In typical electronic devices, temperature is the primary physical variable that controls conductivity. Resistance tends to increase with temperature. However, things are different on the nanoscale. Even at room temperature, the energy difference between quantum levels within a molecule can be much larger than the thermal energy. This means it is possible, in principle, to manipulate the wave function of electrons in a way that tunes the conductive properties of a material on the molecular level.

In a newly published experiment, Constant M. Guédon et al. managed to promote destructive quantum interference between electrons in a single molecule, reducing the molecule's ability to conduct current in the process. They compared the conductive properties of molecules that have an identical primary structure, but have differences in their electronic quantum states. In a molecule where the electrons interfered destructively, it suppressing the flow of electric current. This experiment opens up the possibility of room-temperature molecular devices based on quantum interference.

The researchers' procedure involved depositing five different but chemically-related molecules onto a gold substrate. The molecules, being long chains, create a brush-like layer on top of the gold, with each molecule acting as a wire. An atomic-force microscope (AFM) coated in gold acts as a second electrode. The current flows between the substrate and the AFM through the molecules.

The conductive properties of the molecules depend on whether they are linearly-conjugated or cross-conjugated. Linearly-conjugated means electron orbitals offer only one path for transport across the molecule. In contrast, cross-conjugated molecules effectively offer two paths of different lengths. This latter type exhibits destructive quantum interference. Because the paths are of different lengths, the electron wavefunctions overlap. The effect is to throttle electron flow across the molecule, reducing conductivity.

In the system used by Guédon et al., linearly-conjugated molecules and cross-conjugated molecules differ only in the presence or absence of a small additional structure (an anthraquinone unit, for the chemistry buffs in the audience). The linearly-conjugated molecules exhibited conductivity approximately 100 times larger than their cross-conjugated cousins, even though the energy levels are effectively identical between the two types. In other words, the difference in electron transport isn't from the orbital structure, but from quantum interference.

While many previous nanoscale experiments based on quantum interference require very cold temperatures, this result was obtained at room temperature. Thus, Guédon et al. have shown in principle that molecular electronic devices based on quantum interference may be operated under ordinary conditions, with large changes in conductivity due simply to small chemical differences.

Nature Nanotechnology, 2012. DOI: 10.1038/nnano.2012.37 (About DOIs).