How Mechanically Controllable Break Junctions Trap Molecules

Mechanically controllable break junctions (MCBJs) are precision nanoscale devices used in molecular electronics to trap individual organic molecules between two metallic electrodes. This article provides an overview of how MCBJs utilize mechanical force to fracture a microscopic gold wire, create an adjustable sub-nanometer gap, and capture single molecules using chemical anchoring groups. By routing electrical current through these trapped molecules, researchers can directly measure and analyze quantum conductance at the single-molecule level.

The foundation of an MCBJ device consists of a flexible substrate coated with an insulating layer, upon which a thin gold wire with a microscopic constriction, or notch, is fabricated. Gold is the standard material for the electrodes because it is chemically inert, resists oxidation, and is highly malleable at the atomic scale. The substrate is fixed at both ends inside a three-point bending mechanism, while a piezoelectric actuator is positioned directly beneath the center of the substrate to apply vertical mechanical force.

To create the electrodes, the piezoelectric actuator pushes upward against the bottom of the substrate, causing it to bend. As the substrate bends, the gold wire on the top surface is stretched until it thins at the notched constriction and eventually fractures. This break forms two pristine, atomically sharp gold electrodes facing each other. Because the mechanical motion of the actuator is translated into a much smaller horizontal displacement at the surface of the substrate, researchers can control the distance between the two gold tips with picometer precision, which is smaller than the diameter of a single atom.

Trapping an organic molecule requires introducing the target molecules into the gap via a liquid solution or vapor deposition. The organic molecules are specifically synthesized with chemical "alligator clips," known as anchoring groups, at both ends. Common anchoring groups include thiols, amines, and pyridines, which have a strong chemical affinity for gold. As the gold wire breaks, the freshly exposed gold atoms at the tips are highly reactive, allowing the anchoring groups of a nearby organic molecule to form stable covalent or coordinate bonds with both electrodes simultaneously, bridging the gap.

Investigating quantum conductance involves repeatedly breaking and reforming the gold junction thousands of times while continuously applying a small bias voltage. Before the gold wire completely snaps, the conductance drops in discrete steps, which correspond to multiples of the conductance quantum (\(G_0 = 2e^2/h\)). Once the last gold-to-gold atomic contact breaks, the conductance drops sharply. If a single organic molecule successfully bridges the newly formed gap, this drop is halted, and a distinct plateau appears in the conductance-versus-distance trace at a value significantly lower than \(G_0\).

Because the exact atomic configuration of the gold tips and the orientation of the molecule vary slightly with each break, a single measurement is not sufficient to determine the electronic properties of the molecule. Researchers compile thousands of individual conductance traces into a statistical histogram. The most probable conductance value for the trapped molecule appears as a prominent peak in the histogram. At this scale, electrons do not flow like a continuous fluid as they do in macroscopic wires; instead, they travel via coherent quantum mechanical tunneling through the discrete molecular orbitals of the organic bridge. By adjusting the mechanical gap, scientists can also stretch or compress the trapped molecule, observing how mechanical strain alters its quantum states and electrical conductivity in real time.