Voltage-Modulated van der Waals Interaction in Single-Molecule Junctions.

Understanding how molecular geometry affects the electronic properties of single-molecule junctions experimentally has been challenging. Typically, metal-molecule-metal junctions are measured using a break-junction method where electrode separation is mechanically evolving during measurement. Here, to probe the impact of the junction geometry on conductance, we apply a sinusoidal modulation to the molecular junction electrode position. Simultaneously, we probe the nonlinearity of the current-voltage characteristics of each junction through a modulation in the applied bias at a different frequency. In turn, we show that junctions formed with molecules that have different molecule-electrode interfaces exhibit statistically distinguishable Fourier-transformed conductances. In particular, we find a marked bias dependence for the modulation of junctions where transmission is mediated thorough the van der Waals (vdW) interaction. We attribute our findings to voltage-modulated vdW interactions at the single-molecule level.