Charge conduction without paying for energy

12 April 2018. NUS chemists have found a way to manipulate the movement of charges in solid-state molecular junctions to reduce power consumption.

Understanding and controlling the flow of charges at the nanoscale are important to develop new ways of reducing the power consumption in molecular electronic devices. A molecular junction is made of molecules connecting two metallic electrodes together. For a charge to flow across this junction, it usually needs to overcome an activation energy barrier. This requires energy input which can be in the form of heat. The “Marcus theory” describes the science behind this and explains the effect of temperature on the current flow across molecular junctions. At higher temperatures, the charges can cross the energy barrier more easily.

Prof Christian NIJHUIS from the Department of Chemistry, NUS in collaboration with Prof Enrique del BARCO from University of Central Florida have shown experimentally that under certain conditions, charges can “hop” across molecular junctions without needing to cross the activation energy barrier. In this process, the flow of charges becomes independent of temperature changes and the charges flow as though there is no activation energy barrier. This unique operating condition, known as the “inverted Marcus” region, has not been demonstrated experimentally before. The research findings, which allow the bypassing of the activation energy barrier at molecular junctions, provide a potential new way to reduce power consumption in molecular devices.

Prof Nijhuis said, “Our team has shown experimentally the transition from the “Marcus” to the “inverted Marcus” region in a solid-state molecular tunnel junction using a molecular double quantum dot which we developed. This complex molecule can be visualised as two different units, which are coupled together. When a charge is placed on one location of the molecule, the energy levels of the other part of the molecule change, allowing charges to cross the molecular junction without an energy penalty.”

“Our experimental results also fit well with a theoretical model that combines two theories (Landauer and Marcus theories) which describe the mechanisms of charge transport across molecules. We found an approach where additional power to overcome activation barriers is not required for the charge transport across the molecular junction and this reduces power consumption. The research outcomes provide new insights that are important for the design of molecular junctions,” added Prof Nijhuis.

The figure shows an artist’s impression of molecular junctions where a monolayer of molecules connect the top and bottom electrodes together. Molecular junctions are important active elements for molecular electronics and it is important to understand the physical mechanisms controlling electron transport through them. 



L Yuan; L Wang; AR Garrigues; L Jiang; HV Annadata; M Anguera Antonana; E del Barco*, CA Nijhuis*, “Transition from Direct to Inverted Charge Transport Marcus Regions in Molecular Junctions via Molecular Orbital Gating” NATURE NANOTECHNOLOGY DOI: 10.1038/s41565-018-0068-4 Published: 2018.