Quantum laws rule tiniest circuits

The integrated circuitry of microchips is getting ever smaller. New technologies even promise molecular circuits. But at this length-scale, the normal laws governing conductance have to make way for quantum laws. Ryan Chiechi and co-workers explored some of these quantum effects in molecular circuits in a paper published on 20 December in Nature Communications.

‘We’ve looked at how the specific geometry of molecules can affect the flow of electric charge over distances of single nanometers’, explains University of Groningen associate professor Ryan Chiechi. Usually, single molecule conductance studies are performed by placing one molecule between two electrodes. ‘But the geometry of these molecules is not fixed, which affects their properties.

Chiechi took another approach. He started with an electrode, on which a single layer of molecules can be formed by self-assembly. This way, the geometry of the molecules was fixed. By placing another electrode on top of the monolayer, he created a device on which millions of individual molecules conduct electricity in parallel. ‘We were also able to assess the geometry of the molecules in the monolayer using different experimental techniques and simulations.’

An important effect to be studied was electron tunneling, where an insulating barrier separates two electrodes. Under certain conditions, electrons can ‘tunnel’ through the barrier without changing energy. In the molecules Chiechi studied, this happens when, over their entire length, the so-called pi-orbital s are available.

Switches

Chiechi made devices with such a material and observed quantum mechanical effects. ‘Previous single molecule studies did not reveal the quantum effects of interest, but it turned out this was because in those studies, the molecule was bent.’ This caused a ‘mismatch’ in the pi-orbitals, preventing these quantum effects. The experiments were also done with a variant of his molecule in which a gap was introduced, which interfered with the quantum tunneling. ‘This way, we made what could be called a quantum insulator.’

The studies show how small changes in the geometry of a molecule can affect its characteristics as a quantum conductor, which is important because the self-assembly process fixes that geometry precisely. ‘The changes are minute, less than a tenth of a nanometer.’ The fact that such a small change has such a large influence on conduction raises the possibility that these molecules could be used as switches. ‘But before we can build these tiny electrical switches, we must really understand the quantum effects that can occur.’ Chiechi’s experiments with the self-assembled monolayers appear to be a good way to do just that.

Reference: Marco Carlotti, Andrii Kovalchuk, Tobias Wächter, Michael Zharnikov, and Ryan C. Chiechi: Conformation-driven Quantum Interference Effects Mediated by Through-space Conjugation in Tunneling Junctions Comprising Self-Assembled Monolayers. Nature Communications, online 20 December 2016, DOI 10.1038/NCOMMS13904