In a molecular circuit, a molecule is connected to a metal electrode and the link between these two components is both physical and electrical. While researchers have been able to obtain detailed electrical data on such junctions, measurements that directly probe the physical part of the connection (in particular its strength) have proved much more difficult.

Now, Latha Venkataraman and colleagues have managed to simultaneously perform both electrical and physical strength measurements on a series of molecular junctions. "We have measured the maximum force that a junction can sustain under stress and showed that this force varies systematically with the chemical character of the molecule making up the circuit," Venkataraman of Columbia, told nanotechweb.org.

For example, Venkataraman says that a circuit containing the aromatic molecule pyridine, can sustain a maximum force that is 60% larger than a circuit containing benzenediamine. This is because the lone pair of electrons responsible for binding in pyridine is in the plane of the ring, whereas for benzenediamine a bond forms with electronic states that delocalize into the pi electron of the ring.

Breaking forces
The team employed an atomic force microscope (AFM) to form single molecule junctions between the gold-coated AFM cantilever tip and gold substrates on which a series of molecules had been sputtered. These molecules all had amine or pyridine linker groups that helped them bind to the gold substrate.

The researchers measured the current through the junction while they mechanically pulled the junction apart. The deflection of the cantilever, measured at the same time, monitors the mechanical force. To help, the team developed a new 2D histogram method to statistically evaluate thousands of such force measurements and calculated average breaking forces of between 0.8 nN for 4,4' pyridine to 0.5 nN in 1,4 diaminobenzene.

"To relate our experimental findings with theory, we used density functional theory-based calculations, where bond energy is studied for model structures as they are pulled apart," explained Mark Hybertsen, a theoretical physicist at the Center for Functional Nanomaterials at Brookhaven. The calculations confirm that the gold–nitrogen bond is the weak link that breaks and they provide a quantitative guide to the maximum force that the gold–nitrogen link bond can sustain.

Twice as much information
The new results also provide information on chemical bonding in molecular junctions and the role of stress in bond breaking, say the researchers. Because such junctions cannot be imaged, having probes that can measure both mechanical and electrical properties provides twice as much information than was ever available before.

The team is now investigating the role of thermal fluctuations in bond rupture. "We are also trying to address how electrical properties correlate with bond rupture measurements," revealed Venkataraman.

The work was detailed in Nano Letters.