Transition from Strong to Weak Electronic Coupling in a Single-Molecule Junction

We have investigated charge transport in single-molecule junctions using gold nanoelectrodes at room and cryogenic (10 K) temperatures. A statistical analysis of the low-bias conductance, measured during the stretching of the molecular junctions, shows that the most probable single-molecule conductance is insensitive to the temperature as expected for off-resonant coherent transport. Low-temperature current-voltage measurements show that these junction conformations have a smooth tunnelinglike shape. While separating the electrodes further we find that, in about one-fourth of the cases, the junction switches in an abrupt way to a configuration with $I\text{−}V$ characteristics exhibiting a gap around zero bias and resonances at finite bias. The analysis of the $I\text{−}V$ shape and of the conductance distance dependence suggests a stretching-induced transition from the strong to the weak electronic coupling regime. The transition involves a large renormalization of the injection barrier and of the electronic coupling between the molecule and the electrodes.

Figure 1

(a) Schematic illustrations of a Au-molecule-Au junction formed with mechanically controllable break-junction nanoelectrodes (top) and of the sample with the bending mechanism (bottom). (b) Individual conductance versus displacement traces, recorded in the presence of OPE3, measured at a bias of 100 mV. The blue curves have been measured near the liquid helium temperature (10 K) and the red curves at room temperature (300 K). The traces have been offset along the $x$ axis for clarity. (c) Conductance histogram built from conductance traces that contain molecular junctions at both temperatures.

Figure 2

(a) Low-temperature breaking traces in the presence of OPE3 constructed from $I\text{−}V$’s at low (blue) and high (orange) bias. (b) Current-voltage ($I\text{−}V$) characteristics measured during the breaking event shown in (a). The $I\text{−}V$ curves have been shifted along the $y$ axis for clarity.

Figure 3

(a) Low-temperature breaking traces in the presence of OPE3. (b) Current-voltage ($I\text{−}V$) characteristics measured during the breaking event shown in (a). The $I\text{−}V$ curves have been shifted along the $y$ axis for clarity. (c) $I\text{−}V$’s measured at four different positions close to the switching point. The $I\text{−}V\mathrm{s}$ are offset on the $y$ axis for clarity. (d) Differential conductance numerically calculated from two $I\text{−}V\mathrm{s}$ in (c). Note that the conductance vs bias curves are not offset, emphasizing the finite conductance of $7\times {10}^{-5}$ $G_0$ just before the switching.

Figure 4

Histograms of the level alignment (a) and of the electronic coupling (b) for the strongly (top) and weakly (bottom) coupled configuration of junction 2.