image: This is a schematic representation of a typical conductance-distance trace in the opening process of the two electrodes. The total conductance (G, red solid line) of a single-molecule junction is composed of the through-space tunneling (Gs, transparent black line) and through-molecule tunneling (Gm, transparent red line) contributions. After breaking down of the junction, a gold-molecule-solution-gold channel (Gc, transparent green line) appears. The three gray areas show regions in which the conductance cannot be measured.
Single-molecule break junction techniques offer unique insights into the charge transport at the molecular level. The conductance through single-molecule junction consists of the through-space tunneling and the through-molecule tunneling conductance. However, the existence of through-space tunneling, which is ubiquitous at the single-molecule level, makes the quantitative extraction of the intrinsic molecular signals challenging.
Although its powerful capability to contact individual molecules, single-molecule break junction techniques are intrinsically not capable of intervening the actual signatures of the junction, such as binding geometries, number of molecules in the junction, interaction between the junction and neighbor molecules, etc.
The widely accepted method to extract the conductance information of a single molecule is to obtain statistics in thousands of break junction processes. Typically, a conductance histogram is plotted to find the most probable conductance. However, to which extent should we trust the obtained conductance? Since the result may affected by many stochastic events, such as junction formation probability, early rupture of the molecular junction, etc., in the break junction process.
What's more, as the focus of the state-of-art single-molecule break junction measurements has gradually shifted from strong interaction to weak interaction systems (such as supramolecular junctions) and from static processes to more dynamic processes (such as diffusion and chemical reaction processes), can break junction techniques still offer quantitative information at single-molecule level in these systems?
Very recently, Professor Wenjing Hong's group in Xiamen University, working together with Prof. Jielou Liao's group in University of Science and Technology of China, explored in detail the quantitative characterization capability of break junction techniques in single-molecule systems through an analytical model (Figure 1). This model describes the conductance changes during the opening process of two gold electrodes and validates the capability of conductance and displacement analyses during the break junction experiments of OAE-type molecule junctions. Based on this model, they demonstrated that the break junction technique can be used to detect the conductance of the molecular system with weak interactions under low junction formation probabilities and early rupture of the formed junction before it reaches a fully-stretched configuration. Using the established simulation approach, they further proved that the break junction technique can offer a quantitative understanding of molecular assembly, diffusion and even reaction processes with complementary conductance and displacement analyses.