring groups to move along the surface in a stick-slip-like fashion. The mechanical control of quantum interference effects results in the largest-gauge factor reported for single-molecule devices up to now, which may open the door for applications in, e.g., a nanoscale mechanosensitive sensing device that is functional at room temperature.

Keywords: Quantum interference; density functional theory; mechanically controlled break-junctions; molecular electronics; nanoscale transport; single-molecule.

Figure 1

(a) Schematic illustration of the break-junction experiment of the OPE-linked PC molecule trapped between nanoelectrodes (top panel), together with a presentation displaying, as springs, the combinations of mechanosensitive structures in the molecular design (bottom drawing in this panel). (b) Solid-state molecular structure determined by single-crystal X-ray analysis. (c) Possible behavior of the molecule under applied force: (I) elongation of the molecule under pulling force of the electrodes, (II) junction with the molecule in its relaxed configuration, and (III) compression results in a shortening of the overall junction length. The simulated conductance (in units of the conductance quantum G₀ = 2e²/h) as a function of the applied mechanical stress is displayed as the drawn line; the three cases (I–III) are indicated by the colored circles.

Figure 2

(a) Two-dimensional histogram built from 3000 consecutive breaking traces collected after deposition of the solution containing the molecule displayed in Figure 1. The applied bias is 100 mV, and the electrode speed is 4.0 nm/s. A single breaking trace (black line) has been overlaid as an example. The inset shows the peak-separation distribution. (b) Examples of breaking traces, showing oscillations in conductance as a function of displacement. The traces are taken from four different samples and are offset in the x-axis for clarity. The first trace comes from sample A, shown in panel a; samples B–D are presented in Figures S6–S9 alongside more examples of breaking traces.

Figure 3

(a) Examples of distance-modulation traces (sample B). The blue, orange, and green lines (middle and bottom panel) represent three different conductance measurements, whereas the purple line (top panel) represents the voltage applied to the piezoelectric stack. The applied piezo voltage translates into a peak-to-peak distance of 5.0 Å, and a positive voltage corresponds to a larger electrode distance. The total modulation time is 120 s at a frequency of 0.5 Hz. The conductance responds to the electrode-separation modulation either in-phase (orange), in antiphase (blue), or with double the modulation frequency (green). An example of a full measurement, extending over 120 s, is shown in Figures S17 and S19. (b) Distribution of absolute values of gauge factors obtained from 123 selected traces of the distance-modulation measurements performed on sample B. The number in the figure corresponds to the peak value of the distribution.

Figure 4

(a) Calculated conductance and total energy of the system during gap opening. (b) Selection of snapshots illustrating the stick–slip motion. A video of the simulated stick–slip motion can be found in the Supporting Information.

Figure 5

(a) Frontier orbitals of the molecule. The orbitals are either symmetric (GPH and GPL) or antisymmetric (GPH–1 and GPL+1) with respect to the center of the molecule. The frontier orbitals are shown in the gas phase for clarity (with the sulfur atoms “terminated” with one gold atom each) because they do not change significantly when the gold leads are attached to the molecule. (b) Conductance (horizontal black line in panel c) and total energy. (c) Transmission map of the molecule between two leads from DFT calculations. The horizontal yellow traces in the map arise from orbitals that can be related to the gas-phase frontier orbitals. An anti-resonance is observed between the frontier orbital traces. It shifts in energy as the displacement is varied. The positions at which the pairs of GPH–1 and GPH and of GPL and GPL+1 degenerate are marked with circles.

Figure 6

(a, top panels) Simulation of conductance traces (second panel) when the electrode displacement is periodically modulated (top panel) for three different trapping configurations: pre-stretched (orange), relaxed (green), and pre-compressed (blue). Centers of oscillation at +1, 0, and −1 Å, respectively; amplitude of 0.5 Å. (a, bottom panels) Zoomed-in panel showing the experimental distance-modulation traces presented in Figure 3. (b) Calculated conductance vs electrode displacement. The blue, green, and orange areas (I, II, and III, respectively) show the portion of the curve spanned in the case of different starting positions (pre-compressed, relaxed, and pre-stretched, represented by circles in the same colors). The numbers in the purple circles represent the position of the electrodes along the oscillation period in the case of a relaxed trapping configuration. (c) Schematics of the molecular configurations along a period of electrode distance modulation. Different starting configurations are represented with different colors: pre-stretched in orange, relaxed in green, and pre-compressed in blue. White numbers in purple circles represent the position of the electrodes along the oscillation period. Note that the molecular compression and elongation in the simulation is 0.5 Å, a value smaller than the gap-size variation in the experiment. This can be rationalized by the elastic response of the sulfur–gold connections and of the gold atoms in the electrodes themselves.